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The empirical finding that removal or destruction of the hippocampus can have the opposite effect on behavior from a dysfunctional or disrupted hippocampus, has important implications for interpreting the literature on hippocampal lesions in animals. As noted in the last section, some animal researchers remove the hippocampus surgically. Other researchers, however, attempt to disable the hippocampus by cutting off key subcortical modulating inputs to the hippocampus, thus disrupting its function. Our simulations suggest that these disabling techniques might have very different implications in some situations, despite the fact that many researchers treat the different methods as functionally equivalent. From a clinical perspective, the distinction between disruption and removal of the hippocampus may be especially important for understanding the many different naturally occurring disorders that can damage the human hippocampus and lead to impairments in memory. Given available knowledge about the neural details of different pathologies, it is not clear which should have behavioral consequences more like a hippocampal removal or more like hippocampal disruption, distinctions that our simulations in Figure 12 suggest can be critical. This problem in diagnosis indicates that there are both clinical as well as basic research motives for seeking, within our modeling efforts, a better understanding of the behavioral consequences of different types of hippocampal damage.
Having seen how the random disruption of the hippocampus can have serious consequences for learning, we explore next a related issue: What happens when acetylcholine, a key neurochemical that controls hippocampal function, is either depressed or enhanced?