Service manual

Revision 2/18/05 2 - 5
System Overview
Visual Evoked Potential
A Visual Evoked Potential (VEP) is used to check the
neuro pathways from the eyes to the occipital area in the
brain. Visual evoked potentials are used to diagnose
visual losses due to optic nerve damage, especially from
multiple sclerosis. They are also useful to diagnose
“hysterical blindness,” in which loss of vision is not due
to any nerve damage. For a VEP exam the patient
focuses on a monitor which displays a reversing
checkerboard pattern. For children or others whose
attention may wander, LED goggles are used which
show the pattern to one eye at a time.
Typical system parameters for a recording a VEP are:
• Stimulus=Reverse Checkerboard (32X32)
• Rate=1.9/sec.
• Repetitions=100
• Low Frequency Filter=1Hz
• High Frequency Filter=100Hz
Example of VEP Waveforms
Principles of Signal Averaging
Signal averaging is a method for extracting a small
signal from a noisy environment to improve the signal-
to-noise ratio. This technique is primarily used for
recording evoked potentials
Signal averaging involves applying multiple stimuli and
sampling for the response after each stimulus
presentation. The response is time-locked to the
stimulus, and therefore additive. The ongoing electrical
activity is random, and therefore subtractive.
Averaged Signal=sum of sweeps
number of sweeps
Signal Averaging to Reduce the Effects of
Environmental Noise
Types of Noise
There are several categories of background noise that
can affect the quality of the evoked response as well as
other types of tests done on neurodiagnosic instruments.
System Noise
Stimulus artifact can occur when the patient electrodes
pick up electromagnetic radiation from the headphones.
Route the headphone leads away from the patient
electrode leads to minimize this source of noise. This
noise appears as a large spike at the beginning of the
trace.
Monitor noise can appear on the incoming signal if the
amplifier or patient are too close to display monitor.
Stimulus
Sample Response buried in noise.
1
2
3
4
5
6
Superimposed
samples
Average of
6 samples
Response is time-locked
to the stimulus.
Random noise components
have diminished.