Source: Mohamed Hassan, PixHere, public domain.

What intelligence is and how to measure it are thorny questions.  If the brain’s information processing power could be measured directly with medical instruments, then the problematic assumptions, cultural biases, and pitfalls of IQ tests could be avoided. Moreover, direct measurement of the brain’s information processing power could reveal how specific brain circuits boost intelligence or weaknesses in them impair it.  Monitoring the brain’s electrical activity through electrodes placed on the scalp, termed electroencephalography or EEG, is widely used for diagnosing epilepsy, but a person’s intelligence can also be read in the squiggly traces.

What brainwaves are

EEG recording measures brainwaves, and intelligence can be determined from them in several ways.  To understand how, we must first appreciate what brainwaves are and how they are analyzed. All the astonishing things the brain does is the result of neurons transmitting electrical impulses in neural circuits.  The resulting voltage fluctuations surrounding each neuron combines with others, building up like the roar of applause in a theater from everyone clapping.  Just as you can hear the applause in the lobby outside the theater, and glean a great deal about what is happening on stage, electrodes on the scalp can pick up the combined signals from millions of neurons in the cerebral cortex chattering together.  The signals from individual neurons cannot be discerned, but the combined activity of all the neurons processing information enables researchers to watch how the brain operates as a system, as we think, or dream, or just let our mind wander.  By analyzing electrical traffic flowing through a person’s brain, scientists can determine what the brain is doing moment-to-moment, how it works, and how one person’s brain works in comparison with others.  That’s the general basis for measuring IQ through brainwave analysis.

Surprisingly, the brain’s electrical activity is nothing like the ruckus in a theater or buzzing electrical static in electronic devices.  The brain’s electrical activity oscillates in electromagnetic waves of many different frequencies.  No one predicted this.   Scientists were startled to see these ongoing waves of activity in the brains of animals and humans when methods to detect electricity in the body were first developed around the turn of the 20^th century.  To their surprise, they saw that sensory stimulation or bodily movement was not required to generate brainwaves; waves of electricity were sweeping through the brain spontaneously and unceasingly.  Asleep or awake, and even in a deep coma, brainwaves are always surging.  Brainwaves of different frequencies stream through the brain with frequencies as slow as a heartbeat together with others oscillating faster than the 60Hz frequency of AC electrical current in the US.  Scientists classify brainwaves into 5 frequency bands designated arbitrarily with Greek letters.  For example, brainwaves oscillating at about 10 cycles per second are termed “alpha waves.”

Brainwaves add another and more sophisticated dimension to how the brain operates, because waves of electrical activity streaming through grey matter couple the operation of groups of neurons together, much like ocean waves and tides rock boats in unison.  Even if they are not tethered together by synaptic connections, activity in populations of neurons throughout the brain is coupled together by brainwaves vibrating in different frequency bands.  These waves reflect the brain’s fundamental operation, and so they change with a person’s thoughts, emotions, sensory and motor functions, and intelligence.

Brainwaves are analyzed in several ways.  The different frequency components of brainwaves can be sorted out and compared, much like stockbrokers monitor the volume of rapid day traders versus long-term investors.  Secondly, the synchrony of brainwave oscillations between different parts of the brain can be monitored to determine how activity in different parts of the brain are functionally coupled like the coordination between two sections of an orchestra.  Finally, the speed and amplitude of brainwaves that are provoked by a sensory stimulus can be measured.  These so called ‘evoked potentials’ are like ripples generated by a stone cast into a bay, riding on top of the incessant wind and tidal oscillations.  For example, flashing a picture on a screen will generate a wave of electrical activity in the visual cortex right after the flash.  Brainwaves are also evoked by bodily movement, and according to a study published in May, 2022, brainwaves triggered by simply tapping a finger reveals a person’s intelligence.

“Twilight Zone” brainwaves (readiness potentials) indicate intelligence 

In research by Giuseppe Chiarenza, published in the International Journal of  Psychophysiology,  a curious brainwave called the readiness potential was used to measure IQ.  This strange brainwave, which is still poorly understood by neuroscientists, seems otherworldly, because it appears just before a person makes a limb movement.  Applying our “stone cast into water” analogy for an evoked potential, the readiness potential would be like a ripple erupting on the water just before you cocked your arm to cast the stone.  The “Twilight Zone” readiness potential shakes faith in free will, because this brainwave happens before we are consciously aware of intending to make an action.  It is believed to represent the brain’s unconscious information processing and decision making that drives our behavior– and that, in a nutshell, is IQ.

In this study 110 boys, ranging in age from 6-14, were asked to push a button to stop a beam of light sweeping across a screen within a specified target time shortly after the start of the beam sweep.  EEG recordings detected the readiness potential appearing a fraction of a second before the boys pressed the pushbutton to stop the beam sweeps.   (The boys would think that they had consciously commanded precisely when to press the trigger to stop the beam, but the EEG analysis shows that in fact at a conscious level they are only observers.  Their brain had already sized up the task, decided when and what to do, and launched the complex neural processes to press the button before cluing in the conscious brain.)  When the size of the evoked readiness potential was plotted against scores the boys received on three different tests of IQ (verbal, performance, and full IQ), the amplitude of the brainwave correlated significantly with all three IQ test scores—the bigger the brainwave, the brighter the boy.  How quickly the readiness potential arose before they pushed the trigger also correlated with verbal IQ test scores, but not with the other types of IQ test results.

