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Maarten J. van der Smagt, Chris L. E. Paffen, Frans A. J. Verstraten; Perceived speed and center-surround organization. Journal of Vision 2006;6(6):1054. doi: 10.1167/6.6.1054.
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The perceived speed of a moving stimulus is known to decrease with decreasing contrast as well as increasing stimulus size. Similar effects of contrast and stimulus size have led to the hypothesis that the inhibitory influence of a moving surround acts by decreasing effective contrast in the center. However, at low contrasts center-surround interactions are generally facilitatory. We thus investigated the interaction between contrast, surround size and speed perception.
In a two interval forced choice paradigm, participants judged which of two central moving gratings (radius 1 deg) moved faster. The ‘standard’ moved at 4 deg/s in one of eight directions, had no surround, and had a contrast of 25%. The ‘probe’ had a contrast between 1.5 and 99% and had either no surround or was surrounded by an annulus of 5 or 10 degrees outer radius. The speed of the probe was varied. A staircase procedure determined the point of subjectively equal speed between standard and probe.
Both decreasing contrast and increasing surround size decreased the perceived speed of the moving central grating. The magnitude of the surround effect, however, was much larger than that of the effect of contrast. Interestingly, the effect of adding a surround on perceived speed was similar for very low as well as higher contrast stimuli. This result is at odds with studies on direction discrimination that have shown surround facilitation at low contrasts, and adds to the evidence suggesting independence between direction and speed processing.
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