Behavioural and EEG correlates of forward and backward priming

During affective priming, perception of an emotional “prime stimulus” influences the reaction time of the subsequent emotional “target stimulus”. If prime and target have the same valence (congruent trials), reactions to the target are faster than if prime and target have different valences (incongruent trials). Bem introduced a backward priming paradigm in 2011, where first the target was presented and then the prime after the response. Similar to the classical affective forward priming effects, he found faster reaction times in congruent compared to incongruent trials, and interpreted these results as evidence supporting precognition. In the present study, while measuring EEG, we combined a forward priming paradigm (hypothesis driven analysis) with a related backward priming paradigm (exploratory analysis), following Bem’s study. We analysed the EEG data on a group level (ERPs) and on an individual level (single participants, applying artificial neural networks). We found significantly faster reaction times for congruent compared to incongruent trials in the forward priming experiment (p=0.0004) but no statistically significant differences in the backward priming experiment (p=0.1237). We also found significant differences in ERP amplitude in the forward priming congruent vs incongruent conditions (P8 electrode: p = 0.003). Backward priming results show weaker, shorter, and less significant differences between congruent and incongruent trials, with maxima at electrodes P7, P3, CP5, and CP1. The neural network results were very variable across participants in both the backward and forward priming and on average, the accuracy results were at chance level for both the forward priming as well as the backward priming. Our results replicate behavioral findings and extend the EEG findings for forward priming from the literature. We did not replicate Bem’s backward priming results. The exploratory backward priming EEG results are weak, however they give a good starting point for future studies.