Training can enhance unconscious response priming on fast trials even when measuring consciousness on a trial-by-trial basis

Measuring awareness on a trial-by-trial basis might impose a multi-task cost on the observed effect. Here, we examined this potential cost, asking if it can be mitigated by training. In two experiments, one group of participants reported awareness offline, in a post-test, and another reported it online, in each trial. To test the effect of training, all participants completed two sessions on separate days. When analyzing all trials, we found overall slower reaction times (RTs) in the online group, suggesting a multi-task cost, but no interaction with the priming effect. Notably, this difference was smaller in the second session, implying that the multi-task cost is reduced by training. Critically however, this analysis yielded no convincing evidence for unconscious priming (due to potential threat of regression to the mean). We accordingly analyzed only trials where RTs were fast. Convincing response priming was found, as well as an interaction between priming and session. This suggests that training did increase priming. We also exploratorily tested for individual differences in priming and found between-session consistency mostly for the offline condition. Taken together, our results indicate that although multi-tasking adds noise and prolongs RTs, it does not necessarily diminish unconscious response priming for fast trials, which in turn can be enhanced by training. Costs and benefits of these methodological choices should thus be considered in future studies, as well as targeting only fast responses, where the effects were more compelling. Future work should also test if these patterns apply to other types of priming.