Visual evoked potential interhemispheric transfer time in different frequency bands

Objective: Visual evoked potentials (VEP) have been used to estimate interh emispheric transfer time (IHTT). However, the complex wave of VEP is most p robably formed by different generators of neural populations that act throu gh different frequency channels. If the main peaks of VEP are established b y different types of generators, which can also be connected to each other by a different type of callosal fibres, we would be able to estimate a wide range of various IHTT by measuring the latency between time-locked peaks o f narrow bandpass filtered VEP. This research aimed to test this hypothesis . Methods: Nine right-handed men were presented with a reversal of a checkerb oard pattern as stimuli at RVF or LVF, and EEG was recorded at O1, O2, P3, P4. The grand-averaged VEPs were transformed to the frequency domain by mea ns of the fast Fourier transform to obtain the amplitude frequency characte ristics. Band-pass filters were chosen adequately, according to tuning freq uencies indicated by clear peaks in the amplitude frequency characteristics . The chosen band pass filters (4-8 Hz, 8-15 Hz, 15-20 Hz, 20-32 Hz) were a pplied to the VEP of the subjects, and 4 different components of VEPs for e ach VEP were obtained. The latency of P100 and N160 of unfiltered VEP was m easured. In the band-pass digital filter applied VEPs, positive and negativ e peaks, which are consistent with P100 and N160, were measured for each su bject. Latency differences between hemispheres for digitally unfiltered and filtered VEPs were computed to estimate MTT. Results: In the different frequency bands, different IHTTs were estimated, ranging from 3 ms to 30 ms. Approximately 16 ms for theta band, 11 ms for a lpha band, 6 ms for 15-20 Hz and 3 ms for 20-32 Hz bands were found. Conclu sions: Our findings support the hypothesis which states that unfiltered VEP s provide us with only a rough estimation of MTT. Also, they are consistent with anatomical findings that describe callosal fibres of varying dimensio ns, predicting various velocities between hemispheres. (C) 1999 Elsevier Sc ience Ireland Ltd. All rights reserved.