Memory loss in the early stages of Alzheimer's disease is not due to reduced memory capacity, but an inability to retrieve information that is stored in the brain, researchers have found.

The research team used mice to determine how the brain cells handling memory go about retrieving information. The findings were published March 17 in the British science journal Nature.

Nobel laureate Susumu Tonegawa, who is also director of the Riken Brain Science Institute, led the team.

In the experiment, an electric shock was used to momentarily startle a mouse. When the shock was conducted in a small box, the mouse stored the experience in its memory as a scary moment, and was found to cringe when placed in a similar box later.

A second mouse was genetically altered so that its brain replicated the early stages of Alzheimer's disease. The mouse was apparently unable to recall the scary experience because it did not respond when placed in the box again after one day had passed.

The team conducted further genetic engineering on the second mouse. The change affected the hippocampus, the part of the brain that encodes short-term memories. The change only activated that area of the brain when light was brought into the equation.

The second mouse cringed when it was placed in a box and light was shone on it after a day had passed since the scary experience, much like the first mouse with the unaltered brain. The result showed the researchers that the memory of the experience remained in the rodent's brain.

The team also found that the hippocampus became less active due to shrinkage of the protruded area that connects the hippocampus with other parts of the brain. The team believes that this shrinkage is one factor behind the inability to remember things.