Detailed brain scans of people trying to remember different outdoor scenes and words have revealed that a region of the brain just above the right eye, called the right prefrontal lobe, plays a crucial role in memory formation.
The researchers say that it is the first study to demonstrate which parts of the brain determine whether a specific and current experience will be remembered.
Scientists have long suspected that people remember some things better than others partly because of differences in the way the initial experience is encoded into the brain.
Using powerful brain scanners that can detect small changes in the activity of brain cells, researchers from Stanford and Harvard universities in the United States were able to separate the part of the brain used in processing an image or understanding a word and the part needed to encode a picture or word in the memory.
James Brewer, a neuroscientist at Stanford University, said: ''Several regions of the brain are active when one processes a photograph, such as visual areas and areas that allow a person to make sense of the picture, but the level of activity in most of those regions does not predict whether the photograph will be remembered or not.''
However, in a study published in the journal Science, the researchers found that when the right prefrontal lobe became active, the experience of seeing an image or a word was significantly more likely to be remembered.
The findings may help scientists to understand why sufferers of Alzheimer's disease and other forms of senile dementia often have vivid memories of events early in life but easily forget things that have happened more recently, Dr Brewer said.
''We hope that these findings might give us a tool to examine the earliest effects of Alzheimer's disease so that any treatment that is employed will merely have to spare the neurons [brain cells] that are at risk, rather than replace the ones that have been destroyed,'' he added.
The scientists plan to apply the brain-scanning techniques to older people at risk of Alzheimer's to see if they can predict who will and who will not succumb to the disease.