Alpha and the back of the head
What the 10 Hz bump is, why it lives over visual cortex, why closing your eyes makes it and opening them breaks it, and why it is the first sanity check of every EEG setup on earth.
On this page10 min
You closed your eyes and within two seconds a hill rose in the spectrumSpectrum A plot of how much of a signal lives at each frequency. Related: Fast Fourier transform (FFT)Fast Fourier transform (FFT) The algorithm that breaks a stretch of signal into the sinusoids that would add up to it, giving amplitude and phase at each frequency. , Power spectral density (PSD)Power spectral density (PSD) The spectrum expressed as power per hertz, the standard way to quantify how strong alpha is. . Glossary entry near 10 Hz. You opened them and it fell within one. Hans Berger saw the same thing in 1924 with a galvanometer and a photographic plate, and named it alpha because it was the first thing he saw. A century later it is still the first thing anyone sees, and it is still the way every EEG technologistEEG technologist The person who applies electrodes and runs clinical EEGs, with a certification path of its own. Glossary entry checks that the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry are on a brain.
Where it comes from
The alpha rhythmAlpha rhythm A roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open. Related: Beta rhythmBeta rhythm Roughly 13 to 30 Hz activity, prominent over motor areas and associated with active concentration or holding still., Theta rhythmTheta rhythm Roughly 4 to 8 Hz activity, prominent in drowsiness, some memory tasks, and strongly in the rodent hippocampus., Thalamocortical loopThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms., Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part.. Glossary entry is generated by loops between the thalamus, the brain’s relay station in the middle, and the visual cortex at the back. Thalamic neurons have a built-in tendency to fire in bursts at around 10 per second when they are not being driven by input. Cortex follows. Millions of neurons in visual cortex oscillate together, and their summed currents make a field large enough to reach the scalp at the back of the head. That is why thalamocorticalThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms. Related: Alpha rhythmAlpha rhythm A roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open., Sleep spindleSleep spindle A half-second burst of 11 to 16 Hz activity in light sleep, generated in the thalamus.. Glossary entry is the adjective, why O1, O2, and Oz are the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry, and why the forehead shows almost nothing.
Why closing your eyes makes it
Alpha is what visual cortex does when it has nothing to do. Close your eyes, or stare at a blank wall, and the input drops; the thalamocortical loopThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms. Related: Alpha rhythmAlpha rhythm A roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open. , Sleep spindleSleep spindle A half-second burst of 11 to 16 Hz activity in light sleep, generated in the thalamus. . Glossary entry falls into its idle rhythm and the neurons synchronize. Open your eyes and the flood of input breaks the synchrony: each neuron is busy with its own piece of the scene, they stop firing together, and the summed field shrinks even though the cortex is more active. This is the general rule of EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry rhythms and it is backwards from intuition: more rhythm usually means less processing. The technical term for the rhythm dropping when a region gets busy is event-related desynchronizationEvent-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand. Related: Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part., Motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect.. Glossary entry, and it is the basis of the motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect. Related: Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. , Event-related desynchronization (ERD)Event-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand. , BCI inefficiencyBCI inefficiency The finding that around one in five people cannot control a motor-imagery BCI even after training; once called "BCI illiteracy," a label the field is retiring. . Glossary entry BCIBrain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Closed loopClosed loop A system in which the output feeds back to influence the input, such as a user adjusting their brain activity while watching the cursor it controls. . Glossary entry you will meet in Phase 3.
You close your eyes and start doing hard mental arithmetic. What happens to alpha?
It shrinks. Concentration engages cortex broadly and reduces alpha even with the eyes closed. This is why the instruction for the eyes-closed test is “relax,” and why someone anxious about the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry on their head often shows little alpha at first.
Facts you will use
Alpha’s frequency, called the individual alpha frequency, is between 8 and 12 Hz in almost all adults and is stable within a person for decades. It slows in drowsiness, illness, and old age; it is faster in children only after about age ten (young children have a slower dominant rhythm). It waxes and wanes in spindles a second or two long, which you saw in the trace; a perfectly steady sinusoid would be suspicious. Its amplitude varies enormously between people, from over a hundred microvolts to almost nothing, and a small alpha means nothing about the person.
