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Responses of Neurons in the Medial Superior Temporal Visual Area to Apparent Motion Stimuli in Macaque Monkeys

Howard Hughes Medical Institute, Department of Physiology, W. M. Keck Foundation Center for Integrative Neuroscience, and Neuroscience Graduate Program, University of California, San Francisco, California

Submitted 7 September 2006; accepted in final form 1 October 2006

Monkeys fixated a stationary spot during presentation of dot textures that moved in apparent motion defined by the spatial and temporal separations, x and t, between successive flashes of each dot. For each neuron, we assessed the speed tuning for smooth motion (t = 2 or 4 ms) at speeds 128°/s and the effect of varying the value of t at speeds of 16 and 32°/s. Many medial superior temporal (MST) neurons, like middle temporal (MT) neurons, were tuned for the speed of smooth motion and showed decreases in firing rate as the value of t increased at a constant speed. A subset of MST neurons, however, showed monotonically increasing firing rates as a function of smooth stimulus speed and responses to apparent motion that paralleled a previously discovered illusion where estimates of target speed increase with the value of t. Opponent firing rate, defined as the difference between responses for motion in the preferred and opposite directions, peaked at values of t that were consistent with the behavioral illusion. Comparison with a new sample of MT neurons recorded with the same stimuli failed to reveal comparable effects. Attempts to map the population responses in MT and MST onto the behavioral illusion of increased speed succeeded by averaging the opponent response across MST neurons, but only by applying vector averaging to determine the preferred speed of the most active MT neurons. We suggest that a vector-averaging computation transforms MT's place code for target speed into the rate code of some MST neurons.

This article has been cited by other articles:

				J. Yang and S. G. Lisberger Relationship Between Adapted Neural Population Responses in MT and Motion Adaptation in Speed and Direction of Smooth-Pursuit Eye Movements J Neurophysiol, May 1, 2009; 101(5): 2693 - 2707. [Abstract] [Full Text] [PDF]