Abstract
Older adults typically perform more poorly than younger adults in free recall memory tests. This age-related deficit has been linked to decline of brain activation and brain prefrontal lateralization, which may be the result of compensatory mechanisms. In the present pilot study, we investigated the effect of age on prefrontal cortex (PFC) activation during performance of a task that requires memory associations (temporal vs. spatial clustering), using functional Near-Infrared Spectroscopy (fNIRS). Ten younger adults, ten cognitively high-performing older individuals, and ten low-performing older individuals completed a free recall task, where either a temporal or spatial strategy (but not both simultaneously) could be employed to retrieve groups of same-category stimuli, whilst changes in PFC hemodynamics were recorded by means of a 12-channel fNIRS system. The results suggest PFC activation, and right lateralization specific to younger adults. Moreover, age did not affect use of memory organization, given that temporal clustering was preferred over spatial clustering in all groups. These findings are in line with previous literature on the aging brain and on temporal organization of memory. Our results also suggest that the PFC may be specifically involved in memory for temporal associations. Future research may consider whether age-related deficits in temporal organization may be an early sign of PFC pathology and possible neurodegeneration.
Highlights
An unavoidable consequence of older adulthood is age-related brain decline, which manifests with gradual loss of hemispheric specialization (Cabeza et al, 1997), amongst other symptoms, such as changes in cerebral blood flow and loss of volume of brain tissues (Agbangla, Audiffren, & Albinet, 2017; Bertsch et al, 2009; Jernigan et al, 2001; Pakkenberg et al, 2003)
prefrontal cortex (PFC) haemodynamic response The functional Near-Infrared Spectroscopy (fNIRS) analysis revealed that: 1) younger adults showed an increase of cortical activation in the right anterior PFC, corresponding to BA10, during retrieval; 2) highperforming older participants showed greater cortical activation in left BA10; and 3) low-performing older individuals showed no lateralization
Given that right PFC is usually employed during memory retrieval (Habib et al, 2003), increased activation of left, rather than right, PFC in high-performing older adults suggests that some kind of hemispheric specialization in this group is still present, through engagement of opposite neural pathways
Summary
An unavoidable consequence of older adulthood is age-related brain decline, which manifests with gradual loss of hemispheric specialization (Cabeza et al, 1997), amongst other symptoms, such as changes in cerebral blood flow and loss of volume of brain tissues (Agbangla, Audiffren, & Albinet, 2017; Bertsch et al, 2009; Jernigan et al, 2001; Pakkenberg et al, 2003). Olson et al (2004) found that younger and older adults showed similar performance on the short-term spatial memory test and that both groups encoded locations relative to other locations Taken together, these studies suggest that in older adults experiencing deficits in temporal clustering, spatial contiguity may be an efficient alternative form of memory organization. The present study is the first to investigate age-related changes in functional PFC patterns during associative processes and age-related differences in the use of temporal vs spatial clustering in younger adults and in cognitively high- versus lowperforming older participants. Consistent with previous research presented above, it was hypothesized that: (1) following the HAROLD and CRUNCH models, younger adults would show greater lateralized PFC activation during associative memory, compared to the older groups, and (2) older individuals with lower cognitive functioning would employ greater spatial clustering, compared to younger adults, as an alternative to temporal clustering
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