Deriving behavioural receptive fields for visually completed contours

The visual system is constantly faced with the problem of identifying parti ally occluded objects from incomplete images cast on the retinae. Phenomeno logically, the visual system seems to fill in missing information by interp olating illusory and occluded contours at points of occlusion, so that we p erceive complete objects. Previous behavioural [1-7] and physiological [8-1 2] studies suggest that the visual system treats illusory and occluded cont ours like luminance-defined contours in many respects. None of these studie s has, however, directly shown that illusory and occluded contours are actu ally used to perform perceptual tasks. Here, we use a response-classificati on technique [13-20] to answer this question directly. This technique provi des pictorial representations - 'classification images' - that show which p arts of a stimulus observers use to make perceptual decisions, effectively deriving behavioural receptive fields. Here we show that illusory and occlu ded contours appear in observers' classification images, providing the firs t direct evidence that observers use perceptually interpolated contours to recognize objects. These results offer a compelling demonstration of how vi sual processing acts on completed representations, and illustrate a powerfu l new technique for constraining models of visual completion.