Auditory Sensation

 

            Auditory sensation is the sensation produced by a sound or other auditory stimulus. Auditory sensations are even more important than visual sensations. They have the ability to respond to stimuli from a much longer distance than visual sensations. From the point of view of evolution, auditory sensations are more primary than visual sensations. In fact, in some of the lower organisms where the visual sensation is not fully developed, auditory sensations are fairly well developed. The stimuli for auditory sensations are sound waves.

The Auditory System:

The auditory system or the system of hearing consists of the receptors in the human ear, the auditory nerve and the temporal lobe of the cerebral cortex. The sound waves activates the eardrum in the human ear. These motions of the eardrum or tympanic membrane as it is called are further transmitted through a chain of three bones in the middle part of the ear. These bones are known as ossicles. They are popularly known as the hammer, the anvil and the stirrup because of their shapes. Their technical names are the incus, malleus and stapes. The three bones conduct the disturbances caused by the stimulations to another membrane in the inner part of the ear. This membrane is known as the basilar membrane. The actual receptors which are in the form of hair-like structures, also known as organs of corti varying in length are located along the basilar membrane. The basilar membrane actually forms the lining of a snail-like structure called the cochlea. The basilar membrane is set in vibration by the sound impulses transmitted by the ossicles. These are, in turn, transmitted by the auditory nerve to the temporal lobe of the cerebral cortex. Sound sensations are experienced when the temporal lobe is activated. It can be seen that the human ear consists of three broad divisions; the outer ear, the middle ear and the inner ear. The pinna or the outer ear serves to funnel the sound waves into the middle ear. The middle ear consists of the auditory tube or canal ending in the eardrum or the tympanic membrane followed by the ossicles. The inner ear consists of the cochlea, the basilar membrane and the three semicircular canals. The actual receptors, are in the inner part of the ear.                                                                                                                                                                                                              Theories of Hearing:

Just as in the case of visual sensations, a number of theories have also been developed to explain auditory sensations. We may consider some of them briefly.

Place theory:

According to this theory, different regions or places in the basilar membrane are sensitive to and respond to sound waves of different frequencies. Thus, the receptors in the basal end and near the three bones vibrate more when sound waves of high frequency enter. On the other hand, the receptors at the top end or the end farthest from the three bones respond more to sounds of low frequency. This means that receptors of sound waves of different frequencies are placed at different places of the basilar membrane. The theory also states that the experience of loudness of a sound depends on the total amount of the basilar membrane stimulated. Thus, an intense sound would stimulate the membrane to a greater extent than a less intense sound. The timbre of the sound heard is dependent on the overall patterns of vibrations.

Frequency theory:

Another theory attempting to explain the different characteristics of sound sensations is the frequency theory. No sensation can reach the brain directly. This theory says, the hair cells of basilar membrane respond to the stimulus like the diaphragm of the telephone-vibrating to the frequency of the wave (i.e. pitch). According to this theory, the pitch of a sound sensation depends on the frequency of the impulses reaching the auditory area of the brain rather than anything that happens at the basilar membrane. Loudness would depend on the number of the nerve fibers which participate in the process. Thus a weak sound would involve fewer fibers compared to a stronger sound. This theory, however, faces some difficulty in explaining the hearing of higher levels of pitch because of limitations in the number of impulses that can be transmitted by the nerves. However, it can be seen that this theory emphasizes brain action rather than any activity at the level of the receptors.

Auditory Adaptation:

Another phenomenon which one notices in relation to sound sensation is that of adaptation. If we continue to listen to a sound for some time, it appears to become less and less loud though the stimulus characteristic remains the same. This phenomenon of adaptation, it is a general characteristic of all sensory experiences, though there may be exceptions.

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