Alpha has cousins. The mu rhythmSensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. Related: Motor imageryMotor imagery Imagining a movement without making it, which changes rhythms over motor cortex in a way a classifier can sometimes detect., Event-related desynchronization (ERD)Event-related desynchronization (ERD) The drop in rhythmic power over a cortical area when it becomes active, such as mu weakening when you imagine moving your hand.. Glossary entry over motor cortex is the same frequency and the same idle-loop mechanism, and it drops when you move or imagine moving the corresponding body part. Tau, over auditory cortex, is harder to see on the scalp. Each sensory area has its own idle rhythm and each one breaks when that sense gets busy.
Why it is the sanity check
If you see alpha appear with eyes closed and vanish with eyes open, you know four things at once: the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry are making contact, the amplifier is working, the polarity and gainGain How many times an amplifier multiplies its input; 1000× turns 50 microvolts into 50 millivolts, enough for an ADC to see. Related: Instrumentation amplifierInstrumentation amplifier A precision differential amplifier with very high input impedance and high CMRR, built from three op-amps and sold as a single chip such as the INA128 or AD8232. . Glossary entry are plausible, and the channel is over the back of the head. No other single observation tells you all that in ten seconds. Every EEG technologistEEG technologist The person who applies electrodes and runs clinical EEGs, with a certification path of its own. Glossary entry runs it at the start of every recording. Do the same.
Deep dive Berger, and why it took ten years to be believed 3 min
Hans Berger, a psychiatrist in Jena, recorded the first human EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry in 1924 using a string galvanometer and electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry on his son’s scalp, and published in 1929. The scientific community mostly ignored him; the signals were tiny and the claim that the brain produced a measurable rhythm seemed implausible. In 1934 Adrian and Matthews in Cambridge reproduced the alpha rhythmAlpha rhythm A roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open. Related: Beta rhythmBeta rhythm Roughly 13 to 30 Hz activity, prominent over motor areas and associated with active concentration or holding still. , Theta rhythmTheta rhythm Roughly 4 to 8 Hz activity, prominent in drowsiness, some memory tasks, and strongly in the rodent hippocampus. , Thalamocortical loopThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms. , Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. . Glossary entry, demonstrated its blocking by opening the eyes in a public lecture, and the field existed from then on. Berger’s own motivation was to find a physical basis for telepathy. He did not, but he found the alpha rhythmAlpha rhythm A roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open. Related: Beta rhythmBeta rhythm Roughly 13 to 30 Hz activity, prominent over motor areas and associated with active concentration or holding still. , Theta rhythmTheta rhythm Roughly 4 to 8 Hz activity, prominent in drowsiness, some memory tasks, and strongly in the rodent hippocampus. , Thalamocortical loopThalamocortical loop The two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms. , Sensorimotor rhythm (mu)Sensorimotor rhythm (mu) An 8 to 13 Hz rhythm over motor cortex that weakens when you move, or imagine moving, the corresponding body part. . Glossary entry on the way.
Deep dive What alpha is for, which nobody fully knows 3 min
The idling account above is the classic one and it is incomplete. Alpha over a region often increases when that region needs to be suppressed to protect attention elsewhere: alpha rises over visual cortex when you attend to a sound. So alpha may be an active inhibition signal, a gate, rather than a rest state. The phase of alpha also predicts whether a faint flash will be seen, which suggests it chops perception into ten-per-second frames. These ideas are live research. If you enjoy them, the computational neuroscience spine is calling.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We check our setup by closing our eyes and watching for a hill near 10 Hz that rises within about 2 seconds and falls within about 1 second of opening them. We place electrodes at O1, O2, or Oz over the back of the head, because the rhythm comes from loops between the thalamus and visual cortex idling between 8 and 12 Hz in most adults. We trust the recording when that pattern appears, since it confirms contact, working amplification, sensible gain, and occipital placement at once.
Yours does not have to match. Check that the three ideas are there.