Place theory vs frequency theory: how the ear codes pitch, and where volley theory fits

Place theory and frequency theory give two different answers to one question: how does the ear tell the brain the pitch of a sound? Place theory says that each frequency makes a different place on the basilar membrane in the inner ear vibrate most, and the brain reads pitch from which place, and which nerve fibres, are most active. Frequency theory says that the auditory nerve fires at the same rate as the sound wave, so a 100 Hz tone gives 100 nerve impulses per second. Volley theory joins the two. Groups of nerve fibres fire in turns, so the nerve as a whole can follow faster sounds than one fibre can. The usual exam answer is that timing codes low pitches, place codes high pitches, and both work in the middle range.

That exam answer is a useful rule, but it hides a debate that is more than 150 years old and not closed. Below are the three theories in the words of the people who proposed them. Each comes with the experiments behind it and the parts that short summaries leave out.

• Hermann von Helmholtz compared the ear to a piano with one nerve fibre for each string. His book on the subject first appeared in German in 1863.
• William Rutherford called his rival idea the telephone theory in 1886. His own test of it could not go above 352 impulses per second.
• In 1930, Ernest Glen Wever and Charles Bray heard speech through a cat's auditory nerve. They suggested that fibres fire "in volleys". Later work suggested that much of their signal came from the cochlea, not the nerve.
• Researchers still debate how much each code contributes to pitch. A 2013 review calls the question "somewhat open".
• Cochlear implants give a real test of both codes. The electrode position changes the pitch, and the pulse rate changes it too, but only up to about 300 Hz.

The difference in one table

Place theory: where the basilar membrane vibrates most

The basilar membrane is a strip of tissue that runs along the cochlea. It is about 34 mm long in humans. When sound reaches the cochlea, the membrane vibrates, and the vibration stimulates the hair cells that sit on it. The place of the largest movement depends on the frequency of the sound (APA Dictionary, basilar membrane).

Helmholtz explained the idea with a piano. If you raise the dampers of a piano and a loud musical tone reaches its sounding board, only the strings that match the simple tones in that sound vibrate. He then asked the reader to "suppose we were able to connect every string of a piano with a nervous fibre in such a manner that this fibre would be excited and experience a sensation every time the string vibrated". Tones of different pitch would then go to "different nervous fibres". He believed that the ear works in this way: the end of every fibre of the auditory nerve "is connected with small elastic parts", which are "set in sympathetic vibration by the waves of sound" (Helmholtz, 1885, p. 129). The quotation is from the 1885 English translation. The translator dates the first German edition to 1863.

Georg von Békésy measured what the membrane actually does. He received the 1961 Nobel Prize in Physiology or Medicine "for his discoveries of the physical mechanism of stimulation within the cochlea" (Nobel Prize, 1961). The presentation speech says that he found a wave that travels along the membrane from the stiff base to the flexible tip, the apex. The highest crest of the wave appears "near the apex of the cochlea at low-frequency tones and near its base at high frequencies" (Nobel Prize presentation speech, 1961). So the idea of place held, but the mechanism is a travelling wave, not Helmholtz's set of separately tuned parts.

The frequency map does not stop in the ear. Andrew Oxenham, a hearing researcher at the University of Minnesota, explains that the map goes from low frequencies at the apex to high frequencies at the base. The auditory system keeps this map from the cochlea through the brainstem "to the primary auditory cortex" (Oxenham, 2018). This map is called tonotopic organisation.

The APA Dictionary splits place theory into two claims. The first claim is that different frequencies stimulate different places on the membrane. This claim "is strongly supported by experimental evidence". The second claim is that pitch is coded by the place of greatest stimulation. This claim "remains controversial" (APA Dictionary, place theory). Exams test the theory as one idea, but the two claims have very different support.

Frequency theory: the nerve copies the sound

William Rutherford, a British physiologist, presented the rival theory in a lecture to the British Association in Birmingham on 6 September 1886. The idea came from the telephone. A telephone does not analyse sound. It changes the vibration of a metal plate into an electric current with the same frequency, and a second telephone changes the current back into sound.

Rutherford proposed that the ear works in the same way. He named his idea "the telephone theory of the sense of hearing". In this theory, the hairs of all the auditory cells "vibrate to every tone", and "there is no analysis of complex vibrations in the cochlea". Instead, the hair cells change sound into nerve vibrations "similar in frequency and amplitude to the sound vibrations". The brain then reads the frequency of these vibrations (Rutherford, 1886, p. 167).

