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A. M. McKendrick, R. Karas; The Effect of Normal Ageing on the Surround Modulation of Perceived Contrast. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2004.
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Recent neurophysiology of the aged primate visual system demonstrates a broadening of orientation and direction tuning of V1 neurons. A reduction of inhibition has been proposed as a mechanism for this change in neural function (Leventhal et al, Science, 2003: 300(5620): 812). Psychophysically, surround inhibition can be indirectly measured via the contrast-contrast phenomenon (Chubb illusion), where the contrast perception of a pattern is influenced significantly by its surround. We predicted that older adults would demonstrate less surround modulation of perceived contrast than younger adults (consistent with reduced inhibition).
Eighteen younger adults (aged 18 to 30, mean = 24, stdev = 4.02) and 17 older adults (aged 60 to 72, mean = 66, stdev = 4.47) participated. Contrast discrimination was measured using a method-of-constant stimuli for a small (0.67deg) 40% contrast circular patch of 4 c/deg band-pass filtered noise. Performance was also measured for the same patch embedded in a 95% contrast annulus (4 degrees radius). Psychometric function spread and threshold (point-of-subjective-equality: PSE) were determined by fitting a cumulative Gaussian to the data using a bootstrapping procedure.
The perceived contrast was reduced by the presence of the annulus for both groups. The mean PSE of the older group was significantly less that that of the younger group (t(33) = 2.53, p=0.02), indicating that contrast perception was more affected by the annulus for the older individuals. This finding was opposite to our hypothesis. The mean spread of the psychometric functions did not differ between groups (no surround: t(33) =0.90, p = 0.37; surround: t(33) = -0.70, p =0.49).
Healthy ageing alters surround modulation of perceived contrast. Our findings can not be readily explained by a simple model of reduced inhibition. As brightness induction also has a role in the surround modulation of perceived contrast, further work is required to determine whether our findings are explicable by a decrease in perceptual brightness induction in the elderly group.
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