Parallel detection of Kanizsa subjective figures in the human visual system.

Subjective figures, seen in the absence of luminance gradients (Fig. 1), provide a phenomenal illusion that can be related to the properties of single cells in the visual cortex, offering a rare bridge between brain function and visual awareness. It remains controversial whether subjective figures arise from intelligent cognitive mechanisms, or from lower-level processes in early vision. The cognitive account implies that the perception of subjective figures may require serial attentive processing, whereas on the low-level account they should arise in parallel at earlier visual stages. Physiological evidence apparently fits a low-level account and indicates that some types of subjective contour may be detected earlier than the conventional Kanizsa type. Here we report that even Kanizsa subjective figures can be detected without focal attention at parallel stages of the human visual system.