Author response: Optogenetic dissection of basolateral amygdala contributions to intertemporal choice in young and aged rats

One marshmallow now or two in 15 minutes? That was the choice offered to young children in a classic psychology experiment known as the Stanford marshmallow test. Children who chose to wait went on to do better at school and to show healthier body weights in later life than those who ate the single marshmallow. A brain region called the basolateral amygdala (BLA) helps individuals choose between rewards that differ both in size and in when they will be available. Studies in people and in rodents show that the ability to wait for a larger reward – to delay gratification – increases with age. But whether changes in BLA activity contribute to this change was not known. Choosing between a small reward now versus a larger one later involves several steps. Before a choice, individuals use their previous experience to compare the value of the immediate and the delayed rewards. How they feel at the time can bias this judgment. Someone who is hungry, for example, will assign greater value to receiving a single marshmallow now than someone who feels full. After making their choice, the individual then decides whether the reward they received was better or worse than they expected. This information helps them adjust their expectations for next time. Hernandez et al. set out to examine how the BLA contributes to these different parts of the decision. Young and old rats were given a choice between a small food reward now or a larger reward after a delay. Hernandez et al. used optogenetic tools to temporarily inactivate the BLA either before or after the rats made their choice, and found that the role of the BLA varies across the lifespan. Inactivating the BLA before the choice made both young and old rats more likely to wait for the larger reward. By contrast, inactivating the BLA after a choice made young rats less likely to wait next time round, but had no effect in the older rats. Changes in BLA activity with aging may thus make it easier to delay gratification in later life. But while the willingness of older adults to forego short-term rewards for long-term gain is often viewed as ‘wisdom’, such behavior can also be problematic. A pensioner who decides not to spend some of their savings on heating, for example, may be needlessly reducing their quality of life. Moreover, extreme impulsivity and extreme patience both feature in psychiatric disorders. The former may drive addiction, while the latter is a hallmark of anorexia. Identifying the mechanisms that underlie the ability to delay gratification may therefore help to promote effective decision-making in aging and psychiatric disorders.