Motion selectivity in macaque visual cortex. III. Psychophysics and physiology of apparent motion

We have conducted physiological and psychophysical experiments to identify possible neural substrates of the perception of apparent motion. We used identical sequences of flashed stimuli in both sets of experiments to better compare the responses of cortical neurons and psychophysical observers. Physiological data were obtained from two cortical visual areas, striate cortex (V1) and the middle temporal area (MT). In the previous paper we presented evidence that neuronal thresholds for direction selectivity in extrastriate area MT were similar to psychophysical thresholds for motion perception at the largest effective interflash interval, and thus speed, for a given eccentricity. We now examine physiological and psychophysical thresholds for a broad range of speeds to determine whether such a correspondence exists for speeds below the upper threshold considered in the previous paper. Stimuli were presented in stroboscopic motion of constant apparent speed while the spatial and temporal interflash intervals were systematically varied. For each neuron we measured the largest spatial interval that elicited directionally selective responses at each of several apparent speeds. We calculated the composite performance of neurons in both MT and V1 by averaging the spatial interval necessary for direction selectivity at each apparent speed. We employed the same apparent-motion stimuli for psychophysical experiments with human subjects in which we measured the spatial interval necessary for the perception of motion over a similar range of apparent speeds. We obtained a composite profile of psychophysical performance by averaging thresholds across subjects at each apparent speed. For high apparent speeds, physiological data from MT, but not V1, corresponded closely to the psychophysical data as suggested in the preceding paper. For low apparent speeds, however, physiological data from MT and V1 were similar to each other and to the psychophysical data. It would appear, therefore, that neurons in either V1 or MT could mediate the perceptual effect at low speeds, whereas MT is a stronger candidate for this role at high speeds. We suggest that the neuronal substrate for apparent motion may be distributed over multiple cortical areas, depending upon the speed and spatial interval of the stimulus.