CHICAGO, Nov. 4 (Xinhua) -- A study in monkeys by researchers at Washington University School of Medicine in St. Louis has shown that the activity of neurons in the brain encodes the value of the options and determines the final decision.

To examine the connection between values encoded by neurons and choice behavior, the researchers performed two experiments.

In one experiment, the researchers repeatedly presented monkeys with two drinks and recorded the animals' selections. The drinks were offered in varying amounts and included lemonade, grape juice, cherry juice, peach juice, fruit punch, apple juice, cranberry juice, peppermint tea, kiwi punch, watermelon juice and salted water. The monkeys often preferred one flavor over another, but they also liked to get more rather than less, so their decisions were not always easy. Each monkey indicated its choice by glancing toward it, and the chosen drink was delivered.

Then, the researchers placed tiny electrodes in each monkey's orbitofrontal cortex. The electrodes painlessly stimulate the neurons that represent the value of each option. When the researchers delivered a low current through the electrodes while a monkey was offered two drinks, neurons dedicated to both options began to fire faster. From the perspective of the monkey, this meant that both options became more appealing but, because of the way values are encoded in the brain, the appeal of one option increased more than that of the other. The upshot is that low-level stimulation made the animal more likely to choose one particular option, in a predictable way.

In another experiment, the monkeys saw first one option, then the other, before they made a choice. Delivering a higher current while the monkey was considering one option disrupted the computation of value taking place at that time, making the monkey more likely to choose whichever option was not disrupted. This result indicates that values computed in the orbitofrontal cortex are a necessary part of making a choice.

"In a number of mental and neuropsychiatric disorders, patients consistently make poor choices, but we don't understand exactly why," said senior author Camillo Padoa-Schioppa, a professor of neuroscience, of economics and of biomedical engineering at the university. "Now we have located one critical piece of this puzzle. As we shed light on the neural mechanisms underlying choices, we'll gain a deeper understanding of these disorders."

The study was published Monday in the journal Nature.