Retrospective and prospective coding of information: dissociation of parietal cortex and hippocampal formation

Animals with sham operations, hippocampal formation, or small and large parietal cortex lesions were trained in a task that required memory for short or long lists of items (spatial locations). More specifically, on any one trial, a rat is presented with 2, 4, 6, 8, or 10 items (spatial locations) on a 12-arm radial maze followed 15 min later by a win-shift test that required the animal to choose between a place previously visited and a novel place. Sham-operated animals showed an increase in errors as a function of set size (2 to 8 items) followed by a decrease in errors with a set size of 10 items, suggesting the use of both retrospective and prospective memory codes. In contrast, animals with hippocampal formation lesions made errors for all set sizes, reflecting an inability to use a retrospective and prospective memory code. The failure to use a prospective code is probably a function of the inability to use a retrospective code (i.e., the animal needs to remember first what has occurred before it can determine what information lies ahead). Animals with small or large parietal cortex lesions made most of their errors for the longest list length, reflecting an inability to shift from a retrospective to prospective memory code. This dissociation between the hippocampal formation and parietal cortex might reflect mediation of different memory operations. The hippocampal formation might mediate new incoming information (data-based memory processing) and thus accentuate the importance of a retrospective memory code. The parietal cortex might mediate existing knowledge (expectancy-based memory processing) and thus facilitate the utilization of a prospective memory code.