It has been shown that a moving visual pattern can influence the perceived position of outlying, briefly flashed objects. Using a rotating bar as an inducing stimulus we observed a shift, in the direction of motion, of the perceived position of small bars flashed together on either side of the moving bar. The greatest shift occurred when the 13 ms flashes were presented 60 ms before the rotating bar came closest to their locations. By varying rotation speed we showed that the peak effect was determined by the temporal rather than the spatial interval. The motion induced shift could be attenuated by introducing background flickering dots. The perceived shift decreased with distance from motion when the eccentricity of the flashes was kept constant. We conclude that the shift reflects feedback to primary visual cortex from motion selective cells in extrastriate cortex with receptive fields that overlap the retinal location of the flash.
When cat V1/V2 cells are adapted to contrast at their optimal orientation, a reduction in gain and/or a shift in the contrast response function is found. We investigated how these factors combine at the population level to affect the accuracy for detecting variations in contrast. Using the contrast response function parameters from a physiologically measured population, we model the population accuracy (using Fisher information) for contrast discrimination. Adaptation at 16%, 32%, and 100% contrast causes a shift in peak accuracy. Despite an overall drop in firing rate over the whole population, accuracy is enhanced around the adapted contrast and at higher contrasts, leading to greater efficiency of contrast coding at these levels. The estimated contrast discrimination threshold curve becomes elevated and shifted toward higher contrasts after adaptation, as has been found previously in human psychophysical experiments.
Contextual effects abound in vision. The tilt illusion (TI) is an example-a tilted surrounding annulus causes a vertical central pattern to appear rotated away from the surround. We investigate the dynamics of this effect by presenting components of the stimulus asynchronously. At equal contrast, the largest illusion occurs when centre and surround are presented simultaneously. We vary the spatial gap between centre and surround, the relative contrast and depth and find that these segmentation cues result in a reduced TI upon simultaneous presentation, but not all other times. This reveals the dynamics of orientation and other segmentation cue interactions.
The tendency for briefly flashed stimuli to appear to lag behind the spatial position of physically aligned moving stimuli is known as the flash-lag effect. Possibly the simplest explanation for this phenomenon is that transient stimuli are processed more slowly than moving stimuli. We tested this proposal using a task based upon the simultaneous tilt illusion. When an oriented stimulus is surrounded by another oriented stimulus, the inner stimulus can appear to be rotated away from the orientation of the surround. By flashing central static sinewave gratings at specific phases of an annular gratings rotation cycle, we were able to determine the temporal dependence of the tilt illusion. Our results suggest a small, approximately 20 ms, processing advantage for the rotating stimulus relative to the flashed stimulus. Such a small advantage, if due to differential latencies, is insufficient to account for the flash-lag effect.
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