The reward system and decision-making

This is, perhaps, the most interesting and important study regarding human decision-making to come out over the last year.

Scientists have, for the first time, recorded real time changes in dopamine and serotonin levels in the human brain that are involved with perception and decision-making. Dopamine is the main component in all mammalian reward systems and is also critical to learning and memory.

Their findings are published in the journal Neuron (my favorite mag).

What the researchers say: “This study provides us a unique window into the human brain that has been inaccessible until now,” said the principal investigator. “Almost everything we have known mechanistically about these neurochemicals was from work done in preclinical animal models, not from direct evidence from humans.”

In this observational study, the neurotransmitters dopamine and serotonin were tracked in five subjects using fast scan cyclic voltammetry, an electrochemical technique used to measure dopamine and serotonin. Dopamine and serotonin are chemical messengers used by the nervous system to regulate countless functions and processes in the body. Dopamine is also an integral part of the human reward system.

The subjects performed decision-making tasks similar to playing a simple computer game. As they performed the tasks, measurements of dopamine and serotonin were taken in the striatum, the part of the brain that controls cognition, reward and coordinated movements.

The researchers described the game as a series of dots on a computer screen that moved through a “cross-hair” reference point positioned in the center of the screen. The subjects had to decide which way the dots were moving. Sometimes the dots would move in the same direction and at other times the dots would move more chaotically making the decision harder.

The dots then disappeared, and the subject had to choose which way the dots had moved—clockwise or counterclockwise—relative to a fixed point. This experimental design allowed the team to tease apart different aspects of how the human brain decides what it has perceived.

This sequence was repeated 200 to 300 times per subject varying how the dots moved and thus how difficult it was for the patient to decide what they saw. Occasionally, the patients had to indicate how confident they were in their choices.

The test was designed to track a human’s ability to perceive the dots’ movement and the confidence in correctly identifying the direction of that movement as a way to determine how dopamine and serotonin actually behaved. The trials were randomized so that predictability from one test trial to the next would be minimized.

The findings showed that the more uncertain the patient was about the direction of the dots, the higher the serotonin levels became. When their certainty increased, serotonin levels decreased.

The study also revealed that, prior to the act of choosing, dopamine rose in anticipation of the choice and serotonin levels fell, and when both reached a certain level, the person made their choice. It’s as if dopamine acted like a gas pedal and serotonin acted like a brake and only when both systems were committed was the act of choice (a button press) allowed, the lead author said.

“This study sheds light on the role these neurochemicals play in learning, brain plasticity and how we perceive the environment,” he said. “We now have more detailed insight into how our brains build what we perceive, use those perceptions to make decisions, and interpret the consequences of the choices we make. Dopamine and serotonin appear to be critical in all of these processes.

So, what? The idea of balance between reward and caution in decision-making is both new and, potentially, revolutionary. It will take further research to fully understand this mechanism. Besides the striatum, I would like to know what other areas of the brain—and indeed the wider system—are involved.

The push/pull mechanism described here is also true of the GABA/glutamine/glutamate system. Glutamate is another reward neurochemical, like dopamine, and we know that GABA/glutamine acts as a break there, too. We know that this mechanism affects decision-making in many areas.