WASHINGTON, July 25, 2011 /PRNewswire-USNewswire/ -- Chimpanzees, the closest living relatives to humans, do not experience a decrease in brain volume as they age like humans do, according to a study by George Washington University researcher Chet Sherwood and his colleagues.  There are many similarities between the species, but this discovery reveals an important distinction, demonstrating how humans are unique from other animals.  The study "Aging of the Cerebral Cortex Differs Between Humans and Chimpanzees" is the first study of its kind in this field and will be published in the "Proceedings of the National Academy of Sciences" on July 25, 2011. 

The researchers used magnetic resonance imaging (MRI) to measure the volume of the whole brain and numerous specific internal structures using a sample of 99 chimpanzee brains ranging from 10-51 years of age.  This data were compared to brain structure volumes measured in 87 humans ranging from 22-88 years of age.  Measurements of the neocortical gray and white matter, frontal lobe gray and white matter and the hippocampus were performed.  In contrast to humans, who showed a decrease in the volume of all brain structures over the lifespan, chimpanzees did not display significant age-related changes.  Furthermore, the effects of aging in humans were only evident after the maximum age of chimpanzees.  As a result, the researchers concluded that the brain shrinkage seen in human aging is evolutionarily novel and is the result of an extended lifespan. 

The hippocampus, the area of the brain responsible for encoding new memories and maintaining spatial navigation, was of specific interest to the researchers, as this area is especially vulnerable to age-associated atrophy in humans.  In addition, the hippocampus is the region of the brain most prominently affected by Alzheimer's disease (AD), an illness that is only seen in primarily older humans.  AD is a form of dementia that is associated with a loss of brain function, impacting memory, thinking and behavior.  AD is a result of neurodegeneration, which is the progressive loss of structure or function of neurons, including the death of neurons.  The unique vulnerability seen in humans to develop AD may be in part due to the human tendency to show more pronounced brain atrophy than any other species, even in normal, healthy aging.