The neurophysiological response properties of single neurons were studied quantitatively in four extrastriate areas of the owl monkey: the medial (M), dorsomedial (DM), dorsolateral (DL), and the middle temporal (MT) areas. Directionality was computed by comparing the responses to stimuli moved in the optimal and opposing directions; MT cells had much higher directionality to moving bars than cells in the other areas. Cells in all four areas were sharply tuned to the orientation of stationary flashed bars. Tuning for moving bars was broader than for flashed bars; DM cells were more sharply tuned to moving bars than were cells in the other areas. Tuning was broader to spots than to bars, while directionality was relatively unaffected. A moving array of random dots was the best stimulus for many MT neurons. Random dot stimuli were also effective in M, but evoked weak or no response from DM and DL cells. Extrastriate receptive fields were much larger than striate receptive fields. Eccentricity was correlated with receptive field size, but was uncorrelated with other variables.
Neurons in these four areas were tested for their selectivity to the spatial dimensions, the length and width, of visual stimuli. Cells in DL were much more selective for the spatial dimension than were cells in the other areas. The dimensional selectivity of DL cells is independent of the amount or sign of contrast in the receptive field, and the position of the stimulus within the receptive field. The optimal lengths and widths of visual stimuli are specified independently, and have a wide range of optimal dimensions from 1 to 30° in length, and from 0.25 to 7° in width.
Since many of the neurons of MT show strong directionality, it has been hypothesized that MT contributes to the perception of motion. A well-known aspect of motion perception is the phenomenon of direction-specific adaptation. We tested the neurons of MT for changes in responsiveness due to prolonged adaptation to stimuli moving in various directions. For directional cells, the response to a bar was suppressed following adaptation in the best direction, and enhanced following adaptation in the opposite direction, when compared to the response to a bar following a period of stationary stimulation. For nondirection cells, the effects were much weaker, or absent.
These results support the notion of a localization of function among the various extrastriate areas.
