To finish our stadium analogy: if Single-Unit spikes are one person talking, LFP is the crowd from inside the section, and ECoG is the blimp hovering right over the open field, then a Scalp Surface Field Potential is like listening to the stadium from the parking lot, with the gates closed.
You can't hear individual conversations. You can't even tell exactly which section of the stadium is cheering. But when a massive play happens and 50,000 people scream at the exact same time, you hear a loud, undeniable roar.
In medicine and research, this is known as an Electroencephalogram (EEG).
Here is a breakdown of what EEG is, how it works, and why it is one of the most common tools in neuroscience.
Unlike all the other recording methods we've discussed, EEG is completely non-invasive. There is no surgery, no needles, and no poking the brain.
Instead, electrodes are placed directly on the scalp. Usually, they are embedded in a tight-fitting fabric cap so they sit at precise, standardized locations (known as the 10-20 system).
A standard EEG electrode cap. Source: Ida Palosaari / Getty Images
To make sure the electrical signal can pass from the skin to the metal electrode, researchers squirt a conductive gel under each sensor.
If ECoG (the blimp) measures the synchronized activity of hundreds of thousands of neurons, EEG measures the synchronized activity of millions to tens of millions of neurons.
But there is a major physical hurdle between the brain and the electrode: Volume Conduction.
The signal must travel through fluid, thick bone, and skin.. Source: ResearchGate
Before a neuron's electrical signal can reach the sensor on your scalp, it has to travel through:
Because bone is a terrible conductor of electricity, the skull acts as a massive low-pass filter. It heavily smears, blurs, and weakens the electrical signals.