Shape Processing in Perceptual Decisions of Detection, Identification, and Categorization

With three tasks, it was investigated what kind of shape information was involved in the visual object processing in perceptual decisions, such as detection, identification and categorization. Stimuli taken from Snodgrass and Vanderwart (1980) were modified with different levels of random noise. In the detection task, a modified line-drawing object (target) and a lure of random noise (non-target) were sequentially presented in random order. Observers were asked to choose which stimulus was the target. Detection thresholds (the level of random noise to detect target) were measured. In the identification task, an object description of the target was given before two modified line-drawing objects were presented. Observers were asked to choose the target. The procedure of the categorization task was almost identical to that of the identification task except that the object description was replaced with the category name. Accuracies were calculated in the last two tasks. The results of correlation analyses between the measurements and the shape-variables (e.g., the number of edges, the degree of the circularity or visual complexity) indicated that detection was related to the perception of the local edge, identification involved complexity of shape, and categorization depended on edge detection.