Hanging from the Language Tree

In a famous experiment over 30 years ago, psychology professor Herb Terrace hoped to prove that chimpanzees could acquire speech. Today, as director of the Primate Cognition Laboratory at Columbia, he’s searching for the missing link to cognition.

by Cindy Rodríguez Published Fall 2009
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Since there are five chambers, only five monkeys can work at any given time. They seem eager to step inside, to get their crack at winning a snack. It is Lashley’s turn. The green-eyed monkey scurries into the testing chamber. Before him, a series of photographs appears on the screen: a person, a fish, a bird, and flowers. In this test, Lashley must remember these concepts in that order. The screen is cleared and then displays new photos. One after the next, Lashley gets it right. Natalie Portman, a goldfish, a cardinal, daisies. Then, Halle Berry, a school of fish, a raven, a yellow tulip. And so on.

Photo: Courtesy of Herb Terrace / Ebbinghaus taking a metacognition test in Terrace's monkey lab.  / Courtesy Herb Terrace

Following each correct sequence, a token appears on the edge of the screen, signaling to Lashley that he was correct. After several consecutive correct trials, tokens accumulate to the top of his bank, letting Lashley know that he is about to win. He sticks his hand under the dispenser, and out pops a banana pellet.

Lashley is named for Karl Spencer Lashley, an American psychologist known for his pioneering work in the area of memory. One of Terrace’s star monkeys, he’s been the subject of three papers published in Psychological Science.

So far, it seems the rhesus macaques involved in this test — Augustus, Coltrane, Mozart, and Lashley — are able to distinguish the four groups, which, Terrace says, is remarkable, considering they have never seen birds or fish. Even more surprising is the monkeys’ ability to respond to these categories in a specific order — humans, fish, birds, flowers — even though they are seeing new photographs each time.

The lab’s computers have more than 2000 photographs of each category and include a wide variety of angled shots so the monkeys don’t latch onto one feature, such as beaks, to figure out which one is a bird. The monkeys can pick out a bird even based on a close-up shot of a wing.

While other labs have tested chimps’ understanding of single categories, or concepts, Terrace’s lab is the first to show that monkeys can distinguish among four categories and order them into arbitrarily defined sequences. Their performance has revealed a higher level of cognition than was previously thought possible. Even children can’t do this well until the age of four or five.

Monkeys not only understand concepts, they also can memorize long lists of items and recall them — even months later. “Clearly this is thinking without language,” Terrace says. “This kind of machinery was there as long as 50 million years before humans even appeared in evolution. So when humans developed language, it had this and other nonverbal skills to build upon.”

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