Neural Impulses are transmitted to the brain from neurons in the hair cells of the basilar membrane The main question about hearing is hon these neural impulses are coded to give different kinds of information-for in stance how do we know that a tone is a certain pitch (how do we tell middle C from a note an octave below)? One theory of pitch perception, called the place theory is based on the idea that different sound frequencies (different pitches) actually trigger different neurons It has been found that the frequent of vibrations determines which portion of the basilar membrane is moved or pushed about most (Bekesv, 1955). Thus patch information could correspond to the stimulation of particular neurons on a specific section of the membrane. Electrophysiological studies have in fact, shown that individual neurons in the cats auditory nerve are tuned (most sensitive) to specific frequencies.
One problem with this theory id that not all frequencies seem to cause movement in the basilar membrane more in one place than another. In fact only high and (to some extent) middle frequencies seem to do this pitch information about low frequencies must be transmitted in another way. An alternative theory is that neural activity is coded in terms of the rate (rather the the place) at which neurons are triggered. This is called the frequency theory of pitch perception (Weaver & Bray 1937). In fact, it can be shown that the rate or frequent of pulses traveling up the auditory pent to the brain matches that of a tone over a wide range of frequencies.
Thus is not too surprising for low frequencies since individual nerves can respond over and over at these low rates. Yet is is impossible for a smile nerve to fire, recover and fire again as fast as would be necessary to follow a high-frequency tone. However such high firing rates could be the product of several different sets of nerve fibers each firing in turn at a lower rate, but combining to produce the higher overall rate. (Just as when you listen to ten carpenters hammering, you hear many more hammer blows per minute than any one carpenter could make) This is often referred to as the volley principle, because it is named for the way in which rows of soldiers in the Revolutionary War could load fire and reload one after another to produce more frequent volleys than they could if all the rows fired at the same time.
While pure-tone stimuli are adequate to measure basic properties of human hearing, most of the sounds we hear are more complex. Of special interest are the shifting combinations of tones that make up human speech. In fact, certain regions of our sensory cortex may respond only to such stimuli (Lieberman & Studdart Kennedy, 1978). These same regions may also be involved in speech production. For example, electrical stimulation of certain points in a person’s brain can simultaneously alter perception of speech and cause speechlike movements in facial muscles (Ojemann & Mateer, 1979). We are really just beginning to explore these complex aspects of hearing. (Speech perception will be considered further in another article.)
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