Hippocampal connections and memory : the medial septum and mammillary bodies.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
There are a number of indications that structures associated with the hippocampal region of the mammalian brain have an important role in memory function. Two regions, the medial septum and mammillary bodies, are of particular interest because of their implication in the memory deficits observed in some human neurodegenerative diseases (e.g. Alzheimer's Disease and Korsakoff's Syndrome) and the effects on memory following experimentally applied damage in non-human subjects. The present research investigated the role of the medial septum and mammillary bodies in memory by assessing the effects of damage in these structures on performance of rats in two types of memory task: Serial-Probe Recognition and Delayed-Matching-to-Sample. These two types of task were chosen because of their ability to allow comparisons with some commonly-used human memory procedures and because of their utility in examining several major issues that have not been adequately addressed in previous research. The results from the serial probe recognition studies which assessed memory for list items showed that a Serial Position Effect analogous to that observed in humans, i.e. high accuracy at end list positions compared to middle ones, could be produced with rat subjects tested in a 12-arm maze. Mammillary lesions resulted in a loss of the superior accuracy for both early and late list positions, but no significant reduction in accuracy overall. By contrast, medial septal lesions were very disruptive to overall accuracy and also eliminated the superior accuracy at early list positions. However, medial-septal lesioned rats continued to show superior accuracy at late list positions. The pattern shown by the medial septal group was consistent with the pattern observed in Alzheimer's Disease and Korsakoff's Syndrome subjects. Subsequent studies investigated the effects of Medial Septum and Mammillary Body lesions using an automated delayed-matching-to-sample task. A particular question of interest in the delayed-matching research was whether the disruptive effects of lesions on performance were the result of a change in the rate at which information is 'lost from memory' or, in the subjects' discrimination of to-be-remembered material in the first instance. This issue was addressed using an analytic separation of delay-independent aspects of performance (accuracy at a zero second delay) from the memorial or delay-dependent aspects of performance (rate of accuracy decline over delays). This separation was achieved by examining performance over a range of delays and utilising a bias-free measure of accuracy (Davison & Tustin, 1978) fitted by a quantitative model of memory performance (White & McKenzie, 1982). The results of the delayed-matching studies showed that large medial septal damage, but not mammillary damage or small medial septal damage, produced an increase in the rate of forgetting without a change in the delay-independent aspects of performance. In addition to examining lesion effects on the basic delayed matching task, a systematic investigation was conducted of the interaction between these lesion effects and interference from proactive (behavior and stimuli occurring prior to the current trial) and retroactive sources (behavior and stimuli occurring during the delay of a current trial). This investigation was prompted by indications that Alzheimer's and Korsakoff's subjects display heightened susceptibility to these sources of interference. A heightened susceptibility to proactive interference from previous trials in the large medial septum lesion group was consistent with, and may have been the cause of, the increase in rate of forgetting and the reduction in serial probe recognition accuracy observed in this group. However, the investigation of interference effects also resulted in several findings that were hard to reconcile with other findings. For example, retroactive interference, while not having a differential effect across lesion groups, had an unexpected influence on the delay-independent measure of performance but not on the delay-dependent measure. The consistency of the present findings with human and nonhuman research is discussed. It is concluded that the medial septal region plays an important role in mnemonic function and that damage in this area produces an increased sensitivity to proactive interference effects.