The Universal Sense by Seth Horowitz Page B

What do they do? They roll their heads slightly to one side or the other (humans tend to roll their heads to the left, I’ve found). I’ve never actually seen this written up in any experiments, but in the course of torturing friends, family, students, and pets in the name of auditory science, I’ve found that it’s pretty consistent—so much so that if you look at comics or cartoons of a mammal trying to figure out something it’s just heard, you’ll often see its head tilted to one side. By tilting the head this way, the listener shifts the position of the outer ear and changes both the timing and spectral properties of the sound, which allows the listener to hear it slightly differently when it gets repeated. It’s sort of an auditory equivalent of 3-D movie glasses—rather than seeing slightly visually shifted visual scenes, by tilting your head you hear the sound from a slightly different auditory position, which both gives you more information about where it’s coming from and lets your brain confirm what you heard.
    Given that mammals have this specialization that would let them gather and tweak high-frequency sound, how do we act on it? The sensitivity of the middle and inner ear of other vertebrateslargely tops out at about 4–5 kHz, but we have an evolutionary adaptation of our inner ear, the cochlea, a snail-shell-shaped structure full of sensory hair cells connected to the outer world via the auditory bones of the middle ear and the eardrum. That description is the same general plan of every other vertebrate inner ear as well, but the cochlea is significantly more complex. In the cochlea, the tips of the hair cells are embedded in a tectorial or “ceiling-like” membrane, and the base of the cells is in a long, thin trapezoid-shaped membrane called the basilar membrane. The shape is important—the basal end near the oval window, closest to the outside world, is narrower and stiff, and hence vibrates the most in response to high-frequency sounds. The far or apical end is wider and looser, and more responsive to low-frequency sounds. This variable flexibility causes the hair cells in different regions to vibrate maximally in response to a particular frequency range.
    Because of this arrangement, hair cells don’t have to fire in precise synchrony with the timing of the sound’s phase. Instead, the hair cells’ tuning is defined by their placement on the basilar membrane. Sound enters the fluid-filled chamber of the cochlea and creates a traveling wave with maximum deflection at the place on the basilar membrane corresponding to the particular frequency. This lets the sensory hair cells respond by place coding —it relieves the auditory nerve from the burden of trying to fire tens of thousands of times per second.
    The traveling wave in the cochlea was discovered not by studying animals but rather by studying human cadavers, and won Georg von Békésy the Nobel Prize in Physiology or Medicine in 1961. The problem is, his theory turned out to be at least partially wrong, as it couldn’t explain how complex sounds would actually break up into their component frequencies asthey traveled through the cochlea. This illustrates a basic problem in anatomical science: dead preserved tissue doesn’t work the same way as living tissue, especially in the case of something as dynamic as hearing. Dead guys not only tell no tales, but they also don’t hear so well.
    The mistake in von Békésy’s theory was revealed by looking at another mammalian specialization of the inner ear that is not identifiable in dead tissue. Mammals have an additional set of cochlear hair cells called outer hair cells. For every inner sensory hair cell, there are three outer hair cells. And while sensory hair cells bend and deflect in response to vibration of a specific frequency, outer hair cells do something different. Like the inner sensory hair cells, the tips of the outer hair cells are embedded in the tectorial membrane, while