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Title: Learning memory and the hippocampus.
Author: Habibi, P.
Awarding Body: Birmingham University
Current Institution: University of Birmingham
Date of Award: 1978
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The hippocampus apparently plays an important role in human memory, since damage to either it or its main output pathway, the fornix, results in severe amnesia. Patients with such lesions perform poorly on formal tests of learning and memory, but show almost normal learning and retention in situations where cues are provided to prompt recall. Evidence from psychological studies of these patients had led to the hypothesis that the hippocampus is involved in the retrieval but not the storage of memory. Comparable animal studies have, until recently, failed to show any parallel results. The performance of lesioned animals were indistinguishable from normals on a variety of tasks; impairments were often observed when a reversal of a response strategy was required. In such situations hippocampal animals showed an increase in 'perseverence' tendency and resistance to extinction. The belief that perseverence behaviour was a primary disorder resulting from damage to the hippocampal system led to the formulation of the hypothesis that, in animals, the hippocampus was involved in response inhibition. The fact that hippocampal damage resulted in a selective impairment on reversal tasks with a spatial component, and impaired maze learning, particularly 'place' learning in rats, led to others to suggest a spatial hypothesis of hippocampal function. while some believe that the hippocampus is part of a neural system involved in cognitive maping. A major step towards the reconciliation of the hypotheses of hippocampal function in animals and man was made when recent observations of monkeys with fornix lesions showed a loss of recognition memory but left association memory intact, hence they performed normally on simple discriminvations. However, the results did not explain the many instances of impaired performance on what was becoming an ever increasing list of different tasks. The purpose of the present research has been to define more precisely the nature of this behavioural deficit in animals with hippocampal dysfunction, with the hope of reconciling the role of the hippocampus in animals and man. A preliminary perusal of the literature on the effects of hippocampal and fornix lesions revealed that impaired performance on many different tasks depended upon certain features shared by them all. Performance was normal when cues were associated with availability of reward but impaired when the animal had to remember 'what to do' or 'where to go'. Thus a pattern of behaviour similar to that observed in amnesic patients, began to emerge, with the implication that the hippocampus did serve a similar role in animals and man, namely, the retrieval of information from memory. This hypothesis was then tested in rats and monkeys by 1esioning the fornix and their behaviour was studied and compared to intact controls in a variety of learning situations. The major prediction being that the only critical feature of the tasks was the availability of external cues which could label the presence of reward. In the first experiment rats learned to find food in a Y-maze. Two tasks were learned, each requiring the use of a different strategy. In one, Place Learning, the food remained in the same place in the room which provided landmarks indicating the direction of the reward. In the other, Response Learning, the reward remained in the same relative position to the starting point and features of the room provided no useful information about its position; the animal was required to remember the direction to which it responded. The lesioned group were impaired on Response Learning but not on Place Learning. Experiment 2 was a study of the performance of fornix lesioned monkeys on Spatial Alternation and Object Alternation. There is general agreement that hippocampals are impaired on the former task but no information is available about the latter non-spatial task. Since, in both tasks, no information is available at each trial to indicate the correct choice, the Retrieval hypothesis would predict an impairment on both, while a selective impairment on the spatial task would be predicted by the Spatial hypothesis Unfortunately Object Alternation proved to be very difficult even for normal monkeys. Nevertheless, detailed analysis of the pattern of responses revealed the use of different strategies by each group. The fornix group failing to adopt the correct strategy were perseverating with left-right alternations, while the control group were alternating from object to object, albeit at a low frequency. Experiment 3 studied performance on Object discrimination and two non-spatial alternation tasks, both of which required the animal to remember what to do'. As predicted, the fornix group were not impaired on Object Discrimination. However, performance was only impaired on one of the alternation tasks, Go-Nogo Alternation. Their performance on Response Alternation appeared better than controls and it was suggested that a transfer of strategy from a previous learning situation was responsible. Experiment 4 was a study of Latent Learning and Conditional Reaction. Since Latent Learning involves the memory of recent events it should be absent in hippocampal animals. The results although encouraging were inconclusive owing to the small size of sub-groups. The two Conditional Reaction tasks differed with respect to the availability of external cues. There was no impairment in either learning or reversal of the cued version. Impaired performance of the uncued version was shown to be related to the formation of new, rather than perseverence with old, stimulus-response connections. In experiment 5 long-term retention of a previously learned simple discrimination, Object Reversal and Spatial Reversal were tested. As predicted by the Retrieval hypothesis the fornix group showed poor long-term retention and impaired performance on Spatial Reversal. Object Reversal where cues clearly label the position of reward was unimpaired. The results are discussed in relation to other contemorary hypotheses of hippocampal function and it is suggested that most of the controversies can be resolved if the animal hippocampus serves a role in the retrieval process of memory. The present results lend support to a Retrieval hypothesis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis - Birmingham University. Qualification Level: Doctoral
EThOS ID:  DOI: Not available