Abstract: Perceptual grouping is a function of the human perceptual system to organize discrete entities in the visual field into chunks or perceptual objects for higher order processing (Han et al, 2002). Previous research has found that young adults respond faster to perceptual groups defined by collinearity than by similarity and that the reaction times are shorter when valid cues are employed to engage visual attention in the target orientation. The present study examined the two types of perceptual grouping in different age groups, with the purpose of clarifying cognitive mechanisms underlying age-related decline in perceptual grouping. The processing-speed theory or the sensory theory of aging predicts that the perceptual grouping capacity of the elderly declines more sharply in difficult grouping (grouping by similarity) and in difficult conditions (without cues) than in easy grouping (grouping by colliearity) and in easy conditions (with cues). However, the Hasher-Zacks (1988)’s theory, which attributes the age-related decline to reduced efficiency of inhibitory control, predicts that the cues will benefit the elderly to a less extent than younger adults and that the detection of the two types of grouping will decline by the same rate in the elderly.
To test these predictions, this study employed two types of grouping stimuli under uncued (Exp.1) and cued (Exp.2) conditions. Sixteen old adults and sixteen undergraduate students participated in both experiments. Each experiment consisted of two sections with a single type of grouping in each section. Stimuli were displayed on a gray background (25.1 cd/m2) with 1024×768 graphic resolution. Each stimulus array consisted of a configuration of Gabor patches (see Fig. 1). The central Gabor patch was orientated either 45° or 135° in visual angel and was flanked by two pairs of patches in an ‘X’ configuration. One pair of flankers were either collinear with or similar to the central Gabor patch in the target displays, which were mixed with catch displays in which no perceptual grouping can be detected between Gabor patches. Participants were asked to press spacebar with the index finger when they detected a target Gabor grouping. The response hand and the order of grouping type were counter-balanced over the participants.
A repeated-measures analysis of variance (ANOVA) showed that the older adults responded more slowly than the younger group and that both groups of participants responded faster to grouping by collinearity than by similarity. A significant interaction was detected between the cue benefit and age group, with the cues helping younger group to a greater extent. No interaction was found between grouping type and age group. Moreover, an advantage of perceptual grouping was observed in the orientation of 135°, relative to the orientation of 45°, regardless of the availability of cues, suggesting a probable influence of left-to-right reading habits on perception.
These results demonstrate that while the processing speeds decline by the same rate in the two types of perceptual grouping, the cue to the target orientation benefits age groups differently, with younger adults getting a greater benefit than older adults. These findings are consistent with Hasher & Zack’s theory, suggesting that the reduced inhibitory efficiency contributes to the decline in perceptual grouping.

Key words: Perceptual grouping, Aging, Attention, Directional Bias