Motion streaks provide a spatial code for motion direction

Although many neurons in the primary visual cortex (V1) of primates are dir ection selective(1), they provide ambiguous information about the direction of motion of a stimulus(2,3), There is evidence that one of the ways in wh ich the visual system resolves this ambiguity is by computing, from the res ponses of V1 neurons, velocity components in two or more spatial orientatio ns and then combining these velocity components(2-9). Here I consider anoth er potential neural mechanism for determining motion direction. When a loca lized image feature moves fast enough, it should become smeared in space ow ing to temporal integration in the visual system, creating a spatial signal -a 'motion streak'-oriented in the direction of the motion. The orientation masking and adaptation experiments reported here show that these spatial s ignals for motion direction exist in the human visual system for feature sp eeds above about 1 feature width per 100 ms. Computer simulations show that this psychophysical finding is consistent with the known response properti es of V1 neurons, and that these spatial signals, when appropriately proces sed, are sufficient to determine motion direction in natural images.