The Influence of Masked Stimuli on Response Selection: Evidence from a Semantic Categorization Task

Unconscious visual stimuli can be processed by human observers and influence their behaviour. A striking example is a phenomenon known as “free-choice priming,” where masked “prime” stimuli—of which participants are unaware—modulate which of two response alternatives they are likely to choose. Recent efforts to uncover the mechanisms underlying this intriguing effect have revealed that free-choice priming can emerge even in the absence of automatized stimulus-response (S-R) associations between masked primes and specific motor responses, indicating that free choices can be influenced by a masked prime’s meaning (Ocampo, 2015). It remains unknown, however, whether masked primes bias response selections because they are implicitly classified according to task instructions, or because spreading activation occurs within the prime's semantic network. To adjudicate between these two possibilities, participants in the present experiment categorised targets as either animals or people and selected which of two response alternatives they wanted to make following presentation of a free-choice target. Crucially, while implicit classifications could proceed during processing of both animal and person masked primes, only animal primes could trigger spreading activation within their semantic network. This manipulation modulated free-choice priming; only masked animal primes influenced response selections to free-choice targets. This result indicates that an automatic spreading activation mechanism might underlie a masked prime’s ability to influence free-choice responses.