Representation of perceptually invisible image motion in extrastriate visual area MT of macaque monkeys.
Why does the world appear stable despite the visual motion induced by eye movements during fixation? We find that the answer must reside in how visual motion signals are interpreted by perception, because MT neurons in monkeys respond to the image motion caused by eye drifts in the presence of a stationary stimulus. Several features suggest a visual origin for the responses of MT neurons during fixation: spike-triggered averaging yields a peak image velocity in the preferred direction that precedes spikes by ∼60 ms; image velocity during fixation and firing rate show similar peaks in power at 4-5 Hz; and average MT firing during a period of fixation is related monotonically to the image speed along the preferred axis of the neurons 60 ms earlier. The percept caused by the responses of MT neurons during fixation depends on the distribution of activity across the population of neurons of different preferred speeds. For imposed stimulus motion, the population response peaks for neurons that prefer the actual target speed. For small image motions caused by eye drifts during fixation, the population response is large, but is noisy and does not show a clear peak. This representation of image motion in MT would be ignored if perception interprets the population response in the context of a prior of zero speed. Then, we would see a stable scene despite MT responses caused by eye drifts during fixation.
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