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In the awake macaque, MT neuronal firing to ambiguously rotating cylinders is correlated with the reported direction of rotation when the stimulus is closely matched to the neuronal receptive field. We recently reported the existence of strong choice-related firing in MT, even when the orientation of the cylinder is sub-optimal for the neuron (Curnow et al. 2002, Neurosci. Abs.). It is not clear if the choice signal has the same source in both cases. Here, we have investigated whether the neuronal correlates of the perceptual decision showed similar temporal characteristics over the two second trial for both sub-optimal and optimal stimuli. We separated the trials according to the monkey's choice and the stimulus orientation. For each neuron, all action potentials for each group of 2 second trials were assigned to 60ms bins and the histograms were normalised and averaged over neurons. The emergence of choice-related firing at sub-optimal orientations was similar to that found at optimal orientations. Choice-related differences in firing were not present before stimulus onset but emerged during the first 100-500ms. They then remained stable for the remainder of the trial. The stimulus orientation on the preceding trial did not affect firing rates but there was a relation between choice on the preceding trial and firing rate in the first 500ms of the current trial. The similarity in temporal characteristics between choice-related firing at optimal and sub-optimal orientations suggests that the choice-signals observed in MT stem from the same source. Thus the pool of MT neurons that carries a choice signal (and thus potentially contributes to the decision process) is composed of neurons with a wide range of tuning properties. This has significant implications for models of the neuronal coding of perceptual decisions.

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