Thus, we conclude that a dose of at least 200 mg is required to observe the enhancing effect of caffeine on consolidation of memory," the study authors write.
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.