Optic ̄ow illusion and single neuron behaviour reconciled by a population model

Radial patterns of optic ̄ow contain a centre of expansion that indicates the observer's direction of self-movement. When the radial pattern is viewed with transparently overlapping unidirectional motion, the centre of expansion appears to shift in the direction of the unidirectional motion [Duffy, C.J. & Wurtz, R.H. (1993) Vision Res., 33, 1481±1490]. Neurons in the medial superior temporal (MST) area of monkey cerebral cortex are thought to mediate optic ̄ow analysis, but they do not shift their responses to parallel the illusion created by transparent overlap. The population-based model of optic ̄ow analysis proposed by Lappe and Rauschecker replicates the illusory shift observed in perceptual studies [Lappe, M. & Rauschecker, J.P. (1995) Vision Res., 35, 1619±1631]. We analysed the behaviour of constituent neurons in the model, to gain insight into neuronal mechanisms underlying the illusion. Single model neurons did not show the illusory shift but rather graded variations of their response speci®city. The shift required the aggregate response of the population. We compared the model's predictions about the behaviour of single neurons with the responses recorded from area MST. The predicted distribution of overlap effects agreed with that observed in area MST. The success of the populationbased model in predicting the illusion and the neuronal behaviour suggests that area MST uses the graded responses of single neurons to create a population response that supports optic ̄ow perception.