Information has to be perceived and then processed before a memory is stored. Information can be received through sight, sound, or touch/sensation, and each of these senses has a specialized area of the cerebral cortex which processes the information it receives. The prefrontal cortex then receives the information from the other areas of the cortex and works as a manger, recalling and coordinating between all areas.
Retaining and forming information into long-term memories happens when the neural pathways are regularly stimulated by information. They also become strengthened and associated with the emotional context in which they were formed. When the memory is needed, or when the emotional context is triggered, the prefrontal cortex recalls the memory.
It is a normal part of the aging process to forget. In the late teens the brain's ability to adapt and rewire significantly slows. According to National Geographic, after the age of 30, the loss of neurons increases steadily as people age, and by the age of 75, your memory decreases by 50 percent. In the case of Alzheimers, the loss of neurons and brain tissue occurs more rapidly over a shorter period of time.
Memory mapping, or brain-mapping, is the study of how memories are formed, retained, and forgotten. Neuroscientists in Pittsburgh have worked for years at perfecting a technique called functional MRI (fMRI) which looks at areas in the brain related to working memory. Using an MRI scanner linked to a supercomputer, subjects perform a mental activity and a 3-D image displays which areas of the subject's brain "lit up" during the experiment. This knowledge aids in studying mental illness and Alzheimer's disease. Scientists hope that through brain-mapping, they can gain a better understanding and perhaps prevention of such diseases.