Medial temporal lobe function and recognition memory: a novel approach to separating the contribution of recollection and familiarity.

Human neuroimaging studies of recognition memory have often been interpreted to mean that the hippocampus supports recollection but not familiarity. This interpretation is complicated by the fact that recollection-based decisions are typically associated with stronger memories than familiarity-based decisions. Some studies of source memory controlled for this difference in memory strength and found that hippocampal activity during learning predicted subsequent item memory strength while recollection-based memory (performance on source memory questions) was held at chance. This result suggests that the hippocampus is important for familiarity. However, a difficulty with this approach is that when source memory is assessed by asking specific, task-relevant source memory questions, participants who fail to answer the prescribed questions might nevertheless have available other (task-irrelevant) source information. Accordingly, successful item memory could still be associated with recollection. The present study used a novel method to assess item memory and source memory. Instead of responding to specific source questions, participants rated their source memory strength based on any information about the learning episode that was available to them. When subsequent source memory strength was held constant at the lowest possible level, we identified regions bilaterally in hippocampus, as well as in perirhinal cortex, where activity during learning increased as subsequent item memory increased in strength. In addition, activity in cortical regions (including prefrontal cortex) was related to source memory success independently of item memory strength. These findings suggest that activity in the hippocampus is related to the encoding of familiarity-based item memory, independent of subsequent recollection-based success.