Evidence for unconscious priming using a Bayesian single-subject approach

Many studies have claimed the existence of unconscious priming, but a valid statistical procedure for substantiating such claims is often lacking. Absence of prime awareness is often inferred from non-significant p-values, and priming and awareness are typically tested by comparing two separate tests. In a classic study by Vorberg, D., Mattler, U., Heinecke, A., Schmidt, T., & Schwarzbach, J. (Different time courses for visual perception and action priming. Proceedings of the National Academy of Sciences 2003, 100(10), 6275-6280.), observers were slower to identify the direction of an arrow mask when it followed an arrow prime pointing in the opposite direction, despite being unable to identify the prime direction. Using this study as a starting point, we addressed several methodological shortcomings that are common in the field. First, we used a Bayesian methodological-statistical framework to test priming effects and prime awareness at the single-subject level across multiple sessions. Second, participants received extensive training in the prime awareness task to ensure they understood and executed the task as intended. Third, we directly compared prime awareness and priming. Using this approach, we found an indirect-task advantage consistent with unconscious priming at short prime-mask stimulus onset asynchronies (SOAs) in three out of six participants, under the assumption that the awareness task was at least as sensitive as the priming task. However, unlike in the original study by Vorberg and colleagues, evidence for unconscious priming was not consistently present across all SOAs, and awareness increased with SOA. Priming effects were highly consistent across participants, whereas awareness showed substantial inter-individual variability.