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Monkeys found to wonder what might have been

May 15, 2009
Courtesy Duke University Medical Center
and World Science staff

Record­ings of brain cells have found that mon­keys take note of missed op­por­tun­i­ties and learn from their mis­takes, sci­en­tists say.

“This is the first ev­i­dence that mon­keys, like peo­ple, have ‘would-have, could-have, should-have’ thoughts,” said Ben Hay­den of Duke Uni­ver­s­ity Med­i­cal Cen­ter in Dur­ham, N.C., lead au­thor of the study pub­lished in the re­search jour­nal Sci­ence.

The re­search­ers watched in­di­vid­ual neu­rons in a re­gion of the brain called the an­te­ri­or cin­gu­late cor­tex that mon­i­tors the con­se­quenc­es of ac­tions and me­di­ates re­sult­ing changes in be­hav­ior. The mon­keys were mak­ing choices that re­sulted in dif­fer­ent amounts of juice as a re­ward. 

Their task was like the TV show “Let’s Make a Deal” with the ex­pe­ri­menters of­fer­ing mon­keys choices from an ar­ray of hid­den re­wards. Dur­ing each tri­al, the mon­keys chose from one of eight iden­ti­cal white squares ar­ranged in a cir­cle. A col­or be­neath the white square was re­vealed and the mon­key re­ceived the cor­re­spond­ing re­ward. 

Over many weeks, the mon­keys were trained to as­so­ci­ate a high-val­ue re­ward with the col­or green and the low-val­ue re­wards with oth­er col­ors. Af­ter re­ceiv­ing a re­ward, the mon­key was al­so shown the prizes he missed.

The re­search­ers found that brain cells be­come ac­ti­vat­ed in pro­por­tion to the re­ward—a great­er re­ward caused a high­er re­sponse. They al­so found that these same brain cells, called neu­rons, re­sponded when mon­keys saw what they missed. Most of these neu­rons re­sponded the same way to a real or im­ag­ined re­ward. 

To meas­ure how these re­sponses might help the mon­key to learn, the re­search­ers kept the high re­ward in the same po­si­tion 60 per­cent of the time, or moved it one po­si­tion clock­wise, so that a mon­key could pos­sibly no­tice and adapt to that pat­tern. The mon­keys chose tar­gets next to po­ten­tial high-val­ue tar­gets more than twice as of­ten than those next to low-val­ue tar­gets, ac­cord­ing to the in­ves­ti­ga­tors. This sug­gested the an­i­mals un­der­stood the rela­t­ion­ship be­tween the high val­ue tar­get on the cur­rent tri­al and its likely loca­t­ion on the next tri­al, re­search­ers said; the mon­keys learn­ed the pat­tern and chose the high val­ue more of­ten than by a chance.


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Recordings of brain cells have found that monkeys take note of missed opportunities and learn from their mistakes, scientists say. “This is the first evidence that monkeys, like people, have ‘would-have, could-have, should-have’ thoughts,” said Ben Hayden of Duke University Medical Center in Durham, N.C., lead author of the study published in the research journal Science. The researchers watched individual neurons in a region of the brain called the anterior cingulate cortex that monitors the consequences of actions and mediates resulting changes in behavior. The monkeys were making choices that resulted in different amounts of juice as a reward. Their task was like the TV show “Let’s Make a Deal” with the experimenters offering monkeys choices from an array of hidden rewards. During each trial, the monkeys chose from one of eight identical white squares arranged in a circle. A color beneath the white square was revealed and the monkey received the corresponding reward. Over many weeks, the monkeys were trained to associate a high-value reward with the color green and the low-value rewards with other colors. After receiving a reward, the monkey was also shown the prizes he missed. The researchers found that brain cells become activated in proportion to the reward—a greater reward caused a higher response. They also found that these same brain cells, called neurons, responded when monkeys saw what they missed. Most of these neurons responded the same way to a real or imagined reward. To measure how these responses might help the monkey to learn, the researchers kept the high reward in the same position 60 percent of the time, or moved it one position clockwise, so that a monkey could possibly notice and adapt to that pattern. The monkeys chose targets next to potential high-value targets more than twice as often than those next to low-value targets, according to the investigators. This suggested the animals understood the relationship between the high value target on the current trial and its likely location on the next trial, researchers said; the monkeys learned the pattern and chose the high value more often than by a chance.