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Renee Karas, Allison M. McKendrick; Older Adults Show Both Apparent Increased And Decreased Perceptual Surround Suppression Depending On The Task. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1881.
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The contrast appearance of a central grating patch is reduced in the presence of a high contrast annulus of similar grating, which is thought to reflect centre-surround visual processing. Older adults show an increased magnitude of such suppression (Karas & McKendrick, 2009). However, it has also been shown that older adults can determine the direction of motion of a large high contrast drifting grating in shorter presentation times than younger adults (Betts et al., 2005), suggested to be due to a reduction in cortical inhibitory function. The purpose of our study was to compare surround suppression measures for a drifting version of the perceived contrast centre-surround task, in addition to the motion direction task, in an attempt to clarify the effects of aging on different measures of surround suppression.
Fourteen younger (19-34 years, mean = 25) and 10 older (61 to 75, mean = 69) adults completed two tasks: 1) a contrast discrimination task which measured the point of subjective equality (PSE) for a central patch of drifting sinusoidal grating (0.67 deg, 2c/deg, 2deg/sec, 40% contrast) with and without an annular surround (4 deg, 95% contrast); and 2) a motion direction discrimination task (gabor: 1c/deg, 2deg/sec, 92% contrast) which measured the duration threshold (stimulus presentation duration needed to correctly identify the direction of motion) for stimulus sizes of 2σ = 0.7, 1.3, 2.7 and 5 deg, where σ is the standard deviation of the Gaussian envelope.
The suppression of perceived contrast due to an annular surround was greater for the older adults than younger adults [t(22)=5.91, p<0.001]. For the motion discrimination task, duration thresholds increased with stimulus size for both younger and older adults. A suppression index (SI) was calculated [log(threshold for the 5 deg stimulus)-log(threshold for the 0.7 deg stimulus)] for each individual. There was a trend for a lower SI in older adults that approached statistical significance [t(22)=2.06, p=0.052].
Older observers demonstrate increased surround suppression on one task (contrast discrimination for drifting gratings) while demonstrating apparently decreased surround suppression on the other (motion discrimination task for drifting gratings). These findings do not support a generalised model of reduced cortical inhibition in normal aging, but suggest that other factors such as disrupted signal timing are more likely candidates.
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