but dementia. some people would say this is a normal function of living a very long life. and that this is a pathology. >> we see in brain scans the back half of the brain in people with alzheimer's disease completely deteriorates. the nfl players, though, they don't have alzheimer's disease as a group. it's frontal lobe that's damaged. >> larry: what is dementia? >> dementia is the big category. that's where you lose cognitive function that's abnormal for your age. alzheimer's disease is one of the types. the football dementia that we talked about, that's another one of the types. there's blood vessel dementia, where you have a stroke, aids related dementia. i wrote a book called "preventing alz i'mers." prevent the risk factors. did you know obesity is a rick factor. the more fat on your body, the worse it is. depression is a risk factor.