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Well, I'm noding this under auditory tuning because I really can't think of a better title. There's no generic biological term for what I want to lump into this node, which is mostly the information left over about the auditory system now that I've covered the ear, the cochlea, and auditory localization. Most of what is known about the auditory system comes from the study of auditory localization in organisms such as barn owls and bats. Essentially what this node covers is how the auditory system controls the information it receives from the ear, in order to pick out the salient features of sound.

The first thing I want to talk about is feedback and input modulation. A little information on sound before we proceed. The loudness of a sound depends on the maximum air pressure created by the sound wave. Auditory systems can detect sound over a huge range of sound pressures. That's why volume is measured in decibels (dB). If we didn't use dB to measure sound, we'd have to use numbers with 13 digits, which would get tedious pretty quick. The dB measurement works on a logarithmic scale. That means that an increase in sound volume of 10dB (one Bel) amounts to a 10x increase in sound pressure. Humans can hear sounds that range from 0dB to about 130dB, which is a hell of a range, when you think about it. Above 130dB auditory hair cells (the devices which turn sound energy into neural input) start to become damaged. We can also hear frequencies which range from 20Hz to 20,000Hz, although we are most sensitive in the 1kHz to 4kHz ranges.

Another interesting property of the auditory system is its ability to work around background masking. Simply put, this is why you could hear that guy at the kegger last week bitching to you about his midterms, despite all the background noise. The brain can pluck specific sounds out of a noisy environment, even though the background noise tends to mask the relevant sounds.

So, how does the auditory system manage to detect sounds over such a wide range of volumes? Well, for the quiet sounds, it's simple: Lots of amplification. There's amplification all through the ear, even in the cochlea. But what about the loud sounds? Individual neurons from the cochlea go from resting activity to maximum activity over a range of about 30dB. So how does the auditory system detect sound at ranges of 130dB? I'm glad you asked.

One method involves the tuning of auditory nerve fibers. There are three types of fibers: Low spontaneous rate fibers, medium spontaneous rate fibers, and high spontaneous rate fibers. Individual fibers have a characteristic frequency, a single frequency which the respond strongly to. High SR (spontaneous rate) fibers have one sharp sensitivity around their characteristic frequency and are sensitive to other frequencies to a lesser degree. They are also more sensitive to lower-volume sounds. Low and medium SR fibers are very sharply tuned, and require sounds of higher volume to activate them. So, as sound on a given frequency increases from 0dB the sequence of events probably progress something like this: First, High SR auditory fibers of that CF (characteristic frequency) begin to respond. As volume increases, the less sensitive medium and low SR fibers begin to respond. When the volume increases still further, the high SR fibers with nearby CFs begin to activate.

One of the ways in which sound input is actively altered by the auditory system is via small muscles which are attached to the bones of the inner ear. These muscles can be used to damp the actions of the inner ear bones. This has two effects: First, it lowers the overall amount of sound transmitted through the middle ear; secondly, it decreases transmission of low frequencies more than higher frequencies. This may serve as a mechanism for tuning out low frequency background noise.

There are still other ways that the auditory system can deal with masking and extreme volumes. Remember the Outer hair cells of the cochlea? (see cochlea if you don't) The ones that act as amplifiers? Well, they aren't static amplifiers. Their properties can be changed by feedback from the brain. The outer hair cells amplify sound by pushing along with the sound that stimulates them. Feedback fibers can act to decrease the amount of push they supply. this acts to decrease the mobility of the basilar membrane. This means that louder sounds will be required to move the membrane and stimulate the hair cells (inner and outer). It lowers the sensitivity of the cochlea, which means that the auditory nerve fibers will no longer have saturated responses at their previous maximum limit. All actions have been shifted up in the volume range. Interesting note: The ability for this response decreases with age, which might be why old fogeys like dannye and your parents don't like their music loud.

Back to how your brain works.

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