Do semantic priming and retrieval of stimulus-response associations depend on conscious perception?

What function does conscious perception serve in human behavior? Many studies relied on unconscious priming to demonstrate that unseen stimuli can be extensively processed. However, showing a small unconscious priming effect falls short of showing that the process underlying such priming is independent of conscious perception. Here, we investigated to what extent the retrieval of learned stimulus-response associations and semantic priming depend on conscious perception by using a liminal-prime paradigm that allows comparing conscious and unconscious processing under the same stimulus conditions. The results revealed two striking dissociations. First, S-R priming was entirely independent of conscious perception, whereas semantic processing was strongly enhanced by it. Second, while priming emerged on fast trials for all conditions, only conscious semantic priming was observed on slow trials. The implications of these findings for the time course of response priming and for the contribution of unconscious processes to fast vs. slow responses are discussed.