Effects of adult age and functioning of the locus coeruleus norepinephrinergic system on reward-based learning

Age-related impairments in value representations and updating during decision-making and reward-based learning are often related to age-related attenuation in the catecholamine system. However, it is unclear to what extent age-related declines of norepinephrine (NE) functioning in humans affect reward-based decision-making. In this study, we conducted a probabilistic decision-making task and applied a Q-learning model to investigate individual participants’ anticipatory values and value sensitivities. Task-related pupil dilations and locus coeruleus (LC)-magnetic resonance imaging (MRI) contrast, which served as windows of the LC-NE functions, were assessed in younger and older adults. Results showed that in both choice and feedback phases, younger adults’ pupil dilations negatively correlated with anticipatory values, indicating uncertainty about outcome probabilities. Moreover, only younger individuals who showed higher sensitivities to reward probabilities exhibited larger uncertainty-evoked pupil dilations in both phases. Uncertainty-evoked pupil dilations in older adults were smaller, indicating age-related impairments in value estimation and updating. Although older adults showed lower LC-MRI contrast than younger adults, there was no relationship between value sensitivity and LC-MRI contrast in our present samples. These findings suggest that task-related pupillary responses, which might serve as an indicator of task-related NE functioning, can track age-related deficits in value estimation and updating during reward-based decision-making. Nevertheless, our evidence with the LC-MRI contrast was not able to support the direct association with the age-related decline of LC-NE functioning. Future research using pharmacological intervention or neuromodulator imaging is needed to clarify age-related changes of the LC-NE system in reward-based learning.