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Gregory C. DeAngelis, Yong Gu, Dora E. Angelaki; MSTD population responses account for the eccentricity dependence of heading discrimination thresholds. Journal of Vision 2005;5(8):387. doi: 10.1167/5.8.387.
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© ARVO (1962-2015); The Authors (2016-present)
Psychophysical studies have shown that humans can use optic flow to discriminate differences in heading direction < 1 deg. Maximal heading sensitivity occurs for directions around straight ahead, whereas performance falls off steeply when observers discriminate headings around an eccentric reference. Neurons in area MSTd, which respond selectively to large-field flow patterns, are thought to be a key element of the neural substrate for heading perception. Thus, the population code for heading in MSTd should account for the dependence of psychophysical thresholds on heading eccentricity. It has been suggested that a preponderance of neurons tuned to forward motion (expansion cells) might explain the psychophysics. We examined this issue by recording extracellularly from a large, unbiased sample of MSTd neurons in two fixating macaque monkeys. A 3D heading tuning function was measured for each neuron by presenting large-field optic flow patterns depicting all possible directions of heading separated by 45 deg in both azimuth and elevation. MSTd neurons typically have broad, cosine-like tuning for heading in 3D. Heading preferences are distributed broadly, but neurons preferring fore-aft motion are substantially less common than neurons preferring lateral motion. Thus, a preponderance of expansion cells is not the explanation for better heading discrimination around straight ahead. On the other hand, neurons preferring lateral motion frequently have the steep slopes of their heading tuning functions near forward motion, and this could account for better discrimination. By fitting these 3D tuning functions, along with the variance-mean relationship for each neuron, we computed the performance of a maximum-likelihood estimator of heading over a wide range of heading eccentricities. Heading thresholds for this estimator mimic the psychophysical data, indicating that the MSTd population code can account for increased heading sensitivity around forward motion.
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