You are walking in the city, and your companion cries,"Look at that!". You say,"Where?"
Your companion was looking at a tight-wire walker, crossing between two buildings, at the tenth floor level, while you were watching a dog on the sidewalk. In the meantime, neither of you was paying attention to the street vender or the girl with the pink parasol.
What this indicates to me is that there may be a whole panorama on the retina, but only a limited vision. When I think of it, I can see individual hairs in someone's moustache or the wrinkle of his sleeve. Vision is not limited to the eye or the optic nerves, nor even the visual area of the brain. It involves thought and memory and emotions and who knows what else.
The same thing applies to the sense of hearing. You are walking in the city and your companion cries,"Did you hear that?" Did he mean the siren on the car that was about to be stolen, the ambulance going east, or the fire truck going north? None of the above. He meant the people behind us saying that the Brookyn Bridge will reopen after repairs have been completed.
I was having dinner at a hotel dining room, which had a twenty-feet-high ceiling and a semi-circular wall at one end. It was a huge place with four hundred diners. At any moment, at least a hundred people were speaking. Each one had to raise his or her voice to be heard over the din. Thus the volume of sound increased even more. To make matters worse, the acoustics of the place was the worst possible.
I estimated that every possible frequency of sound wave was reaching my ear. There were sopranos, tenors, altos, baritones, and the cries of infants. In what seemed like a background, I thought I heard a chorus singing "Down By The Old Millstream". To make sure, I walked through the room and found no chorus.
I believe that every note was registering in my ear simultaneously. In the inner ear, there are hair cells, one for each note. I think all of them were vibrating at that time. If I wanted to hear the person across the table from me, I had to direct my attention to his sounds. Similarly I could hear the chorus sing any song I chose, both words and music.
Evidently hearing is not accomplished only by the auditory apparatus. It involves the auditory section of the brain, plus the memory, emotion, and intellectual centers of the brain. Let us not omit the rhythmic, finger tapping, and dancing centers.
For that matter, we have a memory store of melodies and pictures. In general, senses have their data banks.
When there is progressive hearing loss, a hearing aid raises the volume of sounds received. This is a wonderful boon for bird watchers who can hear the songs of the birds again. For the rest of the hearing-impaired crowd, the most noticeable effect is to raise street noise to a painful level. It turns out that, in addition to ample volume, we need an organ to interpret the sounds we hear. Somewhere in the brain we distinguish the vowels and consonants and diphthongs to make speech intelligible. What the user of a hearing aid seeks most is comprehension of spoken language. Alas, the hearing aid is deficient in this respect. The user has a hard time trying to understand what is being said.
The good news is that the new and improved model does remarkably well. It is digital. To understand the digital mechanism, one has to do a lot of reading of the proper books on the subject. Without getting technical, we can say that each component of the sound we receive in a tiny fraction of a second is represented by a short sequence of ones and zeros. In other words, the new hearng aid brings every detail of a sound to one's ear. Whereas the old model brings in less detailed representation of sound. It is broad where the new model is sharp.