Humans are notoriously finicky decision makers, and new research is beginning to elucidate the neural networks that are responsible. For example, we are exquisitely sensitive to framing effects regardless of whether two decisions have mathematically equivalent value – a previous post reviews how this framing effect may arise in the brain. Another famous example is delay of gratification: often we are willing to accept less of something now rather than wait for more of it later. Today’s post summarizes a recent article that begins to explain why.
In their 2004 article, authors McClure, Laibson, Loewenstein and Cohen use fMRI to investigate why people tend to “behave impatiently today but prefer/plan to act patiently in the future.” For example, you will most likely prefer to receive $10 right now rather than receive $11 a full day from now (and thus act impatiently today). However, you will most likely prefer to receive $11 if you must wait 100 days rather than wait only 99 days to receive $10 (and thus act patiently in your plans for the future).
Although economists have successfully modeled this “switch” in behavior using just a single hyperbolic function, the same data can be fitted by the sum of two separate exponential curves: one primarily acting over the short term, and a second acting over the long term with a correspondingly slower rate of decay. McClure et al. suggest that this is not merely a mathematical coincidence; instead, they argue that short and long-term decision making are truly as disparate as they seem.
Short-term reward processing may rely on subcortical circuits involved in reinforcement learning: the dopaminergic ventral tegmental area, and the ventral striatum to which it strongly projects. In contrast, longer-term reward processing may rely on lateral prefrontal cortex. To support these claims, McClure et al had subjects decide whether they wanted less money sooner, or more money later, for a variety of quantities of money (from $5 to $40) and a variety of delays (from immediate to 6 weeks later). Each subject made many such judgments.
As predicted, the results showed that choices involving an immediate reward always more strongly activated the subcortical system than those that involved only later rewards. Conversely, prefrontal areas were more strongly activated when subjects chose a later option relative to an earlier one. These two findings are consistent with the idea that subcortical areas contribute to the part of the decision curve that operates over the short-term, whereas prefrontal areas are involved in the longer-term aspects of decision making.
In summary, then, it seems that an impulsive and reward-driven subcortical system can bias our decisions towards more immediate gains. In contrast, a much more recently involved and late-developing region – the prefrontal cortex – is involved in processing more delayed rewards.