Journal Articles |
2020 |
Dumlu, Seda Nilgün; Ademoğlu, Ahmet; Sun, Wei Investigation of functional variability and connectivity in temporal lobe epilepsy: A resting state fMRI study (Journal Article) Neuroscience Letters, 733 , pp. 135076, 2020, ISSN: 0304-3940. @article{DUMLU2020135076, title = {Investigation of functional variability and connectivity in temporal lobe epilepsy: A resting state fMRI study}, author = {Seda Nilgün Dumlu and Ahmet Ademoğlu and Wei Sun}, url = {https://www.sciencedirect.com/science/article/pii/S0304394020303463}, doi = {https://doi.org/10.1016/j.neulet.2020.135076}, issn = {0304-3940}, year = {2020}, date = {2020-01-01}, journal = {Neuroscience Letters}, volume = {733}, pages = {135076}, abstract = {It is crucial to reveal the variability between patients with epilepsy and healthy subjects to elucidate the underpinnings of the disease pathology. Herein, we assessed the inter-subject variability between patients with temporal lobe epilepsy (TLE) and healthy subjects in terms of estimating the functional connectivity using resting-state functional magnetic resonance (rs-fMRI) scans. According to inter-subject variability results between healthy and TLE population, the latter showed more variability mainly in frontoparietal control, default mode, dorsal/ventral attention, visual and somatomotor networks in line with the broad seizure onset and propagation pathway. As a result of 17-Network parcellation, a significant attenuation is observed in functional connectivity, mostly in bilateral frontoparietal control, somatomotor, default mode and ventral attention networks associated with the functional impairment in attention, long/short term memory, executive functioning. The results are in favor of the argument that the functional disruption in TLE spreads throughout the cortex beyond the temporal lobe with an implication of greater diversity in the TLE population.}, keywords = {}, pubstate = {published}, tppubtype = {article} } It is crucial to reveal the variability between patients with epilepsy and healthy subjects to elucidate the underpinnings of the disease pathology. Herein, we assessed the inter-subject variability between patients with temporal lobe epilepsy (TLE) and healthy subjects in terms of estimating the functional connectivity using resting-state functional magnetic resonance (rs-fMRI) scans. According to inter-subject variability results between healthy and TLE population, the latter showed more variability mainly in frontoparietal control, default mode, dorsal/ventral attention, visual and somatomotor networks in line with the broad seizure onset and propagation pathway. As a result of 17-Network parcellation, a significant attenuation is observed in functional connectivity, mostly in bilateral frontoparietal control, somatomotor, default mode and ventral attention networks associated with the functional impairment in attention, long/short term memory, executive functioning. The results are in favor of the argument that the functional disruption in TLE spreads throughout the cortex beyond the temporal lobe with an implication of greater diversity in the TLE population. |
Assem, Moataz; Blank, Idan; Mineroff, Zachary; Ademoğlu, Ahmet; Fedorenko, Evelina Activity in the fronto-parietal multiple-demand network is robustly associated with individual differences in working memory and fluid intelligence (Journal Article) Cortex, 131 , pp. 1-16, 2020, ISSN: 0010-9452. @article{ASSEM20201, title = {Activity in the fronto-parietal multiple-demand network is robustly associated with individual differences in working memory and fluid intelligence}, author = {Moataz Assem and Idan A. Blank and Zachary Mineroff and Ahmet Ademoğlu and Evelina Fedorenko}, url = {https://www.sciencedirect.com/science/article/pii/S0010945220302720}, doi = {https://doi.org/10.1016/j.cortex.2020.06.013}, issn = {0010-9452}, year = {2020}, date = {2020-01-01}, journal = {Cortex}, volume = {131}, pages = {1-16}, abstract = {Numerous brain lesion and fMRI studies have linked individual differences in executive abilities and fluid intelligence to brain regions of the fronto-parietal “multiple-demand” (MD) network. Yet, fMRI studies have yielded conflicting evidence as to whether better executive abilities are associated with stronger or weaker MD activations and whether this relationship is restricted to the MD network. Here, in a large-sample (n = 216) fMRI investigation, we found that stronger activity in MD regions – functionally defined in individual participants – was robustly associated with more accurate and faster responses on a spatial working memory task performed in the scanner, as well as fluid intelligence measured independently (n = 114). In line with some prior claims about a relationship between language and fluid intelligence, we also found a weak association between activity in the brain regions of the left fronto-temporal language network during an independent passive reading task, and performance on the working memory task. However, controlling for the level of MD activity abolished this relationship, whereas the MD activity-behavior association remained highly reliable after controlling for the level of activity in the language network. Finally, we demonstrate how unreliable MD activity measures, coupled with small sample sizes, could falsely lead to the opposite, negative, association that has been reported in some prior studies. Taken together, these results demonstrate that a core component of individual differences variance in executive abilities and fluid intelligence is selectively and robustly positively associated with the level of activity in the MD network, a result that aligns well with lesion studies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Numerous brain lesion and fMRI studies have linked individual differences in executive abilities and fluid intelligence to brain regions of the fronto-parietal “multiple-demand” (MD) network. Yet, fMRI studies have yielded conflicting evidence as to whether better executive abilities are associated with stronger or weaker MD activations and whether this relationship is restricted to the MD network. Here, in a large-sample (n = 216) fMRI investigation, we found that stronger activity in MD regions – functionally defined in individual participants – was robustly associated with more accurate and faster responses on a spatial working memory task performed in the scanner, as well as fluid intelligence measured independently (n = 114). In line with some prior claims about a relationship between language and fluid intelligence, we also found a weak association between activity in the brain regions of the left fronto-temporal language network during an independent passive reading task, and performance on the working memory task. However, controlling for the level of MD activity abolished this relationship, whereas the MD activity-behavior association remained highly reliable after controlling for the level of activity in the language network. Finally, we demonstrate how unreliable MD activity measures, coupled with small sample sizes, could falsely lead to the opposite, negative, association that has been reported in some prior studies. Taken together, these results demonstrate that a core component of individual differences variance in executive abilities and fluid intelligence is selectively and robustly positively associated with the level of activity in the MD network, a result that aligns well with lesion studies. |
2016 |
Bayram, Ali; Karahan, Esin; Bilgiç, Başar; Ademoglu, Ahmet; Demiralp, Tamer Achromatic temporal-frequency responses of human lateral geniculate nucleus and primary visual cortex (Journal Article) Vision Research, 127 , pp. 177 - 185, 2016, ISSN: 0042-6989. @article{Bayram2016177, title = {Achromatic temporal-frequency responses of human lateral geniculate nucleus and primary visual cortex}, author = {Ali Bayram and Esin Karahan and Başar Bilgiç and Ahmet Ademoglu and Tamer Demiralp}, url = {http://www.sciencedirect.com/science/article/pii/S004269891630102X}, doi = {http://dx.doi.org/10.1016/j.visres.2016.09.001}, issn = {0042-6989}, year = {2016}, date = {2016-01-01}, journal = {Vision Research}, volume = {127}, pages = {177 - 185}, abstract = {Abstract The sensitivity of the sensory systems to temporal changes of the environment constitutes one of the critical issues in perception. In the present study, we investigated the human early visual system’s dependency on the temporal frequency of visual input using fMRI. Blood oxygen level-dependent (BOLD) responses of the lateral geniculate nucleus (LGN) and primary visual cortex (V1) were investigated in a wide frequency range (6–46 Hz) with fine frequency sampling (13 frequencies). Subject-specific functional-anatomic ROIs were derived from the combination of the anatomic template masks and the functional maps derived from multi-session fMRI analyses across all 13 stimulation conditions. Using functional-anatomic ROIs, average responses of LGN and V1 were calculated for each frequency. The V1 surface area was further parsed into 7 eccentricity sectors to detail central and peripheral responses. LGN’s response revealed fluctuations on a background of non-significant decrease of the BOLD response with increasing stimulation frequency, while V1 response displayed similar fluctuations with a global maximum in the range of 8–12 Hz, but a rapid and significant decrease with increasing stimulation frequency especially above 14 Hz. This behavior of V1 response valid for both central and peripheral vision emphasizes that the profound low-pass effect of the visual system to visual input emerges in V1, presumably generated by the intra-cortical circuitry of V1 or projections from extra-striate areas. Besides, the high correlation between LGN and V1 BOLD responses across all visual stimulation frequencies supports the oscillatory tuning in thalamo-cortical interactions as previously claimed in electrophysiological studies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Abstract The sensitivity of the sensory systems to temporal changes of the environment constitutes one of the critical issues in perception. In the present study, we investigated the human early visual system’s dependency on the temporal frequency of visual input using fMRI. Blood oxygen level-dependent (BOLD) responses of the lateral geniculate nucleus (LGN) and primary visual cortex (V1) were investigated in a wide frequency range (6–46 Hz) with fine frequency sampling (13 frequencies). Subject-specific functional-anatomic ROIs were derived from the combination of the anatomic template masks and the functional maps derived from multi-session fMRI analyses across all 13 stimulation conditions. Using functional-anatomic ROIs, average responses of LGN and V1 were calculated for each frequency. The V1 surface area was further parsed into 7 eccentricity sectors to detail central and peripheral responses. LGN’s response revealed fluctuations on a background of non-significant decrease of the BOLD response with increasing stimulation frequency, while V1 response displayed similar fluctuations with a global maximum in the range of 8–12 Hz, but a rapid and significant decrease with increasing stimulation frequency especially above 14 Hz. This behavior of V1 response valid for both central and peripheral vision emphasizes that the profound low-pass effect of the visual system to visual input emerges in V1, presumably generated by the intra-cortical circuitry of V1 or projections from extra-striate areas. Besides, the high correlation between LGN and V1 BOLD responses across all visual stimulation frequencies supports the oscillatory tuning in thalamo-cortical interactions as previously claimed in electrophysiological studies. |
2015 |
