Hippocampal involvement in operant conditioning and auditory discrimination
Gavett, Rebecca Ann
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The hippocampus plays an important role in learning and memory processes. Because hippocampal damage severely disrupts memories for events but spares other forms of learning, it is often portrayed as a specialized brain system mediating only one kind of learning. However, learning abilities "spared" by hippocampal damage may differ significantly from learning in an intact brain. To explore this issue, we compared acquisition and performance of an auditory learning task in rats with hippocampal damage, sham lesions, and intact hippocampi. We performed sham lesions in the dorsal hippocampal regions of one group of rats (n = 4), injected NMDA (an agent known to produce hippocampal lesions) into these same regions of a second group of rats (n = 6), and the last group of rats remained naive (n = 6). After recovery, all sixteen rats learned to discriminate a tone from a train of clicks in a two-choice, lever pressing task; i.e., auditory learning was spared. However, we found that NMDA-treated rats were (1) slower to learn to press a lever during automated shaping, (2) better at responding differentially based on the presence or absence of sound, and (3) faster at achieving high levels of performance in the auditory learning task. Additionally, NMDA-treated rats improved in accuracy more gradually across sessions than sham lesioned rats. These findings indicate that incremental learning abilities spared by hippocampal damage are not normal learning abilities. Our task provides a way to assess how hippocampal dysfunction affects incremental learning processes, thereby facilitating the investigation of effective training strategies for optimizing learning in impaired individuals.