Have you ever been around blind folks “speed-reading” books on tape? I had a professor (of psychology) in college that was blind, and he used a special tape player with a variable playback speed for skimming recorded texts. It sounded like mosquitoes humming to me, but he said that he was so used to it that it sounded normal to him. Now, that’s what can be done consciously, the nervous system is even better at training itself without your conscious interference in the process. Consider the “cocktail party effect” and other audio tricks your nervous system learned without any conscious effort. There is a lot more ambient sound going into the ear and along the auditory nerves than ever gets to the conscious “hearing” centers in your brain. Most of the unnecessary input is evaluated and discarded before bothering “headquarters.” This is a subconscious, or pre-conscious routine, and is probably essential for living in an urban environment.
What you see and hear is not literally what comes in through the nerves. It is processed and interpreted. The literal image on the retinas is upside-down, it is the brain that turns the image 180 degrees for you to “see.” The flashes you “see” when your eyes are closed are not the result of light striking cones and rods. What you “hear” is not literally the waves striking your eardrum, but a condensed report of the important input made by the nerves. Prior to the activity in the brain that is your sensation of hearing, a lot of editing has been done by the nerves. But, in order to do the editing some evaluation of the message is made at a subconscious level to determine whether it is something that should be passed on to the conscious hearing.
This all sounds a little out there. The best way to get a visceral feel for the difference between the raw input and your perception is to try out the Blind Spot exercise. Prove to yourself that you actually have a blind spot on each eye, and your brain automatically fills in the missing part of the picture so that you have to do weird exercises on the Internet to detect them at all. What you see is not what you get. What you “see” is a phenomenon happening in the brain, not what is out in the world.
I wonder what could be missed or hidden in that spot, or how two different people see the same thing… or even the same person at different times.