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Short-term memory and cerebral excitability in elderly psychiatric patients Hannah, Farrell J.

Abstract

The purpose of this thesis was to investigate short-term memory disorder in elderly, psychiatric patients and to attempt to relate this disorder to the concept of "neural excitability". There is no doubt that some elderly, psychiatric patients suffer from a short-term memory deficit. Many investigators have reported that patients with diagnoses of senile psychosis or psychosis with cerebral arteriosclerosis experience serious difficulty with "initial learning" or the immediate recall of new, or at least freshly presented, stimuli. A number of studies have suggested that this short-term memory disorder may be the result of a disruption in one of the mechanisms which has been proposed to account for the ability of young, adult subjects to respond sequentially to stimuli presented simultaneously through different sensory channels. One of these mechanisms has been termed the "p system", which only passes information successively; the other, the "s system", which can contain simultaneously information from two channels. This latter mechanism is the short-term store which is required for the effective handling of simultaneously presented, dichotic stimuli, and which is apparently the defective mechanism in elderly, psychiatric patients with memory disorder. A binaural, simultaneous stimulation experiment was conducted. Two groups of 20 elderly, psychiatric patients (one with clinically ascertained memory disorder and the other without such disorder) were tested using tape-recorded digits. Within these two main groups the conditions of recall were manipulated for the purpose of examining what effect the prescription of the order of recall would have upon the ability of the patients to recall the binaural digits. Half of the subjects from each main group were told before the presentation of the digits which channel they would be required to reproduce first, and the other half was told after the presentation. The general experimental hypotheses were that, under the "before" condition, the memory-disordered subjects would have significantly greater difficulty recalling the stored digits (second channel recalled) than would the non-memory-disordered subjects; this difference would be magnified in the "after" condition. Although the results from some portions of this experiment were not fully in accord with expectation, they all were in the predicted direction. Several possibilities were advanced in an attempt to account for this failure to achieve statistical significance, and for some of the disparities between present results and those from previous studies conducted along similar lines. However, the experimental findings did lend weight to the idea that in some elderly, psychiatric patients there is a breakdown in the short-term storage system of the type proposed by previous investigators. The underlying cause for the short-term memory deficit of some elderly, psychiatric patients may be a reduction in the neural excitability involved in the short-term storage process. A current neuropsychological model proposes a two-stage process of memory functioning: reverberatory activity and permanent changes in the nervous system as a result of this activity. A second experiment was conducted using a cumulative learning paradigm with the aim of examining the efficiency of these stages in elderly, psychiatric patients. Essentially the same two groups of subjects were again tested with a list of repeated and non-repeated series of digits. The hypothesis here was that, in contrast to the control subjects, the experimental subjects would not display cumulative learning of the repeated series of digits. In supporting the experimental hypothesis, the results also indirectly reinforced the notion that elderly, memory-disordered patients suffer from a reduction in the energy of the electrical oscillations and/or an impedance of the neural network, both of which would mitigate against any structural modification based on reverberations. Although the experimental subjects in these studies all displayed a severe short-term memory dysfunction, it was suspected that they were not totally incapable of learning, provided they were given sufficient opportunity for practice. The third and final experiment in this thesis dealt with the serial learning of words which were to be recalled or recognized by the same subjects involved in the first two studies. It was expected that the memory-disordered patients would be inferior to the non-memory-disordered on both a recall and recognition task, but that both groups would show evidence of learning over a series of trials, the control group displaying the greatest amount of improvement under both conditions. The experimental results suggest that at least some learning does occur even where the patients exhibit gross memory disorder but that there is a definite limit to the amount of material which can be learned by these patients. Taken together, the results of these three experiments lend support to the notion that the memory disorder manifested by some elderly, psychiatric patients may be referable to a reduction in cerebral reverberatory activity which makes longer-term learning virtually impossible.

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