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While our brains provide us with a tremendous amount of information about the sounds we hear and what they mean to us, at the most basic level our auditory system answers two major questions about any sound. First, what is the sound? The auditory system must identify what tones or frequencies we are hearing. And second, where is the sound? We must be able to locate the origin of the sound in space. Once we know what sounds we are hearing and where the sounds are coming from, our brains can begin the complex task of assigning meaning to the sounds we hear.
Basilar Membrane
The process of determining what a sound is begins at a flat sheet of tissue (in the cochlea of the inner ear) called the basilar membrane. The basilar membrane detects the component frequencies, or tones, of incoming sound. The special physical properties of the basilar membrane make it particularly good at frequency detection. The membrane is flexible, and vibrates when sound hits it - but it doesn't vibrate evenly all over. One end of the basilar membrane vibrates most at low frequency tones, and the other end of the membrane vibrates most at high frequency tones. This gives the basilar membrane tonotopic organization or organization by tone, similar to a xylophone: tones are arranged from low frequency on one end to high frequency on the other (Figure 1). On a xylophone, if you know which bar is vibrating and where the bar is in the instrument, you can tell what note you will hear. Similarly, if you know that a group of neurons in the basilar membrane is active, and you know where those neurons reside in the membrane, then you can tell what tone you have heard.
When you play an "A" note on a xylophone, the air pulsates 440 times per second, a frequency of 440 Hertz (Hz). Those pulses trigger 440 vibrations per second along the length of the basilar membrane, with the largest vibrations occurring somewhere just past the middle of the membrane - the region of the membrane whose resonant frequency is 440 Hz. Within the resonant region, a group of neurons will begin a chorus of activity, each signaling in turn so that the group collectively signals 440 times a second. This marvelous synchronization of vibrations in the air, in the basilar membrane, and in the activity of neurons in the resonant region is called phase-locking. Phase-locking is an important response mechanism in the auditory system; as we will see, perfect synchronization is critical to detect where a sound came from.
Locating sounds in space, the other fundamental task of the auditory system, is no mean feat; but our brains can determine the origin of a sound with astonishing accuracy, even when we cannot actually see the source. That ability depends on three independent methods for locating sound: a timing method, an intensity method, and a frequency filtering method. By using the results from all three methods, we are able to very accurately pinpoint the origin of sounds in space.
