natus Grass S88X, User & Service Manual

Page: 50 / 64

8-50
Chapter 8
Artifacts, Stimulus Isolation & Constant Current
s
ources
& r
eductioN of
s
timulus
a
rtifacts
Excessive stimulus artifact results when the recording electrodes
unavoidably measure the field distribution of the stimulus voltage
through the preparation. The size of the stimulus and the proximity of
the recording and stimulating electrode pairs contribute to the artifact
amplitude. See Figure 8-2.
Note:
For optimum recording results, use independent stimulating and
recording electrodes and keep recording electrodes perpendicular
to stimulus current flow, if possible.
To alleviate stimulus artifact:
• Isolate the stimulus pulse from ground and thereby reduce the
circulating ground currents between the stimulator, preparation
and recording instrument.
• Space stimulating and recording electrodes as far from each other
as possible and position them for maximum cancellation of field
effects.
• Use as small a stimulator pulse with as short a duration as is possible
(approximately 0.1 ms).
If the field distribution pattern of stimulus current causes substantial
stimulus voltage between the recording electrodes, the resulting artifact
cannot be avoided.
S S
R
R
Equi-
potential
stimulus
voltage
lines
Stimulus
Current
Flow
S = Stimulating Electrodes
R = Recording Electrodes
Figure 8-2: Stimulus Voltage Field Distribution