Appropriately placed electrical impulses can change the preferences of macaques, throwing open debates about the meaning of free will.
"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,” says Professor Vanduffel of Dutch University, KU Leuven. 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 signals were sent to the small ventral tegmental area of the midbrain because it is known to regulate learning and produce dopamine, essential to the brain's reward system. "In this way, this small area of the brain provides learning signals," says Vanduffel. "If a reward is larger or smaller than expected, behavior is reinforced or discouraged accordingly."
In Current Biology Vanduffel reports that stimulation of the ventral tegmental area “increased fMRI activity throughout most of the dopaminergic reward system”. He concludes that the experiments confirm the ventral tegmental's importance in the driving behavior. “This has important implications for research into disorders relating to the brain's reward network, such as addiction or learning disabilities,” he says.
Although the region's connection to the reward system was already well established, scientists could not be certain the ventral tegmental's signals were driving decisions or downstream activity. Studies in rats had shown similar results, but the paper notes, “The primate dopamine system has diverged significantly from that of rodents, exhibiting greatly expanded and uniquely distributed cortical and subcortical innervation patterns.” Consequently it was not safe to conclude the same effects would be seen in macaques, let alone humans.
While the work has exciting possibilities for helping people overcome habits and preferences they want to change, the implications in the wrong hands are frightening. Perhaps fortunately, it will not be easy to change people's desires at will.
“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.” says Vanduffel. Eventually other options may open up, for good or ill, for example using ultrasound.