Abstract
Abstract: :
Purpose: We present a method to identify the temporal onset of cortical activity after visual stimulation using multifocal VEP (mfVEP) recordings. The aim of this study was to compare the interindividual variability of this new latency measure with the variability of the P100 peak implicit time in standard VEP recordings. Methods: 30 visually normal subjects participated in the experiment. Binocular mfVEPs were recorded as voltage difference between two electrodes placed 4 cm above and below the inion. Dartboard patterns with 60 fields were presented within a circular stimulus field with a diameter of 41°. Within the dartboard fields checkerboard patterns with a mean luminance of 86,1 cd/m2 and a contrast of 99.8% were reversed in contrast following m–sequence stimulation. Second order mfVEP traces were squared and averaged across all 60 fields resulting in one multifocal power function (MPF) for each subject. Due to the squaring procedure noise fluctuations do not cancel out but generate a MPF pedestal. Cortical activity after visual stimulation leads to an increase of the MPF regardless of the mfVEP waveforms. Latency was defined by the onset of a sudden MPF rise above the noise pedestal. For comparison VEPs to reversing checkerboard patterns with a check size of 0.4°, a mean luminance of 45 cd/m2, and a contrast of 99.7% were recorded for each subject using a Oz–FPz derivation. The implicit time of the major positive peak near 100ms (P100) was used for analysis. Results: (1) When averaged across all subjects the MPF showed a steep linear rise starting 47ms after visual stimulation and leading to a MPF increase by a factor of 9 during the following 10 ms. (2) Each of the 30 subjects showed a sudden steep MPF rise between 43 and 51 ms. The MPF waveform differed strongly between subjects for later time intervals. (3) Average results (mean ± SD, n=30) were 47.0 ± 2.1 ms for MPF latency and 101.9 ± 5.4 ms for P100 implicit time. Conclusions: MPF analysis of mfVEP recordings clearly indicates the onset of cortical activity after visual stimulation. The low interindividual variability of the MPF latency suggests that this measure of signal transmission time is less modulated by the individual cortical anatomy than the implicit time of the P100 component. The MPF latency data demonstrate that about half of the P100 implicit time is required for signal transmission to the visual cortex while the other half reflects intracortical processing. Thus MPF may help to distinguish between pre–cortical and intracortical involvement of neural dysfunction, e. g., in cases of demyelinating diseases of the visual system.
Keywords: visual cortex • electrophysiology: clinical • electroretinography: non-clinical