Repetition-induced changes in BOLD response reflect accumulation of neural activity.

Recent exposure to a stimulus improves performance with subsequent identification of that same stimulus. This ubiquitous, yet simple, memory phenomenon is termed priming and has been linked to another widespread phenomenon called repetition suppression, which is a repetition-induced reduction in human brain activation as measured using fMRI. Here, competing models of the neural basis of repetition suppression were tested empirically. In a backward masking paradigm, we found that effectively masked object stimuli showed repetition enhancement of brain activation instead of suppression. This finding is consistent with an Accumulation model, but is inconsistent with a Suppression model of neural activity. Enhanced activation and the improved behavioral performance usually associated with priming are both explained by a shift in peak latency of the population neural activity elicited during identification.