Karahan, Esin; Rojas-Lo´pez, Pedro; Bringas-Vega, Maria; Valde´s-Herna´ndez, Pedro; Valdes-Sosa, Pedro Tensor Analysis and Fusion of Multimodal Brain Images (Journal Article) Proceedings of the IEEE, 103 (9), pp. 1531-1559, 2015, ISBN: 1558-2256. @article{Karahan2015, title = {Tensor Analysis and Fusion of Multimodal Brain Images}, author = {Esin Karahan and Pedro A. Rojas-Lo´pez and Maria L. Bringas-Vega and Pedro A. Valde´s-Herna´ndez and Pedro A. Valdes-Sosa}, url = {http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7214360&isnumber=7214335}, doi = {10.1109/JPROC.2015.2455028}, isbn = {1558-2256}, year = {2015}, date = {2015-09-01}, journal = {Proceedings of the IEEE}, volume = {103}, number = {9}, pages = {1531-1559}, abstract = {Current high-throughput data acquisition technologies probe dynamical systems with different imaging modalities, generating massive data sets at different spatial and temporal resolutions-posing challenging problems in multimodal data fusion. A case in point is the attempt to parse out the brain structures and networks that underpin human cognitive processes by analysis of different neuroimaging modalities (functional MRI, EEG, NIRS, etc.). We emphasize that the multimodal, multiscale nature of neuroimaging data is well reflected by a multiway (tensor) structure where the underlying processes can be summarized by a relatively small number of components or “atoms.” We introduce Markov-Penrose diagrams-an integration of Bayesian DAG and tensor network notation in order to analyze these models. These diagrams not only clarify matrix and tensor EEG and fMRI time/frequency analysis and inverse problems, but also help understand multimodal fusion via multiway partial least squares and coupled matrix-tensor factorization. We show here, for the first time, that Granger causal analysis of brain networks is a tensor regression problem, thus allowing the atomic decomposition of brain networks. Analysis of EEG and fMRI recordings shows the potential of the methods and suggests their use in other scientific domains.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Current high-throughput data acquisition technologies probe dynamical systems with different imaging modalities, generating massive data sets at different spatial and temporal resolutions-posing challenging problems in multimodal data fusion. A case in point is the attempt to parse out the brain structures and networks that underpin human cognitive processes by analysis of different neuroimaging modalities (functional MRI, EEG, NIRS, etc.). We emphasize that the multimodal, multiscale nature of neuroimaging data is well reflected by a multiway (tensor) structure where the underlying processes can be summarized by a relatively small number of components or “atoms.” We introduce Markov-Penrose diagrams-an integration of Bayesian DAG and tensor network notation in order to analyze these models. These diagrams not only clarify matrix and tensor EEG and fMRI time/frequency analysis and inverse problems, but also help understand multimodal fusion via multiway partial least squares and coupled matrix-tensor factorization. We show here, for the first time, that Granger causal analysis of brain networks is a tensor regression problem, thus allowing the atomic decomposition of brain networks. Analysis of EEG and fMRI recordings shows the potential of the methods and suggests their use in other scientific domains. |
2014 |
Parlak, Ismail Burak; Egi, Salih Murat; Ademoglu, Ahmet; Germonpré, Peter; Esen, Ozlem Batukan; Marroni, Alessandro; Balestra, Costantino Bubble Stream Reveals Functionality of the Right-to-Left Shunt: Detection of a Potential Source for Air Embolism (Journal Article) Ultrasound in Medicine & Biology, 40 (2), pp. 330 - 340, 2014, ISSN: 0301-5629. @article{Parlak2014330, title = {Bubble Stream Reveals Functionality of the Right-to-Left Shunt: Detection of a Potential Source for Air Embolism}, author = {Ismail Burak Parlak and Salih Murat Egi and Ahmet Ademoglu and Peter Germonpré and Ozlem Batukan Esen and Alessandro Marroni and Costantino Balestra}, url = {http://www.sciencedirect.com/science/article/pii/S0301562913010533}, doi = {http://dx.doi.org/10.1016/j.ultrasmedbio.2013.09.019}, issn = {0301-5629}, year = {2014}, date = {2014-01-01}, journal = {Ultrasound in Medicine & Biology}, volume = {40}, number = {2}, pages = {330 - 340}, abstract = {Abstract The existence of a right-to-left shunt may increase the likelihood of micro-embolism by allowing a flux of bubbles under hyperbaric conditions. The aim of the study was to measure the relationship between these shunts and bubbles in 10 consecutive subjects using trans-thoracic and trans-esophageal echocardiography. In video frames, all cardiac chambers were segmented and bubbles were analyzed by our proposed method and two other methods. The relationship with bubbles and shunts was divided into three classes: no bubbles, 1–20 bubbles, >20 bubbles and measured over 2160 frames. Our sensitivity was 100% and our specificity was between 90.1% and 96.4%. There were 4.32–23.78 bubbles/frame in the left atrium according to our method. After the automatic analysis, shunts were graded double-blinded by two cardiologists. Consequently, we noted that aperture size does not necessarily reflect how active the right-to-left shunt is. Instead, our proposed decay curves constitute a better tool for determining functionality.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Abstract The existence of a right-to-left shunt may increase the likelihood of micro-embolism by allowing a flux of bubbles under hyperbaric conditions. The aim of the study was to measure the relationship between these shunts and bubbles in 10 consecutive subjects using trans-thoracic and trans-esophageal echocardiography. In video frames, all cardiac chambers were segmented and bubbles were analyzed by our proposed method and two other methods. The relationship with bubbles and shunts was divided into three classes: no bubbles, 1–20 bubbles, >20 bubbles and measured over 2160 frames. Our sensitivity was 100% and our specificity was between 90.1% and 96.4%. There were 4.32–23.78 bubbles/frame in the left atrium according to our method. After the automatic analysis, shunts were graded double-blinded by two cardiologists. Consequently, we noted that aperture size does not necessarily reflect how active the right-to-left shunt is. Instead, our proposed decay curves constitute a better tool for determining functionality. |
2013 |
Karahan, Esin; Duru, Adil Deniz; Valdes-Sosa, Pedro; Ademoğlu, Ahmet EEG-fMRI fusion on the cortical surface using Coupled Tensor-Matrix Factorization: A simulation study (Journal Article) Frontiers in Neuroinformatics, 7 (88), 2013, ISSN: 1662-5196. @article{Karahan2013, title = {EEG-fMRI fusion on the cortical surface using Coupled Tensor-Matrix Factorization: A simulation study}, author = {Esin Karahan and Adil Deniz Duru and Pedro A. Valdes-Sosa and Ahmet Ademoğlu}, url = {http://www.frontiersin.org/neuroinformatics/10.3389/conf.fninf.2013.09.00088/full}, doi = {10.3389/conf.fninf.2013.09.00088}, issn = {1662-5196}, year = {2013}, date = {2013-11-03}, journal = {Frontiers in Neuroinformatics}, volume = {7}, number = {88}, abstract = {Although it is a challenging problem in neuroimaging to merge the data coming from different modalities on a common spatial domain, the integration of EEG and fMRI offers us an opportunity to reveal the complex dynamics of brain functions and neuronal interactions (Bayram et al., 2011). In this study, a novel EEG-fMRI fusion approach based on multilinear methods is developed and applied to simulated data. We treat EEG data as a three-way array with temporal, spectral and spatial dimensions. We perform coupled tensor-matrix factorization (CTMF) to obtain common spatial signatures between fMRI and spectral EEG data (Acar et al., 2011). The EEG inverse problem is also incorporated into the merging process to obtain a common image on the cortical surface. We improve the spatial signature estimation by using the alternating least squares algorithm in a hierarchical manner where the noise and factor covariance are also exploited. Unlike conventional CTMF algorithms where a single dimension is considered to be fully coupled between two datasets, we project part of the datasets on a common and part on discriminative subspaces (Liu et al., 2013). This enables us to deal with the cases in which EEG and fMRI sources differ. By this way, our proposed algorithm is able to show both coupled and uncoupled responses at the same time. In the simulation, we generated EEG/fMRI data from one deep and two superficial sources. Deep source is common to both modalities and each of the superficial sources is specific to one modality. Results of the simulation show that CTMF algorithm with inverse problem successfully localizes the sources. (Maximum values of the fMRI spatial components overlap with the real sources; common spatial EEG component overlaps with the real source and the distance between the maximum value of the uncommon EEG spatial component with the real one is 11 mm.) Also, algorithm correctly identifies common and distinct sources (See the figure). Brain activity reflected in different domains is integrated on the same spatial scale by this approach. Our current research focuses on applying the method on real data. Figure 1: Three different source locations are chosen on the cortical surface. Deep source is located in middle temporal gyrus and superficial source specific to fMRI is on the right superior-frontal cortex and superficial source specific to EEG is on the left superior-frontal cortex. fMRI temporal pattern is constituted by convolving hemodynamic function with a boxcar. EEG source signal is generated from sinusoids oscillating at 4 and 12 Hz. Channel EEG is obtained by projecting source signal onto the sensor space with lead field matrix. First two rows shows the fMRI spatial and temporal components found from CTMF algorithm. Last two rows are the EEG components. First column is the spatial signature of the EEG in the source space, second column is temporal signature, third is the spectral signature and final column is the spatial signature of the EEG in the sensor space.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Although it is a challenging problem in neuroimaging to merge the data coming from different modalities on a common spatial domain, the integration of EEG and fMRI offers us an opportunity to reveal the complex dynamics of brain functions and neuronal interactions (Bayram et al., 2011). In this study, a novel EEG-fMRI fusion approach based on multilinear methods is developed and applied to simulated data. We treat EEG data as a three-way array with temporal, spectral and spatial dimensions. We perform coupled tensor-matrix factorization (CTMF) to obtain common spatial signatures between fMRI and spectral EEG data (Acar et al., 2011). The EEG inverse problem is also incorporated into the merging process to obtain a common image on the cortical surface. We improve the spatial signature estimation by using the alternating least squares algorithm in a hierarchical manner where the noise and factor covariance are also exploited. Unlike conventional CTMF algorithms where a single dimension is considered to be fully coupled between two datasets, we project part of the datasets on a common and part on discriminative subspaces (Liu et al., 2013). This enables us to deal with the cases in which EEG and fMRI sources differ. By this way, our proposed algorithm is able to show both coupled and uncoupled responses at the same time. In the simulation, we generated EEG/fMRI data from one deep and two superficial sources. Deep source is common to both modalities and each of the superficial sources is specific to one modality. Results of the simulation show that CTMF algorithm with inverse problem successfully localizes the sources. (Maximum values of the fMRI spatial components overlap with the real sources; common spatial EEG component overlaps with the real source and the distance between the maximum value of the uncommon EEG spatial component with the real one is 11 mm.) Also, algorithm correctly identifies common and distinct sources (See the figure). Brain activity reflected in different domains is integrated on the same spatial scale by this approach. Our current research focuses on applying the method on real data. Figure 1: Three different source locations are chosen on the cortical surface. Deep source is located in middle temporal gyrus and superficial source specific to fMRI is on the right superior-frontal cortex and superficial source specific to EEG is on the left superior-frontal cortex. fMRI temporal pattern is constituted by convolving hemodynamic function with a boxcar. EEG source signal is generated from sinusoids oscillating at 4 and 12 Hz. Channel EEG is obtained by projecting source signal onto the sensor space with lead field matrix. First two rows shows the fMRI spatial and temporal components found from CTMF algorithm. Last two rows are the EEG components. First column is the spatial signature of the EEG in the source space, second column is temporal signature, third is the spectral signature and final column is the spatial signature of the EEG in the sensor space. |
