Speed and direction selectivity of macaque middle temporal neurons

Abstract

1. We tested quantitatively the responses of 147 middle temporal (MT) cells to light and dark bars moving at different speeds ranging over a 1,000-fold range (0.5–512 deg/s). 2. We derived the following quantities from the speed-response (SR) curves obtained for opposite directions of motion. Speed selectivity was characterized by the maximum response, optimum speed, upper cutoff speed, response to slow movement, and tuning width. Direction selectivity was characterized by the direction index (DI) averaged over speeds yielding significant responses (MDI) and by the direction index at optimal speed (PDI). 3. There was an excellent correlation between speed characteristics for light and dark bars. These correlations were stronger than the correlations between direction indexes. The strongest correlations were obtained for maximum response and upper cutoff. 4. SR curves were classified into three groups: low pass (25%), tuned (43%), and broadband (28%), leaving 4% unclassified. 5. In the majority (75%) of MT cells, there was an agreement between the typology of speed selectivity for light and dark bars. Cells were classified as tuned (33%), low pass (22%), broadband (19%), and mixed (22%), leaving 4% unclassified. In addition to differences in speed characteristics, these groups also differed in response level, direction selectivity, and distribution of preferred directions. 6. For tuned cells, there was a very tight correlation of most speed characteristics for light and dark bars. 7. Direction selectivity depended on stimulus speed in most neurons, yielding a tuned average speed-DI curve. 8. Speed characteristics, proportions of speed selectivity types, and direction selectivity indexes showed little dependence on laminar position. 9. Speed characteristics and direction selectivity indexes were not dependent on eccentricity. Proportion of speed selectivity types however, changed dramatically with eccentricity: low-pass cells dominated foveally, tuned cells parafoveally, and broadband cells peripherally. 10. There were also small eccentricity effects on the range of optimal speeds shown by tuned cells and on the speed at which direction selectivity decreases in the slow speed range.