Pick your window into the brain
Imagine you want to know what's happening inside a stadium full of cheering fans. You could stand outside and hear the muffled roar (cheap, easy, but blurry), press your ear to the wall (clearer), or sit in one seat and hear the people right next to you (sharp, but you only catch a tiny patch). Recording the brain works the same way: every method trades off five things at once.
Those five are: spatial detail (how finely you can pinpoint *where* a signal comes from), time detail (how fast a change you can catch), invasiveness (does it stay on your scalp or go inside your skull?), portability (a wearable cap, or a room you can't move?), and cost. No method wins on all five — so the right choice depends entirely on what the application needs.
The electrical family
Most brain recording listens to voltage — the faint electrical chatter of neurons. What separates the members of this family is simply *how close they get* to the cells. The closer you are, the sharper the signal, but the more invasive the setup.
From outside to inside: EEG sits on the scalp — totally non-invasive and cheap, but the skull blurs everything, like hearing the crowd through stadium walls. ECoG places a grid *on the cortical surface*, under the skull — sharper, because nothing thick is in the way. Push electrodes *into* the tissue with a microelectrode array and you get intracortical recording, sharp enough to catch single neurons firing.
In between the slow scalp signal and the crisp single-neuron spikes sits the local field potential (LFP) — the pooled hum of many neurons near an electrode tip. Think of it as the murmur of a whole table of diners rather than one person's words. It carries rich rhythmic information without needing to resolve every individual cell.
Reading magnetism: MEG
Whenever electric current flows, it creates a tiny magnetic field around it — the same physics as an electromagnet. MEG (magnetoencephalography) listens to those minuscule magnetic fields produced by neural currents, instead of the voltage. It's completely non-invasive and has superb timing, catching changes millisecond by millisecond, much like the inside-the-skull electrical methods but without any surgery.
The catch: these magnetic fields are billions of times weaker than the Earth's own field, so MEG must run inside a massive magnetically shielded room full of exquisitely sensitive sensors. That makes it powerful for research but bulky, expensive, and the opposite of portable — you go to the machine, the machine never comes to you.
Reading blood flow: fNIRS
All the methods so far listen to the brain's *electrical activity* directly. fNIRS (functional near-infrared spectroscopy) takes a sideways route: it shines harmless near-infrared light through the scalp and measures how much comes back. Oxygen-rich and oxygen-poor blood absorb that light differently, so the returning light reveals where blood is flowing — and busy brain regions pull in more oxygenated blood.
Because it reads blood instead of voltage, fNIRS is portable and non-invasive — sensors clip into a soft cap you can wear while sitting at a desk. But there's a price: blood flow lags the actual firing by seconds. So fNIRS is slow, the way a thermometer is slow to show that you've started exercising. Great for comfort and mobility, weak for split-second timing.
A cheat-sheet
Here is the whole toolbox on one card. Read each line as: *method — invasiveness, spatial/time detail, portability.*
- EEG — non-invasive (on the scalp); coarse spatial detail but fast timing; very portable and cheap.
- MEG — non-invasive; better spatial detail than EEG and equally fast timing; not portable (needs a shielded room) and costly.
- fNIRS — non-invasive; moderate spatial detail but slow timing (blood lags firing); portable and wearable.
- ECoG — invasive (electrodes on the cortical surface, under the skull); fine spatial detail and fast timing; not portable, needs surgery.
- Intracortical (via a microelectrode array, reading the LFP and single spikes) — most invasive (electrodes inside the tissue); the finest spatial detail and fastest timing; not portable, requires surgery.