The effects of spatiotemporal integration on maximum displacement thresholds for the detection of coherent motion

In a series of nine experiments, observers were required to identify the shapes of moving targets, and to discriminate regions of motion from regions of uncorrelated noise. Maximum displacement thresholds (Dmax) for performing these tasks were obtained under a wide variety of conditions. The stimulus parameters manipulated included the number of distinct frames in the motion sequences, the stimulus onset asynchrony between each frame, the size of the moving dots, and the shape, area and eccentricity of the target regions. For two-frame displays presented in alternation, the area of the target region was the only one of these variables to have any significant effect on Dmax. For longer length sequences, in contrast, Dmax varied dramatically among the different conditions over a range of 10 min arc to 10 deg. In an effort to isolate the specific processes of spatiotemporal integration, we also examined how performance is affected by having overlapping transparent motions in opposite directions, or by the presence of dynamic noise or limited dot lifetimes within the moving target regions. The overall pattern of results suggest that Dmax is primarily determined by the ability of the visual system to isolate motion signals from the noise produced by spurious false target correlations. As a general rule, Dmax will increase as a result of any stimulus manipulation that increases the number of local signal correlations detected relative to those arising from noise, and vice versa.