Variability in memory performance in aged healthy individuals: an fMRI study
Introduction
Decline of episodic memory is an early sign of Alzheimer’s Disease [20] and the hallmark of mild cognitive impairment [36]. However, episodic memory performance also has been described to be impaired in old age without manifest disease [14], [38] even when general health status is controlled [15]. Overall, several studies from the field of experimental aging research have demonstrated a great variability in cognitive and memory performances in the elderly [12], [18], [30], [49], [50]. Recent neuroimaging studies comparing young and older subjects still dispute whether older subjects activate different patterns of the relevant neural substrate or engage additional or alternative areas [1], [8], [22], [23], [41]. More specifically, it is unclear to what extent aged individuals fail to engage critical hippocampal substrates during episodic memory tasks [19].
Given recent observations that activity levels of relevant neural substrate are related to performance [28], [32], we hypothesized that variability of memory performance in a sample of aged individuals correlates with activation levels of hippocampal and extra-hippocampal substrate. To address this question we used a paradigm with repetitive learning and recall of abstract geometric patterns that had revealed robust activation of the hippocampus in young healthy individuals [24]. Our aim was to examine whether there is a relationship between hippocampal activation and successful performance on this memory task in a homogenous age group of older, healthy subjects. Extensive neuropsychological testing and morphometric analyses allowed to assess whether the task specific behavioral and neural variations between aged individuals are related to variations on standard neuropsychological tests and whether variations in observed hippocampal activation can be attributed to inter-individual differences in hippocampal volume.
Section snippets
Study population
The study was carried out in accordance with the Declaration of Helsinki. We obtained permission from the Local Ethical Committee. Twenty-four healthy older normal subjects (13 males, 11 females; age: 64.4±6.7 years; mean±S.D.; range 56–76 years) gave written informed consent prior to the study. All subjects were right-handed according to the Oldfield [34] handedness quotients (range between: +78 and +100). None had any sign of color blindness or visual field defects. From each individual a
Memory performance during fMRI
Multivariate analysis of variance (MANOVA) for repeated measures revealed a significant effect of repetitive recall blocks on mean correct performance (F(4,20)=34.36, P<0.001; Fig. 2A). Post-hoc Newmann-Keuls tests (nominal level of alpha: P<0.05) showed significant increases in performance from the first to the second and from the third to the fourth recall block. Performance did not significantly increase from the second to the third (P=0.054) and from the fourth to the fifth recall block (P
Discussion
The present results were obtained in a large group of healthy older individuals without subjective memory complaints or history of past or present neurological or psychiatric disease and with normal attention and executive functions in neuropsychological testing. Analysis of fMRI activation upon repetitive learning and active recall of abstract geometric patterns revealed significant activation in the hippocampus during initial learning and maximum recall. Extrahippocampal activity was
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