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. The theory was proposed by Ernest Wever and Charles Bray in 1930 as a supplement to the frequency theory of hearing. It was later discovered that this only occurs in response to sounds ranging from about 500 Hz to 5000 Hz.
Description 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 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.
Harmonic spectrums Sounds are often sums of multiple frequency tones. When these frequencies are whole number multiples of a fundamental frequency they create a harmonic. 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. This is the basis of volley theory.
Phase-locking Phase-locking is known as matching amplitude times to a certain phase of another waveform. In the case of auditory neurons, this means firing an action potential at a certain phase of a stimulus sound being delivered. It has been seen that when being played a pure tone, auditory nerve fibers will fire at the same frequency as the tone. Volley theory suggests that groups of auditory neurons use phase-locking to represent subharmonic frequencies of one harmonic sound. This has been shown in guinea pig and cat models. In 1980, Don Johnson experimentally revealed phase-locking in the auditory nerve fibers of the adult cat. 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. 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. This phenomenon was interpreted as the result of a second harmonic, phase-locking to the stimulus waveform. However, at frequencies between about 1000 Hz and 5000 Hz, phase-locking becomes progressively inaccurate and intervals tend to become more random.
Pitch perception Pitch is an assigned, perceptual property where a listener orders sound frequencies from low to high. Pitch is hypothesized to be determined by receiving phase-locked input from neuronal axons and combining that information into harmonics. In simple sounds consisting of one frequency, the pitch is equivalent to the frequency. There are two models of pitch perception; a spectral and a temporal. Low frequency sounds evoke the strongest pitches, suggesting that pitch is based on the temporal components of the sound. Historically, there have been many models of pitch perception. (Terhardt, 1974; Goldstein, 1973; Wightman, 1973). Many consisted of a peripheral spectral-analysis stage and a central periodicity-analysis stage. In his model, Terhardt claims that the spectral-analysis output of complex sounds, specifically low frequency ones, is a learned entity which eventually allows easy identification of the virtual pitch. The volley principle is predominantly seen during the pitch perception of lower frequencies where sounds are often resolved. Goldstein proposed that through phase-locking and temporal frequencies encoded in neuron firing rates, the brain has the itemization of frequencies that can then be used to estimate pitch.
Discovery and history Throughout the nineteenth century, many theories and concepts of hearing were created. Ernest Wever proposed the volley theory in 1937 with his paper "The Perception of Low Tones and the Resonance-Volley Theory". In this paper, Wever discusses previous theories of hearing and introduces volley theory using support from his own experiments and research. The theory was introduced as a supplement to the frequency theory or temporal theory of hearing, which was in contrast to the place theory of hearing.
Place theory The most prominent figure in the creation of the place theory of hearing is Hermann von Helmholtz, who published his finished theory in 1885. Helmholtz claimed that the cochlea contained individual fibers for analyzing each pitch and delivering that information to the brain. Many followers revised and added to Helmholtz's theory and the consensus soon became that high frequency sounds were encoded near the base of the cochlea and that middle frequency sounds were encoded near the apex. Georg von Békésy developed a novel method of dissecting the inner ear and using stroboscopic illumination to observe the basilar membrane move, adding evidence to support the theory.
Frequency theory Ideas related to the frequency theory of hearing came about in the late 1800s as a result of the research of many individuals. In 1865, Heinrich Adolf Rinne challenged the place theory; he claimed that it’s not very efficient for complex sounds to be broken into simple sounds then be reconstructed in the brain. Later, Friedrich Voltolini added on by proposing that every auditory hair cell is stimulated by any sound. Correspondingly, William Rutherford provided evidence that this hypothesis was true, allowing greater accuracy of the cochlea. In 1886, Rutherford also proposed that the brain interpreted the vibrations of the hair cells and that the cochlea did no frequency or pitch analysis of the sound. Soon after, Max Friedrich Meyer, among other ideas, theorized that nerves would be excited at the same frequency of the stimulus.
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