ECoG Grid
A sheet of microphones laid directly on the brain — close enough to overhear thought, gentle enough to never touch a neuron.
Records local field potentials and high-gamma (70–150 Hz) signals from the cortical surface with spatial resolution of hundreds of micrometers to a few millimeters and ~1 ms temporal resolution. It bridges scalp EEG (low resolution) and penetrating microelectrode arrays (more invasive, prone to scarring). High-density µECoG arrays such as Precision Neuroscience's Layer 7 pack 1,024 electrodes into 1.5 cm² and can be inserted through a slit smaller than 1 mm.
Neural populations create local extracellular field potentials as ions flow during action potentials and synaptic activity. An ECoG electrode in contact with the pia mater or arachnoid membrane picks up the summed potential of underlying cortical columns. Standard clinical grids use platinum-iridium electrodes 2–5 mm in diameter on 10 mm centers. High-density µECoG arrays are photolithographically patterned on polyimide or parylene-C, with 50–500 µm electrodes on ~400 µm centers. PEDOT:PSS electrode coatings cut impedance by an order of magnitude versus bare platinum, are optically transparent (allowing simultaneous optogenetic stimulation), and improve charge injection. Layer 7 routes 1,024 channels through 16 ribbon cables to a standard EEG-compatible headstage that records at 20 kHz per channel.
In plain terms
Imagine trying to hear individual conversations in a crowded stadium by holding a microphone outside the building — you'd just get noise. Now imagine sliding a sheet of tiny microphones under the roof, directly above the crowd. Suddenly you can hear specific sections clearly. An ECoG grid is that sheet of tiny microphones, but for the brain: it rests on the brain's surface just beneath the skull and picks up the electrical chatter of the neurons below — without ever poking a single one.
Also called: electrocorticography array, cortical surface array, subdural grid, µECoG.
Why it matters
- Clinical ECoG grids have guided epilepsy surgery since the 1950s by mapping seizure foci with sub-centimeter resolution; high-density arrays now reveal propagation dynamics invisible at coarser pitch.
- High-density ECoG arrays have decoded intended speech at >77% word accuracy in real-time intraoperative sessions, pointing toward communication BCIs for ALS and locked-in patients.
- ECoG offers the highest-bandwidth recording of human cortex available without penetrating the brain.
- PEDOT:PSS coatings make the same array work for both recording and safe stimulation.
Where it lives
- Precision Neuroscience Layer 7-T — FDA 510(k) cleared March 2025; deployed in 37 US patients as of 2025
- Clinical Wyler / Ad-Tech PMT grids — standard epilepsy-mapping tool in hundreds of centers
- Synchron Stentrode — endovascular ECoG variant; 16 platinum electrodes in the superior sagittal sinus; FDA Breakthrough Device
- Soft PEDOT:PSS arrays from research groups (Lacour/EPFL, Minev/Sheffield) for animal and human research
Key numbers
- Layer 7 electrode count: 1,024 (In 1.5 cm²)
- Layer 7 electrode spacing: 400 µm (8× finer than standard clinical grids)
- Standard clinical grid pitch: ~10 mm (Electrodes 2–5 mm in diameter (Ad-Tech))
- Recording bandwidth: 20 kHz / channel
- Speech 4-word classification (intraoperative): 77.5% accuracy (Real-time, Layer 7, 2026 PubMed study)
Surprising facts
- The Layer 7 array deploys 1,024 electrodes through a slit smaller than 1 mm in the skull — no craniotomy required — an 8× resolution improvement on standard grids and a slot narrower than a guitar string.
- A 1.5 cm² ECoG patch sits above roughly 60,000–150,000 neurons, yet the electrodes never touch a single cell body — the array reads the population's collective electric field from outside.
- In 2025 the FDA cleared Layer 7-T under 510(k) as 'substantially equivalent' to a 1990s Ad-Tech subdural strip with eight times fewer electrodes and 25× coarser spacing.
- The largest published Layer 7 series so far covers 37 US patients during routine surgeries, so claims of long-term performance in chronic implants remain a small-sample, near-term picture.
Open questions
- Long-term ECoG signal stability: Subdural ECoG arrays in chronic implants often retain signal quality without the impedance drift seen in penetrating microelectrodes, but the reason is not fully understood. Some datasets show stable high-gamma decoding for more than three years, while others degrade within months — with no clear biomaterial predictor.
A short history
Electrocorticography was developed clinically by Wilder Penfield and Herbert Jasper at the Montreal Neurological Institute in the 1940s and 1950s for epilepsy mapping. Standard Wyler / PMT grids became hospital-standard by the 1980s. Micro-ECoG emerged in academic labs in the 2000s with neural-probe miniaturization. Precision Neuroscience (founded ~2021) received FDA 510(k) clearance for Layer 7-T in March 2025 with 1,024 electrodes. The BCI speech-decoding literature exploded between 2019 and 2025, with real-time phoneme, word, and sentence decoding from ECoG signals.
Sources
- PubMed (2026) — Layer 7 intraoperative speech decoding (https://pubmed.ncbi.nlm.nih.gov/41621103/)
- FDA 510(k) clearance K242618 — Layer 7-T (https://www.accessdata.fda.gov/cdrh_docs/pdf24/K242618.pdf)
- JAMA Neurology (2023) — Stentrode SWITCH study (https://pmc.ncbi.nlm.nih.gov/articles/PMC9857731/)
- Nature Biomedical Engineering (2025) — Layer 7 minimally invasive implantation (https://www.nature.com/articles/s41551-025-01501-w)
- Nature Communications (2024) — origami-inspired soft ECoG with PEDOT:PSS (https://www.nature.com/articles/s41467-024-50597-2)