Researchers Find Direct Evidence of Reward Prediction Error Signal in Human Nucleus Accumbens

Newswise — Miami (April 16, 2012) — According to findings presented today at the 80th Annual Scientific Meeting of the American Association of Neurological Surgeons (AANS) in Miami, researchers have found evidence that, in the subjects studied, nucleus accumbens (NAc) neurons not only encode the difference between expectation and outcome, but in situations with an uncertain outcome, NAc neurons also predicted the subject’s action.

One of the core tenets of intelligent behavior is the ability to connect values with actions, as well as to evaluate expectations relative to final outcomes. These capacities help construct the basis of theories such as reinforcement learning (RL). Equally important, they are critical to uniquely human behaviors, such as financial decision making. Converging evidence suggests that the aforementioned NAc is critical to this process, and ultimately is involved in overseeing the motivational component of goal-oriented behavior.

The researchers performed the study on eight subjects, performing single-neuronal recordings in the NAc using microelectrode recordings in patients undergoing deep brain stimulation for obsessive-compulsive disease and major depression. The results of this study, Reward Prediction Encoded by Single-Neuron Responses in the Human Nucleus Accumbens, will be presented by Sameer A. Sheth, MD, PhD, 10:45-10:59 a.m., on Monday, April 16. Co-authors are Shaun Patel, BS; Matthew Mian, BSE; and Emad Eskandar, MD.

The eight subjects performed a gambling task that required them to make financially contingent decisions. The researchers isolated 19 individual NAc neurons in the subjects. In situations when an unexpected win occurred, NAc activity was potentiated; in outcomes of unexpected loss, NAc activity was attenuated. These neurons therefore encode the difference between expectation and outcome, a reward prediction error (RPE) signal, consistent with RL theory. Furthermore, the researchers found that when the outcome was uncertain, the activity of the NAc neurons predicted the subject’s bet two seconds prior to the subject actually expressing the bet.

The researchers concluded that the results demonstrate for the first-time direct evidence for an RPE signal in the human NAc. This signal encodes the difference between actual and expected outcomes, and is critically important in biasing actions toward rewarding behaviors. These results suggest that there is a role for NAc in encoding or making financial judgments in the absence of any predictive information, a process akin to the subject’s intuition.

Disclosure: the author reports no conflicts of interest.

The 2012 AANS Annual Meeting press kit includes releases on highlighted scientific research, AANS officer and award winners, and National Neurosurgery Awareness Week. These releases will be posted under Media/Press on the 2012 AANS Annual Meeting website page. Additional information about the AANS Annual Meeting and the Meeting Program is available by clicking here.

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Founded in 1931 as the Harvey Cushing Society, the American Association of Neurological Surgeons (AANS) is a scientific and educational association with more than 8,100 members worldwide. The AANS is dedicated to advancing the specialty of neurological surgery in order to provide the highest quality of neurosurgical care to the public. All active members of the AANS are certified by the American Board of Neurological Surgery, the Royal College of Physicians and Surgeons (Neurosurgery) of Canada or the Mexican Council of Neurological Surgery, AC. Neurological surgery is the medical specialty concerned with the prevention, diagnosis, treatment and rehabilitation of disorders that affect the entire nervous system including the spinal column, spinal cord, brain and peripheral nerves. For more information, visit www.AANS.org.