Aging: compensation or maturation?

Abstract

Neuroimaging studies of healthy aging often reveal differences in neural activation patterns between young and elderly groups for episodic memory tasks, even though there are no differences in behavioral performance. One explanation typically offered is that the elderly compensate for their memory deficiencies through the recruitment of additional prefrontal regions. The present study of healthy aging compared magnetoencephalographic (MEG) time-courses localized to specific cortical regions in two groups of subjects (20-29 years and >or=65 years) during a visual delayed-match-to-sample (DMS) task. MR morphometrics and neuropsychological test results were also examined with the hope of providing insight into the nature of the age-related differences. The behavioral results indicated no differences in performance between young and elderly groups. Although there was a main effect of age on the latency of the initial peak in primary/secondary visual cortex, these longer latencies were not correlated with the performance of elderly on the DMS task. The lateral occipital gyrus (LOG) revealed qualitatively different patterns of activity for the two age groups corroborated by neuropsychological test results. Morphometric results for the young versus elderly groups revealed less white (WM) and gray matter (GM) volumes in the frontal lobes of the elderly. When a group of middle-aged subjects (33-43 years) was included in the morphometric analyses, the middle-aged subjects revealed statistically greater WM volumes in frontal and parietal cortex suggesting immature WM tracts in the young. Perhaps our elderly utilized a different strategy compared to the young due to the different brain maturation levels of these groups.

Fig. 1

Experimental design. Subjects were asked to press a button with the right thumb in response to the Match stimuli. The same stimuli were presented in a passive viewing task as a control condition.

Fig. 2

MEG waveforms. This polar plot, viewed from the top down, contrasts averaged waveforms (250 individual responses) evoked by the “Sample” stimuli during the active DMS task (black tracings) and for the passive or control task (red tracings). The frontal sensors are located toward the top of plot, and the left hemisphere is depicted on the left side of the plot. The scale at the upper left portion of the figure denotes the x and y scales of the individual channel waveform plots. The time scale shown is from −100 to 1500 ms. The y axis is in fT. The numbers printed to the left of the waveforms represent sensor numbers.

Fig. 3

MEG source timecourses. The 6 brain regions evaluated (MO, LOG, MP, SMG, DLPF, and AC) are displayed in the MRIs at the right of the figure. Timecourses within each region were averaged across young and elderly subjects for each of 3 task conditions. The left column shows timecourses evoked by the Control stimuli (red tracings) superimposed on averaged timecourses evoked by Sample stimuli (Encoding = blue tracings). The right column shows timecourses evoked by the Control stimuli (red tracings) superimposed on averaged timecourses evoked by Non-match stimuli (Delayed Recognition Memory = blue tracings). Asterisks located by the region label on the y axis indicate brain regions that were significantly affected by task instruction. For each subject, 250 individual neuromagnetic responses were averaged together in response to Sample stimuli and 168 individual responses were averaged together in response to Non-match stimuli. A range of significance probabilities for the main effect of Task and Task × Region interactions are shown above.

Fig. 4

Age-related differences across tasks. Timecourses averaged across the young subjects (red tracings) are shown superimposed on timecourses averaged across elderly subjects (blue tracings) for Encoding (left column) and Recognition tasks (right column). Asterisks noted in the upper left and right panels indicate points in time where significant age-related differences were found across tasks and brain regions. The double asterisks noted for location LOG indicate a significant latency difference for Peak 3 (compare red double asterisks versus blue double asterisks). The “+” indicates a significant Peak 2 amplitude difference between young and elderly, depending upon the task. The key below indicates the significance of these age-related effects.

Fig. 5

GM and WM tissue classification of 5 brain regions. Tissue was classified for 11 young and 9 elderly involved in the visual DMS study (blue and yellow bars, respectively). We also classified tissue from 9 additional subjects in a middle-age range (31–43 years) in order to characterize the shape of the functions (red bars). In these graphs, left and right hemisphere results were averaged together before correlations were conducted. The values shown in this figure are normalized to the whole brain.

Fig. 6

Correlations between subject characteristics of the elderly and WM volumes. WM volumes in the temporal lobes correlated with IQ (top) and years of education in the elderly group only (bottom).