Rutherford knew that his theory had a weak point. A nerve had to carry very fast impulses, and he had no proof that it could. He tested this with the motor nerve of a frog or rabbit. When he stimulated the nerve 200 times per second, the muscle made a sound with the same pitch as his tuning fork. He reached 352 impulses per second, which gave a note of 352 vibrations per second. When he tried faster stimulation, he heard "nothing but a noise". He noted that a honeybee can vibrate its wings 460 times per second, so its nerves must carry signals that fast. But for higher frequencies, he admitted, "the proof there was still wanting" (p. 168).

The modern definition keeps Rutherford's idea and his weak point. According to the APA Dictionary, frequency theory says that a 100 Hz tone gives 100 impulses per second in the auditory nerve. The theory "cannot explain the perception of sounds above 500 Hz". After each impulse, a neuron needs a short recovery time, called the refractory period. In the dictionary's account, this limits a neuron to about 500 impulses per second (APA Dictionary, frequency theory). Humans hear frequencies from about 20 Hz to 20,000 Hz, so a firing rate that copies the sound cannot cover the whole range (OpenStax, Psychology 2e, 2020).

Volley theory: many fibres share the work

The next step came from Princeton in 1930. Ernest Glen Wever and Charles Bray put an electrode on the auditory nerve of an anaesthetised cat. They amplified the electrical signal and sent it to a telephone receiver in a soundproof room. When someone spoke into the cat's ear, the listener in the other room heard the speech. They wrote that "speech was transmitted with great fidelity", and they used the system to talk between the two rooms. They heard tones from 125 Hz up to 4,100 Hz, and a listener could tell a tone of 3,200 Hz from one of 3,300 Hz (Wever & Bray, 1930, p. 345).

Wever and Bray concluded that their result made the telephone theory "more reasonable than ever before". But they saw the same problem as Rutherford. A rate above 4,000 per second was "considerably above that previously found in any nerve", and a single fibre needs time to recover after each impulse. Their solution was this: "it is possible for a high rate to be established by slowly acting fibers going off in volleys" (p. 349). Each fibre fires on some cycles of the sound and rests on others. Other fibres fire on the cycles that it skips. Together, the group fires on every cycle. The APA Dictionary dates the full volley theory to Wever in 1949 and gives an example: a nerve can follow 1,000 Hz, which is faster than any single fibre (APA Dictionary, volley theory).

The famous cat experiment has a twist. Wever and Bray believed that they had recorded impulses from the nerve. In 1934, Charles Hallpike and A. F. Rawdon-Smith in London tested where the signal came from. They found that none of their own tests was decisive, but that "the balance of the evidence is thought to favour a non-neural hypothesis" (Hallpike & Rawdon-Smith, 1934, p. 408). Today the APA Dictionary describes the cochlear microphonic, an electrical signal from the hair cells whose waveform is similar to the sound. Its other name is the Wever–Bray effect, and the dictionary says that Wever and Bray discovered it in 1930 (APA Dictionary, cochlear microphonic). So the clear speech that the listener heard was probably, in large part, this signal from the cochlea, not nerve impulses.

The idea of volleys survived. Its modern basis is phase locking. A nerve fibre does not fire on every cycle of a tone, but when it fires, it fires at about the same point of the wave. According to the APA Dictionary, auditory nerve fibres can phase lock to frequencies below 4 to 5 kHz (APA Dictionary, phase locking). These measurements come from animals. Nobody has recorded phase locking directly from the human auditory nerve (Oxenham, 2013).

How the theories fit together: the exam answer

The AP Psychology course lists all three: "Theories that help explain pitch perception include place theory, volley theory, and frequency theory" (College Board, 2024, p. 45). The OpenStax textbook gives the usual combined answer: "both theories explain different aspects of pitch perception. At frequencies up to about 4000 Hz, it is clear that both the rate of action potentials and place contribute to our perception of pitch. However, much higher frequency sounds can only be encoded using place cues" (OpenStax, 2020).

For an exam, this rule is enough:

• Low pitches: the firing rate, or timing, of the auditory nerve carries the pitch (frequency theory, with volleys).
• Middle pitches, up to a few thousand hertz: timing and place both carry the pitch (volley theory and place theory).
• High pitches: only place carries the pitch (place theory).

