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N Matthews; Correlations among Speed, Direction, and Luminance Discrimination Thresholds . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3867.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To assess the extent to which speed, direction, and luminance discriminations are constrained by overlapping sensory responses. Methods: Thirty naïve observers participated in a within-subjects experimental design. On each trial, two random-dot cinematograms (RDCs) were shown successively. The two RDCs always differed from each other in three ways; speed, direction, and luminance. The speed, direction, and luminance differences were varied independently of each other, and were presented according to the method of constant stimuli. Across trial blocks, the stimulus conditions were held constant while the task was manipulated. Specifically, in separate blocks, each subject judged either the speed, or the direction, or the luminance of the second RDC relative to the first. A discrimination threshold, defined as half the stimulus increment required to alter the response rate from 0.25 to 0.75, was measured for each subject on each task. Results: Thresholds on the speed and direction tasks were significantly correlated with each other ( r(28)=0.50, p < 0.005 ). Luminance thresholds were correlated neither with speed thresholds ( r(28) = 0.024, n.s. ), nor direction thresholds ( r(28) = 0.015, n.s. ). Conclusion: The significant correlation between speed and direction thresholds is consistent with the notion that the two motion tasks are constrained by overlapping sensory responses. Such overlap would likely be only partial, however, as the correlation explained just 25% (r^2=0.25) of the task variability under identical stimulus conditions. It is unlikely that the shared variability in speed and direction thresholds reflects motivational or other non-specific differences across subjects, since luminance thresholds were not correlated with motion thresholds.
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