Aydin, Cagatay; Oktay, Oytun; Gunebakan, Adem Umut; Ademoglu, Ahmet; Ciftci, Rifat Koray Role of Alpha Oscillations During Short Time Memory Task Investigated by Graph Based Partitioning (Journal Article) RADIOENGINEERING, 22 (1, 1, SI), pp. 123-131, 2013, ISSN: 1210-2512. @article{ISI:000318052400014, title = {Role of Alpha Oscillations During Short Time Memory Task Investigated by Graph Based Partitioning}, author = {Aydin, Cagatay and Oktay, Oytun and Gunebakan, Adem Umut and Ademoglu, Ahmet and Ciftci, Rifat Koray}, issn = {1210-2512}, year = {2013}, date = {2013-04-01}, journal = {RADIOENGINEERING}, volume = {22}, number = {1, 1, SI}, pages = {123-131}, abstract = {In this study, we investigate the clustering pattern of alpha band (8 Hz - 12 Hz) electroencephalogram (EEG) oscillations obtained from healthy individuals during a short time memory task with 3 different memory loads. The retention period during which subjects were asked to memorize a pattern in a square matrix is analyzed with a graph theoretical approach. The functional coupling among EEG electrodes are quantified via mutual information in the time-frequency plane. A spectral clustering algorithm followed by bootstrapping is used to parcellate memory related circuits and for identifying significant clusters in the brain. The main outcome of the study is that the size of the significant clusters formed by alpha oscillations decreases as the memory load increases. This finding corroborates the active inhibition hypothesis about alpha oscillations.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this study, we investigate the clustering pattern of alpha band (8 Hz - 12 Hz) electroencephalogram (EEG) oscillations obtained from healthy individuals during a short time memory task with 3 different memory loads. The retention period during which subjects were asked to memorize a pattern in a square matrix is analyzed with a graph theoretical approach. The functional coupling among EEG electrodes are quantified via mutual information in the time-frequency plane. A spectral clustering algorithm followed by bootstrapping is used to parcellate memory related circuits and for identifying significant clusters in the brain. The main outcome of the study is that the size of the significant clusters formed by alpha oscillations decreases as the memory load increases. This finding corroborates the active inhibition hypothesis about alpha oscillations. |
2011 |
Demiralp, Tamer; Bayram, Ali; Karahan, Esin; Bilgiç, Basar; Tahran, Nevzat; Ademoğlu, Ahmet Frequency response characteristics of lateral geniculate nucleus and primary visual cortex (Journal Article) Frontiers in Human Neuroscience, (80), 2011, ISBN: 1662-5161. @article{Demiralp2011, title = {Frequency response characteristics of lateral geniculate nucleus and primary visual cortex }, author = {Tamer Demiralp and Ali Bayram and Esin Karahan and Basar Bilgiç and Nevzat Tahran and Ahmet Ademoğlu}, url = {http://www.frontiersin.org/10.3389/conf.fnhum.2011.207.00080/event_abstract}, doi = {10.3389/conf.fnhum.2011.207.00080}, isbn = {1662-5161}, year = {2011}, date = {2011-11-23}, booktitle = {Front. Hum. Neurosci. Conf. Abs.: 11th Int. Conf. on Cogn. Neuroscience, 2011}, journal = {Frontiers in Human Neuroscience}, number = {80}, abstract = {Studies measuring fMRI BOLD signal have shown that responses of the primary visual area (V1) do not change uniformly or monotonously with the flickering frequencies. Besides, there are evidences that changes in frequency characteristics begin to emerge at the level of the lateral geniculate nucleus (LGN). We therefore set out to investigate BOLD response of the human V1 and LGN with flickering light at systematically varying frequencies between 6 and 46 Hz. The blood oxygenation level-dependent (BOLD) responses to 6, 8, 10, 12, 14, 18, 22, 26, 30, 34, 38, 42 and 46 Hz flicker visual stimulations in the primary visual cortex and lateral geniculate nuclei were investigated in 35 healthy volunteers. fMRI analysis was performed with SPM5 software. Single subject multi-session fixed-effects analysis results of each subject were thresholded at a significance level of p < .05 (uncorrected) and after masking with template LGN and V1 masks, they were binarized. Active LGN and V1 masks specific to subject were used to extract parameter estimates ('beta' images) of each frequency. Normalized average 'beta'-value changes with stimulation frequency for LGN and V1 were calculated and compared. A highly significant correlation between LGN and V1 responses was observed across the stimulation frequencies (p < .0001), reflecting a strong match of the local maxima probably due to the preferred thalamo-cortical interaction around specific frequencies. However, BOLD amplitudes of V1 showed a stronger decline with increasing frequencies, while they were relatively spared in the LGN. This suggests that the thalamus relays a steady visual input at a high frequency with limited output to the cortex. Funding: This study is supported by TUBITAK project #108S101.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Studies measuring fMRI BOLD signal have shown that responses of the primary visual area (V1) do not change uniformly or monotonously with the flickering frequencies. Besides, there are evidences that changes in frequency characteristics begin to emerge at the level of the lateral geniculate nucleus (LGN). We therefore set out to investigate BOLD response of the human V1 and LGN with flickering light at systematically varying frequencies between 6 and 46 Hz. The blood oxygenation level-dependent (BOLD) responses to 6, 8, 10, 12, 14, 18, 22, 26, 30, 34, 38, 42 and 46 Hz flicker visual stimulations in the primary visual cortex and lateral geniculate nuclei were investigated in 35 healthy volunteers. fMRI analysis was performed with SPM5 software. Single subject multi-session fixed-effects analysis results of each subject were thresholded at a significance level of p < .05 (uncorrected) and after masking with template LGN and V1 masks, they were binarized. Active LGN and V1 masks specific to subject were used to extract parameter estimates ('beta' images) of each frequency. Normalized average 'beta'-value changes with stimulation frequency for LGN and V1 were calculated and compared. A highly significant correlation between LGN and V1 responses was observed across the stimulation frequencies (p < .0001), reflecting a strong match of the local maxima probably due to the preferred thalamo-cortical interaction around specific frequencies. However, BOLD amplitudes of V1 showed a stronger decline with increasing frequencies, while they were relatively spared in the LGN. This suggests that the thalamus relays a steady visual input at a high frequency with limited output to the cortex. Funding: This study is supported by TUBITAK project #108S101. |
Kaşıkçı, Itır; Bayram, Ali; Karahan, Esin; Bilgiç, Basar; Ademoğlu, Ahmet; Demiralp, Tamer Hemodynamic correlates of brain electrical oscillations related with working memory (Journal Article) Frontiers in Human Neuroscience, 96 , 2011, ISSN: 1662-5161. @article{Kaşıkçı2011, title = { Hemodynamic correlates of brain electrical oscillations related with working memory}, author = {Itır Kaşıkçı and Ali Bayram and Esin Karahan and Basar Bilgiç and Ahmet Ademoğlu and Tamer Demiralp}, editor = {Ahmet Ademoğlu }, url = {http://www.frontiersin.org/human_neuroscience/10.3389/conf.fnhum.2011.207.00096/full}, doi = {10.3389/conf.fnhum.2011.207.00096}, issn = {1662-5161}, year = {2011}, date = {2011-11-09}, booktitle = {Front. Hum. Neurosci. Conf. Abs.: 11th Int. Conf. on Cogn. Neuroscience, 2011}, journal = {Frontiers in Human Neuroscience}, volume = {96}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Bayram, Ali; Ademoğlu, Ahmet; Karahan, Esin; Bilgiç, Basar; Duru, Adil Deniz; Tahran, Nevzat; Demiralp, Tamer Visual Stimulation Frequency Dependent Changes in BOLD Transients (Journal Article) Front. Hum. Neurosci. Conf. Abs.: 11th Int. Conf. on Cogn. Neuroscience, 2011, 81 , 2011, ISSN: 1662-5161. @article{Bayram2011, title = {Visual Stimulation Frequency Dependent Changes in BOLD Transients}, author = {Ali Bayram and Ahmet Ademoğlu and Esin Karahan and Basar Bilgiç and Adil Deniz Duru and Nevzat Tahran and Tamer Demiralp}, doi = {10.3389/conf.fnhum.2011.207.00081}, issn = {1662-5161}, year = {2011}, date = {2011-11-04}, booktitle = {Frontiers in Human Neuroscience}, journal = {Front. Hum. Neurosci. Conf. Abs.: 11th Int. Conf. on Cogn. Neuroscience, 2011}, volume = {81}, abstract = {Blood oxygenation level dependent (BOLD) signal change is used to map neuronal activity in the brain. In most fMRI experiments, time course of each image voxel is correlated with the stimulus pattern convolved with hypothetical waveform; hemodynamic response function (HRF) which plays an important role in active voxel detection. We investigated experimentally the frequency response characteristics of two BOLD transients, the initial overshoot and the positive BOLD (pBOLD), with flickering light at varying frequencies to test whether a single hemodynamic response function may hold for visual input at different frequencies. The blood oxygenation level-dependent (BOLD) responses to 6, 8, 10, 12, 14, 18, 22, 26, 30, 34, 38, 42 and 46 Hz flicker visual stimulations in the primary visual cortex were investigated in 22 healthy volunteers. Software tools within FSL were used in fMRI data processing. Single subject, single session analysis results were thresholded at a significance level of p < .05 (corrected) and masked with MNI (Montreal Neurological Institute) occipital lobe mask. The percent change of the BOLD response of most significantly active 150 voxels was averaged. Initial overshoot (6-18 s after stimulus onset) and positive BOLD (18-45 s after stimulus onset) were quantified as the average value of the BOLD response in the corresponding intervals. With increasing visual input frequency the pBOLD response declined much stronger compared with the initial overshoot leading to a significant difference between both transients for the frequencies higher than 14 Hz (p < .05). This result suggests that both parts of the BOLD signal correspond to different neuronal activities and that they should be modeled separately for a better description of the process. Funding: This study is supported by TUBITAK project #108S101.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Blood oxygenation level dependent (BOLD) signal change is used to map neuronal activity in the brain. In most fMRI experiments, time course of each image voxel is correlated with the stimulus pattern convolved with hypothetical waveform; hemodynamic response function (HRF) which plays an important role in active voxel detection. We investigated experimentally the frequency response characteristics of two BOLD transients, the initial overshoot and the positive BOLD (pBOLD), with flickering light at varying frequencies to test whether a single hemodynamic response function may hold for visual input at different frequencies. The blood oxygenation level-dependent (BOLD) responses to 6, 8, 10, 12, 14, 18, 22, 26, 30, 34, 38, 42 and 46 Hz flicker visual stimulations in the primary visual cortex were investigated in 22 healthy volunteers. Software tools within FSL were used in fMRI data processing. Single subject, single session analysis results were thresholded at a significance level of p < .05 (corrected) and masked with MNI (Montreal Neurological Institute) occipital lobe mask. The percent change of the BOLD response of most significantly active 150 voxels was averaged. Initial overshoot (6-18 s after stimulus onset) and positive BOLD (18-45 s after stimulus onset) were quantified as the average value of the BOLD response in the corresponding intervals. With increasing visual input frequency the pBOLD response declined much stronger compared with the initial overshoot leading to a significant difference between both transients for the frequencies higher than 14 Hz (p < .05). This result suggests that both parts of the BOLD signal correspond to different neuronal activities and that they should be modeled separately for a better description of the process. Funding: This study is supported by TUBITAK project #108S101. |
