Directionally selective response of cells in the middle temporal area (MT) of the macaque monkey to the movement of equiluminous opponent color stimuli.

Based on the fact that a great majority of cells in the middle temporal (MT) area of the macaque respond to movement of luminance contours with directional selectivity, this area has been thought to be concerned with the analysis of visual motion. However, objects can be discriminated from the background not only by differences in luminance but also by differences in color. It is possible that color signals are also used for motion analysis in MT. In the present study, we examined whether MT cells respond to movement of a pattern composed of pure color-contours. Using a color TV system, a moving color bar was displayed on a uniform background whose color was opponent with that of the bar. The main bar/background color combination we examined was magenta/cyan. Yellow/blue and cyan/magenta combinations were also examined for some cells. The response of MT cells to movements of opponent-color stimuli was recorded while the bar/background luminance ratio was changed from 1/10 to 10/1. In half of 89 cells tested in 3 monkeys, the response decreased considerably (disappeared completely in some cells) at a luminance ratio close to the human equiluminous condition. In the other half, a directional response persisted at any bar/background luminance ratio, though the response decreased to a varied extent (30-90% of the maximum response) near the ratio 1 (human equiluminous condition). The average magnitude of the equiluminous response to the magenta/cyan stimulus for the overall population was about 35% of the maximal response when the length of the bar (0.5 degrees in width) and the movement amplitude were set to be optimal for individual cells, i.e. smaller than 15 degrees and 10 degrees of visual angle, respectively. This fall to 23% when the bar length and movement amplitude were limited to 2 degrees. The same cell responded to pure color-contours of yellow/blue as well as of cyan/magenta combinations. Thus, MT can detect the direction of movement of pure color-contours, although the sensitivity is less than for luminance contours.