Posted by: R. Douglas Fields | December 8, 2014

How is the brain like a guitar? Hint: It is all about rhythm

guitarbearclawtop Typically we are introduced to the nervous system by analogy to an electrical circuit, like a door bell or a telephone line carrying a signal rapidly over long distance to activate a specific process. Never mind that electrical impulses are not transmitted through nerve axons anything like electrons flowing through a copper wire, this electronic circuit analogy is useful up to a point. If you want to understand how the brain works at a more complex level, you are going to need a new analogy, and if you play an acoustic guitar you’ll find it under your fingertips.

Dr. Hans Berger, working at the Psychiatric Clinic at the University of Jena, Germany in the 1920’s was the first person to discover that the human brain radiated waves of electrical energy that could be picked up by electrodes on the scalp. He performed his experiments in secret on hospital patients and on his own son in a small building separated from the rest of the hospital. Initially he believed that he had detected the physical basis for mental telepathy. He told no one in the scientific community about his mysterious findings until after five years of secret experiments.

Fundamentally, Berger, whose daily life was devoted to caring for people with mental illness, was searching for a physical basis for brain function. This was a leap of insight decades beyond his contemporaries. The idea that the human mind and mental illnesses have a physical basis of operation that could be reduced to physical principals and understood by approaching the operation of the mind in the same way a physicist would approach any other phenomenon in nature–by physical measurement–was well outside the realm of thinking among his colleagues in psychiatry.

His brutal approach of stabbing a thermometer deep into the brain of his patients who had survived gunshot wounds leaving holes in the skull, and then provoking various emotional and sensory stimuli to see whether their brain tissue changed its temperature in the process of mental function raises ethical questions by today’s standards. His patients must have had no real understanding of what was being done to them or why, and an attempt to monitor the intricate workings of the human brain in the same way one might use a candy thermometer to monitor the process of making fudge, seems naive in retrospect. However, Berger did not have the advantage of our vantage point. In applying the crude tools available in his day, thermometers and the newly developed electronic amplifier, Berger was doing precisely the same thing that cutting edge neuroscientists today are doing with functional magnetic resonance imaging (fMRI), which allows us to see inside the brain at work and pinpoint where, when, and how its different parts operate. Berger was a man whose scientific ideas were a century ahead of the technology needed to study them.

Brainwaves had been detected in animals, but not in humans before Berger’s experiments. What he observed by attaching electrodes to the scalp and feeding the signals into an electronic amplifier was that the human brain’s electrical activity was not entirely confined to transmission through the wire-like axons connecting neurons into circuits. Instead, the electrical energy of neurons radiated out of the skull like the electromagnetic field radiating from a broadcast antenna or for that matter from any electrical circuit. This is something we all have experienced in hearing the annoying hum of 60 cycle electromagnetic interference that radiates from our electronic devices when it is picked up and amplified unintentionally by sensitive electronic instruments.

Psychiatric hospital in Jena, Germany where Hans Berger first recorded human brain waves

Psychiatric hospital in Jena, Germany where Hans Berger first recorded human brain waves

Moreover, Berger discovered that the electromagnetic energy emanating from the human scalp progressed in waves of certain characteristic frequencies that changed with mental state. The 8-12 Hz brainwaves that he measured, originally called Berger waves and now called alpha waves, swelled in his son’s brain as he sat quietly in that laboratory with his eyes closed, but when his son opened his eyes, his brainwaves abruptly changed. The alpha waves subsided like ocean waves squelched by rain on a windless sea.

Today we know that the electrodes on the scalp are intercepting the combined activity of millions of neurons in the surface layers of the brain, the cerebral cortex, each one sending signals to another neuron in complex circuits. Like the noise of a crowd in a baseball stadium, these electroencephalogram (EEG) recordings are the combined output of all the individual conversations and exclamations going on in the crowd of neurons beneath our skull. These conversations and exclamations wax and wane and sometimes burst with synchronized cheers in response to something that has stimulated them all. But why the oscillations at such characteristic frequencies?

Human brain waves first recorded by Hans Berger

Human brain waves first recorded by Hans Berger

Scientists soon discovered that there are several different characteristic frequencies of brainwave oscillations; each one accompanies different types of mental activities, including attention, consciousness, arousal, meditation, and many other cognitive processes. The question is whether these waves of electrical energy have any function or are simply an epiphenomenon, like the roar of an engine which changes with different states of activity, but the engine’s sound has no impact on the operation of the engine. To glimpse the answer to this question, which is at the forefront of current research in neuroscience, consider the guitar.

The difference between the rich and responsive sound of a classical guitar and the twang of a banjo has nothing to do with the action of the strings that generate the sound when they are plucked. The difference is in the resonance of the guitar’s body, which not only amplifies the sound of the strings vibrating, it blends all the sounds the strings can create into a harmonious and vibrant acoustic radiation. Some frequencies of sound combine and are thus amplified, and others are cancelled out because the two frequencies of oscillation are out of phase, dampening out the sound of the string at that frequency. In the same way that noise-cancelling headphones block out sound by generating the same waveform out of phase to cancel it, that particular frequency generated when the string is plucked makes no sound. If it did, the sound would create an annoying noise because it is not a frequency compatible with the musical scale.

