Reduced sensitivity to immediate reward during decision-making in older than younger adults

Abstract

We examined whether older adults differ from younger adults in the degree to which they favor immediate over delayed rewards during decision-making. To examine the neural correlates of age-related differences in delay discounting we acquired functional MR images while participants made decisions between smaller but sooner and larger but later monetary rewards. The behavioral results show age-related reductions in delay discounting. Less impulsive decision-making in older adults was associated with lower ventral striatal activations to immediate reward. Furthermore, older adults showed an overall higher percentage of delayed choices and reduced activity in the dorsal striatum than younger adults. This points to a reduced reward sensitivity of the dorsal striatum in older adults. Taken together, our findings indicate that less impulsive decision-making in older adults is due to a reduced sensitivity of striatal areas to reward. These age-related changes in reward sensitivity may result from transformations in dopaminergic neuromodulation with age.

Figure 1. Age differences in choice behavior and reaction times.

A) Percentage of delayed choices (y-axis) as a function of delay (x-axis), displayed separately for younger (black) and older adults (grey). B) Percentage of delayed choices (y-axis) as a function of the % difference between rewards, displayed separately for younger (black) and older adults (grey). C) Reaction time in seconds (y-axis) for choice pairs involving immediate rewards (solid) and choice pairs involving only delayed rewards (dashed) (x-axis), displayed separately for younger (black) and older adults (grey). D) Reaction time in seconds (y-axis) as a function of the % difference between rewards, displayed separately for younger (black) and older adults (grey).

Figure 2. Activity in β-areas and δ- areas across age groups.

Left: Significant activations (t-statistics) for choice pairs involving immediate options (β-areas) averaged across age groups. Talairach coordinates: MFG: -8, 57, 19; vmPFC: 0, 39, -4; PCC: 2, −52, 31. Right: Significant activations for all choice pairs (δ- areas). Activations are significant at p<.05, corrected for multiple comparisons. Talairach coordinates: dlPFC: −42, 36, 22; IFG: −45, 7, 32; Ins: −35, 22, 3; SMA; −1, 15, 48; IPL: 30, −48, 40.

Figure 3. Age differences in striatal activity.

A) Left: Significant main effect of age group for choice options involving immediate reward in the ventromedial caudate (t-statistics, significant at p<.05, corrected for multiple comparisons). Right: Time course of BOLD signal change (on the y-axis) for younger (black) and older adults (grey). The x-axis shows time post stimulus onset in seconds. The coordinates refer to Talairach space. B) Left: Significant main effect of age group in the dorsal striatum for all choice options (t-statistics, significant at p<.05, corrected for multiple comparisons). Right: Time course of BOLD signal change (on the y-axis) for younger (black) and older adults (grey). The x-axis shows the repetition time (TR) in seconds. The x-axis shows time post stimulus onset in seconds. The coordinates refer to Talairach space.

Figure 4. Correlation analyses.

A) Correlation between delay discounting (% delayed choices short – long delays) (x-axis) and % BOLD signal change for immediate choice options in the ventral striatum (y-axis). Younger adults are shown in black, older adults are shown in grey. B) Correlation between delay discounting (x-axis) and reaction time for choice options involving delayed reward (y-axis). Younger adults are shown in black, older adults are shown in grey.

Figure 5. Regions of interest (ROI) analysis of BOLD activity in the nucleus accumbens.

A) Time courses for immediate (dashed) and delayed (solid) choice options in the nucleus accumbens (Nacc, as defined using Talairach atlas). Younger adults (top) are shown in black, older adults (bottom) are shown in grey. B) For younger adults (top) BOLD signal change for immediate choice options in the nucleus accumbens correlates positively with discounting. In contrast, BOLD signal change for delayed choice options correlates negatively with discounting. For older adults (bottom) no significant correlations are obtained.