Parkinson's disease slows a person down.

Trust me on that, I possess first-hand knowledge. I was diagnosed with the disease eight years ago.

Actor Robin Williams — who was in the early stages of the disease before his death, according to his widow — was anything but slow. At the apex of his career, the frenetic comic genius had synapses that fired faster than almost any other mortal on this planet. His speed of thought was breathtaking. He was a supernova of creativity.

A synapse, by the way, is a structure that permits a nerve cell (or neuron) to pass an electrical or chemical signal to another cell.

Parkinson's disease murders neurons. Studies have shown that by the time most people are diagnosed they've already lost 60% to 80% of their dopamine producing neurons. A shortage of dopamine causes a range of movement difficulties.

The brain of a newly diagnosed individual has likely been slowing for years.

Parkinson's is a progressive neurological disorder with no known cure. It causes nerve cells to die or become impaired, and patients exhibit such symptoms as tremors or shaking, slowness of movement, rigidity or stiffness, loss of facial mobility, and balance difficulties. Other signs include a shuffling gait, cognitive problems and muffled speech.

The 63-year-old Williams took his life Aug. 11 in his Marin County home. His wife, Susan Schneider, reported that he'd suffered from anxiety and depression for years. Parkinson's patients exhibit high incidents of depressive symptoms.

Williams had also had drug and alcohol abuse problems, but the comedian elected to keep his Parkinson's diagnosis a secret.

A lot of us do that. I did.

Did Parkinson's in any way contribute to Williams' decision to take his life? Who can say? But his incandescent mind was no doubt aware of the debilitating effects Parkinson's can have on the brain. He may already have begun to experience some of those effects.

I'm told that Williams was an indefatigable performer who rarely experienced "down" time. He was "on" 24/7. He had a mind that operated at warp speed. With 60% of his neurons no longer producing dopamine, could he sense that his mind was slowing? Only Williams knows that answer.

Could he live with that? Did he mull the future prospect of living life as someone other than his former brilliant self?

I've asked my less-than-stellar mind that question numerous times. I've noted diminishment in my own thinking processes. Is that a product of normal aging, I've wondered, or decline brought about by Parkinson's? One day I'll know.

What happens if I reach a point where I'm no longer me? Did Williams envision such a scenario?

My father had Parkinson's and wound up with full-blown dementia. It's not a pretty sight. Conversely, I have dozens of friends with the disease who seem almost impervious to cognitive ills. The disease can be agonizingly hit or miss.

Sometimes Parkinson's takes away what's most dear to us.

So far, I haven't had to give up anything I couldn't live without. That's not the case with dear friends, however.

John, who's approaching 70, was a classically trained oboist. It was his identity and passion. But Parkinson's cruelly and with absolute finality took it away from him. He no longer has full use of his hands. He can't finger notes properly.

I can relate. My hands don't operate as they used to, either. I have difficulty buttoning and unbuttoning shirts; my handwriting is shaky and nearly illegible; working a knife and fork is almost beyond my ability; and, though I was able to type 90 words a minute in my prime, I hunt-and-peck today in one-fingered frustration.

Yet, for me, those disabilities are mere inconveniences.

John has lost all capacity for doing the thing he loves most.

"I was devastated when I lost that ability," he told me recently. "I'm no longer the person I was."

An iconic slogan reads: "A mind is a terrible thing to waste." Parkinson's disease routinely ravages many minds.

My heart breaks for Robin Williams. His was truly a beautiful mind.

JIM CARNETT, who lives in Costa Mesa, worked for Orange Coast College for 37 years.