Sunday, August 28, 2005

Some odd things about the brain

In the past month and a half I have had two episodes of sudden, but partial, hearing loss in my left ear. The onset of each was in a period of a few hours, and each lasted several days. The first was more severe than the second. I am hoping there will be no others, but you never know. The doctors certainly don't. My diagnosis is idiopathic sudden hearing loss, which is, as they say, poorly understood. Could be a virus. Could be some kind of auto-immune thing. Could be something else. Frequently it goes away spontaneously. The otolaryngologist wanted to rule out an acoustic neuroma, so I had an MRI, which was negative. During the second hearing-loss episode I got in to see the doctor, and had my hearing tested while the hearing loss was going on. The lab guy made a nice little chart of my hearing acuity. Definitely some hearing loss, he said. I knew that.

The reduction in hearing acuity was for higher pitched sounds. This was very strange because it seemed to me that I was losing sounds in lower registers. The specialist said that such subjective mistakes were not unusual. But it wasn't a mistake. While I was listening to music I couldn't hear, say, a string bass in my left ear that I could clearly hear with my right ear.

After taking a walk, I think I now know why the science was right and I was right too.

When I walk in the neighborhood, I sometimes walk along a busy street for about a quarter mile. During the second hearing loss episode, I was startled when I was was out walking by being beset by loud street noise roars and rumbles, seemingly coming out of nowhere, from indeterminate directions, that I had never heard before. Everything I was hearing seemed, well, really loud, and really strange. I could hear normal things also, but the extraneous noises made the walk really unpleasant. It seemed like I was suffering from increased and unwanted hearing acuity.

But in the quiet of my home, and in the doctor's office, I had hearing _loss_. The tests showed I had diminished left-ear hearing in three or four upper frequencies, and otherwise normal hearing in other frequencies in that ear, and normal hearing in my right ear. So why was I hearing loud noises? Louder than usual.

I kind of think what was going on is that the brain normally suppresses a lot of inessential sensory input. And the brain recognizes normal hearing input by a certain accustomed pattern of bilateral perception that, when disrupted, causes the brain to consider it a _new noise_, potentially dangerous and worthy of my attention. It certainly got my attention. Cars make all kinds of horrible noises going along the street that you just don't notice. But now I was noticing. Loud noises, actually. (Happily, I am back to not hearing them.)

And, in going back and listening to music, I realized that my left ear was simply not interpreting a lot of noises as musical. I think some string bass ovetones got knocked out, and so I was hearing the bass as a background roar, like a passing car. Not music. (Happily, my left ear is once again hearing low musical pitches as music.)

Apparently some people who have this condition have trouble understanding speech, probably for similar reasons. They can hear many of the sounds, but because the brain does get all the customary tonal information, it does not recognize the sounds as words. Fortunately I don't seem to have this problem.

Anyway, I thought it was interesting that the main symptom of my partial deafness, or at least the one that most distressed me, was that everything I heard outside the quiet of my home seemed louder. Unpleasantly so.

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