Detection of divergence in optical flow fields

Psychophysical thresholds for the detection of divergence (expansion and contraction) in the presence of a translational component are determined. Stimuli consist of sparse random dot patterns. Lifetime, number of dots, divergence, and translational velocity are varied over a wide range. Moreover, the two major characteristics of a divergence pattern, namely, the direction of and the velocity along the flow lines, are also varied independently. Lifetime and number of dots have only a small influence on performance. Over a wide range the detection of divergence is independent of the translational component. The results indicate that the direction of flow lines is an important, but by no means the only, characteristic in divergence detection. This provides evidence that, at least in these experiments, no use is made of a mechanism selectively sensitive for divergence.