State-related and item-related neural correlates of successful memory encoding

Neuroimaging studies show that the efficacy of long-term memory encoding of a stimulus is indexed by transient neural activity elicited by that stimulus. Here, we show that successful memory encoding is also indexed by neural activity that is tonically maintained throughout a study task. Using functional magnetic resonance imaging (fMRI), transient and sustained neural activity were dissociated with a mixed event-related and blocked design. In a series of short task blocks, human subjects made semantic or phonological decisions about visually presented words. After statistically removing item-related activity, we found that the mean level of activity across a task block was correlated with the number of words subsequently remembered from that block. These correlations were found in inferior medial parietal and left prefrontal cortex for the semantic task, and in superior medial parietal cortex for the phonological task. Our findings suggest that state-related activity in these brain regions is involved in memory encoding.