Bayram, Ali; Bayraktaroğlu, Zübeyir; Karahan, Esin; Erdoğan, Basri; Bilgic, Basar; Özker, Müge; Kaşıkçı, Itır; Duru, Deniz; Ademoğlu, Ahmet; Öztürk, Cengizhan; Arikan, Mehmet Kemal; Tahran, Nevzat; Demiralp, Tamer Simultaneous EEG/fMRI analysis of the resonance phenomena in steady-state visual evoked responses (Journal Article) Clinical EEG and neuroscience: official journal of the EEG and Clinical Neuroscience Society, 42 (2), pp. 98-106, 2011. @article{A2011, title = {Simultaneous EEG/fMRI analysis of the resonance phenomena in steady-state visual evoked responses}, author = {Ali Bayram and Zübeyir Bayraktaroğlu and Esin Karahan and Basri Erdoğan and Basar Bilgic and Müge Özker and Itır Kaşıkçı and Deniz Duru and Ahmet Ademoğlu and Cengizhan Öztürk and Mehmet Kemal Arikan and Nevzat Tahran and Tamer Demiralp}, url = {https://www.ncbi.nlm.nih.gov/pubmed/21675599}, doi = {10.1177/155005941104200210}, year = {2011}, date = {2011-04-05}, journal = {Clinical EEG and neuroscience: official journal of the EEG and Clinical Neuroscience Society}, volume = {42}, number = {2}, pages = {98-106}, abstract = {The stability of the steady-state visual evoked potentials (SSVEPs) across trials and subjects makes them a suitable tool for the investigation of the visual system. The reproducible pattern of the frequency characteristics of SSVEPs shows a global amplitude maximum around 10 Hz and additional local maxima around 20 and 40 Hz, which have been argued to represent resonant behavior of damped neuronal oscillators. Simultaneous electroencephalogram/functional magnetic resonance imaging (EEG/fMRI) measurement allows testing of the resonance hypothesis about the frequency-selective increases in SSVEP amplitudes in human subjects, because the total synaptic activity that is represented in the fMRI-Blood Oxygen Level Dependent (fMRI-BOLD) response would not increase but get synchronized at the resonance frequency. For this purpose, 40 healthy volunteers were visually stimulated with flickering light at systematically varying frequencies between 6 and 46 Hz, and the correlations between SSVEP amplitudes and the BOLD responses were computed. The SSVEP frequency characteristics of all subjects showed 3 frequency ranges with an amplitude maximum in each of them, which roughly correspond to alpha, beta and gamma bands of the EEG. The correlation maps between BOLD responses and SSVEP amplitude changes across the different stimulation frequencies within each frequency band showed no significant correlation in the alpha range, while significant correlations were obtained in the primary visual area for the beta and gamma bands. This non-linear relationship between the surface recorded SSVEP amplitudes and the BOLD responses of the visual cortex at stimulation frequencies around the alpha band supports the view that a resonance at the tuning frequency of the thalamo-cortical alpha oscillator in the visual system is responsible for the global amplitude maximum of the SSVEP around 10 Hz. Information gained from the SSVEP/fMRI analyses in the present study might be extrapolated to the EEG/fMRI analysis of the transient event-related potentials (ERPs) in terms of expecting more reliable and consistent correlations between EEG and fMRI responses, when the analyses are carried out on evoked or induced oscillations (spectral perturbations) in separate frequency bands instead of the time-domain ERP peaks.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The stability of the steady-state visual evoked potentials (SSVEPs) across trials and subjects makes them a suitable tool for the investigation of the visual system. The reproducible pattern of the frequency characteristics of SSVEPs shows a global amplitude maximum around 10 Hz and additional local maxima around 20 and 40 Hz, which have been argued to represent resonant behavior of damped neuronal oscillators. Simultaneous electroencephalogram/functional magnetic resonance imaging (EEG/fMRI) measurement allows testing of the resonance hypothesis about the frequency-selective increases in SSVEP amplitudes in human subjects, because the total synaptic activity that is represented in the fMRI-Blood Oxygen Level Dependent (fMRI-BOLD) response would not increase but get synchronized at the resonance frequency. For this purpose, 40 healthy volunteers were visually stimulated with flickering light at systematically varying frequencies between 6 and 46 Hz, and the correlations between SSVEP amplitudes and the BOLD responses were computed. The SSVEP frequency characteristics of all subjects showed 3 frequency ranges with an amplitude maximum in each of them, which roughly correspond to alpha, beta and gamma bands of the EEG. The correlation maps between BOLD responses and SSVEP amplitude changes across the different stimulation frequencies within each frequency band showed no significant correlation in the alpha range, while significant correlations were obtained in the primary visual area for the beta and gamma bands. This non-linear relationship between the surface recorded SSVEP amplitudes and the BOLD responses of the visual cortex at stimulation frequencies around the alpha band supports the view that a resonance at the tuning frequency of the thalamo-cortical alpha oscillator in the visual system is responsible for the global amplitude maximum of the SSVEP around 10 Hz. Information gained from the SSVEP/fMRI analyses in the present study might be extrapolated to the EEG/fMRI analysis of the transient event-related potentials (ERPs) in terms of expecting more reliable and consistent correlations between EEG and fMRI responses, when the analyses are carried out on evoked or induced oscillations (spectral perturbations) in separate frequency bands instead of the time-domain ERP peaks. |
Duru, Deniz; Kaşıkçı, Itır; Kurt, Elif; Ademoglu, Ahmet; Demiralp, Tamer Temporal correlation of fMRI and EEG with asymmetric fusion (Journal Article) Frontiers in Human Neuroscience, (97), 2011, ISSN: 1662-5161. @article{Duru2011, title = {Temporal correlation of fMRI and EEG with asymmetric fusion}, author = {Deniz Duru and Itır Kaşıkçı and Elif Kurt and Ahmet Ademoglu and Tamer Demiralp}, url = {http://www.frontiersin.org/human_neuroscience/10.3389/conf.fnhum.2011.207.00097/full}, doi = {10.3389/conf.fnhum.2011.207.00097}, issn = {1662-5161}, year = {2011}, date = {2011-01-05}, journal = {Frontiers in Human Neuroscience}, number = {97}, abstract = {The relationship between the hemodynamic response and the electro-physiological activity of the brain is investigated non-invasively using fMRI and EEG measurements. Simultaneous recordings of these two modalities allow us to analyze the spatiotemporal correlation between the electrophysiological activities and the BOLD signal. We perform an asymmetric EEG-fMRI fusion on a voxel level by using the electrical and hemodynamic steady state visual responses of the brain. A bayesian source localization algorithm is used to project the sensor space EEG data into the source space in which the fMRI BOLD is directly measured. The EEG and fMRI data are spatio-temporally co-registered to obtain statistical parameter maps that show their temporal correlation on a spatial cortical manifold. Flashing light stimuli covering the alpha range at 6, 8, 10 and 12 Hz are presented to healthy subjects in a boxcar design consisting of 3 consecutive rest and stimulation periods and the EEG and fMRI data are simultaneously recorded. The statistical maps of correlations of hemodynamic responses with the source space EEG are determined. In general, a high positive correlation is obtained between the BOLD signal and the EEG power at the stimulation frequency and its harmonics, while at the same time the alpha band power of the EEG showed a significant negative correlation with the BOLD signal. This illustrates that even in a narrow frequency band of the EEG, the relationship between the EEG and fMRI responses contains multiple contrasting components that should be taken into consideration for a reliable EEG-fMRI fusion.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The relationship between the hemodynamic response and the electro-physiological activity of the brain is investigated non-invasively using fMRI and EEG measurements. Simultaneous recordings of these two modalities allow us to analyze the spatiotemporal correlation between the electrophysiological activities and the BOLD signal. We perform an asymmetric EEG-fMRI fusion on a voxel level by using the electrical and hemodynamic steady state visual responses of the brain. A bayesian source localization algorithm is used to project the sensor space EEG data into the source space in which the fMRI BOLD is directly measured. The EEG and fMRI data are spatio-temporally co-registered to obtain statistical parameter maps that show their temporal correlation on a spatial cortical manifold. Flashing light stimuli covering the alpha range at 6, 8, 10 and 12 Hz are presented to healthy subjects in a boxcar design consisting of 3 consecutive rest and stimulation periods and the EEG and fMRI data are simultaneously recorded. The statistical maps of correlations of hemodynamic responses with the source space EEG are determined. In general, a high positive correlation is obtained between the BOLD signal and the EEG power at the stimulation frequency and its harmonics, while at the same time the alpha band power of the EEG showed a significant negative correlation with the BOLD signal. This illustrates that even in a narrow frequency band of the EEG, the relationship between the EEG and fMRI responses contains multiple contrasting components that should be taken into consideration for a reliable EEG-fMRI fusion. |
2009 |
Duru, Adil Deniz; Ademoglu, Ahmet; Demiralp, Tamer Analysis of brain electrical topography by spatio-temporal wavelet decomposition (Journal Article) Mathematical and Computer Modelling, 49 (11-12), pp. 2224- 2235, 2009, ISSN: 0895-7177. @article{Duru2009, title = {Analysis of brain electrical topography by spatio-temporal wavelet decomposition}, author = {Adil Deniz Duru and Ahmet Ademoglu and Tamer Demiralp}, editor = {Ahmet Ademoğlu }, url = {http://www.sciencedirect.com/science/article/pii/S0895717708002367}, doi = {dx.doi.org/10.1016/j.mcm.2008.07.017}, issn = {0895-7177}, year = {2009}, date = {2009-11-11}, journal = {Mathematical and Computer Modelling}, volume = {49}, number = {11-12}, pages = {2224- 2235}, abstract = {Currently, the Functional Magnetic Resonance Imaging (fMRI), Positron Emission Tomography (PET), and Electroencephalography (EEG) recordings are the major techniques of neuroimaging. The \{EEG\} with its highest temporal resolution is still a crucial measurement for localization of activities arising from the electrical behaviour of the brain. A scalp topographic map for an \{EEG\} may be a superposition of several simpler subtopographic maps, each resulting from an individual electrical source located at a certain depth. Furthermore, this source may have a temporal characteristic as an oscillation or a rhythm that extends in a certain time window which has been a basis of assumption for the time-frequency analysis methods. A method for the spatio-temporal wavelet decomposition of multichannel \{EEG\} data is proposed which facilitates the localization of electrical sources separate and/or overlapping on a continuum of time, frequency and space domains. The subtopographic maps asociated with each of these individual components are then used in the \{MUSIC\} source localization algorithm. The validations are performed on simulated \{EEG\} data. Spatio-temporal wavelet decomposition as a preprocessing method improves the source localization by simplifying the topographic data formed by the superposition of \{EEG\} generators, having possible combinations of temporal, frequency and/or spatial overlappings. Spatio-temporal analysis of \{EEG\} will help enhance the accuracy of dipole source reconstruction in neuroimaging.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Currently, the Functional Magnetic Resonance Imaging (fMRI), Positron Emission Tomography (PET), and Electroencephalography (EEG) recordings are the major techniques of neuroimaging. The {EEG} with its highest temporal resolution is still a crucial measurement for localization of activities arising from the electrical behaviour of the brain. A scalp topographic map for an {EEG} may be a superposition of several simpler subtopographic maps, each resulting from an individual electrical source located at a certain depth. Furthermore, this source may have a temporal characteristic as an oscillation or a rhythm that extends in a certain time window which has been a basis of assumption for the time-frequency analysis methods. A method for the spatio-temporal wavelet decomposition of multichannel {EEG} data is proposed which facilitates the localization of electrical sources separate and/or overlapping on a continuum of time, frequency and space domains. The subtopographic maps asociated with each of these individual components are then used in the {MUSIC} source localization algorithm. The validations are performed on simulated {EEG} data. Spatio-temporal wavelet decomposition as a preprocessing method improves the source localization by simplifying the topographic data formed by the superposition of {EEG} generators, having possible combinations of temporal, frequency and/or spatial overlappings. Spatio-temporal analysis of {EEG} will help enhance the accuracy of dipole source reconstruction in neuroimaging. |
