A team of researchers at the University of Pennsylvania have demonstrated that learning in humans can be modified by stimulating dopamine-containing neurons within a particular region of the brain called the substantia nigra.
The substantia nigra is known to play important roles in the control of movement and addiction. This structure, which is found in the midbrain, is rich in dopamine-containing (dopaminergic) neurons; these neurons experience massive degeneration in patients with Parkinson’s disease. By stimulating this region, researchers discovered that the learning process can be altered in humans. In particular, they found that participants became more inclined to repeat actions that led to reward. The team have published their results in The Journal of Neuroscience.
“Stimulating the substantia nigra as participants received a reward led them to repeat the action that preceded the reward, suggesting that this brain region plays an important role in modulating action-based associative learning,” said Michael Kahana, co-senior author of the study and professor of Psychology at the University of Pennsylvania.
Eleven individuals receiving deep brain stimulation for Parkinson’s disease were enrolled in the study. Participants were required to select between pairs of objects on a computer screen, one of which would yield a reward in the form of a cash register noise but the participants were not told which one this was. All they knew was that they had to work out which objects were good choices based on the rewards.
The researchers found that if stimulation was given after a choice resulted in reward, the participants were more likely to repeat this selection. While that may seem an obvious outcome, what is interesting is that they would continue to make this selection even when the reward was no longer associated with that particular choice. This meant that those given stimulation actually did worse than those who were not (48% accuracy vs 67%, respectively).
“While we’ve suspected, based on previous studies in animal models, that these dopaminergic neurons in the substantia nigra play an important role in reward learning, this is the first study to demonstrate in humans that electrical stimulation near these neurons can modify the learning process,” said co-senior author of the study Gordon Baltuch. “This result also has possible clinical implications through modulating pathological reward-based learning, for conditions such as substance abuse or problem gambling, or enhancing the rehabilitation process in patients with neurological deficits.”