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| Vol. 24, No. 8 | October 30, 1998 |
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| Vol. 24, No. 8 | October 30, 1998 |
By Bob Nelson
With a pair of rhesus monkeys and a simple computer game, researchers at Columbia appear to have exploded long-standing ideas about what thinking is and whether humans alone are capable of it.
In an article that ran in Science last Friday (reported in The New York Times on the same day), Herbert S. Terrace, professor of psychology at Columbia and professor of psychiatry at the New York State Psychiatric Institute, and Elizabeth Brannon, a Columbia graduate student in psychology, described experiments with monkeys that they say are the strongest arguments so far for the existence of numerical ability in non-human primates.
"We have ample evidence that animals can think without language," said Terrace, who heads Columbia's Primate Cognition Laboratory. "In our current and previous research, we have shown that animals solved complex problems without help from external cues."
Added Brannon, "Though monkeys do not recognize the word 'two' or the symbol '2,' they share with humans the capacity to master simple arithmetic, on at least the level of a two-year-old child. We don't have direct evidence yet, but it seems likely that these monkeys, and other non-human primates, can count."
The research challenges the prevailing view, which dates to Descartes, that non-human primates cannot think because they cannot use language. It also challenges the views of B.F. Skinner, the noted behaviorist and Terrace's mentor, who held that all examples of animal intelligence were simply conditioned behavior, not thought. Terrace and Brannon believe that cognitive processes-thought-are needed to explain the kind of complex behavior they are studying.
They hope to show that human intelligence, like other human attributes, originated in animals. Brannon and Terrace believe that arithmetic and language evolved separately, and that number skills preceded human speech.
"Language is a complex social skill, whereas counting can be learned by the individual," Terrace said. "Counting is useful in foraging for food, assessing a group of predators or ordering the number of dominant males in one's group."
In their experiments, Brannon and Terrace trained two male rhesus monkeys, Rosencrantz and Macduff, by presenting them with 35 sets of images on a touch-sensitive video screen. Each picture contained a different number of different objects from one through four, for example, one triangle, two bananas, three hearts and four apples. The stimuli appeared in random positions on the screen, to prevent the monkeys from learning the required sequence as a series of fixed motor movements.
Other features of the pictures unrelated to number, such as size, surface area, shape and color, were also varied randomly.
When the monkeys touched the pictures in ascending order, for instance, one square, two trees, three ovals and four flowers, they received a banana-flavored food pellet. If they made an error, the screen turned black for several seconds and a new trial began with different pictures. This "video game" paradigm, which the monkeys clearly enjoy, trained them to perform cognitive serial tasks without verbal instructions-without language, the researchers said.
Over the course of learning 35 different training sets, the monkeys got better and better at responding in the ascending numerical order, one to four. The two psychologists then tested Rosencrantz and Macduff on 150 test trials, in which a new stimulus set, showing numbers of objects from five to nine, was presented on each trial. The monkeys performed just as well as they had on the original 35 training sets.
"There was no way the monkeys could have done this, unless they had learned some numerical rule for ordering the contents of the pictures," Brannon said.
To test whether the monkeys understood the ordinal relations between non-consecutive numbers-that, for example, five is greater than three-Brannon and Terrace gave the monkeys a new set of problems in which they were shown up to nine objects.
The task was to first touch the picture containing the smaller number of objects, then the one with the larger number of objects. For example, if a monkey was shown one picture with five large circles and another containing seven small circles, the correct order was five, then seven. Despite the different tests, Rosencrantz and Macduff responded correctly to all.