Subjective standstill caused by the interaction of moving patterns

In view of recent interest in the mechanisms of movement perception [reviewed in Sekuler (1973)]. it seems worthwhile to mention briefly a phenomenon which I noticed some years ago when working in the Communication Department at Keele, England. The apparatus is described elsewhere (Clarke, 1973). Two similar, moving, visual noise transparencies were alternately illuminated, for equal exposure times, in a field of 6” radius. Both moved at lo”,/sec horizontally, but in opposite directions. There were two subjects. At siow alternation rates. it appeared to each subject as if a single noise pattern was regularly reversing its motion, the change in noise pattern at reversal not being detectable. But when the frequency of alternation exceeded about 12 Hz. the phenomenon of interest occurred: the sensation of reversing motion was abruptly and totally lost, and the subject saw, instead, dynamic (sparkling) visual noise. There was some slight impression of local movement, but this was random, and there was no impression of whole-field movement. It is this “standstill” effect which is of interest. At much higher frequencies of alternation @OH+ or so). the appearance changed again and the two moving noise transparencies were seen as separate patterns, su~rim~sed and moving in opposite directions. The complete toss of whole-field movement percep tion in the intermediate frequency range was subjectively very striking, but, if a single pattern was illuminated continuously and oscillated from side to side in a triangular waveform at similar frequencies, the effect did not occur. Instead, with the single pattern, there was a gradual blurring as frequency increased, till movement perception was lost at about 30Hz. This suggests that the change of pattern at reversal interfered with the movement-processing mechanisms [it is known that such pattern-change suppresses the visual evoked potentials to motion-reversal (Clarke, 1972}], and that this led to cancellation between the separate channels tuned to the two opposite directions, e.g. Levinson and Sekuler (1976). It might be argued that the effect was merely the result of some disruption at the initial level of direction-selective anafysis, rather than of interaction between channels. but I think the latter interpretation is more likely. since standstilf failed to occur if the brightness. speed and exposure time were not all equal in the two patterns. Such a sensitive balancing tends to suggest a subtractive cancellation mechanism rather than simple disruption. Standstill could not be produced with dichoptic stimulation ; binocular rivalry occurred instead. This need not imply that the neural events causing standstill were at an early stage in the visual system before integration of the inputs from the two eyes. for the rivalry could presumably eliminate the velocity-cancellation however late in processing it occurred. The stereoscopic standstill phenomenon of Jufesz and Payne (1968) could, perhaps. have been caused by a related kind of velocity-cancellation, in the depth dimension. In their case, however, the effect occurred at lower frequencies (about S-6 Hz).