We examined age-related differences in figure-ground perception by exploring the effect of age on Convexity Context Effects (CCE; Peterson & Salvagio, 2008). Experiment 1, using Peterson and Salvagio's procedure and black and white stimuli consisting of 2 to 8 alternating concave and convex regions, established that older adults exhibited reduced CCEs compared to younger adults. Experiments 2 and 3 demonstrated that this age difference was found at various stimulus durations and sizes. Experiment 4 compared CCEs obtained with achromatic stimuli, in which the alternating convex and concave regions were each all black or all white, and chromatic stimuli in which the concave regions were homogeneous in color but the convex regions varied in color. We found that the difference between CCEs measured with achromatic and colored stimuli was larger in older than in younger adults. Our results are consistent with the hypothesis that the senescent visual system is less able to resolve the competition among various perceptual interpretations of the figure-ground relations among stimulus regions.

Little is known about how healthy aging impacts figure-ground (FG) perception, which is critical in organizing complex visual input into coherent figures. Inhibitory-suppression is thought to be critical in resolution between competing FG interpretations. Given the age-related changes to inhibitory cortical mechanisms, my thesis investigated the hypothesis that processes underlying FG organization are impaired in aging. We explored the effects of age on the convexity context effect (CCE): the tendency to perceive convex regions as figural [i.e., P(Convex=Figure)] increases with surrounding convex and concave regions. In Chapter 2, we observed that adding context by increasing region number, increased P(Convex=Figure) in younger and older observers, but this CCE was significantly reduced in older adults. Reducing competition between perceptual interpretations by heterogeneously colouring convex regions, thereby invalidating the alternative {Concave=Figure} stimulus interpretation, greatly increased P(Convex=Figure) in older observers. These results supported the notion that aging impairs FG organization, particularly in relatively high competition contexts. In Chapter 3, we explored the possibility that reduced presumption of depth in our stimuli might explain the reduced the CCE in older observers by adding texture motion to region fills consistent with depth in the stimulus. The results were inconsistent with the hypothesis, but supported the reduced inhibitory-suppression hypothesis of the age effect. In Chapter 4, we explored the neural activation associated with the processing mechanisms underlying the resolution of FG competition. The results indicated that the N250 in younger observers but not older observers is sensitive to high vs. low competition contexts. Furthermore, individual differences in this N250-competition effect were correlated with the behavioural effect of competition. Our findings are consistent with the hypothesis that the processing underlying FG organization is impaired in senescence, which may stem from reduced efficacy of the neural architecture supporting this ability. Various implications of this work are discussed. / Dissertation / Doctor of Philosophy (PhD)