Duru, Deniz; Ademoğlu, Ahmet Epileptic Source Localization: Deep Electrode Measurements versus Scalp EEG (Journal Article) International Journal of Bioelectromagnetism, 11 (4), pp. 175-178, 2009. @article{Duru2009, title = {Epileptic Source Localization: Deep Electrode Measurements versus Scalp EEG}, author = {Deniz Duru and Ahmet Ademoğlu}, url = {https://www.researchgate.net/profile/Adil_Duru/publication/255631628_Epileptic_Source_Localization_Deep_Electrode_Measurements_versus_Scalp_EEG/links/0deec53a32b23abde8000000.pdf}, year = {2009}, date = {2009-11-04}, journal = {International Journal of Bioelectromagnetism}, volume = {11}, number = {4}, pages = {175-178}, abstract = {Intracranial recordings and scalp EEG are used with the source imaging techniques to determine the locations and strengths of the epileptic activity. As a source localization method, Low Resolution Electro-Magnetic Tomography (LORETA) is solved for the realistic geometry based on both forward methods, the Boundary Element Method (BEM) and the Finite Difference Method (FDM). Average current source densities are computed using BEM based LORETA scalp EEG and FDM based LORETA deep EEG spikes. Only one of the several sources of the FDM based LORETA method matches with the source obtained using the BEM based LORETA. FDM based deep EEG localization is more sensitive to the sources inside the brain}, keywords = {}, pubstate = {published}, tppubtype = {article} } Intracranial recordings and scalp EEG are used with the source imaging techniques to determine the locations and strengths of the epileptic activity. As a source localization method, Low Resolution Electro-Magnetic Tomography (LORETA) is solved for the realistic geometry based on both forward methods, the Boundary Element Method (BEM) and the Finite Difference Method (FDM). Average current source densities are computed using BEM based LORETA scalp EEG and FDM based LORETA deep EEG spikes. Only one of the several sources of the FDM based LORETA method matches with the source obtained using the BEM based LORETA. FDM based deep EEG localization is more sensitive to the sources inside the brain |
2008 |
Emir, Uzay; Bayraktaroglu, Zubeyir; Ozturk, Cengizhan; Ademoglu, Ahmet; Demiralp, Tamer Changes in BOLD transients with visual stimuli across 1-44 Hz (Journal Article) Neuroscience Letters, 436 (2), pp. 185 - 188, 2008, ISSN: 0304-3940. @article{Emir2008, title = {Changes in BOLD transients with visual stimuli across 1-44 Hz}, author = {Uzay E. Emir and Zubeyir Bayraktaroglu and Cengizhan Ozturk and Ahmet Ademoglu and Tamer Demiralp}, url = {http://www.sciencedirect.com/science/article/pii/S0304394008003157}, doi = {10.1016/j.neulet.2008.03.021}, issn = {0304-3940}, year = {2008}, date = {2008-07-07}, journal = {Neuroscience Letters}, volume = {436}, number = {2}, pages = {185 - 188}, abstract = {The dependency of positive BOLD (PBOLD) and post-stimulus undershoot (PSU) on the temporal frequency of visual stimulation was investigated using stimulation frequencies between 1 and 44 Hz. The PBOLD peak at 8 Hz in primary visual cortex was in line with previous neuroimaging studies. In addition to the 8 Hz peak, secondary peaks were observed for stimulation frequencies at 16 and 24 Hz. These additional local peaks were contrary to earlier fMRI studies which reported either a decrease or a plateau for frequencies above 8 Hz but in line with electrophysiological results obtained in animal local field potential (LFP) measurements and human steady-state visual evoked potential (SSVEP) recordings. Our results also indicate that the dependency of PSU amplitude on stimulus frequency deviates from that of PBOLD. Although their amplitudes were correlated within the 1–13 Hz range, they changed independently at stimulation frequencies between 13 and 44 Hz. The different dependency profiles of PBOLD and PSU to stimulation frequency points to different underlying neurovascular mechanisms responsible for the generation of these BOLD transients with regard to their relation to inhibitory and excitatory neuronal activity.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The dependency of positive BOLD (PBOLD) and post-stimulus undershoot (PSU) on the temporal frequency of visual stimulation was investigated using stimulation frequencies between 1 and 44 Hz. The PBOLD peak at 8 Hz in primary visual cortex was in line with previous neuroimaging studies. In addition to the 8 Hz peak, secondary peaks were observed for stimulation frequencies at 16 and 24 Hz. These additional local peaks were contrary to earlier fMRI studies which reported either a decrease or a plateau for frequencies above 8 Hz but in line with electrophysiological results obtained in animal local field potential (LFP) measurements and human steady-state visual evoked potential (SSVEP) recordings. Our results also indicate that the dependency of PSU amplitude on stimulus frequency deviates from that of PBOLD. Although their amplitudes were correlated within the 1–13 Hz range, they changed independently at stimulation frequencies between 13 and 44 Hz. The different dependency profiles of PBOLD and PSU to stimulation frequency points to different underlying neurovascular mechanisms responsible for the generation of these BOLD transients with regard to their relation to inhibitory and excitatory neuronal activity. |
2006 |
Duru, Adil Deniz; Bayram, Ali; Demiralp, Tamer; Ademoğlu, Ahmet Source Localization of Subtopographic Brain Maps for Event Related Potentials (ERP) (Journal Article) Conf Proc IEEE Eng Med Biol Soc. 2006, 1 , pp. 4832-4, 2006, ISBN: 1557-170X. @article{D.2008, title = { Source Localization of Subtopographic Brain Maps for Event Related Potentials (ERP)}, author = {Adil Deniz Duru and Ali Bayram and Tamer Demiralp and Ahmet Ademoğlu}, url = {https://www.ncbi.nlm.nih.gov/pubmed/17946655}, doi = {10.1109/IEMBS.2006.260824}, isbn = {1557-170X}, year = {2006}, date = {2006-06-01}, journal = {Conf Proc IEEE Eng Med Biol Soc. 2006}, volume = {1}, pages = {4832-4}, abstract = {Localization of the cognitive activity in the brain is one of the major problems in neuroscience. Current techniques for neuro-imaging are based on functional Magnetic Resonance Imaging (fMRI), Positron Emission Tomography (PET), and Event Related Potential (ERP) recordings. The highest temporal resolution is achieved by ERP, which is crucial for temporal localization of activities. However, the spatial resolution of scalp topography for ERP is low. There is a severe limitation for the parametric inverse solution algorithms that they can only perform well for the temporally uncorrelated sources. In this study, a spatial decomposition method is proposed to separate the temporally correlated sources using their topographies prior to their localization.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Localization of the cognitive activity in the brain is one of the major problems in neuroscience. Current techniques for neuro-imaging are based on functional Magnetic Resonance Imaging (fMRI), Positron Emission Tomography (PET), and Event Related Potential (ERP) recordings. The highest temporal resolution is achieved by ERP, which is crucial for temporal localization of activities. However, the spatial resolution of scalp topography for ERP is low. There is a severe limitation for the parametric inverse solution algorithms that they can only perform well for the temporally uncorrelated sources. In this study, a spatial decomposition method is proposed to separate the temporally correlated sources using their topographies prior to their localization. |
Conferences |
2016 |
Assem, Moataz; Alpsan, Meltem Hale; Karahan, Esin; Bayram, Ali; Bilgiç, Basar; Gürvit, Hakan; Ademoglu, Ahmet; Demiralp., Tamer 127 (3), 2016, ISBN: 1388/2457. @conference{Assem2016, title = {ID 138 – Differentiation of attention network deficits in Mild Cognitive Impairment and Alzheimer’s disease by means of auditory oddball fMRI responses}, author = {Moataz Assem and Meltem Hale Alpsan and Esin Karahan and Ali Bayram and Basar Bilgiç and Hakan Gürvit and Ahmet Ademoglu and Tamer Demiralp. }, url = {http://www.sciencedirect.com/science/article/pii/S1388245715013437}, doi = {http://dx.doi.org/10.1016/j.clinph.2015.11.238}, isbn = {1388/2457}, year = {2016}, date = {2016-03-01}, issuetitle = {Neuroimaging}, journal = {Clinical Neurophysiology}, volume = {127}, number = {3}, pages = {e71}, abstract = {Objective To investigate the affection of widespread activations elicited by the auditory oddball paradigm in healthy elderly, Mild Cognitive Impairment (MCI) and Alzheimer’s disease (AD) in an event related fMRI study. Methods 5 controls, 6 MCI and 5 AD subjects participated. Paradigm consisted of standard (1000 Hz, 80%) and target (1500 Hz, 20%) stimuli (200 ms, mean ISI 2.2 s). BOLD images (TR = 2400 ms) and T1 anatomical scans were acquired with a 1.5 Tesla system. Standard preprocessing was applied using SPM8. General linear model design matrix included standard (single regressor), target stimuli (two regressors: correct and wrong responses), realignment parameters and time derivatives of hemodynamic function. Results Areas affected in MCI and AD relative to controls include dorsolateral and ventrolateral prefrontal, anterior cingulate, superior temporal, supramarginal, cuneus, fusiform and lingual gyri. Areas exclusively affected in AD include middle temporal, precuneus gyri, caudate, thalamic nuclei and posterior cingulate cortex. Conclusions Compared to MCI, AD shows progressive affection of attention related regions, especially those of the dorsal network. In addition, basal ganglia recruitment is only affected in AD. Key message fMRI responses to auditory oddball can serve as a potential imaging biomarker to differentiate between attention networks deficits in MCI and AD.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } Objective To investigate the affection of widespread activations elicited by the auditory oddball paradigm in healthy elderly, Mild Cognitive Impairment (MCI) and Alzheimer’s disease (AD) in an event related fMRI study. Methods 5 controls, 6 MCI and 5 AD subjects participated. Paradigm consisted of standard (1000 Hz, 80%) and target (1500 Hz, 20%) stimuli (200 ms, mean ISI 2.2 s). BOLD images (TR = 2400 ms) and T1 anatomical scans were acquired with a 1.5 Tesla system. Standard preprocessing was applied using SPM8. General linear model design matrix included standard (single regressor), target stimuli (two regressors: correct and wrong responses), realignment parameters and time derivatives of hemodynamic function. Results Areas affected in MCI and AD relative to controls include dorsolateral and ventrolateral prefrontal, anterior cingulate, superior temporal, supramarginal, cuneus, fusiform and lingual gyri. Areas exclusively affected in AD include middle temporal, precuneus gyri, caudate, thalamic nuclei and posterior cingulate cortex. Conclusions Compared to MCI, AD shows progressive affection of attention related regions, especially those of the dorsal network. In addition, basal ganglia recruitment is only affected in AD. Key message fMRI responses to auditory oddball can serve as a potential imaging biomarker to differentiate between attention networks deficits in MCI and AD. |
