AN ELECTROPHYSIOLOGICAL CORRELATE OF VISUAL-MOTION AWARENESS IN MAN

It is usually held that perceptual spatial stability, despite smooth p ursuit eye movements, is accomplished by comparing a signal reflecting retinal image slip with an internal reference signal, encoding the ey e movement. The important consequence of this concept is that our subj ective percept of visual motion reflects the outcome of this compariso n rather than retinal image slip. In an attempt to localize the cortic al networks underlying this comparison and therefore our subjective pe rcept of visual motion, we exploited an imperfection inherent in it, w hich results in a movement illusion. If smooth pursuit is carried out across a stationary background, we perceive a tiny degree of illusiona ry background motion (Filehne illusion, or FI), rather than experienci ng the ecologically optimal percept of stationarity. We have recently shown that this illusion can be modified substantially and predictably under laboratory conditions by visual motion unrelated to the eye mov ement. By making use of this finding, we were able to compare cortical potentials evoked by pursuit-induced retinal image slip under two con ditions, which differed perceptually, while being identical physically . This approach allowed us to discern a pair of potentials, a parieto- occipital negativity (N300) followed by a frontal positivity (P300), w hose amplitudes were solely determined by the subjective perception of visual motion irrespective of the physical attributes of the situatio n. This finding strongly suggests that subjective awareness of visual motion depends on neuronal activity in a parieto-occipito-frontal netw ork, which excludes the early stages of visual processing.