Notes

Brain voltage

Everything you experience rides on small voltage changes across neuron membranes. A few numbers that make that concrete.

Parked at minus seventy

A resting neuron sits at about −70 millivolts: the inside is negative relative to the outside. Ion pumps keep it there, pushing sodium out and potassium in, and that upkeep is a large part of why the brain uses about a fifth of the body's energy while making up about two percent of its mass. Rest is not free. It is a charged state held in place, ready to discharge.

The spike

When enough input pushes the membrane up to around −55 mV, sodium channels open and the voltage shoots up past zero within a millisecond, then falls back. That spike, the action potential, is all-or-nothing: neurons signal mostly through when and how often they fire, not how big the spike is.

Reading it through the skull

An EEG picks up voltage at the scalp in the tens of microvolts, roughly a thousand times smaller than the voltage across a single membrane. Most of that signal comes not from spikes but from slower currents in large numbers of aligned neurons in the cortex, changing together. A lone neuron is invisible from outside. Only synchrony shows up.

That is why the EEG is dominated by rhythms. Hans Berger recorded the first human EEG in 1924 and found the alpha rhythm, around 8 to 12 cycles a second. It is strongest when you close your eyes and relax, and it drops when you open them or start paying attention. One of the first things anyone saw in the electrical brain was a signal that tracks what the mind is doing.

Closing the loop

Neurofeedback turns that observation around. Measure a rhythm, show it back to the person as a sound or an image in real time, and they can learn to shift it, without being able to say how. It is a strange kind of introspection: you learn to steer a voltage you can't feel, through a signal you can only hear.