Memory processing of novel experiences in the hippocampus

The hippocampus is a brain structure critical for learning and memory. With limited resources, memory processing in the hippocampus must involve extraction of useful information from daily experiences. A potential factor in this selection may be novelty of incoming information. To avoid redundancy in memory storage, the hippocampus may prefer to process novel stimuli. In this dissertation, I used extracellular tetrode recording techniques in freely behaving rats to demonstrate selective reactivation of novel experiences in the hippocampus. The principle neurons of the hippocampus, known as ‘place cells’, exhibit place-selective activity that is thought to reflect spatial aspects of a memory. In addition, during sleep and rest, ensembles of hippocampal neurons periodically engage in coordinated firing that results in signature patterns in local field potential recordings, called ‘sharp wave-ripples’. I show that during sharp wave-ripples, which are believed to reflect a retrieval mode of the hippocampal network, the place cells that were active in a novel environment fired more often than other place cells that were active in a familiar environment. In addition, firing sequences during sharp wave-ripples primarily reflected trajectories in the novel environment. To investigate how hippocampal representations of a familiar experience incorporate novel information, rats were trained to learn a new goal location within a familiar environment. I found selective modification of spatial representations near a newly-learnt goal in the familiar environment. Moreover, firing sequences during sharp wave-ripples preferentially terminated near the goal location after learning, suggesting preferential memory retrieval of the goal. Together, these results support the hypothesis that novel experiences are preferentially processed in the hippocampus.