Abstract
The hippocampus is essential for the formation of memories for events, but the specific features of hippocampal neural activity that support memory formation are not yet understood. The ideal experiment to explore this issue would be to monitor changes in hippocampal neural coding throughout the entire learning process, as subjects acquire and use new episodic memories to guide behavior. Unfortunately, it is not clear whether established hippocampally-dependent learning paradigms are suitable for this kind of experiment. The goal of this study was to determine whether learning of the W-track continuous alternation task depends on the hippocampal formation. We tested six rats with NMDA lesions of the hippocampal formation and four sham-operated controls. Compared to controls, rats with hippocampal lesions made a significantly higher proportion of errors and took significantly longer to reach learning criterion. The effect of hippocampal lesion was not due to a deficit in locomotion or motivation, because rats with hippocampal lesions ran well on a linear track for food reward. Rats with hippocampal lesions also exhibited a pattern of perseverative errors during early task experience suggestive of an inability to suppress behaviors learned during pretraining on a linear track. Our findings establish the W-track continuous alternation task as a hippocampally-dependent learning paradigm which may be useful for identifying changes in the neural representation of spatial sequences and reward contingencies as rats learn and apply new task rules.
Highlights
The hippocampal formation is essential for creating detailed new memories of experiences [2,3,4,5]
We measured the total volume of remaining tissue within the dentate gyrus and CA fields, as well as the total volume of remaining tissue in retrohippocampal structures
We compared final task performance on day 10 between the two groups, and found that the groups did not significantly differ on either inbound or outbound trials, there appears to be a trend for the lesion group to be skewed towards poorer task performance (Figure 5B; Table S1). These results suggest that hippocampal lesions retard learning of the W-track continuous alternation task but do not prevent eventual fluent task performance
Summary
The hippocampal formation (comprising the dentate gyrus, CA3, CA2, CA1, subiculum, presubiculum, parasubiculum, and entorhinal cortex [1]) is essential for creating detailed new memories of experiences [2,3,4,5]. In non-human subjects such as laboratory rats, lesions of the hippocampal formation as well as non-destructive perturbations of hippocampal neural activity impair learning and memory in a variety of behavioral paradigms [6,7,8,9,10,11,12,13,14]. We feel that it is important to acknowledge that the significance of neural coding phenomena in the hippocampal formation such as place cells, phase precession and sequential replay remains to be established
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