TRANSIENT-SUSTAINED INPUT TO DIRECTIONALLY SELECTIVE MOTION MECHANISMS

The characteristics of the sustained input to directionally selective motion sensors were examined in three human psychophysical studies on directional-motion discrimination. Apparent motion was produced by dis playing a group of dots in two frames (F1 and F2), where F2 was a tran slated version of F1. All stimuli included parts that contained both F 1 and F2 (combined images) and parts containing only F1 or F2 (single images). All displays began with a single image (F1), continued with t he combined image, and ended with F2. Six durations of single and of c ombined images (10, 20, 40, 80, 160, or 320 ms) were crossed factorial ly. As the duration of the single image was increased, perception of d irectional motion first improved, and then declined at longer duration s. This outcome contrasted with the monotonic increment obtained in ea rlier studies under low-luminance conditions. To account for the entir e pattern of results, earlier models of the Reichardt motion sensor we re modified so as to include a mixed transient-sustained input to one of the filters of the sensor. Predictions from the new model were test ed and confirmed in two experiments carried out under both low-luminan ce and high-luminance viewing conditions.