Monkeys, Humans Tie in Memory Game

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April 13, 2007 — Humans pride themselves on their intellectual superiority over other animals. When playing a memory game, however, both humans and rhesus monkeys play equally well or, one might say, equally poorly.

Each species appears to possess a three- to four-item short-term memory limit, meaning that both monkeys and humans often have trouble remembering details about this number of items. This is especially true when the game involves comparing this amount of stuff with another amount.

The mental limit might even be a universal trait possessed by most all creatures.

"It looks like both humans and nonhuman animals use the same type of short-term memory system to represent and count things in their environment, with a strict memory limit of three to four items," lead author Justin Wood told Discovery News.

"To me, this is quite counterintuitive," Wood, a researcher in the Cognitive Evolution Laboratory and the Department of Psychology at Harvard University, added. "Our visual experience of the world is very rich, yet we can only retain a very small portion of it once we close our eyes!"

Wood and colleagues Marc Hauser, David Glynn and David Barner conducted memory game experiments involving free-ranging rhesus macaques living on the island of Cayo Santiago, Puerto Rico.

A single experimenter would approach lone monkeys who were not otherwise occupied. For the first test, the experimenter presented two large opaque containers. He then scooped chopped up carrots from a bucket and poured them into each container, with one containing more than the other. Since the food was not solid, it forced the monkeys to pay attention to the number of scoops.

Prior studies found that both monkeys and humans, when given a choice, almost always go for larger amounts of something desired. So, from the monkeys’ perspective, the goal was to choose the container with the largest amount of carrots, even though they could not see the contents.

The monkeys aced this test, and others involving portion comparisons of one scoop verses two, two verses three and three verses four. But they flubbed on comparisons of four verses five and three verses six scoops.

Earlier memory tests on both human infants and adults came to nearly the exact same conclusions.

Findings have been accepted for publication in the journal Cognition.

Wood said that if the memory element is removed from such a task, both humans and monkeys generally go for the larger number even if the set contains more than three to four portions.

"So, even though monkeys can differentiate between larger numbers that are in clear view, they can’t store all of that information in memory," he explained.

The results show not only that monkeys, like humans, can count non-solid objects, but that this ability is not language dependent.

Humans, for example, can pour rice twice in a glass and quantify it verbally as either "one glass of rice" or "two pours of rice." This then influences how we count non-solid portions. Monkeys can do the same thing, only without naming the amounts.

In the ongoing question, "Which comes first, math or language?" it appears that math is the answer, since at least some quantification and comparison skills are inherent.

Yet another forthcoming study, accepted for publication in the journal Neurobiology of Aging, found that elderly monkeys also perform about the same as humans during long-term motor memory tasks. In this case, the experiment involved old monkeys removing a food reward from a curved rod that required a bit of tinkering to manipulate.

Ashley Walton, a scientist in the Department of Anatomy & Neurobiology at the University of Kentucky Medical Center, and colleagues found that while it took participants a while to figure out the game, once they did, they improved at a rate of around 17 percent. This mirrors comparable tests on elderly humans.

The good news is that while short term memory may be limited and could falter with age, Walton said "motor memory traces are preserved over an extended time interval," indicating that some forms of long-term memory do not always significantly decline with age.