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Nerves, sound and ecphory

I am deeply fascinated by the notion that nerves operate using sound not electricity. There is a theory in psychology of memory retrieval called ecphory based on an analogy with sound. Wouldn't it be something if sound was not an analogy or a metaphor, but the actual vehicle of communication?

Nerves function using sound, not electricity.

Danish scientists challenge the accepted scientific views of how nerves function and of how anesthetics work. Their research suggests that action of nerves is based on sound pulses and that anesthetics inhibit their transmission.

Every medical and biological textbook says that nerves function by sending electrical impulses along their length. "But for us as physicists, this cannot be the explanation. The physical laws of thermodynamics tell us that electrical impulses must produce heat as they travel along the nerve, but experiments find that no such heat is produced...

Physics explains biology

Nerves are 'wrapped' in a membrane composed of lipids and proteins. According to the traditional explanation of molecular biology, a pulse is sent from one end of the nerve to the other with the help of electrically charged salts that pass through ion channels in the membrane. It has taken many years to understand this complicated process, and a number of the scientists involved in the task have been awarded the Nobel Prize for their efforts. But – according to the physicists – the fact that the nerve pulse does not produce heat contradicts the molecular biological theory of an electrical impulse produced by chemical processes. Instead, nerve pulses can be explained much more simply as a mechanical pulse according to the two physicists. And such a pulse could be sound. Normally, sound propagates as a wave that spreads out and becomes weaker and weaker. If, however, the medium in which the sound propagates has the right properties, it is possible to create localized sound pulses, known as "solitons", which propagate without spreading and without changing their shape or losing their strength.

The membrane of the nerve is composed of lipids, a material that is similar to olive oil. This material can change its state from liquid to solid with temperature. The freezing point of water can be lowered by the addition of salt. Likewise, molecules that dissolve in membranes can lower the freezing point of membranes. The scientists found that the nerve membrane has a freezing point, which is precisely suited to the propagation of these concentrated sound pulses. Their theoretical calculations lead them to the same conclusion: Nerve pulses are sound pulses.

Anesthetized by sound

How can one anesthetize a nerve so that feel ceases and it is possible to operate on a patient without pain? It has been known for more than 100 years that substances like ether, laughing gas, chloroform, procaine and the noble gas xenon can serve as anesthetics. The molecules of these substances have very different sizes and chemical properties, but experience shows that their doses are strictly determined by their solubility in olive oil. Current expertise is so advanced that it is possible to calculate precisely how much of a given material is required for the patient. In spite of this, no one knows precisely how anesthetics work. How are the nerves "turned off"? Starting from their theory that nerve signals are sound pulses, Thomas Heimburg and Andrew D. Jackson turned their attention to anesthesia. The chemical properties of anesthetics are all so different, but their effects are all the same - curious!

But the curious turned out to be simple. If a nerve is to be able to transport sound pulses and send signals along the nerve, its membrane must have the property that its melting point is sufficiently close to body temperature and responds appropriately to changes in pressure. The effect of anesthetics is simply to change the melting point – and when the melting point has been changed, sound pulses cannot propagate. The nerve is put on stand-by, and neither nerve pulses nor sensations are transmitted. The patient is anesthetized and feels nothing.

The use of a sound metaphor in memory research is known as selective resonance:

Selective Resonance

Imagine that a 440 Hz tone emitted near the undamped strings of a piano. This act will lead to sympathetic resonance in the strings tuned to 440 Hz and to a lesser extent, in the strings tuned to frequencies that have harmonic relationships to 440Hz (e.g. 880 Hz, 220 Hz).

Memory retrieval can be thought of analogously. Memory traces are 'tuned' to specific frequencies, based on the information encoded into them. At retrieval, they 'resonate' to the extent that they share information with retrieval cues. Retrieval succeeds when the resonance is unique to relevant traces, and not shared with irrelevant traces.

The selective resonance idea can be traced to the little-known memory theorist Richard Semon, who coined the term ecphory in 1921 to describe the process whereby memory traces (or engrams) resonate in response to retrieval cues. This theory contrasts with the notion that memory retrieval success is solely based on strength of association from attention or repetition.

More recently, Roger Ratcliff has demonstrated that the resonance concept forms a realistic basis for formal mathematical models of retrieval. In addition, the principle of selective resonance is a natural property of connectionism.

What is fascinating about the development of sound pulses as the method of data transmission in nerves (in top article) is its potential impact on theories of data transmission in neurons. Perhaps all those sound words in scare quotes will turn out to be literally true.

Schacter, D.L. (1996) Searching for Memory. Basic Books. p.57-59
Semon, R. (1909/1923) Mnemic pyschology. London: Geoge Allen & Unwin.


The 'not electricity' bit is misleading. The paper that the article seems to be based on talks about piezoelectric effects generated by the soliton. So there is an electrical impluse that moves across the neuron, but the electrical impulse is a product of the physical wave.


Solitons as an explanation of nerve impulse

Sound cannot be the impulse along nerves, because impulses are produced even if you were to put a person in a soundproof room, and there was no sound to propagate along nerves.
Wow! This sounds like a similiar challenge to established scientific theory in the field of olfactory science. I just last week read The Emperor of Scent by Chandler Burr, which tells the [very recent, like, 1990s] story of Luca Turin's challenge to the dominant olfactory theory that the shape of smells is what receptors in the nose respond to: his theory is that receptors actually respond to vibration, not shape.

One of the most facinating pieces of evidence was that Dr Turin was asked to give advice regarding a woman who doctors had no idea how to treat: she had a condition in which nearly everything she smelled was absolutely unbearably vile. Turin theorised that, if he was correct regarding vibration, one possible cause for her symptom was minute epileptic fits causing her olfactory receptors to vibrate and percieve smells incorrectly. He recommended anti-epilepsy drugs, and sure enough, the woman's sense of smell [slowly] returned to normal!

I see a huge parallel here. I highly recommend the book, it's a well written and fascinating glimpse into the world of perfumery, olfactory science and physics. As sound is vibration, maybe far more of how we function is due to vibrational processes than science still, currently, assumes!
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August 2016



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