Motivation can have invigorating effects on behavior via dopaminergic neuromodulation. While this relationship has mainly been established in theoretical models and studies in younger subjects, the impact of structural declines of the dopaminergic system during healthy aging remains unclear. To investigate this issue, we used electroencephalography (EEG) in healthy young and elderly humans in a reward-learning paradigm. Specifically, scene images were initially encoded by combining them with cues predicting monetary reward (high vs. low reward). Subsequently, recognition memory for the scenes was tested. As a main finding, we can show that response times (RTs) during encoding were faster for high reward predicting images in the young but not elderly participants. This pattern was resembled in power changes in the theta-band (4–7 Hz). Importantly, analyses of structural MRI data revealed that individual reward-related differences in the elderlies’ response time could be predicted by the structural integrity of the dopaminergic substantia nigra (SN; as measured by magnetization transfer (MT)). These findings suggest a close relationship between reward-based invigoration, theta oscillations and age-dependent changes of the dopaminergic system.

Age-related memory impairments have been associated with structural changes in the dopaminergic system, but the underlying mechanisms remain unclear. Recent work indicates that iron accumulation might be of particular relevance. As iron accumulates, a degeneration of myelin sheaths has been observed in the elderly, but the relationship between both and their impact on memory performance in healthy elderly humans remain important open questions. To address this issue, we combined an established behavioral paradigm to test memory performance [verbal learning memory test (VLMT)] with state of the art quantitative magnetic resonance imaging techniques allowing us to quantify the degree of myelination and iron accumulation via markers of tissue microstructure in a group of young (18 -32 years) and healthy elderly humans (55-79 years). As expected, we observed a decrease in gray matter volume and myelin, and an increase of iron in the elderly relative to the young subjects within widespread brain regions, including the basal ganglia. Furthermore, higher levels of iron within the ventral striatum were accompanied by a negative correlation between myelin and iron specific for the elderly participants. Importantly, both markers of iron and myelin (and their ratio) predicted the performance of the elderly in the VLMT. This suggests that ventral striatum iron accumulation is linked to demyelination and impairments in declarative memory. Together, our data provide novel insights into underlying microstructural mechanisms of memory decline in the elderly.