Phase-locking Factor Research Articles

Cortical source estimates of gamma band amplitude and phase are different in schizophrenia

Reductions in gamma band phase synchrony and evoked power have been reported in schizophrenic subjects in response to auditory stimuli. These results have been observed in the EEG at one or two electrode sites. We wished to extend these results using magnetic field data to estimate the responses at the neural generators themselves in each hemisphere. Whole head magnetoencephalographic (MEG) recordings were used to estimate the phase and amplitude behavior of sources in primary auditory cortex in both hemispheres of schizophrenic and comparison subjects. Both ipsi- and contralateral cases were evaluated using a driving (40 Hz modulated 1 kHz carrier) and a non-driving (1 kHz tone) stimulus. We used source space projection (SSP) to collapse the magnetic field data into estimates of the time course of source strengths in individual trials. Complex wavelet based time–frequency decomposition was used to compute inter-trial phase locking factor (PLF), and mean evoked and induced amplitude for each cortical generator. Schizophrenic subjects showed reduced SSP PLF and evoked source strength for contralateral generators responding to the driving stimulus in both hemispheres. For the pure tone stimulus, only the left hemisphere PLF's in the transient window were reduced. In contrast, subjects with schizophrenia exhibited higher induced 40 Hz power to both stimulus types, consistent with the reduced PLF findings. The method of SSP combined with wavelet based complex demodulation produces a significant improvement in signal-to-noise ratio, and directly estimates the activity of the cortical generators responsible for gamma band auditory MEG evoked fields. Schizophrenic subjects exhibit significant impairment of generation and phase locking of this activity in auditory cortex, suggesting an impairment of GABA-ergic inhibitory interneuronal modulation of pyramidal cell activity.

Early evoked gamma-band responses and phase synchronization elicited by target categorical visual object discrimination

Event Abstract Back to Event Early evoked gamma-band responses and phase synchronization elicited by target categorical visual object discrimination Nikolaos Tsarouchas1, 2, 3*, S. Kostopoulos1, A. Bezeriano1, G. Benedek2 and K. Benedek3 1 Department of Medical Physics, University of Patras, Greece 2 Department of Physiology, University of Szeged, Hungary 3 Department of Neurology, University of Szeged, Hungary Objective: Aim of this study is to investigate whether target categorical visual object discrimination processes modulate cortical gamma-band responses (GBR). Methods: We analyze the amplitude of evoked vs. induced and the phase-locking factor of γ-band responses (≥30Hz) as contrasted to event-related potentials (<30Hz), by means of the continuous wavelet transform, in an ultra-rapid visuocognitive categorical discrimination task. Visual stimuli comprised 500 briefly presented images of complex natural and urban scenes. Subjects had to categorize them as targets (containing different animal objects) by eliciting a motor response vs. equiprobable nontargets (containing only nonanimal objects) by suppressing it. Results: Statistical parametric time-frequency map analysis across subjects and channels identified a characteristic (80-180)ms×(30-50)Hz poststimulus region-of-interest over occipital visual cortex (channels O1/O2), within which significantly higher evoked γ-band amplitudes and phase-locking factors were established for target vs. nontarget categorical stimuli. Conclusions: Enhanced early evoked GBR most likely emerge from the interaction of bottom-up feedforward (visual-feature extracting and binding) with top-down attentional feedback (visual-feature weighting and integrating) processes due to phase synchronization across single-trials in the “resultant” activity of the underlying occipital cortical domains. Significance: Our study provides evidence that early occipital cortical GBR underlie already the representation of target visual objects at the categorical level and the discrimination of different categorical exemplars across higher-order classes of visual objects. Acknowledgments: This work was supported by a PhD Scholarship granted to the author by Greek State Scholarship Foundation (IKY) and a research grant of the University of Patras, Karatheodoris (2004-B411): Functional Laser Speckle Imaging (fLSI). Conference: 10th International Conference on Cognitive Neuroscience, Bodrum, Turkey, 1 Sep - 5 Sep, 2008. Presentation Type: Oral Presentation Topic: Brain Electrical Oscillations in Cognition Citation: Tsarouchas N, Kostopoulos S, Bezeriano A, Benedek G and Benedek K (2008). Early evoked gamma-band responses and phase synchronization elicited by target categorical visual object discrimination. Conference Abstract: 10th International Conference on Cognitive Neuroscience. doi: 10.3389/conf.neuro.09.2009.01.113 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Dec 2008; Published Online: 03 Dec 2008. * Correspondence: Nikolaos Tsarouchas, Department of Medical Physics, University of Patras, Patras, Greece, nick_tsarouchas@yahoo.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Nikolaos Tsarouchas S. Kostopoulos A. Bezeriano G. Benedek K. Benedek Google Nikolaos Tsarouchas S. Kostopoulos A. Bezeriano G. Benedek K. Benedek Google Scholar Nikolaos Tsarouchas S. Kostopoulos A. Bezeriano G. Benedek K. Benedek PubMed Nikolaos Tsarouchas S. Kostopoulos A. Bezeriano G. Benedek K. Benedek Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Out-of-Synch and Out-of-Sorts: Dysfunction of Motor-Sensory Communication in Schizophrenia

