SPATIAL LAYOUT AFFECTS SPEED DISCRIMINATION

We address a surprising result in a previous study of speed discrimina tion with multiple moving gratings: discrimination thresholds decrease d when the number of stimuli was increased, but remained unchanged whe n the area of a single stimulus was increased [Verghese & Stone (1995) . Vision Research, 35, 2811-2823]. In this study, we manipulated the s patial- and phase relationship between multiple grating patches to det ermine their effect on speed discrimination thresholds, In a fusion ex periment, we merged multiple stimulus patches, in stages, into a singl e patch, Thresholds increased as the patches were brought closer and t heir phase relationship was adjusted to be consistent with a single pa tch. Thresholds increased further still as these patches were fused in to a single patch. In a Jission experiment, we divided a single large patch into multiple patches by superimposing a cross with luminance eq ual to that of the background. Thresholds decreased as the large patch was divided into quadrants and decreased further as the quadrants wer e maximally separated. However, when the cross luminance was darker th an the background, it was perceived as an occluder and thresholds, on average, were unchanged from that for the single large patch. A contro l experiment shows that the observed trend in discrimination threshold s is not due to the differences in perceived speed of the stimuli. The se results suggest that the parsing of the visual image into entities affects the combination of speed information across space, and that ea ch discrete entity effectively provides a single independent estimate of speed. Copyright (C) 1997 Published by Elsevier Science Ltd.