Brain–Computer Interfaces - Index of

222 J.E. Huggins
range of sizes from micro-electrodes intended to record single cells (e.g. [1, 2]) to
the more common electroencephalography (EEG) electrodes placed on the scalp
(see Fig. 1). As the distance from the signal source (the individual cells) increases,
the strength of the filtering effect also increases. Observing the brain activity from
the scalp (EEG) can be compared to listening to a discussion from outside the room.
The intervening scalp and bone hide most subtle variations and make the signals
of interest difficult to isolate. To avoid this effect, it is advantageous to at least get
inside the room (inside the skull), even if you remain at a distance from individual
speakers.
ECoG electrodes allow observation of brain activity from within the skull, since
they are surgically implanted directly on the surface of the brain. However, they do
not penetrate the brain itself. ECoG electrodes can be arranged in grids or strips (see
Fig. 2), providing different amounts of cortical coverage. ECoG grids are placed
through a craniotomy, in which a window is cut in the skull to allow placement
of the grid on the surface of the brain and then the bone is put back in place.
ECoG strips can be placed through a burrhole, a hole drilled in the skull through
which the narrow strips of electrodes can be slipped into place. ECoG electrodes are
placed subdurally, (i.e. under the membranes covering the brain), although epidural
placements (above these membranes, but still inside the skull) are sometimes done
simultaneously. In this chapter, the term ECoG will be used to refer to signals from
both subdural and epidural electrodes, with placements solely involving epidural
electrodes specifically noted. ECoG electrodes are commonly 4 mm diameter disks
arranged in either strips or grids with a 10 mm center-to-center distance [e.g. [3–6]].
Smaller, higher density ECoG electrodes are also used [e.g. [7]] and the optimal size
of ECoG electrodes for BCIs remains an open question. Platinum or stainless-steel
ECoG electrodes are mounted in a flexible silicone substrate from which a cable
carries the signals through the skin and connects to the EEG recording equipment.
Standard EEG recording equipment is used for clinical monitoring, although some
units having settings for ECoG.
Fig. 1 Electrode location
options for BCI operation