People who had caffeine after looking at images apparently better at distinguishing them from similar ones the next day

A cup or two of coffee could boost the brain's ability to store long-term memories, researchers in the US claim. People who had a shot of caffeine after looking at a series of pictures were better at distinguishing them from similar images in tests the next day, the scientists found.

The task gives a measure of how precisely information is stored in the brain, which helps with a process called pattern separation which can be crucial in everyday situations.

If the effect is real, and some scientists are doubtful, then it would add memory enhancement to the growing list of benefits that moderate caffeine consumption seems to provide.

Michael Yassa, a neuroscientist who led the study at Johns Hopkins University in Baltimore, said the ability to separate patterns was vital for discriminating between similar scenarios and experiences in life.

"If you park in the same parking lot every day, the spot you choose can look the same as many others. But when you go and look for your car, you need to look for where you parked it today, not where you parked it yesterday," he said.

Writing in the journal Nature Neuroscience, Yassa described how 44 volunteers who were not heavy caffeine consumers and had abstained for at least a day were shown a rapid sequence of pictures on a computer screen. The pictures included a huge range of items, such as a hammer, a chair, an apple, a seahorse, a rubber duck and a car.

When each image flashed up on the screen, the person watching had to say whether the object was normally found indoors or outside, but they were not asked to memorise the pictures. At the end of the task, each volunteer was randomly assigned either a 200mg caffeine pill or a placebo. A typical cup of coffee contains around 150mg of caffeine.

The next day, the scientists brought the volunteers back and sat them down at the computer again. This time, the sequence of images included many they had seen the day before, but some were new and others were similar. The similar images varied in how close to the originals they were. Some showed the same object from a different angle, while others were a similar type of object, such as a different design of hammer from the one they had seen before.

For this part of the study, the volunteers had to say whether each image was either new, old or similar to one they had seen the day before. According to Yassa, the caffeine and placebo groups scored the same except when it came to spotting the similar images. In this task, the caffeine group scored around 10% higher, he said.

"What I've taken from this is that I should keep drinking my coffee," Yassa told the Guardian. "Our study suggests there's a real learning and memory benefit, but other studies suggest caffeine is associated with increased longevity, and a resistance to Alzheimer's disease. In moderate amounts, it could have beneficial effects for health."

Yassa said it was unclear how caffeine might help the storage of memories, but one theory is that it leads to higher levels of a stress hormone called norepinephrine in the brain, which helps memories to be laid down.

Some scientists, however, say they need more evidence to believe the effect. George Kemenes, a neuroscientist who studies memory at Sussex University, said the statistical techniques used in the paper were not good enough to prove the effect was real. "I have reservations. If the statistics aren't right the whole story, beautiful as it is, unravels," he said.

"Even if this was solidly true, which in my view it isn't, it wouldn't prove that caffeine has a memory-enhancing property. It wouldn't call this an improvement in long-term memory."

Jon Simons, who works on memory at Cambridge University, said the study was interesting and carefully designed, but the effect needed to be shown in a larger number of people. "The claim that caffeine affects the consolidation of memories is based on quite a small effect that would really benefit from replication in a larger sample to be convincing," he said.