Theories of Hearing

 APPRECIATION OF PITCH OF THE SOUND – THEORIES OF HEARING

Many theories are postulated to explain the mechanism by which the pitch of the sound is appreciated or the frequency is analyzed. These theories are generally classified into two groups. According to the first group, the analysis of sound frequency is the function of cerebral cortex and the cochlea merely transmits the sound. According to the second group of theories, the frequency analysis is done by cochlea, which later sends the information to cerebral cortex.

Theories of Hearing


THEORIES OF FIRST GROUP

1. Telephone Theory of Rutherford

Telephone theory was postulated by Rutherford in 1880. It is also called frequency theory. According to this theory, the cochlea plays a simple role of a telephone transmitter.

In telephone, sound vibrations are converted into electrical impulses, which are transmitted by cables

to the receiving end. There the receiver instrument converts the electrical impulses back into sound waves. Similarly, cochlea just converts the sound waves into electrical impulses of same frequency. Impulses are transmitted by auditory nerve fibers to cerebral cortex, where perception and analysis of sound occur. It is believed that, the nerve fibers can transmit maximum of 1,000 impulses per second. Thus, the telephone theory fails to explain the transmission of sound waves with frequency above 1,000 cycles per second. So, a second theory was postulated.

2. Volley Theory

In 1949, Wever postulated this theory. Volley means groups. According to this theory, the impulses of sound waves with frequency above 1,000 cycles per second are transmitted by different groups of nerve fibers. However, this theory has no evidence to prove it. Thus, these two theories were not accepted by many physiologists.

THEORIES OF SECOND GROUP

1. Resonance Theory of Helmholtz

Resonance theory was the first theory of hearing to emerge in 1863. According to Helmholtz, analysis of

sound frequency is the function of cochlea. Basilar membrane contains many basilar fibers. Helmholtz

named these basilar fibers resonators and compared them with the resonators of piano.

When a string in piano is struck, sound with a particular note is produced. Similarly, when the sound

with a particular frequency is applied, the basilar fibers in a particular portion of basilar membrane are

stimulated. Resonance theory was not accepted because the individual resonators could not be identified in cochlea. Gradually, this theory was modified into another theory called the place theory, which is more widely accepted.

2. Place Theory

According to this theory, nerve fibers from different portions (places) of organ of Corti on basilar membrane give response to sounds of different frequency. Accordingly, corresponding nerve fiber from organ of Corti gives information to the brain regarding the portion of organ of Corti that is stimulated. Many experimental evidences are available to support place theory. Experimental evidences supporting place theory

i. If a person is exposed to a loud noise of a particular frequency for a long period, he becomes

deaf for that frequency. It is found that the specific portion of organ of Corti is destroyed as

in the case of boilermaker’s disease.

ii. In experimental animals, destruction of a portion of organ of Corti occurs by exposing the animal

to loud noise of a particular frequency

iii. In human high-tone deafness, there is degeneration of organ of Corti near the base of

cochlea or degeneration of nerve supplying the cochlea near the base

iv. During exposure to high-frequency sound, cochlear microphonic potentials show greater

voltage in hair cells near base of the cochlea. Also, during the exposure to low-frequency

sound, cochlear microphonic potentials show greater voltage in hair cells near apex of the

cochlea.

v. There is point-to-point representation of basilar membrane in auditory cortex.

3. Traveling Wave Theory

From place theory, emerged yet another theory called the traveling wave theory. This theory explains

how the traveling wave is generated in the basilar membrane. Development, generation, movement and

disappearance of traveling wave are already described.

APPRECIATION OF LOUDNESS OF SOUND

Appreciation of loudness of sound depends upon the activities of auditory nerve fibers. Intensity or loudness of sound correlates with two factors:

1. Rate of discharge from the individual fibers of auditory nerve

2. Total number of nerve fibers discharging. When loudness of sound increases, it produces

large vibrations, which spread over longer area of basilar membrane. This activates large number of hair

cells and recruits more number of auditory nerve fibers. So, the frequency of action potential is also increased.

LOCALIZATION OF SOUND

Sound localization is the ability to detect the source from where sound is produced or the direction through which sound wave is traveling. It is important for survival and it helps to protect us from moving objects such as vehicles. Cerebral cortex and medial geniculate body are responsible for localization of sound

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