Textbooks give different numbers for the borders. The APA Dictionary puts the limit of simple frequency theory at 500 Hz, and OpenStax puts the end of the shared range at about 4,000 Hz. Learn the order of the ranges, not one exact number.

Where the exam answer is too simple

Research on pitch does not agree with every part of the tidy division. Oxenham's reviews point to evidence on both sides.

Some evidence favours timing. Phase locking in animal auditory nerves fades at higher frequencies and stops above about 4–5 kHz. This is close to the frequency where people lose a clear sense of musical pitch. The highest note on a grand piano is 4,186 Hz. Oxenham notes that place theory "provides no explanation" for this loss at high frequencies, because the cochlea's place tuning becomes sharper, not weaker, at high frequencies. People can also hear very small changes in frequency, and these changes are smaller than simple place theory predicts (Oxenham, 2018).

Other evidence favours place. In one study, listeners heard pitches between 1 and 2 kHz when all the parts of the sound were above 6 kHz. At those frequencies, timing information is probably not useful. Oxenham concludes that either the brain does not need timing for musical pitch, or the human nerve phase locks to higher frequencies than researchers think (Oxenham, 2013).

The brain also changes the code. Timing information becomes less exact at each stage after the ear. In the auditory cortex, phase locking is generally not seen above about 100 Hz. The place map, on the other hand, continues up to the primary auditory cortex. So if the ear uses timing, the brain must change it into a different code early in the auditory pathway (Oxenham, 2018).

Oxenham's conclusion for pure tones is that "it may be safest to assume that the auditory system uses both place and timing information". After more than a century of dispute, he writes, "the question remains somewhat open" (Oxenham, 2013).

The missing fundamental: a problem for simple place theory

Most real sounds are not pure tones. A violin that plays the A at 440 Hz makes a wave that repeats 440 times per second. The sound also has energy at 880 Hz, 1,320 Hz, 1,760 Hz and higher multiples, called harmonics. We hear one pitch, at 440 Hz. We still hear that pitch if the 440 Hz part is removed or hidden by noise (Oxenham, 2018). This effect is called the missing fundamental.

Simple place theory has difficulty here. If pitch is the place of greatest vibration, then a sound with no energy at 440 Hz should not give a pitch of 440 Hz. The APA Dictionary lists this as a clear flaw of Helmholtz's theory: it cannot account for the pitch of a missing fundamental. The dictionary adds that "the essential notion remains viable" (APA Dictionary, Helmholtz theory). Oxenham notes that researchers now explain the pitch of such sounds with place information, timing information or a combination of the two (Oxenham, 2018).

What cochlear implants show

A cochlear implant is a natural test of both codes. The device has a row of electrodes inside the cochlea. Each electrode stimulates the auditory nerve at one place. Oxenham notes that 12 to 24 electrodes replace the work of about 3,500 inner hair cells. Even so, many users understand speech in quiet places (Oxenham, 2018).

Implant users report two kinds of pitch:

• Place pitch: the pitch goes down when the active electrode is nearer to the apex of the cochlea. This matches place theory.
• Rate pitch: the pitch goes up when the electrical pulses come faster, but only up to about 300 Hz. This matches frequency theory, with a much lower limit than the 4–5 kHz of natural phase locking.

The two kinds of pitch do not combine into one scale. Studies find them on separate dimensions. Oxenham, citing earlier work, notes that place pitch in implant users may be closer to timbre, the quality of a sound, than to pitch. Pitch is still a major difficulty for implant users. Speech needs little frequency detail, but pitch needs much more. Better pitch would help users with music and with tone languages (Oxenham, 2018).

Keeping the terms apart

The hearing theories come with many names: place theory, resonance theory, Helmholtz theory, frequency theory, telephone theory, temporal theory, volley theory, basilar membrane, tonotopic organisation, phase locking and missing fundamental. In Psychology World you can save terms like these to one list. Then you can drill the list as flashcards, as multiple-choice questions once it has four or more terms, or by typing the answer. What each study mode does is set out on the home page.

Hearing is one part of the sensation topic. Our article on habituation vs sensory adaptation separates two other sensation terms. Our guide to memorising psychology terminology explains which study methods have the strongest evidence.