Using a novel method for testing recall, researchers have shown that rhesus monkeys can recall and reproduce simple shapes (Curr. Biol. doi:10.1016/j.cub.2011.03.044; published online 28 April 2011). The monkeys appeared to use a procedure for recall that is similar to that used by humans.

In this study, Benjamin Basile and Robert Hampton of Emory University in Atlanta, GA, devised a recall test for monkeys that is modeled on the Rey-Osterrieth Complex Figure Test. When taking the Rey-Osterrieth Complex Figure Test, humans reproduce a complex line drawing from memory. For their version of the test, Basile and Hampton trained five adult male rhesus macaques to use a computer touchscreen to reproduce a simple figure from memory. In the study phase, a monkey saw a shape consisting of two colored boxes on a 5 × 5 grid on the touchscreen. During the test phase, the monkey saw one of the boxes on the screen (this box appeared in a new location). The monkey could then touch the screen to reproduce the absent box. If a monkey correctly reproduced the shape, he received a food reward. If a monkey made an error, he was put in 'time out' and received no reward.

The monkeys learned to reproduce two-box shapes more accurately than expected by chance. The researchers then tested whether the monkeys could reproduce new three-box shapes. The monkeys completed the novel three-box shapes with much higher accuracy than would be expected by chance, suggesting that the monkeys had learned a general reproduction rule rather than inflexible stimulus-response rules.

Credit: Nico Smit

The authors note that virtually all memory tests used with nonhumans are recognition tests, in which animals must recognize something that is familiar. In contrast, to solve this memory test, the monkeys needed to use recall, or retrieve information about something (the shape) that was not present. Recognition and recall solve different problems. Recognizing food as familiar is quick and resistant to distraction. Recollection, such as recalling the location of distant food, is slower and vulnerable to distraction but supports a more detailed memory.

“[W]e've shown that for simple shapes, monkeys have a pattern of performance for recognition and recall that mirrors that of humans,” said Basile in a press release. “And their ability to immediately transfer their performance to new shapes suggests we're tapping into some general cognitive capacity. With this type of information, we are moving closer to better diagnosing and developing treatments for memory impairments in humans.”