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 ≥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.

Functional Magnetic Resonance Imaging of Compensatory Neural Recruitment in Aging and Risk for Alzheimer’s Disease: Review and Recommendations

“…In addition, postmortem evidence suggests that myelination of the hippocampus continues into the '5th and 6th decades of life' [53] . In fact, both Aine et al [42] and Raz et al [54] have reported an inverted U-shaped function for white matter volume in frontal cortex in which young and older adults have reduced volumes relative to middle-aged adults. It is possible that older adults utilize a different strategy than the young adults in part as a result of different maturational stages.…”

“…As Aine et al [42] suggest, methods used to examine age-related changes have limitations including use of the subtraction method, differences in resting state physiology, and differences in neurovascular coupling. Given this heterogeneity across studies, we further operationalize our proposed definition of compensatory mechanism recruitment by proposing a set of criteria to be considered when assessing whether a study's results are reflective of compensation.…”

“…48 ], and may relate to alterations in CBF or cerebrovascular responsivity [49] . As Aine et al [42] note, when examining age-related differences in functional neuroanatomy, many groups do not consider the plastic nature of the brain that is influenced by development and life experience through childhood and adulthood [50,51] . Studies of aging should consider both maturational and degenerative changes that occur during adulthood and aging, and as Aine et al [42] suggest, brain maturation (e.g., myelination) continues into middle age 'when maximal WM volume and myelination are reached in frontal lobes and association areas, regions necessary for higher cognitive functions' [see also ref.…”

“…Another possible approach involves using the simplest version of a task so that all individuals perform well [42] . As Nielson et al [56] suggest, it must be clear whether there is a direct benefit of increased activation in order to consider activation as compensatory.…”

A close relationship between verbal memory and SN/VTA integrity in young and older adults

“…Hence, vascular factors cannot fully explain the relationship between MTR and CVLT in the young. However, there are a number of maturational brain changes that continue into late adolescence (for a review see Paus, 2005), including proliferation of myelin into the peripheral cortical neuropil (Aine et al, 2006;Sowell et al, 2004;Yakovlev & Lecours, 1967), the formation and usage-dependent selective elimination of synapses (Hensch, 2004;Huttenlocher & Dabholkar, 1997), and metabolic changes (Chugani, Phelps, & Mazziotta, 1987), which could contribute to a reversed relationship between MTR and cognition in the young adults. Indeed, we observed that there was an age-related increase in MTRs of both regions within the group of the young adults (Fig.…”