Zubeyir Bayraktaroglu

Comparative analysis of event-related potentials during Go/NoGo and CPT: Decomposition of electrophysiological markers of response inhibition and sustained attention

Neuropsychological tests target specific cognitive functions; however, numerous cognitive subcomponents are involved in each test. The aim of this study was to decompose the components of two frontal executive function tests, Go/NoGo (GNG) and cued continuous performance task (CPT), by analyzing event-related potentials (ERPs) of 24 subjects both in time and time-frequency domains. In the time domain, P1, N1, P2, N2 and P3 peak amplitudes and latencies and mean amplitudes of 100 ms time windows of the post-P3 time period were measured. For GNG, the N1 amplitude and for both GNG and CPT N2 amplitudes were significantly higher in the NoGo condition compared with the Go condition. P3 had a central maximum in the NoGo conditions of both paradigms in contrast to a parietal maximum in the Go conditions. All peaks except P1 and mean amplitudes of the post-P3 period were more positive in CPT compared to those of GNG. N1, N2 and P3 latencies were longer for the NoGo condition than the Go condition in the CPT. In time-frequency analyses, the NoGo condition evoked higher theta coefficients than the Go condition, whereas the CPT and GNG paradigms differed mainly in the delta band. These results suggest that theta component reflects response inhibition in both GNG and CPT, whereas delta component reflects the more demanding sustained attention requirement of the CPT. The latency prolongation observed with the NoGo condition of the CPT paradigm was thought to be due to perseverance/inhibition conflict enhanced by the primer stimuli in CPT.

Simultaneous EEG/fMRI Analysis of the Resonance Phenomena in Steady-State Visual Evoked Responses:

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.

Changes in BOLD transients with visual stimuli across 1–44 Hz

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.

Implementation of Low Resolution Electro-Magnetic Tomography with fMRI Statistical Maps on Realistic Head Models

Functional neuroimaging studies can be performed by combining the modalities of fMRI and Electroencephalography because of their complementary properties. The main advantage of EEG imaging among other modalities is the high temporal resolution while fMRI has high spatial resolution. So, usage of these procedures is going to help us to gain more information about the functional organization of the brain. In this study, changes in the relationship between steady state visual evoked potentials (SSVEP) generators and BOLD responses during visual stimulation have been systematically studied with 5 stimulus presentation rates (2, 4, 6, 8,10) between 2-10 Hz. fMRI Analysis was carried out using statistical parametric mapping (SPM). The result of fMRI analysis is used as a localization mask for SSVEP localization process. SSVEP generators are localized using low resolution electro magnetic tomography (LORETA) which is implemented on a realistic head model. Then, for each stimulus frequency voxel by voxel correlation values of the active regions are computed.

TASK-RELATED AND TASK-FREE FUNCTIONAL NEUROIMAGING IN APATHETIC ALZHEIMER'S DISEASE

were evaluated through the Memory Complaint Scale (MCS), forms A and B3, Addenbrooke’s Cognitive Examination– Revised (ACER)4, Mini Mental State Exam (MMSE)4, Isolated Cognitive Test (TCI), Timed Up and Go Test (TUG) e Timed Up and Go Test associated a to dual task of phone dialing (TUGDT). Results: 51 elders were evaluated, with majority of women, median of 71 years old, median of schooling of 4 years, median of TCI time of 16.0 seconds, median of TUG time of 11.9 seconds and median of TUGDT time of 26.6 seconds. With Spearman’s test were not found correlations between MCSA and TCI time (p1⁄40.836), TUG time (p1⁄40.536) and TUGDT time (p1⁄40.922), neither between MCSB and TCI time (p1⁄40.873), TUG time (p1⁄40.534) and TUGDT time(p1⁄40.188). On the other hand were ascertained correlations between ACER and TCI time (p1⁄40.000/r1⁄4.490), TUG time (p1⁄40.009/r1⁄4.364), and TUGDT time (p1⁄40.000/r1⁄4.553) and also MMSE and TCI time (p1⁄40.000/r1⁄4.561), TUG time (p1⁄40.038/r1⁄4.291) and TUGDT time (p1⁄40.000/r1⁄4.526). Conclusions: The memory complaints do not contemplate possible functionality debts, in contrast with the TUG and TUGDT performances that are affected by cognition. Therefore, including mobility tests in elders’ cognitive evaluations showed to be of great value for improving diagnostics reliability.

Source concordance analysis of simultaneously recorded steady state visual evoked potentials and fMRI

1) School of Physical Education and Sport, Sport Health Sciences Department, Marmara University, Istanbul, 34800 2) Department of Physiology, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey 3) Department of Neuroscience, Institute of Experimental Medicine, Istanbul University, Istanbul, Turkey 4) Hulusi Behcet Life Sciences Research Laboratory, Istanbul University, Istanbul, Turkey 5) Department of Biomedical Engineering, Istanbul Arel University, Istanbul, 34537 6) Department of Computer Engineering, College Of Engineering And Natural Sciences, Istanbul Sehir University, Istanbul, Turkey

Steady state visual evoked potential informed fMRI analysis for alpha, beta and gamma bands

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.

Projection of the steady state visual evoked potentials on prefrontal cortex

The aim of this experiment was to investigate the impact of steady-state visual stimulation on electrophysiology of the human brain. The study was performed on 6 subjects and the measurements were made by the non-invasive technique, EEG (Electroencephalography). The changes in the electrophysiological brain signals that had been obtained by EEG during visual stimulation were analyzed. The amount of changes in the recorded signals exhibited dependencies to the frequency of the applied visual stimulation. In this study, the frequency dependent electrophysiological response of the frontal cortex of the brain was analyzed and compared with the response of the occipital cortex. In both occipital and prefrontal cortexes, the highest EEG signal energy was observed at the 10–13 Hz stimulation band. Other than this frequency band, there were no similarities between the two regions in terms of local maximum and minimum signal energy values. On the other hand, when the EEG response to the whole stimulation frequency was considered, the correlation coefficient of the signal powers of both regions was found to be 0.84.

Interactions of Gamma and Theta Oscillations in the Electroencephalogram (EEG) during Memory Processes

Human subjects typically keep about 7plusmn2 items in short-term memory (STM). A theoretical neuronal model has been proposed to explain this phenomenon with physiological parameters of brain oscillations in the gamma and theta frequency range, i.e. roughly 30-80 and 4-8 Hz, respectively. In that model, STM capacity equals the number of gamma cycles (e.g. 25 ms for 40 Hz), which fit into one theta cycle (e.g. 166 ms for 6 Hz). The model is based on two assumptions: 1) theta activity should modulate gamma activity and 2) the theta/gamma ratio should correlate with human STM capacity. The first assumption is supported by electrophysiological data showing that the amplitude of gamma oscillations is modulated by the phase of theta activity. However, so far this has only been demonstrated for intracranial recordings. We analyzed human event-related EEG oscillations recorded in a memory experiment in which 13 subjects perceived known and unknown visual stimuli. The paradigm revealed event-related oscillations in the gamma range, which depended significantly on the phase of simultaneous theta activity. Our data are the first scalp-recorded human EEG recordings revealing a relationship between the gamma amplitude and the phase of theta oscillations, supporting the first assumption of the abovementioned theory. Interestingly, the involved frequencies revealed a 7:1 ratio. However, this ratio does not necessarily determine human STM capacity. Since such a correlation was not explicitly tested in our paradigm, our data is not conclusive about the second assumption. Instead of theta phase modulating gamma amplitude, it is also conceivable that focal gamma activity needs to be downsampled to theta activity, before it can interact with more distant brain regions