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Observers are faster to find and discriminate a target pair of adjacent, same-shaped objects that are within the same perceptual group (within group) versus within adjacent perceptual groups. Manipulating the probability of a within-group pair modulated this grouping effect such that it increased as the percentage of within-group trials (25%, 50%, or 75%) increased. This probability effect was stronger for extrinsic grouping factors (common region and connectedness) than for intrinsic factors (proximity and color similarity). Further experiments, however, suggested that this extrinsic-intrinsic difference was due to differences in overall speed with which the target pair was found. The results are interpreted as indicating that grouping is sensitive to top-down influences as long as there is sufficient time for feedback to operate.
Beck et al. (Tue,) studied this question.