Abstract
Primate visual system can integrate local motion vectors into a global motion pattern and segment multiple visual stimuli based on motion cues. Previous psychophysical studies showed that when random dots of overlapping stimuli moving in two directions were locally paired, human subjects perceived an integrated vector-averaged (VA) direction of the stimuli; whereas when the dots were unpaired, subjects perceived motion transparency and two component directions. The mechanism underlying this drastic perceptual change is unknown. We recorded from neurons in the middle-temporal (MT) cortex of fixating monkeys. Visual stimuli were random dots moving at 5⁰/s in two directions separated by 90⁰. We varied the VA direction of the stimuli to characterize the direction tuning. In the “paired-dot” condition, two dots moving in different directions were locally paired within a path of 0.4⁰ (lifetime of 80 ms). In the “unpaired-dot” condition, dots moving in two directions were unpaired and had the same lifetime. We found that in response to the unpaired-dot stimuli, neurons showed bimodal tuning which represented two component directions, similar to our previous finding using stimuli that had a much longer lifetime and elicited strong motion transparency. Remarkably, MT neurons showed a unimodal tuning to the paired-dot stimuli and the peak was reached when the VA direction of the stimuli was near a neuron’s preferred direction. The shape of the unimodal tuning closely matched the tuning to single motion directions, suggesting MT responses to the paired-dot stimuli correlate with perceptual integration. Moreover, when the paired-dot stimuli were placed closer to fovea, they appeared to contain two directions and the MT tuning changed from unimodal to bimodal, again correlated with the perceptual change. Our findings reveal neural correlates of an intriguing perceptual phenomenon and have implications for the roles of local computation in early visual areas on motion integration and segmentation.