Somatosensory Dedifferentiation in the Aging Brain

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PROJECT SUMMARY Normal aging is associated with pervasive deficits in sensorimotor function. According to the dedifferentiation hypothesis of aging, these age-related impairments are partially attributable to decreases in the distinctiveness of neural representations. Simply put, as we age, different mental states increasingly rely on shared neural substrates and thus evoke relatively similar (rather than distinct) patterns of brain activation. These less distinctive neural representations lead to less efficient neural processing and undermine behavioral performance. Consistent with this hypothesis, neural activation patterns in older adults are often more bilateral than those in young adults (a reduction in hemispheric asymmetry in older adults). Furthermore, our group has found that neural activation patterns in response to different visual stimuli or motor actions are significantly less distinctive in older compared to younger adults. We also found that older adults with more distinctive neural representations perform significantly better than others on a range of behavioral tasks. Informed by this previous work, we propose to investigate the neural and behavioral consequences of age-related dedifferentiation in the somatosensory system. First, we will use functional MRI to examine the distinctiveness of somatosensory representations during tactile stimulation in both young and old adults (Aim 1). We predict that these representations will be less distinctive (more dedifferentiated) in the older participants. Second, we will collect a battery of behavioral measurements of tactile performance in the same participants to examine the relationship between somatosensory neural distinctiveness and behavior (Aim 2). We predict that reduced somatosensory distinctiveness will be associated with impaired tactile performance. Third, we will use repetitive transcranial magnetic stimulation (rTMS) to test whether age-related recruitment of ipsilateral somatosensory cortex is helpful or harmful to behavioral performance (Aim 3). If reduced hemispheric asymmetry in older adults is compensatory, then stimulation of ipsilateral somatosensory cortex should impair their tactile performance. The proposed studies will provide novel insight into how the brain changes with age, and whether those changes are linked to impaired behavior. Findings from this research could provide the basis for novel interventions to prolong functional independence for older adults.