A NEW PROCEDURE FOR THE REGISTRATION OF THE VISUAL-EVOKED CORTICAL POTENTIAL BY MULTICHANNEL RECORDING OF THE GRADIENT DISTRIBUTION

A new bipolar multichannel visual-evoked cortical potential (VECP) pro cedure is presented, which can supply objective information on visual field losses. The electrical potential is recorded from 11 electrodes applied to the back of the head in the form of an equidistant rectangu lar grid. Each neighbouring pair of electrodes feeds one of 14 bipolar channels. The adjacent horizontal and vertical channels are used to c alculate an approximation to the direction and amplitude of the electr ical field gradients. The gradient distribution is represented by a ma p of arrows (gradient map) for every instant of the sweep, so that the whole sweep can be plotted as a time series of gradient maps. The map s are easy to scale and are well suited for visual evaluation. Twenty normal subjects were stimulated using checkerboard reversal, partial f ield patterns to simulate visual field defects. The stimulated area va ried between full-, half- and quarter-field, and the particular area s timulated could be clearly seen in the resulting gradient maps. Additi onally, we developed a computerized classification procedure that dete cted 86% of the disturbed visual fields from the gradient recordings.