Luthiers, who build acoustic stringed instruments like guitars and violins, are masters at crafting all the wooden components that go into constructing the instrument such that the instrument as a whole resonates harmoniously at just the right combination of frequencies, while dampening the sounds that interfere with the optimal operation of the instrument. The luthier achieves this result by carefully evaluating the weight, dimensions, and stiffness of all the wooden components that go into making the guitar, shaving them down to precise thickness and dimensions such that he or she can consistently produce a beautifully sounding and responsive instrument. This cannot be done by following a blueprint, because every piece of wood is slightly different in weight, stiffness, grain tightness and orientation. Different types of woods vibrate at different frequencies and reflect or absorb sound waves slightly differently. There are far too many variables to consider, which is why making fine guitars is an art, not a science.

Shaving the top braces of an acoustic guitar

Shaving the top braces of an acoustic guitar

The luthier fusses carefully over the top surface of the guitar, because this plate of wood acts like a speaker cone to radiate the sound, while the back and sides of the guitar operate like the speaker cabinet to further modify and direct the sound energy. The luthier will brace the top with slips of wood and shave them with a chisel to force it to vibrate in very precise ways that will combine all of the bass and treble sounds desired into a harmonious tone. He will shave the top plate of wood down carefully with a hand plane and test the acoustic effect periodically by lifting the plate to his ear by pinching it between thumb and finger, and tapping it briskly with his knuckle. A good sound board will ring with a complex and brilliant sound when it is tapped, so this process of carefully thinning down the wood is called “tap tuning.”

As the luthier carefully shaves the top thinner the tap tone lowers in frequency because the stiffness of the wooden plate is being reduced gradually, making it better at vibrating at low frequencies and worse at vibrating at high frequencies. The decision of when to stop thinning the top is critical and somewhat mysterious, because it is not a precise frequency that the luthier’s ear is listening for; it is a certain rich complexity of tones radiating with clarity and brilliance. If the top plate is thinned too much, it suddenly becomes useless. Knowing when to stop thinning is what separates the master luthier from a furniture maker.

Guitar resonating at 220Hz

Guitar resonating at 220Hz

Guitar resonating at 600 Hz.

Guitar resonating at 600 Hz.

It is possible to see the acoustic action of a guitar top by scattering glitter over its surface and playing specific frequencies of sound through a loudspeaker. Sweeping slowly from low frequency to higher frequency the glitter covering the top suddenly begins to vibrate and dance over the surface. Suddenly at a certain frequency of sound, all the particles of glitter dancing on the surface suddenly form a tight circle. This indicates that the entire top of the guitar is waving up and down, throwing the glitter off the vibrating surface and into the stationary borders, or node. This is the fundamental frequency of the guitar’s top. When the strings generate this frequency the guitar will broadcast a strong, loud tone at this frequency–the lowest frequency the guitar can generate effectively.

Now as the luthier slowly sweeps the sound from the loudspeaker to a higher frequency, the glitter suddenly begins to dance again, and like band members at halftime the glitter comes together, re-arranges, and forms two circles on the top of the guitar as shown in the accompanying photo. The guitar top is now vibrating at higher frequency such that the left and right sides of the guitar top are each oscillating simultaneously. This only occurs at this precise higher frequency. This same process will occur at other specific resonant frequencies making the guitar vibrate in different ways and revealed by different geometric patterns of glitter formed on its vibrating surface.

Now imagine that the top of this guitar in the figure accompanying this article were the top of your brain: the cerebral cortex. If the population of neurons in the sheet of brain cells in our cerebral cortex were firing at this specific frequency what would happen? Both parts of the cerebral cortex, separated by long distances, would suddenly begin to function in synchrony, just like the left and right sides of the guitar top shown above vibrating in synchrony at 600 Hz. This is what brainwaves do. Neurons fire electrical impulses when the voltage of their cell membrane reaches a specific threshold. If the membrane voltage is fluctuating up and down slightly below the threshold for generating an electrical impulse, a signal that arrives when that neuron is close to threshold will make it fire an electrical impulse, but the same signal that arrives when the oscillating voltage is at its trough would fail to reach threshold and the neuron would remain silent. Thus, brainwaves can couple activity of large population of neurons into functional groups, just as ocean waves move all boats at anchor in a harbor in synchrony even though they are not directly tethered to each other. In this way, the transmission and operation of neurons in the brain become coupled into functional assemblies.

Such oscillation is what couples activity of neural circuits into groups, providing a way to couple neural circuits together over long distances so that they operate simultaneously, even though they are not directly tethered together. This resonance is what combines mental activities together and tunes our level of attention and other mental states. Consider, for example, how all the diverse aspects of an experience, sights, sounds, emotions, time and place, somehow get coupled together to form a memory. Without the simultaneous activity of all the neurons in different regions of the cerebral cortex all firing together, the scene (or schema) evoked by a memory could not develop. This is much like the unique sound of say, a C# minor chord, making the entire guitar operate as a system to evoke a rich and specific tone that evokes a very specific emotional and cognitive response in our brain when we hear it.

More to explore
1. For an excellent book on brain waves see: Buzaki, Rhythms of the Brain, Oxford University Press, 2011.

2. I was able to visit Hans Berger’s laboratory and go through his notebooks in researching the chapter on his research for my book The Other Brain, where interested readers can find more on Hans Berger and brainwaves. http://theotherbrainbook.com.

3. Those interested in guitar building may find this article written for the Washington Post Sunday Magazine of interest. The title of the article is “The Last Guitar,” but it was changed by the publisher to “Guitar Hero.” http://www.washingtonpost.com/wp-dyn/content/article/2007/09/04/AR2007090401785.html

The guitar images accompanying this article are from the author’s workshop.

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