Abstract
In vivo neuroimaging studies have generally indicated a greater involvement of posterior cortical areas in early-stage dementia of the Alzheimer type (DAT) relative to frontal involvement. By contrast, some recent neuropsychological studies have shown that DAT patients perform poorly in frontal lobe tasks even in the early stages of the disease, although there is disagreement as to whether this necessarily implicates frontal pathology. The main aim of this study was to examine the hypothesis prompted by both neuroimaging studies and the traditional clinical conception of the disease, that, compared with the functioning of posterior association cortex, executive functions (thought to depend on frontal lobe integrity) are relatively spared in the early stages of DAT. A group of patients with a diagnosis of early-stage, probable DAT (n= 17) was compared with age- and IQ-matched controls (n= 17) across a range of neuropsychological tasks presumed to exercise frontal or temporoparietal functions. A profile of strengths and weaknesses was observed across ‘anterior’ and ‘posterior’ cognitive tests, including dissociations among some tests of temporoparietal function, in particular visual object perception (impaired) and spatial analysis skills (intact). Thus there was little support for the notion that the disease progresses cortically in a posterior-to-anterior direction. Possible reasons for the discrepancy between neurophysiological and neuropsychological observations are discussed, including the possibility that neuropsychological tests do not provide a valid indication of regional brain function when used in the context of DAT. Caution is urged in the clinical application of ‘frontal lobe tests’ for the differential diagnosis of DAT.
Highlights
Dementia of the Alzheimer type (DAT) is characterized by a progressive deterioration of memory and intellect
In somewhat over half of the tasks administered the performance of the DAT group was comparable to that of the controls, suggesting that poor performance on the other tests cannot be attributed to generalized cognitive impairment
The pattern of results appears at first sight not to support the suggestion prompted by evidence from in vivo neuroimaging studies that executive functions may be spared in the early stages of DAT relative to temporoparietal functioning
Summary
Dementia of the Alzheimer type (DAT) is characterized by a progressive deterioration of memory and intellect. According to the prevailing clinical conception (see, for example, Huppert and Tym 1986) certain functions are prominently affected at an early stage in the disease process, such as aspects of recent memory, while others appear for some time to remain grossly intact, e.g. praxis, language functions ( it should be noted that appropriate examination may reveal signs of impairment in these areas even in the absence of symptoms). Microscopic observations have shown cell loss in certain brain regions, including basal forebrain and brain stem structures, especially nucleus basalis, dorsal raphe nucleus, and locus coeruleus (Tomlinson et al., 1981; Whitehouse et at., 1981; Bondareff et at., 1982; Iversen et ai, 1983; Wilcock et ai, 1988).
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