2014 |
Assem, Moataz; Alpsan, Meltem; Karahan, Esin; Bayram, Ali; Bilgiç, Başar; Gürvit, Hakan; Ademoglu, Ahmet; Demiralp, Tamer fMRI responses of Alzheimer's disease and mild cognitive impairment patients during target detection (Conference) 20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014 2014. @conference{Assem2014, title = {fMRI responses of Alzheimer's disease and mild cognitive impairment patients during target detection}, author = {Moataz Assem and Meltem Alpsan and Esin Karahan and Ali Bayram and Başar Bilgiç and Hakan Gürvit and Ahmet Ademoglu and Tamer Demiralp }, url = {https://www.researchgate.net/publication/260676060_fMRI_Responses_of_Alzheimer's_Disease_and_Mild_Cognitive_Impairment_Patients_during_Target_Detection}, doi = {DOI: 10.13140/RG.2.1.1377.8641}, year = {2014}, date = {2014-11-01}, organization = {20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014}, abstract = {We investigated differences in brain activation between healthy, mild cognitive impairment (MCI) and Alzheimer's disease (AD) groups in an event related fMRI auditory oddball task. This is the first study to investigate AD patients in a fMRI auditory oddball paradigm. AD, compared to MCI, show similar but stronger patterns of decreased activation relative to the healthy group especially for caudate nuclei, posterior cingulate gyrus and left superior temporal gyrus.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } We investigated differences in brain activation between healthy, mild cognitive impairment (MCI) and Alzheimer's disease (AD) groups in an event related fMRI auditory oddball task. This is the first study to investigate AD patients in a fMRI auditory oddball paradigm. AD, compared to MCI, show similar but stronger patterns of decreased activation relative to the healthy group especially for caudate nuclei, posterior cingulate gyrus and left superior temporal gyrus. |
Bayram, Ali; Karahan, Esin; Bilgiç, Başar; Ademoğlu, Ahmet; Demiralp, Tamer Temporal frequency responses of human lateral geniculate nucleus and primary visual cortex in fMRI (Conference) 20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014 2014. @conference{Bayram2014, title = {Temporal frequency responses of human lateral geniculate nucleus and primary visual cortex in fMRI}, author = {Ali Bayram and Esin Karahan and Başar Bilgiç and Ahmet Ademoğlu and Tamer Demiralp }, url = {https://www.researchgate.net/publication/266156287_Temporal_frequency_responses_of_human_lateral_geniculate_nucleus_and_primary_visual_cortex_in_fMRI}, year = {2014}, date = {2014-08-27}, organization = {20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014 }, abstract = {The thalamo-cortical loop in the early visual system plays important roles in visual perception and attention. In this sense, investigating temporal frequency characteristics of primary visual cortex (V1) and lateral geniculate nucleus (LGN) could help to reveal their intrinsic frequency tuning properties. The presented work is based on the BOLD fMRI responses of LGN and V1 during a wide range of visual stimulation frequencies (6-46 Hz). LGN and V1 showed highly correlated BOLD responses with similar local frequency selectivities. Besides, V1 responses decreased dramatically above 22 Hz, while LGN responses decreased by a much smaller amounth with increasing frequencies.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } The thalamo-cortical loop in the early visual system plays important roles in visual perception and attention. In this sense, investigating temporal frequency characteristics of primary visual cortex (V1) and lateral geniculate nucleus (LGN) could help to reveal their intrinsic frequency tuning properties. The presented work is based on the BOLD fMRI responses of LGN and V1 during a wide range of visual stimulation frequencies (6-46 Hz). LGN and V1 showed highly correlated BOLD responses with similar local frequency selectivities. Besides, V1 responses decreased dramatically above 22 Hz, while LGN responses decreased by a much smaller amounth with increasing frequencies. |
2011 |
Sevgi, Meltem; Karahan, Esin; Bayram, Ali; Duru, Deniz; Öztürk, Cengizhan; Ademoğlu, Ahmet; Demiralp, Tamer A Group Study on BOLD Change to the Steady State Visual Stimuli With Bayesian Inference (Conference) Human Brain Mapping Conf., Quebec City, Canada, 2011 2011. (BibTeX) @conference{Sevgi2011, title = {A Group Study on BOLD Change to the Steady State Visual Stimuli With Bayesian Inference}, author = {Meltem Sevgi and Esin Karahan and Ali Bayram and Deniz Duru and Cengizhan Öztürk and Ahmet Ademoğlu and Tamer Demiralp}, editor = {Ahmet Ademoğlu }, year = {2011}, date = {2011-11-22}, organization = {Human Brain Mapping Conf., Quebec City, Canada, 2011}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Karahan, Esin; Özker, Müge; Bayram, Ali; Bayraktaroğlu, Zübeyir; Erdoğan, Basri; Kaşıkçı, Itır; Öztürk, Cengizhan; Ademoğlu, Ahmet; Demiralp, Tamer Simultaneous EEG/fMRI Analysis of Steady-State Visual Evoked Responses (Conference) Human Brain Mapping 2011, 2011. @conference{Karahan2011, title = {Simultaneous EEG/fMRI Analysis of Steady-State Visual Evoked Responses}, author = {Esin Karahan and Müge Özker and Ali Bayram and Zübeyir Bayraktaroğlu and Basri Erdoğan and Itır Kaşıkçı and Cengizhan Öztürk and Ahmet Ademoğlu and Tamer Demiralp}, editor = {Ahmet Ademoğlu}, url = {https://www.researchgate.net/publication/272733975_Simultaneous_EEG-fMRI_Analysis_of_Steady-State_Visual_Evoked_Responses}, year = {2011}, date = {2011-11-16}, booktitle = {Human Brain Mapping 2011}, abstract = {EEG-fMRI fusion allows the investigation of the BOLD signal and the underlying neural activity. The correlation between the synchronization patterns of EEG and BOLD is an important aspect of EEG-fMRI integration. Steady-state evoked potentials (SSVEPs) enable the generation of synchronized EEG patterns that stay stationary along the time duration of the BOLD response and are observed as oscillations at the stimulation frequency and its harmonics. Suspending the neural activity during the time course of the vascular response allows the comparison of various aspects of neurovascular coupling. The aim of this study is to investigate the behavior of BOLD and EEG responses to the synchronization of neuronal activations.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } EEG-fMRI fusion allows the investigation of the BOLD signal and the underlying neural activity. The correlation between the synchronization patterns of EEG and BOLD is an important aspect of EEG-fMRI integration. Steady-state evoked potentials (SSVEPs) enable the generation of synchronized EEG patterns that stay stationary along the time duration of the BOLD response and are observed as oscillations at the stimulation frequency and its harmonics. Suspending the neural activity during the time course of the vascular response allows the comparison of various aspects of neurovascular coupling. The aim of this study is to investigate the behavior of BOLD and EEG responses to the synchronization of neuronal activations. |
2010 |
Duru A.D., Akdeniz Kara Ozkara Uzan Ademoglu Demiralp Epileptic source localizations based on EEG and SDE measurements (Conference) 2010 15th National Biomedical Engineering Meeting, BIYOMUT, 2010. (BibTeX) @conference{Duru2010, title = {Epileptic source localizations based on EEG and SDE measurements}, author = {Duru, A.D., Akdeniz, G., Kara, E., Ozkara, C., Uzan, M., Ademoglu, A., Demiralp, T.}, editor = {Ahmet Ademoğlu }, year = {2010}, date = {2010-11-11}, booktitle = {2010 15th National Biomedical Engineering Meeting, BIYOMUT}, journal = {2010 15th National Biomedical Engineering Meeting, BIYOMUT}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
AD Duru G Akdeniz, Özkara Uzan Ademoğlu Demiralp Comparison of Epileptic Source Localizations Based on EEG and SDE Measurements (Conference) 16th Annual Meeting of the Organization for Human Brain Mapping, Barcelona, 2010, 2010. (BibTeX) @conference{Duru2010b, title = { Comparison of Epileptic Source Localizations Based on EEG and SDE Measurements }, author = {AD Duru, G Akdeniz, Ç Özkara, M Uzan, A Ademoğlu, T Demiralp}, editor = {Ahmet Ademoğlu }, year = {2010}, date = {2010-11-03}, booktitle = { 16th Annual Meeting of the Organization for Human Brain Mapping, Barcelona, 2010}, journal = {16th Annual Meeting of the Organization for Human Brain Mapping, Barcelona, 2010}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
2009 |
E. Karahan, Öztürk fMRG İmgelerinin Çok Değişkenli Sınıflandırılması (Conference) BİYOMUT, İzmir, 2009, 2009. (BibTeX) @conference{Karahan2009, title = {fMRG İmgelerinin Çok Değişkenli Sınıflandırılması}, author = {E. Karahan, C. Öztürk}, editor = {Cengizhan Öztürk}, year = {2009}, date = {2009-07-08}, booktitle = {BİYOMUT, İzmir, 2009}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
2008 |
Duru A. D., Ademoglu Demiralp Radial Basis Function Decomposition of Topographic EEG Maps (Conference) 10th International Conference on Cognitive Neuroscience (ICON X), September 1st-5th, 2008, Bodrum, Turkey. Absract Book, p. 446., 2008. (BibTeX) @conference{Duru2008, title = { Radial Basis Function Decomposition of Topographic EEG Maps}, author = {Duru, A. D., Ademoglu A. Demiralp T. }, editor = {Ahmet Ademoğlu}, year = {2008}, date = {2008-09-04}, booktitle = {10th International Conference on Cognitive Neuroscience (ICON X), September 1st-5th, 2008, Bodrum, Turkey. Absract Book, p. 446.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Bayram A. Bilgiç B., Arslan Baran Tanridag Demiralp Crossmodal interaction with the auditory modality affects the BOLD response to visual stimuli (Conference) 10th International Conference on Cognitive Neuroscience (ICON X), Bodrum, Turkey, (2008), p. 333., 2008. (BibTeX) @conference{A.2008, title = {Crossmodal interaction with the auditory modality affects the BOLD response to visual stimuli}, author = {Bayram A., Bilgiç B., Arslan A.B., Baran B., Tanridag O., Demiralp T., }, editor = {Tamer Demiralp }, year = {2008}, date = {2008-09-03}, booktitle = {10th International Conference on Cognitive Neuroscience (ICON X), Bodrum, Turkey, (2008), p. 333.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Yildiz G. Duru A. D., Ademoglu Demiralp Combined ICA-Bayesian MCMC Reconstruction of EEG Sources (Conference) 10th International Conference on Cognitive Neuroscience (ICON X), September 1st-5th, 2008, Bodrum, Turkey. Abstract Book, p. 132., 2008. (BibTeX) @conference{G.2008, title = {Combined ICA-Bayesian MCMC Reconstruction of EEG Sources}, author = {Yildiz G., Duru A. D., Ademoglu A., Demiralp T., , }, editor = {Ahmet Ademoğlu }, year = {2008}, date = {2008-09-01}, booktitle = {10th International Conference on Cognitive Neuroscience (ICON X), September 1st-5th, 2008, Bodrum, Turkey. Abstract Book, p. 132.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Emir U.E. Bayraktaroglu Z., Öztürk Ademoglu Demiralp Dependency of BOLD transients on the frequency of flickering visual stimuli (Conference) 10th International Conference on Cognitive Neuroscience (ICON X), Bodrum, Turkey. (2008), p. 324, 2008. (BibTeX) @conference{U.E.2008, title = {Dependency of BOLD transients on the frequency of flickering visual stimuli}, author = {Emir U.E., Bayraktaroglu Z., Öztürk C., Ademoglu A., Demiralp T., , }, editor = {Ahmet Ademoğlu }, year = {2008}, date = {2008-06-09}, booktitle = {10th International Conference on Cognitive Neuroscience (ICON X), Bodrum, Turkey. (2008), p. 324}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
2007 |
