Abstract
Temporal integration in the visual system causes fast-moving objects to leave oriented ‘motion streaks’ in their wake, which could be used to facilitate motion direction perception. Temporal integration is thought to occur over 100 ms in early cortex, although this has never been tested for motion streaks. Here we compare the ability of fast-moving (‘streaky’) and slow-moving fields of dots to mask briefly flashed gratings either parallel or orthogonal to the motion trajectory. Gratings were presented at various asynchronies relative to motion onset (from to ms) to sample the time-course of the accumulating streaks. Predictions were that masking would be strongest for the fast parallel condition, and would be weak at early asynchronies and strengthen over time as integration rendered the translating dots more streaky and grating-like. The asynchrony where the masking function reached a plateau would correspond to the temporal integration period. As expected, fast-moving dots caused greater masking of parallel gratings than orthogonal gratings, and slow motion produced only modest masking of either grating orientation. Masking strength in the fast, parallel condition increased with time and reached a plateau after 77 ms, providing an estimate of the temporal integration period for mechanisms encoding motion streaks. Interestingly, the greater masking by fast motion of parallel compared with orthogonal gratings first reached significance at 48 ms before motion onset, indicating an effect of backward masking by motion streaks.
Highlights
Visual perception may seem compellingly real and immediate, but it does not arise instantaneously
Consistent with our reasoning that this effect is due to iso-orientation masking, when the target grating was oriented orthogonally to the motion streaks, masking was greatly attenuated, by about 8–10 dB
The orientation effect for the fast ‘streaky’ motion mask is large, with target grating thresholds showing 10 dB more masking when they are oriented parallel with the direction of motion than when oriented orthogonally. This is the critical comparison for our streak hypothesis, as streaks should only be present in fast motion masks, and the masking effect should only occur when the test grating is parallel to the streaks
Summary
Visual perception may seem compellingly real and immediate, but it does not arise instantaneously. The neurons underlying our visual experience operate on an integrate-and-fire principle, and so it takes time for their response to build up Within their integration period, information is accumulated, and the eventual response is a function of the summed inputs over that time span. One obvious benefit is that a weak signal is more likely to be detected, as its sum over the integration period may exceed a neuron’s threshold, even though the instantaneous signal may be weak and sub-threshold. For example, that two brief signals of duration t are equivalent to a single signal of duration 2t Another problem is that any movement of the stimulus during the integration period will lead to blurring of the summed image. Thresholds should reach a plateau when the asynchrony matches the temporal integration period, as beyond this point the oriented streak information will not accumulate any further. We predict that masking will be strongest for fast motion parallel to the target grating, as fast motion will leave long motion streaks that will effectively mask the orientation of the parallel target grating
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