Biomedical Engineering Reference
In-Depth Information
no amplification. This makes this circuit a buffer circuit, and the idea is that the opera-
tional amplifier has a high input impedance matching that of the scalp signal but also
gives low output impedance for the transmission of the signal from the electrode plate
to the amplifier input stage. This low impedance immunizes the electrode lead against
capacitively coupled ambient noise and thus helps to achieve a good SNR.
Using active electronics on an electrode requires extra leads for the supply of
power to the electronics, which potentially increases the bulk of electrode cabling.
This can be alleviated by the use of microcabling and is easily made up for by the
possibilities the availability of electrical power on the electrode affords. The actiCap
system made by Brain Products GmbH (Gilching, Germany) is shown in Figure
2.4(c, d). This system uses an additional data line in order to implement an optical
impedance check directly on the electrode itself. During impedance mode a
three-color LED (red-yellow-green) shows the impedance state of each electrode
directly on the head of the subject, thereby eliminating the need to check the com-
puter display for suboptimal impedances. This new feature allows for extremely
efficient and fast electrode preparation.
Another feature, which the actiCap shares with the EasyCap Active by EasyCap
GmbH (Herrsching, Germany) shown in Figure 2.4(b), is the ability to connect to
virtually any existing EEG amplifier system. Both active electrode systems give all
advantages of active electrodes at the sensor level, but also convert the signals so
that they can be measured by any connected standard amplifier. This allows for the
added advantage of using active electrodes without having to invest in a complete
“active” EEG amplifier system such as the ActiveTwo system manufactured by
Biosemi (Amsterdam, Netherlands), which was among the first active electrode sys-
tems for EEG recordings on the market and is also widely used. A 256-channel
Biosemi active electrodes system is shown in Figure 2.5(c).
2.2.2 Electrode Caps and Montages
Traditionally, the International 10-20 system defined by [10] has been used to
describe the locations of EEG scalp electrodes relative to anatomic landmarks on the
64 channel
quick cap
68 channel
customized cap
256 channel
customized cap
(a) (b) (c)
Figure 2.5
Electrode caps and montages. (a) Commercially available 64-channel electrode cap
based on the 10-10 layout (Compumedics, El Paso, Texas). (b) Customized infracerebral 68-channel
electrode cap (EasyCap, Herrsching, Germany). (c) 256-channel infracerebral electrode cap devel-
oped at the Swartz Center for Computational Neuroscience, San Diego, California, by A. Vankov and
S. Makeig, in collaboration with L. Smith (Cortech Solution, Wilmington, North Carolina).
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