G Yıldız AD Duru, Ademoğlu EEG Dipol Kaynaklarının MZMC Yöntemleri ile Yerelleştirilmesi (Conference) BİYOMUT, İstanbul, 2007., 2007. (BibTeX) @conference{Yıldız2007, title = { EEG Dipol Kaynaklarının MZMC Yöntemleri ile Yerelleştirilmesi}, author = {G Yıldız, AD Duru, A Ademoğlu, }, editor = {Ahmet Ademoğlu }, year = {2007}, date = {2007-11-06}, booktitle = {BİYOMUT, İstanbul, 2007.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Ademoğlu A., Duru Cairo Spatiotemporal Analysis Of Event Related Brain Potentials (Conference) Euro-Mediterranean Conference on Biomathematics, 2007. (BibTeX) @conference{Ademoğlu2007, title = {Spatiotemporal Analysis Of Event Related Brain Potentials}, author = {Ademoğlu, A., Duru A.D.,, Cairo, 2007. }, editor = {Ahmet Ademoğlu}, year = {2007}, date = {2007-11-05}, booktitle = {Euro-Mediterranean Conference on Biomathematics}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Duru A. D. Ademoglu A, Demiralp Yıldız Bayesian EEG Dipole Source Localization Using Simulated Annealing-Reversible Jump Markov Chain Monte Carlo Method on Realistic Head Model (Conference) Proceedings of the 29th Annual International Conference of the IEEE EMBS Cité Internationale, Lyon, France, August 23-26, 2007, pp 4268-4272., 2007. (BibTeX) @conference{D.2007, title = {Bayesian EEG Dipole Source Localization Using Simulated Annealing-Reversible Jump Markov Chain Monte Carlo Method on Realistic Head Model}, author = {Duru A. D., Ademoglu A, Demiralp T., Yıldız G,}, editor = {Ahmet Ademoğlu }, year = {2007}, date = {2007-08-25}, booktitle = {Proceedings of the 29th Annual International Conference of the IEEE EMBS Cité Internationale, Lyon, France, August 23-26, 2007, pp 4268-4272.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
H. Eryilmaz AD Duru, Parlak Ademoğlu; Demiralp, Neuroimaging Of Event Related Brain Potentials Using fMRI § Dipole Source Reconstruction (Conference) Proceedings of the 29th Annual International Conference of the IEEE EMBS, France, p. 3384-3387, 2007., 2007. (BibTeX) @conference{Eryilmaz2007, title = {Neuroimaging Of Event Related Brain Potentials Using fMRI § Dipole Source Reconstruction}, author = {H. Eryilmaz, AD Duru, B Parlak, A Ademoğlu and T Demiralp }, editor = {Ahmet Ademoğlu}, year = {2007}, date = {2007-04-05}, booktitle = {Proceedings of the 29th Annual International Conference of the IEEE EMBS, France, p. 3384-3387, 2007.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
A.D. Duru H. Eryılmaz, Emir Bayraktaroğlu Demiralp Ademoğlu Implementation of Low Resolution Electro-Magnetic Tomography With fMRI Statistical Maps On Realistic Head Models (Conference) Proceedings of the 29th Annual International Conference of the IEEE EMBS, France, 5239-5242, 2007., 2007. (BibTeX) @conference{Duru2007, title = {Implementation of Low Resolution Electro-Magnetic Tomography With fMRI Statistical Maps On Realistic Head Models}, author = {A.D. Duru, H. Eryılmaz, U. Emir, Z. Bayraktaroğlu, T. Demiralp, A. Ademoğlu, }, editor = {Ahmet Ademoğlu }, year = {2007}, date = {2007-04-04}, booktitle = {Proceedings of the 29th Annual International Conference of the IEEE EMBS, France, 5239-5242, 2007.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
G Yıldız AD Duru, Ademoğlu; T Demiralp, Bayesian EEG Dipole Source Localization using SA-RJMCMC on Realistic Head Model (Conference) Proceedings of the 29th Annual Int. Conf. of the IEEE EMBS, France, p. 4268-4272., 2007. (BibTeX) @conference{Yıldız2007b, title = {Bayesian EEG Dipole Source Localization using SA-RJMCMC on Realistic Head Model}, author = {G Yıldız, AD Duru, A Ademoğlu and T Demiralp, }, editor = {Ahmet Ademoğlu }, year = {2007}, date = {2007-04-04}, booktitle = {Proceedings of the 29th Annual Int. Conf. of the IEEE EMBS, France, p. 4268-4272.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
2006 |
AD Duru Eryılmaz H., Bayram Demiralp Ademoğlu Istanbul Subtopographic EEG Source Localization After Spatio-Temporal Wavelet Decomposition (Conference) 13th World Congress of Psychophysiology, Istanbul 2006., 2006. (BibTeX) @conference{Duru2006b, title = {Subtopographic EEG Source Localization After Spatio-Temporal Wavelet Decomposition}, author = {AD Duru, Eryılmaz H., Bayram A., Demiralp T., Ademoğlu A.,, Istanbul 2006. }, editor = {Ahmet Ademoğlu }, year = {2006}, date = {2006-06-01}, booktitle = {13th World Congress of Psychophysiology, Istanbul 2006.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
ve A.D. Duru A. Bayram, Demiralp Ademoğlu Olaya İlişkin Potansiyellere Ait Beyin Alt Harita Bileşenlerinin Kaynaklarının Yerelleştirilmesi (Conference) BİYOMUT, İstanbul, 2006. , 2006. (BibTeX) @conference{Duru2006, title = {Olaya İlişkin Potansiyellere Ait Beyin Alt Harita Bileşenlerinin Kaynaklarının Yerelleştirilmesi}, author = {A.D. Duru, A. Bayram, T. Demiralp ve A. Ademoğlu}, editor = {Ahmet Ademoğlu }, year = {2006}, date = {2006-04-05}, booktitle = {BİYOMUT, İstanbul, 2006. }, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
ve H. Eryilmaz, Duru Ademoğlu Olaya İlişkin Süreçlerin iMRG ve EEG Kaynak Yerelleştirimi ile Görüntülenmesi (Conference) BİYOMUT, İstanbul, 2006., 2006. (BibTeX) @conference{Eryilmaz2006, title = {Olaya İlişkin Süreçlerin iMRG ve EEG Kaynak Yerelleştirimi ile Görüntülenmesi}, author = {H. Eryilmaz, A.D. Duru ve A. Ademoğlu}, editor = {Ahmet Ademoğlu}, year = {2006}, date = {2006-04-04}, booktitle = {BİYOMUT, İstanbul, 2006.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
2005 |
U. Emir A.D. Duru, Ademoğlu Ata Akın Coregistration of fNIRS Data on to the Realistic Head Model (Conference) Proceedings of the IEEE Engineering in Medicine and Biology 27th Annual Conference , 2005. (BibTeX) @conference{Emir2005, title = {Coregistration of fNIRS Data on to the Realistic Head Model}, author = {U. Emir, A.D. Duru, A. Ademoğlu, Ata Akın }, editor = {Ahmet Ademoğlu}, year = {2005}, date = {2005-05-12}, booktitle = {Proceedings of the IEEE Engineering in Medicine and Biology 27th Annual Conference }, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
A. Ademoğlu A. D. Duru, Demiralp Havana Cuba Source Localization Of Topographic EEG Maps Prefiltered By Spatio-Temporal Wavelets (Conference) IX. International Conference on Cognitive Neuroscience, Havana Cuba, 2005., 2005. (BibTeX) @conference{Ademoğlu2005, title = {Source Localization Of Topographic EEG Maps Prefiltered By Spatio-Temporal Wavelets }, author = {A. Ademoğlu, A. D. Duru, T. Demiralp,, Havana Cuba, 2005. }, editor = {Ahmet Ademoğlu}, year = {2005}, date = {2005-04-04}, booktitle = {IX. International Conference on Cognitive Neuroscience, Havana Cuba, 2005.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
A. Ademoğlu A. D. Duru, Bayram Demiralp Source Localization Of Topographic EEG Maps Prefiltered by Spatio-Temporal Wavelets (Conference) Brain Dynamics and Cognition Conference, İzmir, 2005., 2005. (BibTeX) @conference{Ademoğlu2005b, title = {Source Localization Of Topographic EEG Maps Prefiltered by Spatio-Temporal Wavelets}, author = {A. Ademoğlu, A. D. Duru, A. Bayram, T. Demiralp }, editor = {Ahmet Ademoğlu}, year = {2005}, date = {2005-04-04}, booktitle = {Brain Dynamics and Cognition Conference, İzmir, 2005.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Emir U. Duru A.D., Ademoğlu Akın İşlevsel Yakın Kızıl Ötesi Spektroskop Verilerinin Gerçek Kafa Modelinin Üzerine Çakıştırılması (Conference) BİYOMUT, İstanbul, 2005., 2005. (BibTeX) @conference{U.2005, title = {İşlevsel Yakın Kızıl Ötesi Spektroskop Verilerinin Gerçek Kafa Modelinin Üzerine Çakıştırılması}, author = {Emir U., Duru A.D., Ademoğlu A., Akın A., }, editor = {Ahmet Ademoğlu }, year = {2005}, date = {2005-04-01}, booktitle = {BİYOMUT, İstanbul, 2005.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
2004 |
Ademoğlu A. Duru D., Demiralp Istefanopulos; B., Baykan Source Localization Of Electrical Dipoles Using Wavelet Prefiltering § MUSIC Scanning (Conference) Evoked Potentials International Conference XIV 3th National Neuroscience Congress, Leipzig, 2004. (BibTeX) @conference{Ademoğlu2004b, title = {Source Localization Of Electrical Dipoles Using Wavelet Prefiltering § MUSIC Scanning}, author = {Ademoğlu A., Duru D., Demiralp T., Istefanopulos Y., and Baykan B. }, editor = {Ahmet Ademoğlu }, year = {2004}, date = {2004-11-02}, booktitle = {Evoked Potentials International Conference XIV 3th National Neuroscience Congress, Leipzig}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
A. Ademoğlu A.D. Duru, Ergen Istefanopulos Baykan Demiralp EEG Ve Olaya İlişkin Potansiyellerde Gerçekçi Kafa Modeli İle Kaynak Yerelleştirimi (Conference) III. Ulusal Sinir Bilimleri Kongresi, Denizli, 2004. (BibTeX) @conference{Ademoğlu2005b, title = {EEG Ve Olaya İlişkin Potansiyellerde Gerçekçi Kafa Modeli İle Kaynak Yerelleştirimi}, author = {A. Ademoğlu, A.D. Duru, M. Ergen, Y. Istefanopulos, B. Baykan, T. Demiralp }, editor = {Ahmet Ademoğlu }, year = {2004}, date = {2004-11-01}, booktitle = {III. Ulusal Sinir Bilimleri Kongresi, Denizli}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Ademoğlu, 3D Localization Of The Dipoles Of Event Related Potential Components (Conference) Modelling Mental Processes and Disorders, Kuşadası, 2004., 2004. (BibTeX) @conference{Ademoğlu2004, title = {3D Localization Of The Dipoles Of Event Related Potential Components}, author = {A. Ademoğlu }, editor = {Ahmet Ademoğlu}, year = {2004}, date = {2004-07-01}, booktitle = {Modelling Mental Processes and Disorders, Kuşadası, 2004.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } |
Inproceedings |
2014 |
Sahin, Duygu; Duru, Adil Deniz; Ademoglu, Ahmet Preprocessing effects on group independent component analysis of fMRI data (Inproceeding) Biomedical Engineering Meeting (BIYOMUT), 2014 18th National, pp. 1-5, IEEE, 2014, ISBN: 978-1-4799-7572-3. @inproceedings{Sahin2014, title = {Preprocessing effects on group independent component analysis of fMRI data}, author = {Duygu Sahin and Adil Deniz Duru and Ahmet Ademoglu}, url = {http://ieeexplore.ieee.org/document/7026333/}, doi = {10.1109/BIYOMUT.2014.7026333}, isbn = {978-1-4799-7572-3}, year = {2014}, date = {2014-11-04}, booktitle = {Biomedical Engineering Meeting (BIYOMUT), 2014 18th National}, pages = {1-5}, publisher = {IEEE}, abstract = {Functional connectivity networks (FCN) might differ due to tissue loss in brain. Spatially independent components can be gathered with the group independent component analysis, which is one of the methods that can extract FCNs from fMRI data. Comparison of spatial or temporal results of group-wise data is possible for the differences in parameters of the preprocessing or processing steps. In this study, after the fMRI data, which is taken from Alzheimer's disease and mild cognitive impairment patients during an oddball paradigm, is preprocessed by two different methods; a group independent component analysis is done. Both of the preprocessing methods include slice time correction, motion correction, coregistration, normalization and spatial smoothing while they differ in normalization step as the chosen algorithm varies. After the preprocessing, group independent component analysis is applied with the same parameters for both of the methods. As a consequence, the effect of the difference between the two preprocessing methods are investigated. Depending on the results, stability, power spectrums and spatial maps of the components show an alteration by the algorithm used in normalization step.}, keywords = {}, pubstate = {published}, tppubtype = {inproceedings} } Functional connectivity networks (FCN) might differ due to tissue loss in brain. Spatially independent components can be gathered with the group independent component analysis, which is one of the methods that can extract FCNs from fMRI data. Comparison of spatial or temporal results of group-wise data is possible for the differences in parameters of the preprocessing or processing steps. In this study, after the fMRI data, which is taken from Alzheimer's disease and mild cognitive impairment patients during an oddball paradigm, is preprocessed by two different methods; a group independent component analysis is done. Both of the preprocessing methods include slice time correction, motion correction, coregistration, normalization and spatial smoothing while they differ in normalization step as the chosen algorithm varies. After the preprocessing, group independent component analysis is applied with the same parameters for both of the methods. As a consequence, the effect of the difference between the two preprocessing methods are investigated. Depending on the results, stability, power spectrums and spatial maps of the components show an alteration by the algorithm used in normalization step. |
