Brain cell mechanism for decision making also underlies judgment about certainty

Countless times a day people judge their confidence in a choice they are about to make -- that they now can safely turn left at this intersection, that they aren't sure of their answer on a quiz, that their hot coffee has cooled enough to drink.

University of Washington (UW) researchers who study how the brain makes decisions are uncovering the biological mechanisms behind the belief that a choice is likely to be correct. Their most recent results will be published in the May 8 edition of Science.

"Choice certainty," noted one of the researchers, Dr. Roozbeh Kiani, "allows us to translate our convictions into suitable actions." Several other research projects have shown that choice certainty is closely associated with reaction time and with decision accuracy.

Kiani and the co-author of the May 8 Science article, Michael N. Shadlen are members of the UW Department of Physiology and Biophysics and of the National Primate Research Center. Shadlen is also an investigator in the Howard Hughes Medical Institute.

The researchers tested the possibility that the same brain cell mechanism that underlies decision making might also underlie judgments about certainty. In their study, rhesus monkeys played a video game in which they watched a dynamic, random dot display. They then had to determine the direction of motion. The difficulty of the task was varied by both the percentage of moving dots and the viewing time. After a short delay, the fixation point faded. This cued the monkey to indicate its choice of direction by moving its eyes toward one of two targets. The monkey would receive a reward for each correct choice, and no reward for an incorrect choice.

On a random half of the trials, the monkey could pass on making a choice and instead pick a third, fixed-position target that guaranteed a small reward. While watching the moving dots, the monkeys didn't know whether this third option would be offered. The sure bet was shown during the short delay.

"The monkeys opted for the sure target when the chance of making a correct decision about the motion direction was small," the researchers noted. They picked the sure bet more frequently when the visual evidence was weaker and duration shorter.

According to the researchers, when the monkeys waived the sure-bet option, they more accurately picked the correct direction than when the wager wasn't offered. This occurred at all levels of difficulty, suggesting that the monkeys chose the sure bet because of uncertainty, not because that round of the game was too hard.