VSP Visually evoked potential
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A patterened voltage change embedded in the ongoing EEG (electroencephalogram) that reflects a process occurring as a response to a visual stimulus.
Visually evoked potential/visually evoked cortical potential/visually evoked response
A VEP is an INDIRECT measurement of ___ function.
foveal (mainly central 10 degress of VI retinotopic projection
VEP records potentials generated in the ___ ____ that represent the end stages of visual processing elicited by retinal stimulation.
If you don't have or have a damaged ___ you won't have a VEP.
fovea or if there is something in the brain blocking the visual pathway
Why is the EEG on order of +50 microvolts?
because there is a lot of neural noise going on in retina and brain, need to filter out (amp of waves is ~50uv)
What is intrinsic and extrinsic activity? which one is usually greater?
Intrinsic activity is normal activity in the brain and extrinsic activity is the activity that is caused by a change in the enviro; usually more intrinsic, but can vary (ex: tired)
VEPs are on order of _____ microvolts
VEP signal is small compared to the overall ___ signal.
extrinsic or intrinsic?
VEP signal =
- VEP signal = extrinsic
- EEG= intrinsic + extrinsic
During a VEP you usually want to detect differences of ____ microvolts?
What are some ways to detect the small differences needed for a VEP; filter out intrinsic noise?
- -Signal averaging
- -pattern recognition techniques
If you have a bunch of signals, the noise will average out to zero and the signal itself will be amplified?
if we know the freq of a VEP signal, we can filter out other frequencies
there are computers that can recognize and separate out the pattern VEP produced from the patterns that the noise produces
pattern recognition techniques
Looking at the activity of the brain b4 you show stimulus, then you can show stimulus and "lock in" what time you showed the stimulus. Then you can compare the activity w/out and stimuls and during the stimulus.
Time locked activity extraction by averaging
Stimulus related processing (extrinsic)
tonic background activity related to ongoing processes (level of arousal (intrinsic)
This is the most common method of extracting the signal...
For statistically random signals, signal averaging reduces the random noise by the ______
inverse of the square root of the number of signals averaged
period of time that contains the response to the signal
Signal and noise in each epoch sum ____ to produce the recorded waveform for each epoch
There is some ___ in waveform latencies. Ex on one epoch, the waveform may appear a little sooner or little later than on another epoch.
The latency jitter is significantly higher in what dz?
What is the setup of a VEP?
- -electrodes: Gold cup
- -amplification: ~10, 000x/AC
- -typical filtering: 1-100 Hz bypass; 60 Hz notch
- (used to minimize electrical interference from AC in facility)
Where is the placement of the electrodes in a VEP?
top of head(reference), occipital and earlobe (ground)
Placement of pt during a VEP
A distance from the monitor that will allow the monitor to subtend about 10 degress at the pts eye. (foveal)
VEP test distance is dependent on ..
size of monitor
Be careful about using a small screen and having a pt close, bc this can stimulate ___ in younger pts.
- accommodation; can flaw results.
- Make sure pts are refracted for the test difference ( a blurry image will give you inaccurate results)
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