the hippocampus

The hippocampus' key function is to process recent memory and store it as short-term memory. Dementia, in particular Alzheimer's disease, kills nerve cells and causes subsequent tissue loss, initially in the hippocampus and subsequently throughout the brain. The brain shrinks dramatically, affecting nearly all its functions, and in particular its role in the formation of new short-term memories. Further memory loss, disorientation and Amnesia follow with an inability to recognize time and location.

As the disease progresses the cortex shrivels up and brain shrinkage, especially severe in the hippocampus, results in expansion of the ventricles (fluid-filled spaces within the brain).

See: Alzheimers' Association Braintour