The pigeons and baboons learned to associate images with a response. The pigeons pecked the left or right side of an image shown on a screen. Baboons used a joystick to move a cursor to the left or right side.
The appropriate response (arbitrarily selected for each image) was rewarded with food; an incorrect response was met with a brief delay before moving on to the next image.
Over days more images were added for the animals to learn. Testing these images allowed the scientists to gauge long-term memory.
The pigeons started to show a decline in the number of images remembered at around a thousand.
"As items get older, they begin to disappear from memory as new items come in," Cook said.
The baboons never showed a marked decline after remembering at least 3,500 images.
Edward Wasserman is a psychologist at the University of Iowa in Iowa City. He said the memory capacities of pigeons and baboons demonstrated in the experiment are "quite noteworthy."
"Beyond the question of memory capacity is the question, What does this have to do with the animals' daily lives? Is such a prodigious capacity helpful to them?" he continued.
"You could imagine that by storing so much individual information you could get lost in the trees and miss the forest."
The pigeons and baboons both responded quickly to the images they knew, suggesting their long-term memories are organized in parallel. That means the animals can access any memory they need at any time and at the same speed—as long as they haven't forgotten it.
"When they know it, they know it," Fagot said. But when the animals had forgotten an image, it apparently wasn't clear to them right away that this was the case.
"For some images they are not sure about, they have to search their memories, and that takes time. When they are wrong, their response time is longer than when correct," he added.
The similarity between how the two species appear to remember and process memories suggests that the basic brain mechanisms evolved early in the history of life, the researchers note.
Wasserman, the University of Iowa psychologist, said the finding of a similar memory mechanism in both species is "one of the greatest strengths of the paper."
"It suggests that the similarity in the performance of the baboons and pigeons is not skin deep. It cuts much deeper. It really refers to some fundamental mechanism of the nervous system, not some trivial superficial similarity," he added.
According to study authors Fagot and Cook, increasing memory capacity may underlie the development of higher intelligence—the ability to accomplish more complex cognitive tasks.
For example, a larger capacity for stored memories allows the brain to make connections between those experiences and "recognize patterns that might generate abstract thought," Cook said.
Fagot said the experiments demonstrate "memory has been expanding over evolution, and this might mirror intelligence in general."
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