The team also found that memory performance was not improved if subjects were given caffeine 1 hour before carrying out the picture identification test.
They investigators say there are many possibilities as to how caffeine may enhance long-term memory.
For example, they say it may block a molecule called adenosine, preventing it from stopping the function of norepinephrine - a hormone that has been shown to have positive effects on memory.
They note that further research should be conducted to better understand the mechanisms by which caffeine affects long-term memory.