TEMPORAL ANALYSIS OF THE CHROMATIC FLASH VEP - SEPARATE COLOR AND LUMINANCE CONTRAST COMPONENTS

Temporal analysis of the chromatic flash visual evoked potential (VEP) was studied in human subjects with normal and anomalous colour vision using a deterministic pseudo-random binary stimulus (VERIS). Five exp eriments were carried out on four normal subjects investigating hetero chromatic red-green exchange and single colour/achromatic (either red/ grey or green/grey) exchange over a wide range of luminance ratios for the two stimuli, the effects of lowered mean luminance on the chromat ic VEP and the effects of colour desaturation at constant mean luminan ce and constant luminance contrast. Finally, the performance of three dichromats, a protanope and two deuteranopes, on heterochromatic excha nge VEP and on colour desaturation were investigated. In contrast to t he chromatic electroretinogram, which shows great symmetry with respec t to luminance ratio on opposite sides of the isoluminant point, the c hromatic VEP demonstrated a distinct asymmetry when the colours exchan ged included red. On the red side of isoluminance (red more luminant t han green), a wave with longer latency and altered waveform became dom inant. The effects of green stimulation were indistinguishable from th ose of achromatic stimulation at the same luminance contrast over the whole range of chromatic contrast and for all levels of desaturation s tudied. Desaturation of red with constant luminance contrast (desatura ted red/grey stimulation:, resulted in a systematic alteration in the evoked waveform. Subtraction of the achromatic first- and second-order responses from responses recorded in the red desaturation series resu lted in remarkably uniform waveforms; with peak amplitudes growing lin early with saturation. The absence of interaction between achromatic a nd coloured components for all (including the most intense colour) sti mulus parameters used suggests that the generators of these components are separate. Recordings from the dichromats showed that the contrast response minimum shifted from the point of photopic isoluminance to t he point of zero cone contrast (at the silent substitution point) for the remaining cone type. The waveforms recorded with a series of lumin ance ratios were much simpler than those recorded from trichromats and symmetrical with respect to their isoluminant points. Despite the ind ication of the presence of L cones of apparently normal spectral sensi tivity in the deuteranopes ton the basis of flicker photometry), there was no evidence for a red-sensitive component in the desaturation or heterochromatic stimulation series. The results are discussed in terms of the possibility of separate generation of chromatic and achromatic contributions to the VEP. (C) 1998 Elsevier Science Ltd. All rights r eserved.