The Effects of Attention on Cortical Reinstatement in the Aging Brain

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A defining feature of episodic memory is the vivid recollection of previously experienced events. Models of episodic memory propose that the same neural representations present at encoding are engaged during memory retrieval, a process termed cortical reinstatement. The hippocampus plays a crucial role in stabilising memory traces and facilitating retrieval by reactivating the same neural patterns in sensory-specific cortical regions that were present at encoding. Studies show that neural patterns of reinstatement in sensory-specific cortical regions such as early visual cortex are associated with the degree of encoding related hippocampal activity [1] and corresponds with higher memory accuracy [2]. These findings indicate that the hippocampal-cortical connections supporting the binding of complex representations at event encoding underlie the experience of 're-living' a previous event.Normal aging is marked by episodic memory decline, which is thought to be related to age-related structural and functional alterations in the hippocampus and surrounding medial temporal lobe structures [3]. Current literature has focused on whether age-related memory decline emerges from changes in sensory-specific cortical reactivation supporting memory retrieval. Older adults are found to exhibit reduced activity and decreased specificity in posterior cortical regions involved in sensory processing, combined with increased activity in frontal regions [4]. This neural pattern is believed to be a compensatory mechanism resulting from reduced neural specialisation. Such domain-general neural patterns may contribute to the difficulty in distinguishing between similar items stored in memory [5]. Additionally, it could also reflect deficits in attentional control that may lead to an over-recruitment of frontal rather than posterior-sensory regions [6].Indeed, older adults commonly display deficits in attentional control in behavioural paradigms [6] and over-recruitment of frontal regions has been linked to greater impairments in attentional control [7]. One possibility is that the greater reliance on frontal rather than posterior sensory regions in older adults may reflect impairments in directing attention to sensory information, thus reducing the extent to which sensory representations are bound into an episodic memory trace by the hippocampus, and the subsequent reactivation of cortical representations at retrieval.I propose to examine whether older adults recruit sensory processing brain regions during memory retrieval less effectively than younger adults, or if changes in attentional control and encoding strategies affect the nature of encoding sensory information. To do this, I propose to use a task designed to manipulate the orientation of attention of visual and semantic information during memory encoding. The aim is to determine whether orienting attention to visual relative to semantic stimuli will reduce age differences in cortical reinstatement at retrieval.To measure neural activity, I intend to use fMRI with a particular focus on using representational similarity analysis (RSA) [8]. RSA utilises representational dissimilarity matrices to assess whether neural activation in a region of interest is similar or dissimilar for encoding and retrieval conditions. Thus, greater similarity of neural patterns for both conditions indicates greater reinstatement and better episodic memory recollection. Applying the outlined neuroimaging technique allows the comparison of encoding-retrieval pattern similarity between young and older adults.I anticipate that the study findings will advance our understanding of the neurocognitive mechanisms underlying memory deficits in the aging brain. It will contribute to the current models of episodic memory by examining whether attentional allocation can facilitate cortical reinstatement effects in older adults and how this relates to improved recollection for previously encoded information.