Multiple signals of recognition memory in the medial temporal lobe

The medial temporal lobe (MTL) is known to play an essential role in recognition memory (the ability to judge the prior occurrence of a stimulus). Electrophysiological studies in nonhuman primates have suggested the presence of more than one type of recognition signal in the medial temporal lobe (e.g., novelty, familiarity, and recency). It has also been suggested that the perirhinal cortex plays an essential role in visual recognition memory. Here, we present fMRI results from 16 college-aged participants who underwent a continuous yes/no recognition task of novel and familiar pictures with multiple stimulus presentations. Our goal was to understand the dynamics of recognition in the MTL over multiple trials. We hypothesized that we could see changes in signal with repeated exposure that carry information related to novelty, familiarity, and recency. Whole brain activation maps demonstrated a strong novelty effect, marked by activity in several frontal and occipital regions that decreases with increasing number of presentations. The opposite pattern was observed in several other regions that include the supramarginal gyrus and inferior parietal lobule. In the MTL region, we observed monotonic decreases in activity across trials in the parahippocampal cortex as well as the anterior perirhinal cortex. We also observed monotonic increases in activity in the posterior perirhinal cortex with increasing memory strength. In addition, we observed clear effects of pre-experimental familiarity with the stimulus in several regions. Consistent with previously reported electrophysiological data, we found evidence for several medial temporal lobe signals carrying recency, familiarity, and novelty information.