Devices
Two questions sort almost every device: does it read, write, or both — and does it sit outside the skull, on the brain, or in it?
| Method | Direction and placement | What it is used for |
|---|---|---|
| EEG | Reads · on the scalp | Epilepsy diagnosis, sleep studies, research; low-bandwidth brain–computer interfaces |
| MEG | Reads · outside the head | Mapping activity before surgery; research. Measures magnetic fields, so the skull does not blur the signal |
| fMRI and fNIRS | Read · outside the head | Locating function through blood oxygenation. Slow — seconds, not milliseconds |
| ECoG | Reads · on the surface of the brain | Finding the source of seizures; speech-decoding research |
| Intracortical arrays | Read and write · in the cortex | High-bandwidth brain–computer interfaces for paralysis. Resolve single neurons |
| Endovascular electrodes | Read · inside a blood vessel | Brain–computer interfaces placed without open brain surgery |
| Cochlear implant | Writes · auditory nerve | Hearing. The most widely used neural implant: about a million fitted worldwide by 2022 |
| Deep brain stimulation | Writes · deep brain nuclei | Parkinson’s disease, essential tremor, dystonia |
| Spinal cord and vagus nerve stimulation | Write · spinal cord, peripheral nerve | Chronic pain; epilepsy and depression |
| TMS and tDCS | Write · outside the head | TMS has been cleared in the United States for depression since 2008; the first tDCS device for depression was approved there in December 2025 |
| Responsive neurostimulation | Reads and writes · on and in the cortex | Epilepsy: detects abnormal activity and responds to it |
Why invasiveness matters
The skull and scalp smear electrical signals, so a scalp electrode averages the activity of millions of neurons. Moving the electrode inward buys resolution at the cost of surgery, infection risk and the body’s reaction to a foreign object: over months, scar tissue forms around an implant and the signal degrades. Much of the engineering in this field is an attempt to get the resolution without the damage.