Maybe you never hit your head, but you do remember early brain trauma. That trauma was polio. The poliovirus damaged neurons in your brain stem responsible for activating your brain and focusing attention whether or not it damaged your spinal cord motor neurons. But the poliovirus neither damaged neurons in the hippocampus, which is responsible for storing memories, nor in the cortex, the thinking "computer" of your brain. So why, in our 1990 International Post-Polio Survey, did nearly 85 percent of polio survivors report memory problems, 70 percent report difficulty thinking clearly and 79 percent report difficulty "thinking of words I want to say"?

First, it makes sense that the more fatigue polio survivors report, the more memory and thinking problems they have. But even our most severely fatigued polio survivors had no trouble when we gave them tests of memory and thinking. However, severely fatigued polio survivors took up to two-thirds more time to complete attention tests, and had more errors, than did polio survivors without fatigue. All of the fatigued polio survivors' scores on these tests were abnormally low.

Second, word-finding difficulty isn't a memory problem. Trouble with word finding happens when the part of your brain that "thinks" of a word has difficulty talking to the part of the brain that "says" the word. Our studies found that word-finding difficulty is related to both fatigue and trouble with attention. We found word-finding difficulty, fatigue severity and attention problems were all related to polio survivors' brains making less dopamine. Dopamine is the brain-activating neurochemical. You may know that low levels of dopamine cause Parkinson's disease. We found that polio survivors and people with Parkinson's had identical levels of word-finding difficulty--no surprise since it is dopamine that connects the word "thinking" and word "saying" parts of the brain.

So, it's early brain trauma, due to the poliovirus killing dopamine-producing and brain-activating neurons, combined with the natural death of remaining neurons with age, that reveal attention and word-finding problems in midlife. This is the same process that is thought to be responsible for midlife attention and word-finding problems in individuals who had an early brain injury: The normal age-related loss of neurons reveals that they already have a reduced number of brain neurons.

It will be a relief to know that polio survivors don't develop Parkinson's disease any more frequently than do other individuals. What's more, polio survivors may actually be protected against getting Alzheimer's disease. The gene that makes the poliovirus receptor--which grabs the poliovirus and pulls it inside neurons, where it does its dirty deeds--is found on chromosome 19. The poliovirus receptor gene shares space on chromosome 19 with another gene, the one that makes a protein called APOE-4, which is thought to damage the brain in ways that cause Alzheimer's disease. But the two genes have an "either/or" relationship: you can't have one with the other. If you have the APOE-4 gene, you can't make poliovirus receptors, and vice versa. With the APOE-4 gene, a person would be less susceptible to polio but more likely to get Alzheimer's. Without the gene, you would make poliovirus receptors and be susceptible to polio, but be less likely to get Alzheimer's disease.