Abstract
Neurons in the extrastriate visual area MT respond selectively to both the direction and the speed of a given stimulus. However, these response properties may depend upon the shape, or form, of the stimulus. We measured the dependence of speed tuning on spatial frequency by systematically varying the spatial and temporal frequency of sine gratings moving in the preferred direction of the isolated cell. In spatial and temporal frequency space, a speed-tuned cell must have an oriented response profile, such that the optimal temporal frequency of the cell shifts in proportion to changes in spatial frequency. Although we found that some MT cells exhibited oriented response profiles, their shifts in temporal frequency preference as a function of the spatial frequency of the stimulus were too small to be consistent with form-invariant speed tuning. We have now extended this work by measuring the response profiles of the cells to both low (8%) and high (32%) contrast gratings. MT cells' preferences for spatial and temporal frequency did not change as a function of contrast, although the responses to lower contrast gratings tended to be less oriented in spatial-temporal frequency space. These results indicate that MT neurons are tuned to specific spatial-temporal frequency combinations, rather than to speed. However, speed tuning was more strongly evident for stimuli containing more than one spatial frequency. When a stimulus contained two spatial frequencies moving at a single speed the response of the MT cell was greater than when the two components moved at different speeds. These data suggest that the absence of true speed-tuning in area MT for sine wave gratings may not pose a problem for representing the speed of real-world objects.