Your brain runs on electricity
Picture your brain and you might imagine thoughts floating around like clouds. The reality is more down-to-earth: your brain is a vast, busy electrical network. It holds roughly tens of billions of cells called neurons, and these cells talk to one another using tiny electrical pulses, the way one phone signals another across a wire.
Here is the key idea to carry through everything that follows: every feeling, every plan to move your hand, every memory shows up as a pattern of electrical activity. Because those patterns are physical and electrical, a sufficiently sensitive machine can, in principle, listen in on them. That single fact is what makes the whole field possible.
The spike: a neuron's word
When a neuron decides to send a message, it fires a brief electrical blip called an action potential, or simply a spike. Think of it as a single tap of Morse code: a quick pulse, then silence, then maybe another. The neuron does not whisper louder or softer to mean different things. It either taps or it doesn't.
That is what "all-or-nothing" means: a spike either happens at full strength or not at all, with nothing in between. So how does a neuron say more than one thing? Through timing and rhythm. A flurry of rapid taps means something different from a single lonely tap, just as a fast burst of Morse means a different letter than a slow one.
From one neuron to a chorus
One neuron tapping is a soft voice in a stadium. The brain's real work happens when huge crowds of neurons fire together. When millions rise and fall in step, their combined activity forms steady waves called brain rhythms (you may have heard them called brainwaves), like an audience clapping in unison until a beat emerges.
If you place a tiny sensor among that crowd, you don't hear each voice cleanly; you hear the pooled hum of everyone nearby. That blended signal is called the local field potential, the murmur of a whole neighborhood of neurons rather than one shouting individual.
Some of these choruses are wonderfully convenient to read. The motor cortex, a strip of brain just over the top of your head, lights up in a characteristic way the moment you so much as plan to move, even before the muscle twitches. That reliable, repeatable pattern is exactly the kind of signal a brain-computer interface loves to grab onto.
What we can actually measure
Now for the catch that shapes everything to come. How clearly you can hear the brain depends entirely on how close you sit to it. Place a hair-thin electrode right next to neurons, and you can pick out individual spikes, the crisp taps of single cells. That clarity is precious, but it usually means surgery to get the sensor inside.
Move outside the skull and stick sensors on the scalp, and the picture softens dramatically. Bone and skin smear the signal, so individual spikes vanish and only the big, blurry chorus survives. This scalp recording is called EEG (electroencephalography). It is safe, painless, and as easy as putting on a cap, but it trades sharp detail for comfort.