AN ELECTROPHYSIOLOGICAL METRIC OF ACTIVITY WITHIN THE ON-PATHWAY AND OFF-PATHWAY IN HUMANS

Several animal studies have shown an anatomical and functional separat ion between the ON- and OFF-pathways in the retina and in the lateral geniculate nucleus. Psychophysical studies in humans have also documen ted separate pathways that process increments and decrements of light, However, at the level of the visual cortex, there is electrophysiolog ical evidence of interactions between the ON- and OFF-pathways. In add ition, psychophysical studies have shown that these pathways can exhib it differential sensitivity and be differentially adapted. These findi ngs motivated an electrophysiological study to gather further evidence of processing within the ON- and OFF-pathways in the human visual sys tem. Using sawtooth stimulus modulation, we measured the visual evoked potential (VEP) before and after adaptation to both rapid-on and rapi d-off sawtooth stimuli. The effect of adaptation was determined by com paring the VEP response in three test conditions: without adaptation, after adaptation to the same sawtooth polarity, and after adaptation t o the opposite sawtooth polarity, The results reveal a selective adapt ation effect, which provides physiological evidence for separate proce ssing of increments and decrements in the human visual system. We conc lude that with appropriate stimulus parameters, the VEP can serve as a n objective measure of processing within the ON- and OFF-pathways in h umans. (C) 1997 Elsevier Science Ltd.