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Primates may think, learn like humans

Rhesus macaque monkeys show not only the ability to learn ordered images but also an abstract, flexible knowledge of learned material, a new study reports--suggesting that these supposedly human-specific characteristics trace back to other primates.

In a recently published collaborative study involving four rhesus macaque monkeys, researchers from Duke, Columbia University and Barnard College investigated the precursors of human cognition in non-human primates.

"This research is important in the sense that we know very little about how intelligence evolved. We don't even know how language evolved," said Herbert Terrace, professor of psychology at Columbia. "So we're interested in [the] evolution of cognition and the precursors of precognitive learning."

In the study, researchers showed the monkeys a touch-sensitive monitor screen with a certain number of randomly placed images of simple, everyday objects, such as animals, people, scenery and cars. Choosing the images on a trial-and-error basis, the monkeys eventually learned the correct order to touch the objects on the screen in order to receive a banana-pellet reward.

When a monkey pressed the wrong image, the screen turned black for a while, and the monkey would have to start pointing to the images in the correct order from the beginning, as in a video game.

"Monkeys have this incredible ability to learn sequences," Terrace said. "I believe that this is the most complicated sequence learning ever reported by an animal."

The monkeys began with lists that consisted of three images to place in order, eventually graduating to four and finally seven images. In this study, researchers used seven lists with three images, 11 lists with four images and four lists with seven images. In order for the monkeys to move on to lists with more images, they had to complete 65 percent of the earlier trials correctly.

One of the study's most peculiar findings is that the monkeys were able to complete the lists better and more quickly the longer they played the image-ordering game. The improvement in learning strategy suggests that the monkeys developed what psychologists call expertise for sequencing the images.

"What was so interesting about this research is that not only were the monkeys able to learn the sequence, they got better and better at it," said Elizabeth Brannon, assistant professor of psychological and brain sciences at Duke. "Basically, we thought that seven-item lists were going to be challenging for the monkeys, and they weren't."

Not only did the monkeys learn how to order the images from a given list, they were also able to discern images from different lists in pair-wise tests. For example, if a car was the first image in list D and a bridge was the third image in list A, and the monkeys were presented with two images, a car and a bridge, they would first point to the car, said Lisa Son, assistant professor of psychology at Barnard College.

"The pair-wise tests suggest that [the monkeys] really have an abstract concept and a flexible knowledge of the lists," Son explained. "They knew where each item was in each particular list."

This abstract knowledge of the ordinal position of the pictures is in some ways considered a hallmark of what some human psychologists call declarative memory--the ability to have a very explicit memory of something, Brannon said. Claims of monkeys possessing declarative memory, however, are certainly suspect.

"We can't verbally test the monkeys for specific memories, but the fact that they have an abstract knowledge of the lists is getting closer to a suggestion that monkeys may have something similar to declarative memory," Brannon said.

The monkeys involved in the study were observed at the Psychiatric Institute of Columbia University, and the research was funded by the National Institute of Mental Health and the National Science Foundation.


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