How EEG Picks Up Brain Activity at the Scalp
An electroencephalogram, or EEG, records tiny changes in voltage using electrodes placed on the scalp. Those changes begin with electrical currents from many brain cells acting in coordinated patterns. A single neuron’s activity is far too small to register at the skin; an EEG becomes useful when large groups of cells produce signals that overlap in time.
The Skull Blurs the Signal
Brain tissue, fluid, the skull, and skin all conduct electricity differently. As currents pass through these layers, the voltage spreads out and weakens. That makes the signal at the scalp a blurred mixture of activity from broad areas, rather than a crisp reading from one neuron or a precise point deep in the brain.
The electrodes do not peer through the skull like miniature microphones. They measure voltage differences between locations on the scalp. An amplifier boosts those differences, while filtering and careful electrode contact help distinguish brain rhythms from interference such as eye movements, muscle tension, or electrical equipment.
Sensitivity Comes With a Tradeoff
Because the signal is weak, EEG is especially good at tracking changes over time: researchers can see whether activity shifts within milliseconds. But locating exactly where a signal began is harder, since the skull has already spread it before it reaches the electrodes.
That tradeoff explains why EEG caps can reveal the timing of brain activity without providing a sharply detailed map of its source. The “whisper” reaches the scalp, but by then it has blended with other voices.
