Time course of free-choice priming effects explained by a simple accumulator model

Unconscious visual stimuli can be processed by human observers and modulate their behavior. This has been shown for masked prime stimuli that influence motor responses to subsequent target stimuli. Beyond this, masked stimuli can also affect participants' behavior when they are free to choose one of two response alternatives. This finding demonstrates that an apparently free-choice between alternative behaviors can be subject to influences that are outside of awareness. We report three experiments which exhibit that the temporal dynamic of free-choice priming effects corresponds to that of forced-choice priming effects. Forced-choice priming effects were relatively robust against variations of prime stimuli but sensitive to physical features of target stimuli. Free-choice priming effects, in contrast, depended largely on the stimulus-response compatibility of the prime. A simple accumulator model which accounts for forced-choice response priming can also explain free-choice priming effects by the assumption that unconscious stimuli can initiate motor responses when participants are engaged in a speeded choice-reaction time task. According to our analyses free-choice priming results from a response selection mechanism which integrates conscious and unconscious information from external, stimulus driven sources and also from internal sources.