Neural processes during encoding support durable memory

The ability to form durable memory is critical for human survival and development, but its underlying cognitive and neural mechanisms have not been well understood. In particular, existing studies have not clearly dissociated the neural processes supporting short- and long-duration memories. The present study addressed this issue with functional MRI and a modified subsequent memory paradigm. Participants were asked to make semantic judgment on a list of 320 words in the scanner. Half of the words were tested after a short delay (i.e., 1day, T1) and again after a long delay (i.e., 1week, T12), whereas the other half were tested only once after the long delay (T2). Materials forgotten during T1 were categorized as forgotten trials, and those remembered during T2 were categorized as long-duration trials. In contrast, trials remembered during T1 but not during T12 were categorized as short-duration trials. We found that compared to forgotten trials, short-duration trials showed decreased activation in the posterior cingulate cortex (PCC) and precuneus, which is consistent with many previous observations. Importantly, long-duration trials showed stronger activity in the left inferior frontal gyrus (LIFG) but less deactivation in the PCC relative to short-duration trials. Psychophysiological interactions (PPI) analysis revealed stronger functional connectivity between LIFG and PCC for long-duration trials than for forgotten trials. Our results suggest that strong PCC activity, in combination with strong LIFG activity, supports long-lasting memory.