VISUAL PROCESSING DELAYS ALTER THE PERCEIVED SPATIAL FORM OF MOVING GRATINGS

This study shows that there are delays in processing high spatial freq uencies relative to low frequencies, and that these may affect the per ceived brightness profile of drifting waveforms. The stimuli were comp lex waveforms consisting of 2-3 sinusoidal components, either drifting or stationary. The phase of the components was varied until the brigh tness profile of the waveform appeared as a square, triangle, ramp or bar. The results indicate that stationary waveforms are perceived veri dically, but drifting waveforms are not. The harmonics of a drifting c omplex wave must be phase advanced, relative to the fundamental, in or der to cancel motion-induced waveform distortions. This suggests that during visual processing the harmonics must be phase delayed, indicati ng that they are being processed more slowly than the fundamental. The most significant delays appear to be those between the fundamental an d its second and third harmonic. Furthermore, the results show that th e magnitude of the delays is dependent on the phase relationship betwe en the components at perceptually significant points in the waveform: delays are less when the components are in sine phase than when they a re in cosine phase. Separate experiments show that the detectability o f phase shifts is least when the components are in sine phase. Togethe r, these results may explain why drifting ''sharp-edged'' stimuli are not perceptually distorted: the human visual system appears to be rela tively insensitive to phase shifts around square-wave phase and may th erefore tolerate differences in the processing times of certain harmon ics. A discussion of the possible origin of these processing delays is presented, together with the hypothesis that frequency dependent dela ys may reflect the spatiotemporal inseparability of cortical visual un its.