Phase synchronization of neural activity preceding a motor act may reflect an efference copy of the motor plan and its expected sensory consequences (corollary discharge), which is sent to sensory cortex to herald the arrival of self-generated sensations and dampen the resulting sensory experience. We performed time-frequency decomposition of response-locked electroencephalogram (EEG) to examine phase synchronization of oscillations across trials (phase-locking factor [PLF]) to self-paced button presses. If prepress PLF reflects the activity in motor cortex, it should be contralateralized. If it reflects the action of the efference copy, it should be related to subsequent sensory suppression. If efference copy/corollary discharge mechanisms are abnormal in schizophrenia, it should be reduced in patients with schizophrenia. Electroencephalogram was collected while 23 patients (20 schizophrenia; 3 schizoaffective) and 25 age-matched control subjects pressed a button, at will, every 1 to 2 sec. Phase-locking factor preceding and following button presses was calculated from single-trial EEG; averaging single trials yielded response-locked event-related potentials (ERPs) to the tactile response associated with button pressing. Consistent with its hypothesized reflection of efference copy/corollary discharge signals, prepress gamma band neural synchrony was 1) maximal over the contralateral sensory-motor cortex in healthy subjects, 2) correlated with the ipsilateralized somatosensory ERP amplitude evoked by the press, and 3) reduced in patients. Prepress neural synchrony in the beta band was also reduced in patients, especially those with avolition/apathy. These data are consistent with dysfunction of forward model circuitry in schizophrenia and suggest that the specific motor-sensory system affected is selectively linked to symptoms involving that system.

The effect of selective attention on the gamma-band auditory steady-state response

Studies have demonstrated that selective attention can modulate the steady-state evoked potential to repetitive visual and tactile stimulation. However, examinations of the effect of attention on the auditory steady-state response (ASSR) have proven equivocal. The current experiment therefore utilized EEG to examine the effect of attention on the ASSR in healthy humans (n=15). Auditory click trains in the beta (20 Hz) and gamma (40 Hz) ranges were randomly presented binaurally in an oddball discrimination paradigm (each frequency served as the oddball (target) in each of two blocks). A Fast Fourier Transform was used to assess the effect of attention on the ASSR (signal power), and phase consistency across trials was assessed using the phase-locking factor (PLF). As expected, both 20 and 40 Hz targets elicited a robust P300 response, with maximal amplitudes over parietal regions. For the ASSR, it was found that EEG signal power was larger to 40 Hz targets compared to 40 Hz frequent stimuli across all frontocentral electrodes. No differences in signal power were observed during 20 Hz stimulation. Finally, increased PLF values were observed for 40 Hz targets compared to frequent trials. These results provide evidence that selective attention can enhance signal power and phase-locking of the ASSR, particularly to auditory stimulation in the gamma range.

The effect of cannabis use and gender on the visual steady state evoked potential

Objective Cannabis use often induces subjective distortions of perception. However, little work has been done examining the electroencephalographic (EEG) correlates of early sensory processing in cannabis users. The present study therefore examined visual function in cannabis users as assessed via the steady state visual evoked potential. (SSVEP). Methods SSVEPs were examined in current cannabis users ( n=17; 59% male; mean age=23.2 (S.D.=5.3)) and drug-naive controls ( n=16; 38% male; mean age=21.3 (S.D.=3.1)) to periodic photic stimulation presented at 18 and 25 Hz. The visual SSVEP was quantified via spectral power and the phase-locking factor (PLF) at each frequency of stimulation. The transient N160 event-related potential (ERP) was also evaluated at stimulus onset. Results The results showed that for both frequencies, female subjects in general displayed larger visual SSVEPs. A significant gender X group interaction also occurred at 18 Hz of stimulation, and age of onset of cannabis use positively correlated with 18 Hz spectral power values. Finally, the transient N160 component was shown to be reduced in cannabis users, regardless of gender. Conclusions The present study was the first to demonstrate altered visual SSVEPs in cannabis users, and extends previous research demonstrating increased steady state responses in female subjects. While decreased SSVEPs provide initial evidence of altered oscillatory properties in primary visual circuits, reduction of the transient N160 component suggests disruption of later-stage visual processing in cannabis users. Significance These data provide evidence of cannabinoid modulation of sensory/perceptual function in the visual system, and indicates that cannabis use may affect the oscillatory properties of sensory cortical circuits.