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Jing Liu, William T Newsome; Correlation between MT activity and behavioral judgment of visual speed in macaque monkeys. Journal of Vision 2003;3(9):199. doi: 10.1167/3.9.199.
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© ARVO (1962-2015); The Authors (2016-present)
The middle temporal area (MT) has been implicated in the visual perception of the speed of moving objects. MT of macaque monkeys contains neurons tuned for the speed of moving stimuli, and monkeys with MT lesions exhibit impaired speed perception. The human homolog of MT exhibits elevated activity when human subjects perform speed-related tasks, as revealed in imaging studies. In this study, we investigated whether fluctuations in MT activity correlate with a monkey's judgments of speed on a trial-to-trial basis.
We trained two monkeys to perform a fine speed discrimination task. During each trial, the monkey fixated on a central spot while viewing peripherally moving random dots in two apertures symmetric around the fixation point. Dots within each aperture moved at the same direction and speed, but the two groups of dots differed slightly in their speeds. At the end of each trial, the monkey had to make a saccade to the aperture containing the faster dots. We positioned one aperture within the MT receptive field and recorded multi-unit responses as the monkey performed the task. Using methods based in signal detection theory, we computed a ‘choice probability’ for each site. The distribution of choice probabilities shows that the trial-to-trial fluctuations of neural activity in MT are correlated with perceptual judgments of speed. In other words, by monitoring the activity of MT neurons, one can predict with greater than chance probability the monkey's speed judgment on individual trials. This finding strongly implies that MT contributes to the monkey's perceptual decisions on speed. A comparison of neurometric and psychometric thresholds revealed, however, that MT multi-units are generally less sensitive to small speed differences than are the monkey psychophysically. Pooling may be necessary to account for psychophysical sensitivity. We have been unable to bias speed judgments using electrical microstimulation, presumably due to a lack of speed columns in MT.
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