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P. B. Delahunt, J. L. Hardy, J. S. Werner; Senescence of Human Orientation Discrimination. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5502.
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Recent electrophysiological studies demonstrate that the number of highly tuned orientation sensitive cells in macaque V1 and V2 is dramatically reduced with age [Schmolesky et al. (2000); Yu et al. (2006)]. Similar changes are likely to occur in the human visual cortex. Our goal was to examine the extent to which these putative changes impact orientation discrimination sensitivity at the perceptual level.
Ten younger (mean age 26.7) and ten older (mean age 71.5) observers participated in this study. The stimuli consisted of 1 and 4 cpd Gabor patches presented on a calibrated CRT. Thresholds were obtained using 2 randomly interleaved adaptive staircases (QUEST). In Experiment 1, baseline contrast sensitivity measurements were obtained for vertically orientated stimuli. In Experiment 2, orientation discrimination thresholds were obtained by asking participants to indicate whether a Gabor patch was tilted to the left or right. In Experiment 3, orientation tuning curves were obtained using a masking paradigm where contrast sensitivity was measured for a vertical Gabor patch superimposed on sine-wave maskers of various orientations.
All three experiments show large differences in performance between young and old. However, when the data are plotted as multiples of contrast sensitivity, the differences largely disappear. The bandwidth of orientation channels estimated from the masking experiment was similar for younger and older observers.
Despite the putative losses in the number of orientation-tuned cells in the visual cortex, orientation discrimination of the older participants was similar to the younger group when contrast sensitivity differences were taken into account. In addition, the perceptual orientation tuning curves are similar in shape. This suggests a re-organization of the visual system to adapt to changing inputs so that the perceptual performance remains relatively constant throughout the life span.References:Schmolesky et al. (2000), Nature, 3, 385-390.Yu et al. (2006), Neuroscience, 140, 1023-1029.
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