Short-term spatial memory is impaired in early Alzheimer's disease but not in temporal variant frontotemporal lobar degeneration

The aim of this study was to determine the ability of a test of spatial memory, designed specifically to reflect hippocampal function, to differentiate between patients with early Alzheimer's disease (AD) and those with frontotemporal lobar degeneration (FTLD). The “4 Mountains Test” was applied to 28 patients with cognitive impairment. This test involved presentation of a computer-generated landscape containing a configuration of four mountains and is designed to assess allocentric spatial memory function. The patient groups were as follows: amnestic MCI (aMCI, n = 6); early AD (n = 7); frontal FTLD (n = 5); left temporal FTLD (n = 3); right temporal FTLD (n = 4). Three individuals with subjective memory impairment (SMI) were also assessed. Control data were obtained from 25 cognitively intact subjects. The test was divided into four sections assessing spatial perception, spatial memory, non-spatial perception and non-spatial memory. The memory tests involved recall of information after a two-second delay. Comparison was made with performance on established tests of recognition and recall (the Doors and Peoples tests). Patients with aMCI and early AD were equally impaired on the spatial memory subtest. There was no difference in performance between controls, subjects with SMI and patients with temporal variant FTLD regardless of the laterality of pathology in the latter groups. Topographical perception was unimpaired in all patient groups. On the Doors and Peoples tests, performance on the recall subtests was impaired in all patient groups but with no significant difference in performance across groups. Patients with aMCI and early AD are unable to retain information on spatial information over a short delay. By contrast this function is preserved in patients with temporal variant FTLD. Individuals with subjective memory complaints also perform normally on this test. This impairment of spatial memory is considered to be a reflection of the early pathological involvement of the hippocampus in AD. The preservation of spatial memory function in temporal variant FTLD may be due to the relative sparing of the posterior hippocampus in these patient groups. This test of spatial memory may have utility as a sensitive and specific test for detection of early AD.