A team led by Dominique Muller from the University of Geneva in Switzerland has used an electron microscope to photograph changes occurring at the connection points between two nerve cells or neurons when long-term memory is established during its active phase.

By analysing successive cross-sections of rat brains, the researchers studied the `synapses' or contact points that link two neurons.

From the microscope images, the scientists constructed three-dimensional images of these minuscule connections. In doing so, they realised that during the memory process, there is duplication of certain synapses, a report in the journal Nature says.

Each of the 100 billion neurons in a human brain can have up to 10,000 contact points or synapses with its surrounding neurons. But only a few dozen of these synapses undergo memory transformation, which explains why it has taken scientists so long to capture images of the modifications that occur when a neuron is stimulated and acts as if it "remembers" the stimulation.