2011 |
Duru, Deniz; Erdoğan, Basri; Kaşıkçı, Itır; Bayram, Ali; Ademoğlu, Ahmet; Demiralp, Tamer Investigaton of the Neuronal Efficacy and EEG Source Power under Steady-State Visual Stimulation (Inproceeding) 2011 Annual International Conference of the IEEE Engineering in Medicine and Biology Society, pp. 6576-6579, 2011, ISSN: 1094-687X. @inproceedings{Duru2011, title = { Investigaton of the Neuronal Efficacy and EEG Source Power under Steady-State Visual Stimulation}, author = {Deniz Duru and Basri Erdoğan and Itır Kaşıkçı and Ali Bayram and Ahmet Ademoğlu and Tamer Demiralp}, doi = {10.1109/IEMBS.2011.6091622}, issn = {1094-687X}, year = {2011}, date = {2011-07-06}, booktitle = {2011 Annual International Conference of the IEEE Engineering in Medicine and Biology Society}, pages = {6576-6579}, abstract = {Understanding the nature of the link between neuronal activity and BOLD signal plays a crucial role i) for improving the interpretability of BOLD images and ii) on the design of more realistic models for the integration of EEG and fMRI. The aim of this study is to investigate the neural mechanism underlying hemodynamic behavior in a series of visual stimulation frequencies and explore possible implications for the neurovascular coupling. We studied the relationship between electrophysiological and hemodynamic measures by performing simultaneous steady state electroencephalography (EEG) and fMRI recordings in a healthy human subject during a series of visual stimulation frequencies (6 Hz, 8 Hz, 10 Hz, 12 Hz). BOLD amplitudes were computed for voxels within an anatomical mask which was obtained by mapping the significantly active voxels using general linear modelling (GLM) on fMRI data. On the same anatomical map, EEG power time series belonging to the fundamental frequency and its harmonics due to the stimulation are estimated using a distributed source imaging technique. The neuronal efficacies which represent the vascular inputs driving the BOLD response are estimated by use of an extended version of Balloon model. A nonlinear relationship is demonstrated between the mean EEG source powers and the neuronal efficacies driving the BOLD response. The result suggests that BOLD signal which is an indicator of the metabolic demand of both synchronized and non-synchronized neuronal activities; changes independent of EEG activity which is a measure sensitive to the synchronicity of neuronal activity.}, keywords = {}, pubstate = {published}, tppubtype = {inproceedings} } Understanding the nature of the link between neuronal activity and BOLD signal plays a crucial role i) for improving the interpretability of BOLD images and ii) on the design of more realistic models for the integration of EEG and fMRI. The aim of this study is to investigate the neural mechanism underlying hemodynamic behavior in a series of visual stimulation frequencies and explore possible implications for the neurovascular coupling. We studied the relationship between electrophysiological and hemodynamic measures by performing simultaneous steady state electroencephalography (EEG) and fMRI recordings in a healthy human subject during a series of visual stimulation frequencies (6 Hz, 8 Hz, 10 Hz, 12 Hz). BOLD amplitudes were computed for voxels within an anatomical mask which was obtained by mapping the significantly active voxels using general linear modelling (GLM) on fMRI data. On the same anatomical map, EEG power time series belonging to the fundamental frequency and its harmonics due to the stimulation are estimated using a distributed source imaging technique. The neuronal efficacies which represent the vascular inputs driving the BOLD response are estimated by use of an extended version of Balloon model. A nonlinear relationship is demonstrated between the mean EEG source powers and the neuronal efficacies driving the BOLD response. The result suggests that BOLD signal which is an indicator of the metabolic demand of both synchronized and non-synchronized neuronal activities; changes independent of EEG activity which is a measure sensitive to the synchronicity of neuronal activity. |
2010 |
Özker, Müge; Karahan, Esin; Erdogan, Basri; Bayraktaroglu, Zübeyir; Bayram, Ali; Ademoglu, Ahmet; Demiralp, Tamer Steady State Visual Evoked Potential Informed fMRI Analysis for Alpha, Beta and Gamma Bands (Inproceeding) 2010 15th National Biomedical Engineering Meeting, pp. 1-3, 2010. @inproceedings{Özker2010, title = {Steady State Visual Evoked Potential Informed fMRI Analysis for Alpha, Beta and Gamma Bands}, author = {Müge Özker and Esin Karahan and Basri Erdogan and Zübeyir Bayraktaroglu and Ali Bayram and Ahmet Ademoglu and Tamer Demiralp}, url = {http://ieeexplore.ieee.org/abstract/document/5479787/}, doi = {10.1109/BIYOMUT.2010.5479787}, year = {2010}, date = {2010-04-09}, booktitle = {2010 15th National Biomedical Engineering Meeting}, pages = {1-3}, abstract = {In this study, simultaneous EEG and fMRI measurements are recorded from 18 healthy subjects during steady state visual stimulation. The stimulation frequencies that belong to the alpha, beta and gamma bands, at which the highest EEG amplitudes were observed, were determined for each subjects. The fMRI responses at those specific stimulation frequencies were anlayzed. Both EEG amplitudes and the fMRI parameters, namely the extent of the active area and the mean percentage positive BOLD change r were observed to be the highest at the alpha band and the lowest at the gamma band.}, keywords = {}, pubstate = {published}, tppubtype = {inproceedings} } In this study, simultaneous EEG and fMRI measurements are recorded from 18 healthy subjects during steady state visual stimulation. The stimulation frequencies that belong to the alpha, beta and gamma bands, at which the highest EEG amplitudes were observed, were determined for each subjects. The fMRI responses at those specific stimulation frequencies were anlayzed. Both EEG amplitudes and the fMRI parameters, namely the extent of the active area and the mean percentage positive BOLD change r were observed to be the highest at the alpha band and the lowest at the gamma band. |
2008 |
Duru, Adil Deniz; Ademoğlu, Ahmet Source Localization of Subtopographies Decomposed by Radial Basis Functions (Inproceeding) Medical Imaging and Augmented Reality: 4th International Workshop Tokyo, Japan, August 1-2, 2008 Proceedings, pp. 108-115, Springer Berlin Heidelberg, Berlin, Heidelberg, 2008, ISBN: 978-3-540-79982-5. @inproceedings{Duru2008, title = {Source Localization of Subtopographies Decomposed by Radial Basis Functions}, author = {Adil Deniz Duru and Ahmet Ademoğlu}, url = {http://link.springer.com/chapter/10.1007/978-3-540-79982-5_12}, doi = {10.1007/978-3-540-79982-5_12}, isbn = {978-3-540-79982-5}, year = {2008}, date = {2008-11-04}, booktitle = {Medical Imaging and Augmented Reality: 4th International Workshop Tokyo, Japan, August 1-2, 2008 Proceedings}, journal = {Medical Imaging and Augmented Reality: 4th International Workshop Tokyo, Japan, August 1-2, 2008 Proceedings}, pages = {108-115}, publisher = {Springer Berlin Heidelberg}, address = {Berlin, Heidelberg}, abstract = {Functional neuroimaging methods give the opportunity of investigating human brain functioning. Mostly used functional neuroimaging techniques include Electroencephalogram (EEG), functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and optical imaging. Among these techniques EEG has the best time resolution, while fMRI has the best spatial resolution. High temporal resolution of EEG is an attractive property for neuroimaging studies. EEG inverse problem is needed to be solved in order to identify the locations and the strength of the electrical sources forming EEG/ERP topographies. Low spatial resolution of the scalp topography causes this localization problem more complicated. In this paper, a spatial preprocessing method, which separates a topography into two or more subtopographies is proposed. The decomposition procedure is based on defining a spatial map with radial basis functions which forms the subtopographies. A simulated data is used to exhibit the advantage of using this decomposition technique prior to EEG source localization. It is shown that the accuracy of the source localization problem is improved by using the subtopographies instead of using the raw topography.}, keywords = {}, pubstate = {published}, tppubtype = {inproceedings} } Functional neuroimaging methods give the opportunity of investigating human brain functioning. Mostly used functional neuroimaging techniques include Electroencephalogram (EEG), functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and optical imaging. Among these techniques EEG has the best time resolution, while fMRI has the best spatial resolution. High temporal resolution of EEG is an attractive property for neuroimaging studies. EEG inverse problem is needed to be solved in order to identify the locations and the strength of the electrical sources forming EEG/ERP topographies. Low spatial resolution of the scalp topography causes this localization problem more complicated. In this paper, a spatial preprocessing method, which separates a topography into two or more subtopographies is proposed. The decomposition procedure is based on defining a spatial map with radial basis functions which forms the subtopographies. A simulated data is used to exhibit the advantage of using this decomposition technique prior to EEG source localization. It is shown that the accuracy of the source localization problem is improved by using the subtopographies instead of using the raw topography. |
Duru, Adil Deniz; Ademoğlu, Ahmet Realistic head model preparation for EEG forward problem (Inproceeding) 4th IET International Conference on Advances in Medical, Signal and Information Processing - MEDSIP 2008, pp. 1-4, 2008, ISSN: 0537-9989. @inproceedings{Duru2008, title = {Realistic head model preparation for EEG forward problem}, author = {Adil Deniz Duru and Ahmet Ademoğlu}, url = {http://ieeexplore.ieee.org/abstract/document/4609099/}, doi = {10.1049/cp:20080470}, issn = {0537-9989}, year = {2008}, date = {2008-06-05}, booktitle = {4th IET International Conference on Advances in Medical, Signal and Information Processing - MEDSIP 2008}, journal = {Advances in Medical, Signal and Information Processing, 2008.}, pages = {1-4}, abstract = {Electroencephalography (EEG) forward solution computes the electrical potentials over the electrodes that are lying on the scalp surface. Forward solution requires a numerical head model to work on. The head model can be spherical or a realistic one. Accuracy of the forward problem can be improved by using realistic head models. Triangulated realistic head model is an input for the Boundary Element method (BEM) which solves the EEG forward problem. In this paper, a software is introduced which constructs 3-D triangulated realistic head models from the Magnetic Resonance images.}, keywords = {}, pubstate = {published}, tppubtype = {inproceedings} } Electroencephalography (EEG) forward solution computes the electrical potentials over the electrodes that are lying on the scalp surface. Forward solution requires a numerical head model to work on. The head model can be spherical or a realistic one. Accuracy of the forward problem can be improved by using realistic head models. Triangulated realistic head model is an input for the Boundary Element method (BEM) which solves the EEG forward problem. In this paper, a software is introduced which constructs 3-D triangulated realistic head models from the Magnetic Resonance images. |
Presentations |
2007 |
AD Duru T Demiralp, Ademoğlu Subtopographic EEG Source Localization After Spatio-Temporal Wavelet Decomposition (Presentation) 05.11.2007. @misc{Duru2007b, title = {Subtopographic EEG Source Localization After Spatio-Temporal Wavelet Decomposition}, author = {AD Duru, T Demiralp, A Ademoğlu, }, editor = {Ahmet Ademoğlu}, year = {2007}, date = {2007-11-05}, abstract = {International Workshop on Image Analysis in the Life Sciences Theory and Applications (IEEE), Johannes Kepler University, Austria, 2007}, keywords = {}, pubstate = {published}, tppubtype = {presentation} } International Workshop on Image Analysis in the Life Sciences Theory and Applications (IEEE), Johannes Kepler University, Austria, 2007 |