Electrocorticography · Grey Matter

Electrocorticography (ECoG) records the brain's electrical activity from electrodes laid directly on the surface of the cortex, under the skull, and it is used mainly to find the exact origin of seizures before epilepsy surgery and, increasingly, as the signal for brain-computer interfaces that decode speech or movemen

Electrocorticography. Electrocorticography (ECoG) records the brain's electrical activity from electrodes laid directly on the surface of the cortex, under the skull, and it is used mainly to find the exact origin of seizures before epilepsy surgery and, increasingly, as the signal for brain-computer interfaces that decode speech or movement.

The electrodes are discs or contacts on a thin flexible sheet (a grid or a strip), placed through an opening in the skull onto the brain's surface, beneath the dura (subdural) or on top of it (epidural). They pick up the same kind of signal as scalp EEG, the summed synaptic currents of the cortex beneath, but without the skull in the way.

Removing the skull changes the signal. Bone conducts poorly and spreads the potential, so ECoG sees a patch of millimetres where EEG sees centimetres, with amplitudes many times larger and far fewer muscle and eye artefacts.

It reaches higher frequencies. Activity above about 70 Hz (high gamma) tracks local neuronal firing well and is strong in ECoG, while the skull filters most of it out of the scalp EEG; that band carries much of what speech decoders use.

It still records populations. Standard clinical contacts are millimetres wide and centimetres apart; high-density grids shrink both, but single neurons need penetrating electrodes such as Neuropixels or Utah arrays.

It costs surgery. Clinical ECoG is placed for days to weeks in patients who need it for seizure mapping, which is also why so much of human intracranial research comes from people with epilepsy.

ECoG removes the skull and keeps the population.

It is the middle step between reading from outside and reading single cells, with the inverse problem much reduced and the surgical cost real.

Questions: What does moving the electrodes from the scalp to the surface of the brain gain? It removes the skull, the layer that most attenuates and blurs the signal. Electrodes on the cortex record from a patch millimetres across instead of centimetres, see amplitudes several times larger, pick up far fewer muscle and eye artefacts, and keep the high-frequency activity (above about 70 Hz) that tracks local firing and that the skull filters out. The price is surgery and limited coverage, since a grid only sees the cortex it lies on, and it still records populations of neurons, never single cells.