Immediate, large rewards cause animals to make fast, correct decisions. But the real world is fettered with choices that don't lead to timely, big bonuses--such as having to slog through cleaning our plate to get to dessert. A new study fingers a region of the brain called the thalamus, which may give us the motivation to eat the green beans of life so we can have our cheesecake.
Scientists trying to understand the circuits in the brain that reward animals for making choices have long focused on two regions: the cortex, where the higher functions such as paying attention reside, and the basal ganglia, where the neurotransmitter dopamine acts to make the brain feel rewarded. A third region, the thalamus, connects to both areas, but neuroscientists have long thought that it functions mainly in relaying sensory information. Neuroscientist Minoru Kimura at the Kyoto Prefectural University of Medicine in Japan wondered if the thalamus might play a role in a decision-making that led to smaller, less immediate rewards.
To find out, Kimura and colleagues hooked up two rhesus monkeys to electrodes that recorded signals coming from a specific part of the thalamus. Then the team had the monkeys hold down a lever until a button lit up as red or green. When the light was red, the animals were trained to let go of the lever and press the button. When they did this, they got a large amount of water--a big reward. But when the button shone green and the monkeys were trained to hold the lever down, they received significantly less water--a small reward. In addition, a less-compensated task was usually followed by a task that rewarded the big drink of water. The team found that the neurons in the thalamus stayed silent when the red light came on (i.e., when the monkeys anticipated a large reward) but flared when the green light came on (i.e., when the monkeys were resigned to a small reward). To see if the neurons controlled the animals' behavior, the researchers stimulated the nerve cells via implanted electrodes just as the button lit up. Even if the animal was supposed to perform the large reward task, stimulating the neurons caused them to perform the small reward task instead, the team reports 17 June in Science.
"The finding is interesting and provocative," says neuroscientist Leo Sugrue of Stanford University in California. He says the neurons fire to give the monkeys the motivation to perform less rewarding tasks so that they will eventually receive larger rewards. It's "like a coach yelling at you to keep going," he says.