![]() ![]() 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. 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. First, it has been demonstrated that different portions of the basilar membrane are damaged by prolonged exposure to loud tones of different frequency. 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. The nerve fibers connected to different parts of the membrane then transmit stimuli of different frequency to the brain. 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. This theory was originated by Helmholtz in the middle of the nineteenth century. ![]() ![]() 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. 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. 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. ![]()
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