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Oliver Braddick, Rory Trevelyan-Thomas, Catherine Manning; CAN SPEED BE JUDGED INDEPENDENT OF DIRECTION?. Journal of Vision 2017;17(10):936. doi: 10.1167/17.10.936.
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The ability to judge speed is a fundamental aspect of visual motion processing. Speed judgments are generally assumed to depend on signals in motion-sensitive, directionally selective, neurons in areas such as V1 and MT. Speed comparisons might therefore be expected to be most accurate when they use information within a common set of directionally tuned neurons. However, there does not appear to be any published evidence on how well speeds can be compared for movements in different directions. We tested speed discrimination judgments between pairs of random-dot stimuli presented side-by-side. 12 participants judged which appeared faster of a reference stimulus moving vertically upwards or downwards at 6 deg/sec, and a comparison stimulus moving either in the same direction or in a direction differing by 180º, ±45º or ±90º. The point of subjective equality (bias) and sensitivity (slope of the psychometric function) were estimated from individual psychometric functions fitted for each condition. We found no systematic differences in sensitivity across different directional combinations. However, when the comparison stimulus moved orthogonally (±90º) to the reference stimulus, but not in other combinations, it was judged to move faster than the reference stimulus. We conclude that while there are directional biases in speed information, it can be equally efficiently compared whether it is derived across or within direction channels. These results inform our understanding of how information is extracted from the joint representation of speed and direction, and guide methods for testing motion processing in typical and atypical development.
Meeting abstract presented at VSS 2017
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