Monkeys Show Sophisticated Learning Abilities

Not only can the monkeys choose which image came first in the same list, but they can also compare the order of pictures that came from different lists, found the researchers. The scientists said they have not yet found the limits of the monkeys' learning capacity.

The researchers -- Herbert Terrace of Columbia University, Lisa Son of Barnard College and Elizabeth Brannon of Duke University -- reported their findings in an article in the January 2003 Psychological Science. Son and Brannon were graduate students at Columbia when the study was conducted. The scientists' research was sponsored by the National Institute of Mental Health and the National Science Foundation. Brannon is an assistant professor in the Department of Psychological and Brain Sciences and a member of Duke's Center for Cognitive Neuroscience.

"While quite a lot of research has been done in monkeys addressing specific cognitive abilities such as numerical cognition or concept formation, very little has been done up to this point on the development of expertise," said Brannon.

To explore how monkeys learn, the researchers decided to teach four rhesus macaque monkeys -- named Benedict, Macduff, Oberon and Rosencrantz -- to distinguish the order of images displayed on a video touch screen. In the experiments, the monkeys were first presented with sets of images such as animals, people, scenery, cars and bridges. In return for banana-pellet rewards, the monkeys were then required to touch the images in a particular arbitrary but consistent order. The pictures were presented in random positions on the screen in each trial but the monkeys needed to ignore their physical locations and touch them in the correct order. When the researchers began their study, they gave the monkeys three- and four-image lists, and once the monkeys showed proficiency in these lists, graduated to seven-image lists.

Finally, the monkeys were asked to order pairs of images originally learned in the same list, and also pairs that contained items from two different lists.

The monkeys showed very high abilities to learn and manage four, seven-item lists, found the researchers. In addition, the monkeys were able to order pairs that had come from the same list and pairs that had come from different lists with similar high accuracy, responding correctly to 94 percent and 91 percent of these trials respectively. Such high performance indicates that the researchers have not yet reached the limits of the animals' abilities, said Brannon.

"It was only an initial guess on our part that seven-item lists would be particularly challenging, but it didn't turn out be all that challenging, and it seems possible that we could test longer lists." Importantly, the researchers observed that the monkeys' learning process exhibited the same basic properties as human learning.

"For example, we found that the monkeys were learning each additional new list much quicker than they learned their first list within any given list length," said Brannon. "So, they weren't just learning about the particular photographs within a sequence; they were learning a strategy of how to learn. And that's what we call expertise." Also, said Brannon, in the paired imaging tests, the monkeys showed the ability to apply their knowledge in a flexible way as humans do.

"In learning the lists, they had not learned anything about the relationship between picture A from list 1 and picture C from list 4," said Brannon. "And yet, their performance was pretty much indistinguishable on pairs that were within one list, versus pairs that were between lists.

"I was very surprised that there was no significant difference in accuracy between these two types of comparisons -- that the animals really treated them as the same problem," said Brannon. "They showed logical acquisition of an incredible amount of flexible knowledge, which is the hallmark of what in humans is called declarative knowledge."

Finally, the researchers found two distinctive properties -- called "distance" effects and "magnitude" effects -- in the monkeys' responses that mimicked human responses to memory tasks and number discrimination tasks. In the distance effect, the monkeys' responses were slower when two images were close together in the original list (e.g., A and B) and faster when the images were farther apart in the original list (e.g., A and D).

"Another example of a distance effect is if you're comparing two line lengths," said Brannon. "If two lines are very similar to each other, it's a very difficult discrimination. So, your accuracy is lower and your reaction time is slower. If, on the other hand, you're comparing a very short line and a very long line, it's a much simpler discrimination. And the distance is bigger and your accuracy is better and your reaction time is much faster."

Similarly, the magnitude effect is an increase in time and decrease in accuracy when the initial item in a comparison is farther down in the original list. In analyzing the results, the scientists found such an effect in the monkeys.

"These effects tell us that there seems to be some kind of common code in both monkeys and humans for distinguishing order -- perhaps some sort of spatial map that represents the essence of order," said Brannon. According to Brannon, the researchers' findings offer important lessons to guide future studies of primate cognition.

"For one thing, we don't typically give animals these kinds of complicated sequences that involve many responses within a given trial," she said. "And now when we do, we see the development of expertise.

"Also, when we measure cognitive capacities in adult humans, it is in the context of a huge amount of life experience. Our experiments took two years, and although that might seem like a long time, it doesn't compare to the kind of experience that adult humans have had.

"So, when we're trying to understand the limits of cognitive capacities in animals, it may not be fair to compare these naïve animals to humans."