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Lia E. Tsotsos, Allison B. Sekuler, Patrick J. Bennett; The effects of aging on the bandwidths of directionally-selective mechanisms. Journal of Vision 2008;8(6):1023. doi: 10.1167/8.6.1023.
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Psychophysical studies show that motion detection thresholds are elevated, and the accuracy of perceived direction is diminished, in older human subjects (Bennett et al., 2007, Vision Res, 47, 799–809). This result is consistent with the hypothesis that the bandwidth of directionally-selective channels widens with age. The current experiment examined this hypothesis directly by estimating directional selectivity in groups of younger (n=7; mean age = 20.6) and older (n=7; mean age = 67) subjects using a notched-noise masking technique. The stimuli were 500 ms presentations of random dot kinematograms that drifted coherently to the right (0 deg) or left (180 deg) at 5 deg/sec. The subject's task was to identify the direction of motion, and thresholds were estimated by varying dot contrast. The dots were embedded within a mask comprised of high-contrast dots that moved in random directions on each frame. The direction of each mask dot was drawn from a random distribution spanning 360 deg, with the restriction that the direction could not fall within the ranges of 0±n and 180±n deg. The width of the noise notch, 2n, was varied from 0 to 179 deg in separate blocks. In both groups, thresholds increased as 2n increased from 0 to 10, and then decreased gradually as 2n increased to 179 deg. However, thresholds decreased more rapidly in younger subjects: specifically, the quadratic trend in the threshold-vs-notch function was significantly greater in younger subjects (F(1,96)=5.03, p=.027). This result is consistent with the claim that directionally-selective channels are more broadly tuned in older subjects. We currently are extending this result to other dot speeds and a broader range of older subjects.
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