Dynamics of hippocampal and cortical activation during consolidation of a nonspatial memory.

Observations of temporally graded retrograde amnesia after hippocampal damage suggest that the hippocampal region plays a critical, time-limited role in memory consolidation. However, these observations do not indicate where permanent memory is stored, nor do they clarify whether the hippocampus normally remains involved in a nonessential way. Evidence from multiple neural imaging studies indicate the time-limited role of the hippocampus and suggest that the anterior cingulate cortex is a critical storage site of different types of long-term memory. However, each of the previous studies examined spatial memory, leaving open the question of whether different cortical areas support long-term memory for other types of material. We characterized the course of involvement of cortical and hippocampal areas in animals trained in an explicitly nonspatial task. First, we confirmed previous findings that hippocampal damage produces temporally graded retrograde amnesia for the social transmission of a food preference (STFP) within our experimental protocol. Damage to the hippocampal region 1 d, but not 21 d, after training impaired subsequent recall of STFP. Then, we characterized the anatomical patterns of activation of the immediate early gene c-fos during retrieval of STFP immediately and 1, 2, and 21 d after training. The ventral subiculum was activated during retrieval shortly after learning, but the level of activation declined at successive times. In contrast, olfactory recipient regions including piriform, entorhinal, and orbitofrontal cortex showed the opposite pattern, increasingly greater activation in successively later retrieval tests. These findings support the view that different cortical networks support long-term memory for different types of information.