Stress modulates reinforcement learning in younger and older adults

Abstract

Animal research and human neuroimaging studies indicate that stress increases dopamine levels in brain regions involved in reward processing, and stress also appears to increase the attractiveness of addictive drugs. The current study tested the hypothesis that stress increases reward salience, leading to more effective learning about positive than negative outcomes in a probabilistic selection task. Changes to dopamine pathways with age raise the question of whether stress effects on incentive-based learning differ by age. Thus, the present study also examined whether effects of stress on reinforcement learning differed for younger (age 18-34) and older participants (age 65-85). Cold pressor stress was administered to half of the participants in each age group, and salivary cortisol levels were used to confirm biophysiological response to cold stress. After the manipulation, participants completed a probabilistic learning task involving positive and negative feedback. In both younger and older adults, stress enhanced learning about cues that predicted positive outcomes. In addition, during the initial learning phase, stress diminished sensitivity to recent feedback across age groups. These results indicate that stress affects reinforcement learning in both younger and older adults and suggests that stress exerts different effects on specific components of reinforcement learning depending on their neural underpinnings.

Figure 1

Probabilistic selection task. Three symbol pairs were presented during training; each symbol was associated with a unique probability of positive and negative feedback (e.g., symbol A: 80% Correct! and 20% Incorrect). In the test phase, participants selected symbols from previously learned and novel pair combinations. Critical outcome measures from the test phase included accuracy at selecting the most rewarding symbol ( Choose -A) from novel pair combinations and accuracy at avoiding the least rewarding symbol (Avoid-B) from novel pairs.

Figure 2

Mean cortisol levels by stress condition across sample intervals. Analysis of log-transformed cortisol levels revealed that baseline values (t0) did not differ between control and stress groups, but diverged after the stress manipulation. Raw cortisol values used in display; error bars represent SEMs.

Figure 3

Stress by valence interaction for learning performance in younger and older adults. Error bars represent SEMs. Results indicate that stress-related improvements to reward learning were significant for both younger and older adults. Decreases in negative feedback learning under stress were not significant.