Rapid activation of motor responses by illusory contours.

Whereas physiological studies indicate that illusory contours (ICs) are signaled in early visual areas at short latencies, behavioral studies are divided as to whether IC processing can proceed in a fast, automatic, bottom-up manner or whether it requires extensive top-down intracortical feedback or even awareness and cognition. Here, we employ a response priming paradigm to assess two measures of IC processing using identical stimuli: response priming by ICs, which can occur independently of visual awareness, and a measure of visual awareness of the ICs. In three experiments, participants responded to the orientation of illusory and real-contour targets preceded by illusory and real-contour primes at stimulus-onset asynchronies from 35 to 106 ms. Both illusory and real-contour targets were strongly primed by ICs, and this effect was independent of prime visibility. The effect was fully present in the fastest responses and cannot be explained by responses to stimulus features other than the ICs. Results indicate that ICs, regardless of whether they are consciously perceived, activate fast motor responses, indicating that they are processed without time-consuming intracortical feedback. We conclude that conflicting studies were based on qualitatively different measures of IC processing, some depending on visual awareness and others independent of it.