In Vivo Brain Research

ECoG Electrodes for Brain Research Applications

Surface of the skull: Electroencephalography (EEG)

• Pros: non-invasive, stable recordings over time

• Cons: low spatial and temporal resolution

Surface of the brain: Electrocorticography (ECoG)

• Pros: less invasive than penetrating electrodes, high signal-to-noise, stable recordings over time

• Cons: more invasive than EEG

Inside the brain: Penetrating electrodes 

• Pros: high signal-to-noise ratio (close to the neurons)

• Cons: highly invasive (Inserted into the brain), signal/noise and neuronal environment deteriorate over time, accessible cortex area is limited

Penetrating electrodes are inserted into the brain tissue. They are made of (i) metals such as platinum/iridium, tungsten, stainless steel, (ii) micromachined/microfabricated silicon such as the Utah Array and the Michigan Probe, or (iii) flexible polymers. Penetrating electrodes are positioned close to the neurons inside the brain parenchym, thus affording high signal-to-noise ratio and the capability to record from individual neurons or small neural populations. However, their major disadvantage is that, in most cases, their recording characteristics (signal/noise, neuronal environment) deteriorate over time, as a matter of weeks or months, due to the formation of an encapsulation layer around the electrodes. In addition, penetrating electrodes are small, thus the accessible cortex area is limited.

Electrodes can also be placed either on the surface of the scalp (Electroencephalography, EEG), or on the surface of the brain (ECoG), i.e., they do not pierce the blood-brain barrier. The tissue response is therefore negligible for EEG and reduced for ECoG and the long-term stability of neural recordings is improved for both techniques compared to penetrating electrodes. EEG electrodes have lower spatial resolution and signal-to-noise ratio than ECoG electrodes due to current spread through the dura, skull, and skin. 

ECoG electrode arrays that are used clinically in epilepsy treatment need to be large, typically about 80mm x 80mm, because of the requirement to record from a large area of the brain surface. Large area electrode arrays are also required in brain-machine-interfaces (BMI). Most commercial ECoG arrays have electrodes that are 3-5 mm in diameter with an inter-electrode spacing of ≈10 mm (center-to-center). It has been demonstrated, however, that sub-millimeter electrode spacing would be desirable for both, seizure localization and BMI. Recording with today’s ECoG arrays therefore falls into one of two categories:

• large brain regions (80mmx80mm) at low spatial resolution (10mm electrode spacing): conventional ECoG

• small brain regions (10mmx10mm) at high spatial resolution (0.5-1mm electrode spacing): μECoG

Our goal is the development of a μECoG electrode array that allows the recording/stimulation of large regions of the brain at high spatial resolution.

BMSEED is developing a microelectrode array for electrocorticography (ECoG) for research and clinical applications that (i) has a higher spatial resolution than commercial ECoGs, (ii) accesses a larger area of the cortex, and (iii) has improved grid compliance and therefore improved safety compared to current micro ECoGs (μECoG).

A major clinical application of ECoG electrodes is the pre-surgical localization of epileptogenic zones in the brains of patients with epilepsy. Epilepsy is the 4th most common neurological problem in the United States with an estimated 150,000 new cases each year and a prevalence estimated at about 2.3 million adults and 470,000 children age 0-17. About 30% of patients with epilepsy have seizures that cannot be controlled by medication, and therefore require surgery, and 80–84% of epilepsy surgery centers around the world perform ECoG in some or all of their patients with partial epilepsy.  

ECoG Electrode Array