"Stimulating dopamine-containing neurons in a deep brain structure known as the substantia nigra can modify human learning," said neuroscience student Ashwin Ramayya from University of Pennsylvania and Amrit Misra from California-based Drexel University.
They suggest that the stimulation may have altered learning by biasing individuals to repeat physical actions that resulted in reward.
"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," explained co-senior author Michael Kahana, a professor of psychology in Penn's school of arts and sciences.
During the study, 11 participants went deep brain stimulation (DBS) treatment for Parkinson's disease.
During an awake portion of the procedure, participants played a computer game where they chose between pairs of objects that carried different reward rates (like choosing between rigged slot machines in a casino).
The objects were displayed on a computer screen and participants made selections by pressing buttons on hand-held controllers.
When stimulation was provided in the substantia nigra following reward, participants tended to repeat the button press that resulted in a reward.
"This is the first study to demonstrate in humans that electrical stimulation near these neurons can modify the learning process," said Gordon Baltuch, a professor of neurosurgery at University of Pennsylvania.
The report was published in the Journal of Neuroscience.