Abstracts

Rationale: Recent work has shown that increasing the spatial resolution of intracranial EEG (iEEG) reveals signal features that may be critical in understanding epileptic networks. The work has led to several companies offering FDA-approved microelectrode grids that connect to preexisting EEG equipment. Our group recently measured the impedances several of these grids and demonstrated that, due to their large impedance, microelectrodes will attenuate EEG signals if connected to commonly-used EEG amplifiers. In this study we demonstrate the clinical implications of this attenuation. Methods: Raw iEEG data from macroelectrodes implanted on a deidentified patient were digitally filtered to simulate the signal recorded by a commercial hybrid grid (2 macro-, 8 microelectrodes) and a commercial EEG amplifier. The filtering method was verified by recording from the hybrid grid and EEG acquisition amplifier in a saline bath. The filtered EEG data were read by three trained epileptologists, who marked the time and location of seizures, spikes, and slow activity. A well-known automated detector for High Frequency Oscillations (HFOs) was used on each data set. Several sample hybrid grids were imaged with scanning electron microscopy (SEM). Results: Macroelectrodes did not alter the original signal because their impedance is much lower than the amplifier's. Microelectrodes attenuated low frequencies, which led to several difficulties in clinical interpretation: theta and delta slowing were not recognized; identification of spikes and seizures was disrupted, leading to false negative as well as false positive markings; and the HFO detector produced false positives. SEM demonstrated marked variability in exposed electrode surface area, as well as frequent sharp edges that could damage tissue. In addition, during the experimental recording the higher impedances generated significantly greater noise and were especially sensitive to motion of nearby observers, which led to large baseline fluctuations and false HFOs. Conclusions: The high impedance of microelectrodes increases recorded noise and attenuates brain signals when recorded on standard EEG amplifiers, greatly disrupting clinical interpretation. Some of these changes can be mitigated by using higher impedance amplifiers. Great care must be taken when comparing results from microelectrodes.