Our brains assess the pros and cons of thousands of situations every day, and this mostly occurs without our conscious brains being involved. Breaking research looks at how long waiting times influence reward pathways.
The nucleus accumbens is a tiny group of cells that lie deep in the center of our brains, and it releases the neurotransmitter dopamine. This clump of neurons assesses "how good a reward will be in the future."
Previous research has shown that the amount of dopamine released here provides information about the size of a reward, the likelihood that it will be received, and how long before it is likely to be delivered.
A new study, which was published recently in the journal Cell Reports, investigates how these dopamine cells signal the passing of time.
The research was carried out at the University of Texas at San Antonio (UTSA) by a team led by Dr. Matthew Wanat, an assistant professor of biology. According to the authors, they wanted to find out whether or not dopamine release could "convey retrospective reward-related information, such as elapsed time."
Dopamine release and a long wait
Using traditional Pavlovian conditioning, several rats were taught that a particular sound signaled the arrival of a reward. Once this relationship was solidly paired in the rats' brains, the sound of the signal (with or without the treat) caused a surge in dopamine, which steadily declined once the treat had been received and consumed.
A technique called voltammetry allowed Dr. Wanat and his team to measure the tiny amounts of dopamine being released in the core of the nucleus accumbens with each reward cue.
For this experiment, the researchers used two different tones: the first was presented after a short wait (15–25 seconds), while the second was presented after a longer wait (65–75 seconds). Both tones signaled the same reward, with the same delay from signal to reward. The only difference was the length of time since the previous signal/reward.
They found that more dopamine was released into the brain when the short-wait tone sounded. In other words, as far as the reward system is concerned, a short wait is better than a long wait.
On a basic level, the results are what one might expect: the brain is more excited by the prospect of waiting less time to be rewarded.
Reward and addiction
Dr. Wanat explains the rationale behind his research, saying, "The big question that we're focusing on is to identify the brain signals that influence the decisions we make."
"Many decisions," he adds, "are based upon comparing the value between cues associated with different rewards. There is a lot of evidence to suggest that these dopamine signals and external cues provide useful value-related signals that could inform our decisions to engage in a behavior."
Drug addiction is one area of study that will eventually benefit from Dr. Wanat's findings. Addiction is a behavior fueled by the dopamine reward pathway; the drug "hijacks" these reward centers.
"By figuring out how the dopamine system works in normal and abnormal circumstances," says Dr. Wanat, "we could potentially identify important changes and the ways that could target the dopamine system to rectify the consequences of those behaviors."
"A lot has been said about the role of dopamine in reward, but reward is only really important in the context of making choices. Dr. Wanat's experiments allow direct measurement of dopamine acting in the brain during the process of choosing, and reveals how the brain decides the values of our choices."
Charles Wilson, Ewing Halsell Distinguished Chair in Biology, UTSA
Dr. Wanat's ongoing research projects focus on the interplay between memory, stress, and drug addiction. Studies such as this most recent publication are vital steps toward understanding how drug addiction works, and, ultimately, how it might be curtailed.