Abstract
Normative aging is known to affect how decisions are made in risky situations. Although important individual variability exists, on average, aging is accompanied by greater risk aversion. Here the behavioral and neural mechanisms of greater risk aversion were examined in young and old rats trained on an instrumental probability discounting task. Consistent with the literature, old rats showed greater discounting of reward value when the probability of obtaining rewards dropped below 100%. Behaviorally, reward magnitude discrimination was the same between young and old rats, and yet these same rats exhibited reduced sensitivity to positive, but not negative, choice outcomes. The latter behavioral result was congruent with additional findings that the aged ventral tegmental neurons (including dopamine cells) were less responsive to rewards when compared to the same cell types recorded from young animals. In sum, it appears that reduced responses of dopamine neurons to rewards contribute to aging-related changes in risky decisions.
Highlights
According to the world health organization (WHO), in almost every country, the proportion of people aged over 60 years is growing faster than any other age group as a result of both longer life expectancy and declining fertility rates
When examining all young rats’ ventral tegmental area (VTA) neurons response to reward, or first pellet cue, we found that the firing rate was significantly higher during this cue period compared to baseline (two-way ANOVA, significant main effect of time, F(1,568) = 10.87, p = 0.001; Figure 6A), but there was no significant difference between probability blocks to this cue (no main effect of probability, F(1,568) = 0.13, p = 0.72; Figure 6A)
The aged rats’ VTA neurons did not distinguish between the cues that predict the probabilistic rewards with high and low probabilities while the young rats VTA neurons did. This is evident even in individual neurons’ responses to the lever cue in young and aged rats (Figures 6C,F). These results indicate that the increase in probability discounting in aged rats may be due to the blunting of the dynamic range of the VTA neurons’ responses, which might prevent the aged rats from correctly assessing the value of the risky option
Summary
According to the world health organization (WHO), in almost every country, the proportion of people aged over 60 years is growing faster than any other age group as a result of both longer life expectancy and declining fertility rates. While studies exploiting the power of in vivo neuroimaging in humans have yielded important insights, the approaches available in animal models are necessary for a comprehensive cell biological account of the neural basis of cognitive aging. The latter is especially relevant since similar behavioral and cognitive patterns of change are observed in old rodents and primates (Gallagher et al, 2011)
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