The effects of objective and perceived size properties on visual form matching.

The present experiments examined the effect of perceived size on the time it takes to make veridical form judgments. Recent studies have demonstrated an increase in the time required to match forms, as a function of increasing size, orientation, contrast, or color differences between the forms to be matched. The increase in reaction time to match forms has been explained by suggesting that form perception and matching is dependent on the degree of difference that exists in the objective size of the forms to be matched. This conflicts with the generally accepted view that form matching is independent of irrelevant proximal stimulus properties. The key to this issue seems to be whether form perception depends on objective or perceived stimulus properties. Studies in this area all involve manipulations of the objective stimulus properties, thus making it difficult to examine the effects of perceived properties. The current series of studies decoupled objective and perceived size differences by employing an illusion. The results demonstrated that the time to match forms is a function perceived differences in the stimulus forms, not the differences that exist in the objective size of the forms to be matched. In Experiment 1 three conditions were employed: a plain background condition in which the forms to be matched were objectively and subjectively equal in size, a double-illusion condition in which the forms were objectively and subjectively equal in size, and an illusion condition in which the forms were objectively equal in size but perceived as unequal in size. The results indicated a significant increase in reaction time for the illusion condition in which the forms were perceived to be different sizes, compared to the other two conditions in which perceived and objective size were equal. Experiment 2 results converged with and replicated those of Experiment 1. In Experiment 2 perceived size was equated for each subject in the context of the size illusion, thus varying the objective size of the stimuli. This manipulation led to a significant decrease in reaction time in this added condition, as compared to the illusion condition from Experiment 1 in which objective size was equal and perceived size differed. The results of Experiment 2 were replicated in a third experiment. Experiment 3 replicated the decrease in reaction time noted in the perceptually equal illusion condition. The decrease between illusion conditions was also approximated by an increase in a plain background condition with objective size differences. Thus the effect of perceived size accounts for the additional time to match forms of unequal size. The conclusion drawn from this series of experiments is that the time required to make veridical form judgments depends on perceived differences in the forms to be judged, not on the objective properties of the proximal stimulus. Implications of these results on models of pattern recognition are examined.