Spatial summation and its interaction with the temporal integration mechanism in human motion perception

The combination of visual motion information over visual space (spatial summation) and stimulus duration (temporal integration) was investigated using a random-pixel array (spatiotemporally broad-band) apparent motion stimulus designed to isolate specific populations of visual motion detectors. The results indicate that, in agreement with results from spatiotemporally narrow-band stimuli, spatial summation follows the form of linear probabilistic summation rather than non-linear probabilistic summation. Linear probabilistic summation holds for a wide range of stimulus parameters and when changing either motion stimulus height or width. Linear probabilistic summation breaks down when the motion display region approaches a height and/or width that is related to the spatial displacement size, not the speed, of the random-pixel array. This height and width (termed the critical height and width, or critical dimension), increases with spatial displacement size and can be interpreted as a measure of the basic dimensions of the selected motion detector population's receptive field. The critical height is smaller than the critical width, a result that is consistent with a motion detector receptive field that is elongated in the direction of motion. Perhaps most importantly, the mechanisms of temporal integration and spatial summation can work independently under a wide range of conditions. Finally, the results provide evidence for a short-term inhibitory phenomenon from the edges of the useful display area that affects the visibility of the motion.