Brain–Computer Interfaces - Index of

244 K.J. Miller and J.G. Ojemann
have splitters built in to the clinical wire ribbons, and these should be used whenever
possible. (f) Amplifiers. These will vary widely by institution, and, also depending
on the institution, will have to have, for instance, FDA approval (USA) or a CE
marking (EU) (the process of obtaining this approval is associated with both higher
cost and lower quality amplifiers). Many amplifier systems will have constrained
sample rates (A/D rates), built in filtering properties, and large noise floors which
obscure the signal at high frequencies. Regardless of which system is used, it is
important to characterize the amplifiers independently using a function generator.
The ECoG recording is, by necessity, in the context of clinical amplification and
recording, so the experimental recording must take place in the context of clinical
amplification with commercially available amplifiers (eg, XLTEK, Synamps, Guger
Technologies, Grass). Most clinically relevant EEG findings are detected visually
and classically the information explored is between 3 and 40 Hz, so the settings
on the clinical amplifiers may be adequate to obtain clinical information, but not
for research purposes. Recent advances have suggested that faster frequencies may
be clinically relevant so many newer systems include higher sampling rate (at least
1 kHz) as an option to allow for measurement of signals of 200 Hz or higher, but
this varies by institution, and the clinical recording settings will vary even within
institutions, depending upon the clinical and technical staff managing the patient.
Experimentalists must obtain either the clinically amplified signal, or split the signal
and amplify it separately. Using the clinical signal has the advantage that less
hardware is involved, and that there are no potential complications because of the
dual-amplification process. Such complications include artifact/noise introduction
from one system to the other, currents between separate grounds if the two do not
share a common ground. Splitting the signal has the advantage that the experimenter
can use higher fidelity amplifiers and set the amplification parameters at
will, rather than having to use the clinical parameters, which typically sample at
a lower frequency than one would like, and often have built in filtering properties
which limit the usable frequency range. The ground chosen, which must be the
same as the clinical ground to avoid complication, will typically be from the surface
of the scalp. Most amplifiers will have a built in choice of reference, which
each electrode in the array will be measured with respect to. These may also be
from the scalp, as they often are clinically, or they may be from an intra-cranial
electrode.
The experimenter will often find it useful to re-reference the electrode array
in one of several ways. Each electrode may be re-referenced with respect to a
single electrode from within the array, chosen because it is relatively “dormant,”
each may be re-referenced to a global linear combination of electrodes from
the entire array, or each may be referenced to one or more nearest neighbors.
Re-referencing with respect to a single electrode is useful when the one in the experimental/clinical
montage is sub-optimal (noisy, varies with task, etc), but it means
that the experimenter has introduced an assumption about which electrode is, in fact,
appropriate. The simplest global referencing is a common average re-reference: the
average of all electrodes is subtracted from each electrode. The advantage of this
is that it is generic (unbiased, not tied to an assumption), and it will get rid of