Statistical analysis of short-latency flash visual evoked cortical potentials with noncephalic reference.

Most of the reports on visual evoked cortical potentials have dealt with peak latency or amplitude. An early negative deflection at about 20 ms, however, was reported to coincide with the initial signal to the monkey cortex. We studied the initial signal to the human cortex in nine volunteers by taking into account prestimulus fluctuation. Visual evoked cortical potentials were recorded for an interval 10 ms before and 92.4 ms, after a flash stimulus, by means of a neck reference. The amplitudes of the response before the flash stimulus and at each succeeding 5-ms interval after the stimulus were tested with a t-test. The grand average of the visual evoked cortical potentials showed an initial negative peak at 22.3 ms over the frontal poles, a second negative peak at 52.4 ms over the central area, and a third negative peak at 74.7 ms over the occipital areas. The topographic maps corresponding to the peaks of the visual evoked cortical potentials were the same as those of the dynamic t-values. The visual evoked cortical potential and t-values with the bipolar and source derivations showed a marked response beginning around 40 ms over the posterior head. This statistical analysis is an easy method to standardize evoked potentials and to identify deflections.