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Bart Krekelberg, Richard J. A. Wezel, Thomas D. Albright; Speed adaptation in macaque MT. Journal of Vision 2004;4(8):82. doi: 10.1167/4.8.82.
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© ARVO (1962-2015); The Authors (2016-present)
Prolonged exposure to a moving stimulus changes speed perception in two salient ways. First, the speed of a subsequently presented stimulus is underestimated. Second, the ability to discriminate speeds is improved. This can be interpreted as a sacrifice in accuracy of absolute speed perception in favor of the ability to discriminate speeds. We investigated the neural correlates of this perceptual phenomenon in the macaque middle-temporal area. We used circular patches of random dots both as adaptation and test stimuli. In the experimental condition, a patch with random dots moving in the cell's preferred direction was positioned in a cell's receptive field for 2s. After this adaptation period and a brief (0.5s) blank period, test stimuli were shown at 5 speeds around the adaptation speed (80%, 40% or 0% slower or faster than the adaptation speed). In the control condition, the dots were stationary during the adaptation period. This allowed us to control for the effect of contrast adaptation. We recorded from 83 cells in two monkeys that fixated a central dot during stimulus presentation. The effects of adaptation on the cell's responses were clear-cut; nearly all cells reduced their firing rate. Assuming a code in which higher firing rates correspond to higher perceived speeds, this could explain the underestimation of perceived speed after adaptation. The reduction in firing rate was not constant for all test speeds; it was strongest when the test speed was most different from the cell's preferred speed. This implies that the population speed-tuning curve became narrower. Such a narrower tuning curve may explain why motion discrimination improves with adaptation.
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