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Lia E. Tsotsos, Allison B. Sekuler, Patrick J. Bennett; The effect of aging on directional tuning when making judgments about horizontal and vertical motion. Journal of Vision 2011;11(11):468. doi: https://doi.org/10.1167/11.11.468.
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Motion direction identification is impaired in older subjects (Bennett et al., Vis Res, 2007). One explanation for this effect is that the bandwidth of directionally selective mechanisms broadens with age. Although neurophysiological work with senescent primates has found evidence of broader directional tuning in V1 neurons, it is not known if directional tuning changes similarly as a function of age in human observers. To investigate this issue, we measured direction discrimination thresholds in six younger and six older adults. The stimuli were random dot kinematograms (RDK; 400 dots), and the dependent variable was the percentage of coherently moving dots needed to discriminate left-right or up-down motion. RDKs were embedded in a mask consisting of 100 dots moving coherently in four directions: d ± δ deg and (d + 180) ± δ deg, where d is the target direction and δ is the difference between target and mask directions. For both left-right and up-down target motion, thresholds in both age groups declined monotonically as δ increased from 5 to 90 deg. For left-right target motion, neither the slope of the masking function nor threshold in a no-mask baseline condition differed between age groups. For up-down target motion, the masking function obtained with older subjects had a slope that was one-half of the slope obtained with younger subjects, and threshold in the no-mask condition was, on average, twice as high in older than younger subjects. These preliminary results are inconsistent with the idea that there is a non-specific decrease in the selectivity of directionally-tuned mechanisms with age. They suggest, instead, that the effect of aging on the selectivity of directional masking may vary with the target direction. Currently we are replicating this result on a larger sample of subjects, and generalizing it to different dot densities and speeds.
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