Correlate of Surface Electroencephalogram (EEG) With Implanted EEG Recordings (ECOG)

Electroencephalography (EEG) is an electrophysiological monitoring method to record
electrical activity of the brain. It is typically noninvasive, with the electrodes placed on
the scalp, although invasive electrodes are sometimes used, also referred to as
electrocorticography (ECoG). EEG measures voltage fluctuations resulting from extracellular
ionic currents summed across a large population of neurons. In clinical contexts, EEG refers
to the brain's spontaneous electrical activity over a period of time, as recorded from
multiple electrodes placed on the scalp. EEG is most often used to diagnose epilepsy, which
can be associated with abnormalities brain's electrical activity. It is also used to diagnose
sleep disorders, depth of anesthesia, coma, encephalopathies, and brain death.

Electrocorticographic activity (ECoG) is composed of synchronization of the postsynaptic
potentials (PSPs), which mainly occur in cortical pyramidal cells. To record ECoG activities
on the scalp as electroencephalographic activities (EEG), ECoG activities must be conducted
through cerebral spinal fluid (CSF), arachnoid, dura matter, skull bone, and subcutaneous
tissue; these potentials form the EEG. Electrical potentials generally decrease in inverse
proportion to the square of the distance from their origin and depend on the conduction rate
(Gauss' law). The conduction rate of CSF is quite high, and potentials arising from the
cortex spread widely into the CSF. On the other hand, the conduction rate of skull bone is
much lower, and electrical potentials attenuate quite rapidly in bone. As a result of the
varied conduction potentials, the EEG is recorded over wide areas of scalp which results in
reduced amplitude; this effect is called the "smearing effect". Therefore, the EEG recording
does not express all the ECoG potentials and the spatial resolution of the EEG is relatively
low.

Ever since the implementation of invasive EEG recordings in the clinical setting, it has been
perceived that a considerable proportion of epileptic discharges present at a cortical level
are missed by routine scalp EEG recordings. Several in vitro, in vivo, and simulation studies
have been performed in the past decades aiming to clarify the interrelations of cortical
sources with the participants scalp and invasive EEG correlates. The amplitude ratio of
cortical potentials to the participants scalp EEG correlates, the extent of the cortical area
involved in the discharge, as well as the localization of the cortical source and its
geometry have been each independently linked to the recording of the cortical discharge with
scalp electrodes. The need to elucidate these interrelations has been particularly imperative
in the field of epilepsy surgery with its rapidly growing EEG-based localization
technologies. Simultaneous multiscale EEG recordings with scalp, subdural, and/or depth
electrodes, applied in presurgical epilepsy workup, offer an excellent opportunity to shed
light on this fundamental issue. Novel computational tools may serve to provide surrogates
for the shortcomings of EEG recording methodology and facilitate further developments in
modern electrophysiology.

One of the major clinical uses of surface EEG recordings are to describe the level of
sedation during anesthesia to guide the anesthesiologist in drug titration. The hypothesis is
that EEG can help assure proper depth of anesthesia by confirming an unconscious state during
a general anesthetic while at the same time ensuring a patient is not given too much
anesthetic. The current protocol will allow the simultaneous recording of surface and depth
electrodes to better describe consciousness and illuminate the shortcomings of EEG to record
brain activity transitions with high fidelity. The protocol will also allow the investigators
to implement novel analyses based on better understanding of the electrophysiological
acquisition technology capabilities. In many cases, the goal of anesthesia is to not only
change consciousness but to also impede memory formation. The investigators hope to better
describe the interplay between consciousness and memory formation while using the EEG as an
objective measure.

Inclusion Criteria:

- Male or Female subjects 18 years of age or older.

- English or Spanish Speaking

- Subjects who have been previously diagnosed with intractable epilepsy and require
implantation of invasive electrophysiological recordings as part of their routine
clinical presurgical workup.

Exclusion Criteria:

- Patients with skin abnormalities at the planned application sites that would interfere
with sensor or electrode applications.

- Patients with pre-existing conditions and/or co-morbidities that would prohibit them
from participating in the study due to unacceptable risks to their health and
well-being, as determined by the Principal Investigator (PI).

- Patients who the PI deems ineligible at the PI's discretion

- Pregnant patients