Frequency theory explains how the brain experiences sound waves, with pitch encoded by the firing rate in the primary auditory fiber. The theory explores how sound is experienced by the mind and accounts for hearing through the anatomical structure of the ear.
Frequency theory attempts to explain how the brain experiences sound waves. While frequency theory is primarily a physiological theory that seeks to explain how the anatomical structure of the ear accounts for hearing, it is also a psychological theory that explores how sound is experienced by the mind.
Before the theory of frequencies can be fully understood, a brief description of the physiological structure of the ear is required. Sound is picked up by the outer ear, which includes the pinna and the external auditory canal. At this stage, the sound is a beep. Separating the outer and middle ear is the tympanic membrane or eardrum. When the acoustic signal enters the middle ear, the acoustic signal becomes mechanical due to the oscillatory motion of the ossicular chain, which transmits the signal and increases the signal gain by approximately 22 decibels (dB) to the inner ear, where the sound will be enter a fluid-filled cavity, called the cochlea.
The cochlea is located in the inner ear and is made up of three fluid-filled chambers: the scala tympani, the scala vestibulo, and the scala media. The scala media contains the organ of corti, known as the organ of hearing. The organ of corti houses hair cells that become excited when the signal enters the cochlea, which is now a hydraulic signal, and moves fluid. When the fluid is moved, it excites the hair cells, which in turn cause them to convert the hydraulic signal into a mechanical signal. This causes the auditory nerve to fire, sending an electrical signal to the brain’s auditory system, which the brain perceives as sound.
Frequency theory states that pitch is encoded by the firing rate in the primary auditory fiber. The basilar membrane moves up and down due to the displacement of the perilymph and endolymphatic fluid in the cochlea, caused by each individual sine wave. The movement of the membrane causes the excitation of the hair cells in the cochlea. Each nerve is related to a specific frequency. Once that specific wave enters the cochlea, its frequency and intensity are sensitive to a specific nerve and it causes that nerve to be activated. The nerve cannot send another message until the message has been sent and the nerve recovers. Each nerve fiber in the auditory nerve sends information to the auditory cortex where it assembles the information and puts it together to perceive and interpret the auditory signal.
Frequency theory, in more simplistic terms, explains how the human brain basically experiences a representational system of hearing. Frequency theory essentially states that humans do not actually experience sound waves, but rather vibrations on the auditory nerve, the frequencies of which are identical to the frequencies of sound waves arriving in the ear.
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