One spike is invisible from afar
When a single neuron fires, it sends out a brief electrical pulse called an action potential, or spike. Picture one firefly blinking once in a vast, dark field. Up close, the flash is obvious. But step back a few hundred meters, put a hill and some fog between you and the firefly, and that single blink simply vanishes. That is the situation an electrode on your scalp faces.
Two things work against you here. First, distance: the electrical field from one spike falls off sharply with every millimeter, and your scalp sits centimeters away from the action. Second, the spike is fast and brief — it lasts only about a millisecond, so even at close range it is a faint, fleeting blip rather than a steady glow.
Summation: the crowd's roar
Now imagine not one firefly but tens of thousands of them, all blinking at the same instant, over and over in rhythm. Suddenly the field glows in pulses you can see from the hilltop. The same thing happens in your brain: when large populations of neurons become active together, in sync, their tiny individual fields add up into something far larger. This adding-up is called summation.
There is a crucial catch: summation only works when the neurons are in step. A stadium crowd chanting in unison makes a roar you can hear from outside; the same number of people each muttering a different word at a different moment cancels out into a meaningless murmur. Synchrony is what turns a crowd of whispers into a single shout — and that synchronized, rhythmic activity is what we call a brain rhythm.
Volume conduction and the smearing skull
Even a strong, synchronized rhythm still has to travel from deep in the brain out to the surface. On the way it spreads through brain tissue, fluid, the skull, and the scalp — a process called volume conduction. Imagine shining a flashlight through a frosted shower door: the light gets through, but it arrives dim and spread out, with the sharp edges of the beam smeared into a soft glow.
Two things happen by the time the signal reaches a scalp electrode. It is weak — on the order of microvolts, that is, millionths of a volt, roughly tens of microvolts for typical rhythms. And it is smeared: each scalp electrode picks up a blurry blend from a wide patch of underlying cortex, so neighboring electrodes see overlapping, similar-looking signals. You lose the fine spatial detail of exactly which small group of neurons was active.
What survives to the scalp
Put it all together and a clear rule emerges about what kind of brain activity actually makes it out to a scalp electrode. Slow, large, synchronized rhythms survive. Because they are produced by many neurons in step, summing into strong fields, and because they change gradually, they ride through the smearing skull and still show up as EEG — the electroencephalogram, the wiggly lines recorded from the scalp.
Crisp single-neuron spikes do not survive. They are too small, too fast, and too local; by the time they reach the scalp they have been attenuated and blurred into oblivion. This is exactly why invasiveness buys sharpness: the closer you can put an electrode to the source — on the surface, or even inside the cortex — the more of that fast, fine-grained detail you recover, at the cost of a more involved procedure.