Electrocorticography

Electrocorticography

Electrocorticography (ECoG) involves placing electrodes directly onto the exposed surface of the brain to capture electrical activity from the cerebral cortex. This procedure can be carried out either during surgery (intraoperative ECoG) or outside of surgery (extraoperative ECoG), requiring a craniotomy to implant the electrode grid, making it an invasive technique. Currently, ECoG is considered the standard method for identifying epileptogenic zones in clinical settings.

ECoG signals consist of synchronized postsynaptic potentials (local field potentials) recorded directly from the cortex's exposed surface. These potentials primarily originate in cortical pyramidal cells and traverse several layers of the cerebral cortex, cerebrospinal fluid (CSF), pia mater, and arachnoid mater before reaching subdural recording electrodes positioned just below the dura mater (outer cranial membrane). In contrast, to reach the scalp electrodes of an electroencephalogram (EEG), electrical signals must also traverse the skull, where they rapidly weaken due to bone's low conductivity. Consequently, ECoG boasts a superior spatial resolution compared to EEG, offering crucial advantages for presurgical planning. ECoG provides a temporal resolution of approximately 5 milliseconds and a spatial resolution of 1 centimeter.

Depth electrodes enable the measurement of neural populations within a sphere with a radius of 0.5-3 millimeters around the electrode tip. With a sufficiently high sampling rate (above 10 kHz), depth electrodes can also capture action potentials, achieving spatial resolutions down to individual neurons, with each electrode's field of view spanning approximately 0.05-0.35 millimeters.

During ECoG recording, electrodes are placed directly on the exposed cortex's surface. To access the cortex, a surgeon performs a craniotomy, removing a section of the skull to expose the brain's surface. This procedure may be conducted under general anesthesia or, if patient interaction is necessary for functional cortical mapping, under local anesthesia. Electrodes are then surgically implanted on the cortex's surface, guided by preoperative EEG and magnetic resonance imaging (MRI) results. These electrodes can be positioned outside the dura mater (epidural) or beneath it (subdural). ECoG electrode arrays typically feature sixteen sterile, disposable electrodes made of stainless steel, carbon tips, platinum, or gold balls, each mounted on a ball-and-socket joint for easy positioning. These electrodes are affixed to an overlying frame in a "crown" or "halo" arrangement.

Additionally, subdural strip and grid electrodes are commonly used, featuring various dimensions with electrode contacts ranging from 4 to 64. These grids are transparent, flexible, and numbered at each electrode contact, with a standard spacing of 1 centimeter between grid electrodes, and individual electrodes typically measuring 5 millimeters in diameter. These electrodes rest gently on the cortical surface, designed with enough flexibility to withstand normal brain movements without causing injury. Strip and grid electrode arrays can be maneuvered under the dura mater to access cortical regions not directly exposed by the craniotomy. Furthermore, depth electrodes may be employed to record activity from deeper brain structures like the hippocampus.