Color this chimp amazing

Psychologist suggests synthesthesia may underlie apparent memory feats

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UP FOR THE COUNT After seeing numbers 1 through 9 for only a fraction of a second (left), Ayumu touches them in order while they are obscured by white squares (right). His ability could stem from synesthesia, a recent proposal suggests. T. Matsuzawa, Primate Research Institute

In what seems like a blow for humanity, a very smart chimpanzee in Japan crushes any human challenger at a number memory game.

After the numbers 1 through 9 make a split-second appearance on a computer screen, the chimp, Ayumu, gets to work. His bulky index finger flies gracefully across the screen, tapping white squares where the numbers had appeared, in order. So far, no human has topped him.

Ayumu’s talent caused a stir when researchers first reported it in 2007 (SN: 12/8/2007, p. 355). Since then, the chimp’s feat has grown legendary, even earning him a starring role in a recent BBC documentary.

But psychologist Nicholas Humphrey says the hype may be overblown. In an upcoming Trends in Cognitive Sciences essay, Humphrey floats a different explanation for Ayumu’s superlative performance, one that leaves humans’ memory skills unimpugned: Ayumu might have a curious brain condition that allows him to see numbers in colors. If Humphrey’s wild idea is right, the chimpanzee’s feat has nothing to do with memory.

“When you get extraordinary results, you need to look for extraordinary ideas to explain them,” says Humphrey, of Darwin College at Cambridge University in England.

The idea came to him after listening to two presentations at a consciousness conference in 2011. Tetsuro Matsuzawa of the Primate Research Institute at Kyoto University in Japan described his research on the memory skills of Ayumu, his mother Ai, and two other mom-offspring pairs. And neuroscientist David Eagleman of Baylor College of Medicine in Houston talked about the brain condition known as synesthesia, which causes people to attach sensory experiences to letters or numbers. A synesthete might always see the number four as blue, for instance.

Because synesthesia usually applies to strings of symbols such as letters or numbers, there was no reason to think that animals other than humans would experience it. No reason, that is, until Ayumu and his chimp colleagues learned numbers, Humphrey says.

If Ayumu does perceive the numbers on the screen in colors, then when the digits disappear each white square that replaces them would, in his mind, have a distinct aftereffect color. Ayumu could simply be ordering these colors in a learned sequence without having to remember the original numbers, Humphrey proposes.

Humphrey’s explanation is “speculative, in the best sense of the word,” says neuroscientist V.S. Ramachandran of the University of California, San Diego. Work in his lab has found that synesthesia can give people an edge on visual tasks — the cross-wiring in the brain helps them remember better.

Matsuzawa, who has worked closely for decades with Ayumu and other chimps that excel on these number tasks, is convinced that the animals really do have a superior working memory compared with humans. “I don’t deny that humans are wonderful creatures,” he says, but they simply can’t compete when it comes to rapid storage and recall. Maybe the human lineage lost this memory ability as it gained abstract reasoning skills and language, Matsuzawa proposes.

A simple experiment, Humphrey says, could reveal whether Ayumu is synesthetic: Changing the white square to colored squares would throw him off if he was relying on colors to order the numbers. Matsuzawa, who declined to comment directly on Humphrey’s theory, has no plans to test this.

After touching the white circle to start a round, Ayumu breezily reconstructs the order of briefly flashed numbers.
Credit: T. Matsuzawa, Primate Research Institute

Laura Sanders

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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