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.
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You closed your eyes and within two seconds a hill rose in the spectrum 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 technologist checks that the electrodes are on a brain.
Where it comes from
The alpha rhythmAlpha rhythmA roughly 8 to 12 Hz oscillation over the back of the head that appears when the eyes close and fades when they open. 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 loopThe two-way circuit between the thalamus and the cortex whose rhythmic activity produces alpha, sleep spindles, and other EEG rhythms. Glossary entry is the adjective, why O1, O2, and Oz are the electrodes, 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 loop 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 EEG 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. Glossary entry, and it is the basis of the motor imagery BCI 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 electrodes 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. 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 electrodes are making contact, the amplifier is working, the polarity and gain 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 technologist 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 EEG in 1924 using a string galvanometer and electrodes 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 rhythm, 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 rhythm 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.
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