The effect of normal aging on closed contour shape discrimination.

Our experiments explore whether contour processing of closed shapes is altered by healthy aging. Contour processing was measured using a closed contour (circle or ellipse) constructed of Gabor elements. The contour was presented either on a blank background or embedded in noise (identical Gabor elements of random orientation). Twenty-one older (age range: 61-80 years) and 21 younger (age range: 22-38 years) adults participated in three experiments: 1) the number of Gabors comprising the contour was fixed (10, 12 or 15) and the threshold aspect ratio required to discriminate the shape (circle versus ellipse) was measured; 2) orientation jitter was added to the Gabor elements comprising the contour and shape aspect ratio discrimination thresholds were measured; and 3) the aspect ratio was fixed (three times the individual threshold aspect ratios) and the threshold number of elements required to determine the shape was measured. Older adults required a larger number of elements to discriminate the global contour shape (F(1, 41) = 15, p < 0.001), even when stimulus saliency was matched for contrast sensitivity and aspect ratio threshold. This finding is consistent with other recent work showing deteriorations in cortically mediated visual processing with age.