Older adults are impaired at implicit associative learning (IAL), or the learning of relationships between stimuli in the environment without conscious awareness. These age effects have been attributed to differential engagement of the basal ganglia (e.g. caudate, globus pallidus) and hippocampus throughout learning. However, no studies have examined gray matter diffusion relations with IAL, which can reveal microstructural properties that vary with age and contribute to learning. In this study, young (18-29 years) and older (65-87 years) adults completed the Triplet Learning Task, in which participants implicitly learn that the location of cues predict the target location on some trials (high frequency triplets). Diffusion imaging was also acquired and multicompartment diffusion metrics were calculated using neurite orientation dispersion and density imaging (NODDI). As expected, results revealed age deficits in IAL (smaller differences in performance to high versus low frequency triplets in the late learning stage) and age-related differences in basal ganglia and hippocampus free, hindered, and restricted diffusion. Significant correlations were seen between restricted caudate diffusion and early IAL and between hindered globus pallidus diffusion and late IAL, which were not moderated by age group. These findings indicate that individual differences in basal ganglia, but not hippocampal, gray matter microstructure contribute to learning, independent of age, further supporting basal ganglia involvement in IAL.

Whereas adults across the lifespan can accurately recognize previously encountered items, they are worse at remembering precisely which items were previously seen together, and this associative memory deficit is exacerbated in older adults. However, the literature is dominated by studies of pair-wise associations, with very few examinations of higher associative loads and none in older adults. In the present study, 190 adults (18-83 years) completed a novel recognition task in which they studied word pairs, triplets, and quadruplets and were later tested on their memory for repeated, recombined, and novel word sets. Results revealed significantly more incorrect responses to recombined sets that increased from pairs to triplets to quadruplets, with older age groups making significantly more of these associative memory errors than younger age groups. Participants also made significantly more correct responses to repeated pairs compared to triplets and quadruplets, but these recognition memory effects did not vary by age group. These findings replicate and extend the paired-associative memory literature by demonstrating that associative memory deficits increase both with demands on binding processes at higher associative loads and with aging. They further demonstrate the feasibility of manipulating and assessing associative memory load using our novel QuadMax task.

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