Different scalp localization of pattern onset and reversal visual evoked potentials

Visual evoked potentials were recorded monopolarly from seven electrodes on a horizontal line from T3 to T4 and bipolarly in one channel from left of the midline to right of the midline. Stimulation of the right and left half-fields was performed with a checkerboard generator of 10 degrees x 13.5 degrees and checks of various sizes at high contrast. The results may be summarized as follows: 1. Source-sink analysis for lateral half-field pattern onset with small checks shows C1, peaking about 80 msec, to have a source on the scalp contralateral to the half-field stimulated with a sink on the midline. 2. To half-field pattern reversal, bipolar recording across the scalp midline indicates an early phase reversal peaking about 80 msec followed by another phase reversal peaking at about P100 latency. Source-sink analysis indicates the first reversal has a contralateral source, while the second has a source on the scalp midline or ipsilateral to the field stimulated. 3. Pattern reversal results vary with check size, with a smaller check size increasing the amplitude of the first phase reversal and decreasing the amplitude of the second.