PSYCHOPHYSICAL EVIDENCE OF A SUSTAINED INPUT TO DIRECTIONALLY SELECTIVE MOTION MECHANISMS

Human psychophysical evidence congruent with neurophysiological findin gs of a sustained input to directionally selective motion sensors in c at visual cortex is reported. Apparent motion was produced by displayi ng a group of dots in two frames (F1 and F2), where F2 was a translate d version of F1. All stimulus sequences included a period during which F1 and F2 were displayed concurrently (combined images) and a period during which only F1 or F2 was on display (single images). There were three stimulus sequences: a display beginning with combined and ending with single image, a display beginning with single and ending with co mbined image, and a display beginning with F1, continuing with combine d image, and ending with F2. Six durations of single and of combined i mages (10, 20, 40, 80, 160, and 320 ms) were crossed factorially in ea ch stimulus sequence. Directional motion was seen easily at long durat ions of the single image in all stimulus sequences, as would be expect ed on the basis of a sustained input to the directional-motion-sensing mechanisms. Perception of directional motion improved with the durati on of single images, but declined as the duration of combined images w as increased. Baker and Cynader's model could account for the effect o f duration of single images, but not for the effect of duration of com bined images. An elaborated version of the model provides a good quali tative match to all empirical findings.