When electrical pulses are applied to the ventral tegmental area of their brain, macaques presented with two images change their preference from one image to the other. The study by researchers Wim Vanduffel and John Arsenault (KU Leuven and Massachusetts General Hospital) is the first to confirm a causal link between activity in the ventral tegmental area and choice behaviour in primates.

The ventral tegmental area is located in the midbrain and helps regulate learning and reinforcement in the brain's reward system. It produces dopamine, a neurotransmitter that plays an important role in positive feelings, such as receiving a reward. "In this way, this small area of the brain provides learning signals," explains Professor Vanduffel. "If a reward is larger or smaller than expected, behavior is reinforced or discouraged accordingly."

Causal link

This effect can be artificially induced: "In one experiment, we allowed macaques to choose multiple times between two images -- a star or a ball, for example. This told us which of the two visual stimuli they tended to naturally prefer. In a second experiment, we stimulated the ventral tegmental area with mild electrical currents whenever they chose the initially nonpreferred image. This quickly changed their preference. We were also able to manipulate their altered preference back to the original favorite."

The study, which will be published online in the journal Current Biology on 16 June, is the first to confirm a causal link between activity in the ventral tegmental area and choice behaviour in primates. "In scans we found that electrically stimulating this tiny brain area activated the brain's entire reward system, just as it does spontaneously when a reward is received. This has important implications for research into disorders relating to the brain's reward network, such as addiction or learning disabilities."

Could this method be used in the future to manipulate our choices? "Theoretically, yes. But the ventral tegmental area is very deep in the brain. At this point, stimulating it can only be done invasively, by surgically placing electrodes -- just as is currently done for deep brain stimulation to treat Parkinson's or depression. Once non-invasive methods -- light or ultrasound, for example -- can be applied with a sufficiently high level of precision, they could potentially be used for correcting defects in the reward system, such as addiction and learning disabilities."