Auditory nerve fibers12/24/2023 ![]() ![]() Historically, there have been many models of pitch perception. Low frequency sounds evoke the strongest pitches, suggesting that pitch is based on the temporal components of the sound. There are two models of pitch perception a spectral and a temporal. In simple sounds consisting of one frequency, the pitch is equivalent to the frequency. Pitch is hypothesized to be determined by receiving phase-locked input from neuronal axons and combining that information into harmonics. Pitch is an assigned, perceptual property where a listener orders sound frequencies from low to high. However, at frequencies between about 1000 Hz and 5000 Hz, phase-locking becomes progressively inaccurate and intervals tend to become more random. This phenomenon was interpreted as the result of a second harmonic, phase-locking to the stimulus waveform. Johnson observed that during frequencies below 1000 Hz, two peaks are recorded for every cycle of the stimulus, which had varying phases according to stimulation frequency. In the presence of -40 to -100 decibel single tones lasting 15 or 30 seconds, recordings from the auditory nerve fibers showed firing fluctuations in synchrony with the stimulus. In 1980, Don Johnson experimentally revealed phase-locking in the auditory nerve fibers of the adult cat. This has been shown in guinea pig and cat models. ![]() Volley theory suggests that groups of auditory neurons use phase-locking to represent subharmonic frequencies of one harmonic sound. It has been seen that when being played a pure tone, auditory nerve fibers will fire at the same frequency as the tone. In the case of auditory neurons, this means firing an action potential at a certain phase of a stimulus sound being delivered. Phase-locking is known as matching amplitude times to a certain phase of another waveform. When groups of auditory neurons are presented with harmonics, each neuron fires at one frequency and when combined, the entire harmonic is encoded into the primary auditory cortex of the brain. When these frequencies are whole number multiples of a fundamental frequency they create a harmonic. Sounds are often sums of multiple frequency tones. Harmonic waveform of a fundamental frequency L/2 Harmonic spectrums The reason for this is that neurons can only fire at a maximum of about 500 Hz but other theories of hearing did not explain for hearing sounds below about 5000 Hz. The volley theory was explained in depth in Ernest Wever's 1949 book, Theory of Hearing Groups of neurons in the cochlea individually fire at subharmonic frequencies of a sound being heard and collectively phase-lock to match the total frequencies of the sound. The total response corresponds with the stimulus. ![]() Volley Theory of Hearing demonstrated by four neurons firing at a phase-locked frequency to the sound stimulus. ![]() It was later discovered that this only occurs in response to sounds that are about 500 Hz to 5000 Hz. The theory was proposed by Ernest Wever and Charles Bray in 1930 as a supplement to the frequency theory of hearing. Volley theory states that groups of neurons of the auditory system respond to a sound by firing action potentials slightly out of phase with one another so that when combined, a greater frequency of sound can be encoded and sent to the brain to be analyzed. ![]()
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