Age related changes in fronto-striatal connections underlying intertemporal choice

Previous research has produced highly conflicting results on differences in intertemporal preferences with age. Some studies report increased patience with age, others increased impulsivity, and still other a non-linear relationship. These have collected fMRI data from 28 subjects engaged in an intertemporal choice task in an effort to help resolve this conflict using neuroscience data. Half of the subjects were age 20-29 and the other half were 70-79, with both groups matched for education and relative socioeconomic status. The researchers found no evidence of differences in choice behavior by age. This could be due to the limited time periods that they have investigated (longest time delay was 2 months) or small range of dollar values (mean $20).

Despite the null finding with respect to behavior, the researchers did find a significant difference in brain activity underlying intertemporal choice that was anticipated. Specifically, brain areas associated with the mesolimbic dopamine system are more greatly involved in intertemporal valuation in older subjects. Furthermore, neural responses in these brain areas show reduced sensitivity to the time at which rewards are offered relative to younger participants. Thus, while there is no apparent difference in intertemporal preferences with age, they do find evidence that older participants account for delay using brain structures associated with more crystallized intelligence. These results were recently published: Samanez-Larkin, G.R., Mata, R., Radu, P.T., Ballard, I.C., Carstensen, L.L., McClure, S.M. (2011) Age differences in striatal delay sensitivity during intertemporal choice in healthy adults. Frontiers in Decision Neuroscience 5: 126.

Based on these findings the researchers hypothesized that (1) greater experience with age allows for direct associative learning of reward values at delays of at least several weeks, and (2) this may result is more rigid responding in different choice contexts. The researchers have been investigating this second hypothesis in behavioral experiments with 28 participants recruited to date (14 in each age group). A number of contextual factors are known to affect an individual's discount rate. They have been investigating the magnitude effect (smaller discount rates with larger dollar values), the date/delay effect (smaller discount rates when times expressed as dates, e.g. May 28, instead of delays, e.g. 2 weeks), and the hidden zero effect (smaller discount rates when stating null outcomes, e.g. nothing now but $20 next week). As hypothesized, these framing and contextual effects all have greater impact on intertemporal preferences in younger relative to older subjects.