Researchers from Johns Hopkins University looked at caffeine's impact on memory while excluding its other brain-enhancing factors. The study showed that caffeine enhances certain memories for up to 24 hours after it's consumed.
"The finding that caffeine has an effect on this process in humans – the process of making memories more permanent, less forgettable – was one of the big novelties," said study author Michael Yassa, an assistant professor of neurobiology and behaviour at the University of California, Irvine, who conducted the research while at Johns Hopkins.
The study, which was funded by the US National Institutes of Health and the US National Science Foundation, included more than 100 participants who were "caffeine naive", meaning they were not big coffee, tea or cola drinkers, Yassa said.
"We picked people who were getting less than 500 milligrams of caffeine a week," he said. "Most weren't coffee drinkers. Most had a soda once or twice a week."
Coffee's caffeine content varies greatly. Most average-size cups contain about 160 milligrams (mg), Yassa said. But a 16-ounce cup of Starbucks coffee packs 330 mg of caffeine, according to the Centre for Science in the Public Interest.
A dose of at least 200 mg of caffeine was needed to enhance memory consolidation, the researchers said.
For the study, which was published online in the journal Nature Neuroscience, the researchers asked the participants to look at hundreds of common, everyday images on a computer screen: shoes, a chair, a rubber duck etc.
"We asked them to tell us if it was an indoor or an outdoor object, but we didn't really care about what they said," Yassa said. "We just wanted them to attend to the object, to get that object into their brains."
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Five minutes after the participants looked at the images, half were given 200 milligrams of caffeine and half received a placebo. They returned 24 hours later, after the caffeine was out of their system, and looked at more images of objects. They were asked to label the pictures as either old, new or similar to the original images they'd seen (for example, a picture of a duck they viewed the day before, but taken from a slightly different angle).
People who had taken the caffeine were better at distinguishing the similar pictures from the original ones, and those who had received the placebo were more likely to incorrectly identify the similar images as the old images, the researchers said.
Yassa said the caffeine-induced ability to recognise similar, but not identical, images did not occur when people were given smaller doses of caffeine or when caffeine was ingested an hour before the picture test.
"On caffeine, the participants were more likely to identify the similar items correctly as similar and not old," he said. "In doing so, this demonstrates that the caffeine enhanced the brain's consolidation process – the process of making those items more permanent in their memory."
The study didn't actually prove that caffeine improves memory, however. One limitation of the study is that participants knew they were involved in caffeine research, the researchers said.
In the United States, 80% of adults consume caffeine every day, according to the US Food and Drug Administration.