Attentional processing and the subjective contour illusion.

Leading explanations of the subjective contour illusion can be classified as being either "bottom-up" or "top-down." Bottom-up explanations assert that peripheral, physiological mechanisms often associated with the perception of real contours also account for subjective contour (SC) perception. In contrast, top-down explanations posit a more central locus of SC perception and are formulated on a molar, psychological level. A major aspect of bottom-up perceptual processing is that it is largely automatic. On the other hand, top-down processing implies a greater role for selective attention. In an effort to distinguish between bottom-up and top-down accounts of SC perception, the present investigation used a dual-task paradigm to test the relative attentional demands of real contour perception versus SC perception. In the primary task, subjects made speeded same-different discriminations of either paired SC forms or their real contour analogues. Half the subjects performed this primary task in conjunction with a six-digit short-term memory load secondary task. If subjective forms indeed impose a greater limited-capacity processing load than real forms, then the need to share processing capacity with a secondary task was expected to produce a greater increment in reaction time (RT) for subjective relative to real forms. The results indicated that the expected enhanced RT increment for subjective relative to real forms with the addition of a concurrent memory load was limited to same trials. This result implies that the nature of response indicators must be considered in assessing capacity requirements with the sort of dual-task paradigm used in the present investigation. Nevertheless, the fact that the increment in same RT with the addition of a concurrent memory load was greater for subjective relative to real forms accords with expectations derived from the notion that the perception of SCs is more attention demanding than that of real contours. If the interpretation of the present study is correct, then a comprehensive theory of SC perception will most likely be formulated within the top-down perspective of conceptually driven visual information processing.