Electrocorticography · Lobeworks/17

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: When do epilepsy teams choose depth electrodes over a grid on the surface of the cortex? A grid needs a large opening in the skull and sees densely the surface it lies on; depth electrodes go in through small holes and sample sparse points along lines that can reach deep structures in both hemispheres. Teams choose depth electrodes when the suspected onset is deep (the hippocampus, the insula, the bottom of a sulcus), when it could lie on either side, or when several distant regions are candidates. A grid keeps its place when the focus is near the surface and the cortex around it must be mapped finely, next to language or motor areas for example. Many centres that once relied on grids now start with depth electrodes, partly because their complication rate is lower. 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. Why did a brain implant that worked for six years in a woman with ALS stop working? Because the disease reached the cortex under it. Electrode strips placed on the surface of her sensorimotor cortex in 2015 let her signal her carers with attempted hand movements, and by 2020 it had become her only way to call them, even at night. After about six years the signals shrank and her use of it fell, until it ended in 2023; scans taken eight years apart showed her brain tissue wasting, while the hardware had no fault. A device that reads the cortex lasts only as long as the cortex it reads.