Detection of vorticity in optical flow fields

Psychophysical thresholds for the detection of vorticity in the presence of a translational component are determined for human observers. Stimuli consist of sparse random-dot flow patterns. The detection of vorticity depends critically on the translational component. The curvature of the flow lines, however, cannot be the only factor limiting human performance. On the basis of a comparison with an ideal detector, it is found that for small vorticities (<0.5 rad/s) humans typically use the stimulus information in an optimal manner. For higher vorticities performance gets worse, possibly owing to matching problems. Lifetime and number of dots have hardly any influence on performance. Although this indicates that the human observer can already perform the task by using only local information, it does not imply that global information is disregarded. If the stimulus is disturbed locally, global information is used to full extent.