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Memory Disorders

Introduction

Memory enters into nearly all cognition and memory dysfunction is one of the most common sequelae of neurological disorders. It seems therefore highly implausible to consider a unitary research or assessment on memory. Thirty years ago, Tulving's (1972) contribution to the organization of memory consisted of dividing long-term memory in terms of content, i.e. episodic and semantic memory. Episodic memory refers to memory for specific events within a spatial and temporal context. Semantic memory, on the contrary, holds information that is independent of the context in which that information was learned. More recently, Tulving's view of memory as multiple systems (1995) comprises five types of memory: procedural memory, perceptual representation system, semantic memory, primary memory and episodic memory. A great deal of research indicates that even severely amnesic patients show preservation in the procedural system (conditioning, motor-skill acquisition, perceptual learning, verbal facilitation and rule learning).

A further memory construct, which had an important impact on memory research, concerns declarative/non-declarative memory (Squire, 1993). Declarative memory is defined in terms of facts and events acquired through learning and retrieved intentionally. Non-declarative memory refers (mainly but not only) to priming and skills learning. Declarative/non-declarative memory is parallel to Schacter's (1992) concept of explicit/implicit memory.

Characteristics of memory loss depend on the locus of lesion. Very broadly speaking, damage to parts of the limbic circuit (hippocampus, fornix, mamillary bodies, anterior thalamus and cingulated gyrus) affects memory performance in different ways (Mishkin, 1982). Moreover, memory deficits arising from temporal or frontal lesions have been reported, 25 years ago, as showing qualitative differences.

Neuropsychological Assessment

The aim of the neuropsychological assessment of memory is threefold: (i) Characterization of the brain damaged patient's memory for both clinical and research purposes. (ii) Rehabilitation, whenever possible, on the bases of the results obtained in (i). (iii) Measurement of change by comparing, for instance, the patient's performance on two occasions, pre- and post-rehabilitation programme or pre- and post-neurosurgery. The present entry develops the first goal, which comprises the assessment of both memory loss and memory preservation. Moreover, within the frame of memory viewed as an ensemble of subsystems, this entry deals with the explicit contents of memory only.

Memory deficits can be triggered by impaired intellectual, attentional and/or motivational functioning. However, loss of memory cannot be accounted for diffuse deficits of cognitive capacities since patients presenting memory loss are well able to perform normally on tests of general intellectual abilities. Memory assessment is best achieved by obtaining a comprehensive cognitive profile and by interpreting the memory performance on the basis of the patient's general cognitive capacities. The neuropsychological examination should include the patient's verbal and non-verbal IQs and an estimation of the premorbid cognitive level of functioning in order to find if there is a significant difference between pre- and post-illness mental status. Besides general abilities, it is important to assess language functions, visuoperceptual and visuospatial capacities, executive functions and, particularly, attentional functions. Indeed, attentional impairments preclude any conclusions on memory deficits since explicit memory depends on the integrity of the attentional processes. To get round this difficulty in the examination of memory, the clinician may try to minimize the influence of attentional factors by selecting the tests, when feasible. A further way to circumvent this is by controlling the patient's attention. Thus, for instance, on Warrington's (1984) verbal and non-verbal Recognition Memory Tests, the patient is asked if the items shown by the examiner (words or faces) are pleasant or unpleasant. On the contrary, some other memory tests may both rely openly on intact attention, particularly during the encoding part of the task, or they may be designed to assess attention during a learning task. Finally, an adequate understanding of memory deficits would also require assessment of current levels of motivation.

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