Abstract
Many perceptual and motor processes can occur in the absence of consciousness, as evidenced by recent subliminal priming studies. However, which cognitive processes can be triggered by unconscious information, and which cannot? It has been suggested that cognitive control processes, associated with prefrontal cortices, can only operate on consciously perceived stimuli. To put this claim to a direct test, we used a masked stop-signal paradigm to study response inhibition. Participants responded as fast and accurately as possible to go-signals, but tried to inhibit their responses when an occasional stop-signal was presented after the go-signal. By varying the SOA between stop-signals and masks, behavioral measures on unconscious and conscious stop trials could be compared with behavioral measures on trials containing no stop-signal (go trials). After sufficient practice, participants inhibited more responses to go-signals in unconscious stop trials, than in perceptually comparable go trials. Additionally, unconscious stop-signals significantly slowed down the speed of responses. The latter effect was only present for good inhibitors, i.e. participants with a short duration of the inhibitory process. In this study, we show that response inhibition can be triggered unconsciously, but that it depends heavily on S-R automation and task performance. These results increase the traditional range of cognitive processes known to operate outside awareness and allow inferences about the depth of processing of unconscious stimuli in the human brain.