GPN Research Feature
If I offered to give you either $100 today or $200 tomorrow, which would you choose? What if it were a choice between either $100 today or $200 in five years? If you would be willing to wait for one day, but not for five years (and I certainly fall into that category), at what point does the extra waiting time balance out the extra money you would receive for waiting? The way that we make that kind of decision, called “delay discounting,” is an important question at the interaction of neuroscience and economics. The explanations proposed for the delay discounting effect range from simple economic reasoning (eg, I could invest my $100 today and make a profit) to an inability to empathize with your future self. Regardless of the reasons we value future rewards less than immediate ones, delay discounting decisions can be very difficult when your perceived cost of waiting is close to value of bigger reward you would receive for choosing to delay gratification.
This difficult choice was the subject of a recent paper out of Professor David Redish’s lab, published in the September Issue of Cognitive, Affective, & Behavioral Neuroscience. Dr. Redish studies behavior and cognitive processes in rats, with a focus on spatial navigation and decision making. In a previous study Adam Johnson, a former graduate student with Dr. Redish, found that when rats paused to make a decision about which way to turn to find food, they performed what initially looked like “mental time travel” to Johnson, where the rat’s brain seemed to navigate a path it was about to travel. Instead of time travel, Johnson and Dr. Redish concluded that the non-body centered brain activity was instead a form of imagination where the rat thought about taking each possible path before choosing one. Pausing to imagine two possible outcomes closely resembled a phenomenon psychologists in the 1930s referred to as “vicarious trial and error” (VTE), in which rats pause to look down two possible paths they could take before making a decision.
In a project led by neuroscience graduate student Andrew Papale, Dr. Redish’s lab used a delay discounting task to set up difficult choices and measured VTE when the rats were forced to choose between either a small reward immediately or a larger reward later. The twist in their experimental design was to modify the length of the wait for the delayed gratification option depending on the rat’s choices in each session. One of Papale’s important findings was that rats would spend the early part of each session modifying the delay for the larger reward until the delayed gratification penalty balanced out the extra reward received for waiting. Later in the session, the rats spent less time choosing between the options and behaved in a more habit driven way where they tried to keep the balance of waiting and reward consistent.
During the early period when rats were tweaking the delay for the larger reward, they performed much more VTE than they did later in the experiment when they had reached a point where both options were approximately equally rewarding. This finding clashes with one neuro-economic theory of VTE that suggested that the more equal two choices are, the more VTE should be performed. Instead, according to Dr. Redish, “we think that VTE is reflecting an internal deliberation process in which the animal is considering the possible options.” Once a balance had been reached in the later stages of the experiment, the need for deliberation decreased and VTE became less common.
Which brings us to a final question: do rats really deliberate? “Yes,” says Papale. “Rats almost certainly evaluate outcomes of important decisions.” And while choosing between two rewards may require less deep thinking than choosing a career or writing a novel, the process of making hard choices is probably something with which we can all relate.
Johnson A, Redish AD. Neural ensembles in CA3 transiently encode paths forward of the animal at a decision point. J Neurosci. 2007 Nov 7;27(45):12176-89.