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According to researchers from Massachusetts General Hospital (MGH), activating a particular region of the brain affects the decision-making progress. The team was able to change monkeys' choices between two images by artificially stimulating the ventral tegmental area (VTA), which consists of a group of neurons located at the base of the midbrain.

"Previous studies had correlated increased activity in the primate VTA with positive events experienced by the animal but could not prove that VTA activity actually caused behavioral changes," stated study author, Wim Vanduffel, PhD, of the Martinos Center for Biomedical Imaging at MGH and head of the Laboratory for Neuro- and Psychophysiology at the University of Leuven. "Studies in rodents have shown that artificially manipulating VTA activity strongly influences behavior, and our work has bridged the gap between rodent and primate."

In this study, the team placed microelectrodes in the VTAs of macaque monkeys using high-resolution magnetic resonance imaging as a guide. In the beginning of the experiment, the monkeys were showed two images. The researchers recorded which image the monkeys preferred by observing their eye movements. The monkeys had been trained to look initially at a white square and then at either picture. The picture that the monkeys looked at most frequently was considered the favored one.

After knowing which image the monkeys preferred, the researchers showed the monkeys the picture pairs once again. This time when the monkeys looked at the less favored picture, the team administered mild stimulation to the VTA. The researchers noted that the stimulation caused the monkeys to change their preference. The stimulation was then applied when the monkeys glanced at their preferred image. The monkeys' preference changed back to normal.

In the second part of the study, the researchers showed the monkeys a 20-minute clip. The clip contained the two images that randomly showed up on the screen every five seconds. Every time the non-preferred image showed up, the researchers administered stimulation to the VTA. Once again, the team discovered that repeated stimulation changed the monkeys' preferences.

"Our study showed that the timing of VTA stimulation is important - when stimulation happens immediately after an action is performed, the monkey is more likely to perform that action - and that it applies 'value' to a particular stimulus and motivates future behavior," said Vanduffel, who is also an assistant professor of Radiology at Harvard Medical School. "Other studies have implicated the VTA in learning based on negative, as well as positive reinforcement, and a recent rodent study found that increasing VTA activation can relieve depression, possibly by increasing motivated behavior. Our findings lay the groundwork for further investigation of the role of the VTA in reinforcing and regulating motivated behavior."

The study was published in Current Biology.