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Stimulus Systems
8 Somatosensory stimulation
8.1 Electric stimulation
There is 2-channel transcutaneous constant current electric stimulator inside the
stimulator cabinet. The electrical pulse generator (LUCIUS & BAER GmbH) for
the stimulators is located in a stimulus cabinet outside the magnetically shielded
room (between the magnetically shielded room and the corridor wall).There is
also a Neuromag stimulus I/O box identical to the one beside the stimulus
computers. Connect the BNC cables from the stimulator to the corresponding
output of the Neuromag stimulus I/O. Check that the triggers meant for the right
and left sides are connected correctly. For example, use trigger 1 for left side (red)
and trigger 2 for the right side (blue).
Electrical stimulation can be generated internally with Acquisition Control
software or externally with the stimulus PC. When generating the stimuli with the
Acquisition software, the main computer sends the trigger directly to the
Neuromag stimulus I/O box. If the sequence is generated with the stimulus PC,
connect the PC trigger output to the Neuromag stimulus input with BNC cables.
The Acquisition Control software generates simple stimulus sequences. To do
that, select the “External Stimulus Sequence” in Settings and define the trigger
channel and the inter-stimulus interval (ISI), i.e. the time difference between the
previous trigger and the new one. For more complex stimulus sequences use the
Presentation software (see sections 3.1 and 4.1).
It is possible to adjust
both the amount of the
current and the duration
of the electrical pulse
with the knobs on the
stimulator front panel
(Figure
11).
The
stimulator generates only
square wave pulses. The
recommended
pulse
duration is 0.2 ms. The
digital display shows the
current intensity in mA
Figure 11 The electrical pulse generator.
for the left and right
stimulators. Common current values for median nerve stimulation are 6-7 mA.
Note that these values depend on the subject’s sensitivity as well as on the
stimulated nerve (or skin). As a normal procedure, always start by stimulating at
lower intensity and only then increase the value gradually.
If a trigger pulse does not occur for a long time, for instance during a long break
in the measurement, the current value on the display starts blinking and no current
pulse is sent inside. This is a safety property of the stimulator and in this case
follow the instructions:
•
•
Start running the stimulus sequence.
Turn the current to zero.
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