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Romesh D. Kumbhani, Najib J. Majaj, Golbarg T. Saber, J. Anthony Movshon; Influence of contrast on the pattern direction selectivity of macaque MT neurons. Journal of Vision 2009;9(8):739. doi: 10.1167/9.8.739.
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© ARVO (1962-2015); The Authors (2016-present)
Neurons in area MT can integrate the motion of 1D image elements to signal the 2D motion of complex patterns. This pattern direction selectivity is typically studied using plaids composed of two identical, high-contrast gratings. We wondered how pattern motion computation would be affected when we reduced the contrast of one of the plaid's components to make “hybrid” plaids.
We recorded from 73 MT neurons in anesthetized, paralyzed macaques while presenting drifting gratings and plaids with several component contrast ratios. The tuning of MT neurons to gratings was unaffected by contrast, but their preferred directions for plaids were strongly dependent on the ratio of component contrasts. When the component contrasts were unequal, even when each component evoked a strong response, plaid responses were dominated by the higher contrast component. At contrast ratios of 4:1 or greater, the lower contrast component had little or no influence on the plaid response, even though it was effective when presented alone. This “winner-take-all” interaction between component gratings may be partly mediated by cross-orientation interactions in earlier visual areas, but appears also to involve interactions in MT.
In human observers, we measured the perceived direction of plaids with different component contrasts and found a similar trend. Subjects veridically perceived the direction of pattern motion when the component contrast ratio was 2:1. At higher contrast ratios, subjects showed a strong bias towards the direction of the higher contrast component. We conclude that the pattern motion computation in MT neurons depends on contrast, and may underlie the perceived direction of plaids composed of unequal contrast components.
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