ANTAGONISTIC COMPARISON OF TEMPORAL FREQUENCY FILTER OUTPUTS AS A BASIS FOR SPEED PERCEPTION

The prevailing view of motion detection in human vision is that the re tinal image is convolved with each of a set of spatiotemporal filters and that perceived speed emerges from a process of pooling the outputs of these filters. Such a system can operate only if multiple filters exist; ideally the filters should also be fairly narrowly tuned in bot h spatial and temporal frequency. These constraints are met in the cas e of spatial frequency. But several studies suggest that multiple, fin ely tuned temporal filters do not exist; instead just two (perhaps thr ee) broadly-tuned temporal mechanisms can be identified. We report som e experiments concerning the effects of adaptation to motion on percei ved speed. It is shown that perceived speed is increased by adaptation in some circumstances and decreased in others. We then present a comp utational model in which a temporal frequency code, on which perceived speed is presumed to be based, is derived by a process of antagonisti c comparison of the responses of two psychophysically-plausible, broad ly-tuned temporal mechanisms. The model, which includes the effects of adaptation to motion upon the sensitivities of the filters and the su bsequent comparison of their sensitivities, is shown to give a good fi t to the empirical data.