Biomedical Engineering Reference
In-Depth Information
should be suited to the paradigm used. If the difference between the experimental
conditions is very small, such as in EEG gamma-band studies with effects at or
below 1
V, then the resolution should be high enough to resolve this difference with
at least 10 steps to ensure proper quantification of peak values. Also, and equally
important, low amplifier gains may not raise the signal level sufficiently above the
noise level present at the input stage of the amplifier, resulting in lower SNR. To
summarize, the gain should be set as high as possible without risking saturation of
the amplifier.
μ
Highpass Filter
The choice of the highpass edge frequency and steepness (order) depends on the
measures to be acquired. Measurements of slow potentials such as contingent nega-
tive variations (CNV) or of lateralized readiness potentials (LRPs) require, or at least
benefit from, the use of dc coupled recordings. For all other recording purposes the
time constant of this filter should be set long enough to allow passage of the slowest
components expected without significant alteration due to the filter.
Lowpass Filter
The usable spectrum acquired is generally bounded by the effective bandwidth of the
amplifier, which is typically enforced by analog hardware filters in the amplifier
input stage. Further, digital lowpass filters can be used to limit the frequency band
acquired to the spectral content of interest.
Mains Notch Filter
The mains notch filter is a very steep filter designed to specifically filter out a very
narrow band of frequency content around the mains frequency. One would gener-
ally use a notch filter in EEG recordings, unless its use interferes with the target spec-
trum. This would be the case, for example, for recordings of gamma-band activity. It
would be difficult to give examples of recording parameters that are representative
beyond the scope of a specific modality or paradigm since the exact recording
parameters have to be honed for the recording task at hand. However, if the parame-
ters are chosen according to the preceding general rules of thumb and with the task
and EEG components of interest in mind, it should be easy to achieve the temporal,
amplitude, and spectral resolution and accuracy required for the research at hand.
2.3.2 EEG Artifacts
As outlined earlier, it is important to create a recording environment that minimizes
the potential for ambient artifacts. Typical steps taken to ensure optimal recording
environments include the use of an acoustically and electrically shielded EEG cabin
and the installation of a separate earth ground band for the laboratory. Further steps
should include ensuring that all interconnected devices use the same mains phase
and ground and that either centrally or locally installed mains noise filters are used,
for example, through a high-quality uninterruptible power supply. However, even
though steps taken to reduce artifact sources in the recording environment are usu-
ally quite effective, they can only help to reduce, but cannot totally avoid, external
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