Visuell evozierte Potentiale (VEP) und Elektroretinogramm (ERG)

A method is proposed to determine components of evoked scalp potentials, in terms of times of occurrence (latency) and location on the scalp (topography). The scalp field distributions were evoked by checkerboard reversal and were recorded simultaneously in 47 channels. Component latencies are defined as times of maximal values of the electrical power of the evoked field (a measure of the amount of field relief); this measurement is independent of the choice of the reference electrode. In 10 subjects, two evoked components were found consistently: at 100 and at 140 msec. Plots of scalp locations of the extreme field values (i.e., reference-free data) at the occurrence times of the components showed occipitally positive and anteriorly negative extreme values at 100 msec, and vice versa at 140 msec. The occipital extreme values were surrounded by steep field gradients suggesting occipital generator processes. The polarity reversal of the evoked field distributions between 100 and 140 msec was a quick, jump-like location change of the extreme values in the field. The locations of the extreme field values were stable for long periods around peak times of the power curve. During these periods, the shape of the field remained constant (assessed by the average standard deviation of voltages per electrode between successive field distributions), suggesting also a stable localization of the generating process in depth. The field distributions tended to be concentric around the extreme field values. The major characteristics of the observed scalp fields showed no wave fronts, and no continuous 'traveling' of extreme values over larger distances.

Visual evoked potentials (VEPs) were recorded from normal adult subjects performing in a visual discrimination task. Subjects counted the number of presentations of the numeral 4 which was interposed rarely and randomly within a sequence of tachistoscopically flashed background stimuli (numeral 2s). Intrusive, task-irrelevant (not counted) stimuli were also interspersed rarely and randomly in the sequence of 2s; these stimuli were of two types: simples, which were easily recognizable (e.g., geometric figures), and novels, which were completely unrecognizable (i.e., complex, colorful patterns). It was found that the simples and the counted 4s evoked posteriorly distributed P3 waves (latency 380-430 msec) while the irrelevant novels evoked large, frontally distributed P3 waves (latency 360-380 msec). These large, frontal P3 waves to novels were also found to be preceded by large N2 waves (latency 278 msec). These findings indicate that "the P3" wave is not a unitary phenomenon but should be considered in terms of a family of waves, differing in their brain generators and in their psychological correlates. These late positive components are discussed in terms of task-relevance, recognition and Pavlov's "what is it" response.