Abstract
The responsivity of primary sensory cortical neurons is reduced following prolonged adaptation to an effective stimulus, but such adaptation has been little studied in higher sensory areas. Many psychophysical studies have described strong perceptual effects of adaptation to visual motion, so we studied the effect of prolonged visual stimulation on the responses of neurons in the macaque's area MT, a cortical area whose importance to visual motion perception is well established. We adapted MT neurons with sinusoidal gratings drifting in the preferred or null direction. Preferred adaptation strongly reduced the responsiveness of MT cells, primarily by changing their contrast gain, and this effect was specific to the adapted location within the receptive field. Null adaptation did not affect responses to preferred motion, but reduced the ability of null gratings to inhibit the response to a simultaneously presented preferred stimulus. These adaptation effects may be due to adaptation-induced changes in contrast gain earlier in the visual pathway, and may not reflect processing changes within MT itself.
We next examined the effect of adaptation on stimulus selectivity. In primary visual cortex, responsivity is usually reduced most when the adapting and test stimuli are well matched. Theories about the functional benefits of adaptation have relied on this specificity, but the resultant changes in neuronal tuning are of the wrong kind to account for well-documented perceptual aftereffects. We studied the effect of adaptation on the direction tuning of MT neurons. In striking contrast to V1, responsivity in MT is best maintained in the adapted direction, but strongly reduced for nearby directions. Consequently, adaptation in the preferred direction reduces direction tuning bandwidth, while adaptation at near-preferred directions causes tuning to shift toward the adapted direction. This novel effect of adaptation is consistent with perceptual aftereffects and suggests that different cortical regions adjust to constant sensory input in distinct ways.