It is difficult to say why this brainwave response reveals intelligence, given the uncertainty of exactly what neural processing produces this electrical activity just before a movement is made.  But clearly the mysterious data crunching ongoing in the brain unconsciously in planning the movement and making the decision to act is evident in the electrical activity these circuits produce prior to executing this task. More robust neural processing, indicative of more intelligence, produces a larger brainwave.

Other ways brainwaves measure IQ and illuminate what intelligence is

In other studies the connection is clearer between IQ, neural network processing, and brainwaves.  People with higher intelligence solve complex problems faster and more effortlessly compared to those with lower IQ who struggle harder to find the answer.  In theory, this would mean that higher IQ would be associated with faster neural processing and increased neural efficiency, especially in the frontal lobes where higher-level cognitive functions are carried out.  Many different types of EEG measurements support this theory of intelligence.

How quickly an evoked brainwave potential appears after a stimulus is one indication of information processing speed.  For example, about one third of a second after an unexpected event, a brainwave with positive polarity will arise, called the P300 or “odd ball” signal.  This happens, for example, when we hear a musician hit a bad note.  That voltage peak appears before the person becomes consciously aware of the musician’s slip up.  Consider how much high-speed complicated information processing and pattern recognition must be going on to generate this odd ball brainwave response faster than a thought. Higher intelligence should mean faster information processing, and several studies show that the quicker this wave peaks, the higher a person’s IQ.

The speed of information processing can also be measured by the amplitude or power of different frequency bands of brainwaves.  Often the ratio of power between two frequency bands is insightful, for example the relative power of alpha waves (10 cycles per second) to theta waves (about 5 cycles per second).  These EEG measures of IQ will depend on where on the scalp the measurements are made, because different sensory, motor, and cognitive functions are carried out in different parts of the cerebral cortex.  The power of different brainwave frequency bands also depends on a person’s state of arousal and what they are doing, for example, sitting with eyes closed or performing a task or asleep.  In general, the theory that higher IQ is associated with faster neural network function is supported by many different types of EEG studies of brainwave power in association with different types of intelligence.

One intriguing insight from such studies is that intelligence can be achieved in different ways.  For example, during REM sleep the power of delta waves oscillating at a frequency of about 3.5 cycles per second in frontal brain regions, correlates negatively with IQ in women, but no correlation is found in men.  There is no difference in IQ between men and women, but this finding shows that, just like horsepower in an automobile engine, high performance can be achieved through different mechanisms.

Delta brainwave power during REM sleep measures IQ in women but not in men. Source: Ujma,P.P. et al.,(2017) Scientific Reports 7:18070; Creative Commons license

In another approach, the synchrony of brainwave activity among populations of neurons can be measured; that is, how well the peaks and troughs of two brainwaves coincide in time.  This can be determined for populations of neurons communicating in neural networks inside one brain region, for example within the prefrontal cortex, or the synchrony of oscillations between two brain regions, say between the frontal and parietal lobes.

Such tests can reveal intelligence without any mental challenges.  They can be performed on people sitting quietly with their eyes closed, doing nothing but letting their mind wander.  This approach reveals the innate functional wiring of an individual’s brain.  EEG measurements can also be made while performing specific cognitive tests, such as doing mental arithmetic, to reveal intelligence.  It is difficult to briefly summarize the many ways that IQ can be determined by EEG, because of the different experimental designs and types of EEG measurements, but in general, aspects of EEG readings that reflect faster and more efficient information processing correlate with higher IQ.

For example, a 2005 study by Robert Thatcher and colleagues at the University of South Florida College of Medicine measured how synchronized the peaks and troughs of brainwaves were in people as they sat quietly with their eyes closed.  The results of just two 5-minute EEG recordings on each person showed that the higher the synchrony was between brainwaves within the prefrontal cortex (called phase delay), the higher the IQ. This fits with the concept that general intelligence reflects faster processing and better neural network coordination.  Interestingly, longer phase lags between brainwaves in the frontal lobes and other cortical regions, the higher the person’s IQ.  The authors conclude that the longer lag in brainwaves between the two brain regions reflect more neural network complexity in the brains of people with higher IQ.  To understand this result, imagine that the whole brain was engaged in a cognitive task, brainwaves everywhere would tend to be in synchrony.  But in that case, you might as well have only one neuron in the brain!  In higher IQ individuals, smaller brain regions can accomplish the task more efficiently.  In people with higher intelligence, the prefrontal lobes quickly perform the cognitive task and orchestrate other cortical regions that follow the lead with a phase lag.

This week I will give an invited lecture at the annual meeting of the high IQ society, MENSA, where membership requires scoring in the top 2% on IQ tests.  Preparing for that talk is what inspired me to write this article to share with you.  But beyond the fascinating science this subject involves, the capability to measure a person’s intelligence directly by EEG has profound scientific, medical, and social implications.  We are on the brink of grappling with these transformational questions as advances in brainwave research promise to make reporting IQ scores from medical instruments as routine as cholesterol levels.