Eeg Of Poor Quality - Geodesic Sensor Net Technical Manual

Other Large Electric Fields
Anything that generates a large electric or magnetic field (for example, ceiling fans,
freezers, centrifuges, or transformers) can be a source of environmental noise. Such
sources can sometimes be elusive, especially if hidden in a wall, ceiling, or floor, or in
a different room.
• Check that all obvious sources (fans, copiers, fluorescent lights) are eliminated,
disabled, or placed at a greater distance from the vicinity of the subject, all data
cables, and the amplifier. The contribution of electric fields to noise decreases as
the square of the distance between the devices increases.
• Avoid subject contact with antenna sources (metal desks or chairs).
Configuration Issues
Channels with floating inputs affect the amplifier's ability to perform common mode
rejection (CMR) and, hence, increase susceptibility to environmental noise.
• To improve CMR, short unused channels to isolated common. Do not short
unused channels to reference.
• Check for sensors close to, or touching, common. Sensors near common will
normally exhibit increased noise.

EEG of Poor Quality

The previous section discussed electrical and magnetic noise as a source of degraded
EEG signals. This section discusses the issue of poor EEG more generally and
provides guidelines for remedying it.
Disregarding noise for the moment, poor EEG can be characterized by one of more of
the following phenomena:
• flatline or relatively low-amplitude waveforms at a single channel, a group of
neighboring channels, a set of non-neighboring channels, or the entire sensor
array
• certain channels being steadily or intermittently stuck at maximum amplitude,
also called railing
Geodesic Sensor Net Technical Manual
S-MAN-200-GSNR-001 • January 31, 2007
11: Troubleshooting
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