HEARING

Theories of hearing must explain both the wide range of pitch—from 20 to 20,000 cycles per second—and the full range of intensity or loudness audible to the human being, from about 15 to 160 decibels. The two outstanding explanations, the place theory and the frequency theory, both accept the fact that the basilar membrane, within the snail-like cochlea of the inner ear, responds to changes transmitted from the outer ear to the oval window—and both recognize that the organ of Corti, situated on this membrane, contains sensitive cells which are activated by deflections of the basilar membrane, and which connect with the auditory nerve that transmits the impulses to the brain. However, the place theory holds that the basilar membrane actually sorts out the different tones, high and low, while the frequency theory holds that it merely transmits the different impulses to the auditory nerve and the sorting out process is done in the brain itself.Place or Piano Theory. This theory was originated by Helmholtz in the middle of the nineteenth century. He noted that the basilar membrane is wide at one end and narrow at the other, and suggested that fibers in the narrow end respond to high tones and those in the wide end to low tones, just as the strings of a piano or harp vibrate in resonance to differently pitched tones. The nerve fibers connected to different parts of the membrane then transmit stimuli of different frequency to the brain. In this theory, loudness is explained by the amount of the basilar membrane activated by a given sound—that is, a loud sound would activate a wider range of fibers than a soft sound.The place theory is supported by three types of evidence. First, it has been demonstrated that different portions of the basilar membrane are damaged by prolonged exposure to loud tones of different frequency. Second, electrical and mechanical measurements made within the normal functioning ear show that different portions of the membrane are actually sensitive to tones of different frequency. And third, “mapping” of the temporal lobe of the brain indicates that different regions represent different parts of the cochlea and respond more to one frequency than to another. Recently, however, Bekesy (1960), has shown that since the ear is filled with a viscous fluid, the basilar membrane is not free to vibrate like the strings of a piano. While still adhering to the place theory, he has developed the “traveling wave theory,” which states that when a sound enters the ear, a wave travels along the basilar membrane and displaces it a maximum amount at a point which corresponds to its frequency. The theory is supported by experiments performed on both animals and human beings. Frequency or Telephone Theory. According to this interpretation, the basilar membrane is essentially a transmitting instrument, like a telephone or microphone, which vibrates as a whole at the frequency of the incoming sound. These vibrations are then translated into nervous impulses according to their initial frequency, and different portions of the auditory nerve have been found to transmit impulses of different frequencies. This has been proved by amplifying the electrical impulses picked up directly from the auditory nerve of a cat. These impulses will produce a sound which is similar to the sound that is striking the ear of the cat, just as if its ear were behaving as a telephone instrument.Evidence of this type, however, seems to indicate that the auditory nerve does not respond to frequencies above 4000 to 5000 cycles per second. Moreover, the basilar membrane itself appears to be limited in its capacity to respond to the higher frequencies within the range of human hearing, and it is an accepted fact that a single neuron can transmit no more than 1000 impulses per second. These objections have led Wever (1949) to introduce the “volley” theory, which holds that fibers respond to the higher frequencies by firing in groups or squads. One group may fire at the first condensation of the sound wave, remain in a refractory phase while another group discharges, and then fire again at the third condensation. For a tone of 3000 cps there would be a spurt of activity in the auditory nerve every three thousandth of a second, with different groups of fibers responding each time. Pitch would therefore depend on the frequency of the volleys and not on the frequency of discharge of individual nerve fibers. Loudness, on the other hand, is believed to depend on the number of impulses that occur in each spurt—that is, intense stimulation activates more fibers than less intense stimulation, and also produces more frequent responses in each fiber. A loud sound would therefore produce more impulses per volley than a soft sound. At present the tendency is to favor a combination of the two theories. As Hilgard and Atkinson point out (1967), “Even with the help of the volley theory, however, it is difficult to apply the frequency notion to account for frequencies above 5000 cps. Consequently, most experts agree that a two- process theory of pitch is necessary: the frequency principle is used to account for frequencies up to about 5000 cps and a place theory takes over at higher frequencies.”

Cite this page: N., Pam M.S., "HEARING," in PsychologyDictionary.org, November 28, 2018, https://psychologydictionary.org/hearing/ (accessed June 15, 2019).
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