Final: VEP Flashcards
What is a VEP? What part of the eye does it measure? What does it record?
Visual Evoked (Cortical) Potential
A patterned voltage change embedded in the ongoing EEG that reflects a process occurring as a response to a visual stimulus.
It measures the foveal function mainly 10 degrees of VI retinotopic projection.
-It records potentials generated in the occipital cortex that represent the end stages of visual processing elicited by retinal stimulation.
Intrinsic activity is ______ than extrinsic activity
greater than
How do you fix the signal to noise problem in VEP?
Signal averaging
Filtering
Pattern recognition
EEG is on order of + ___ microvolts
50
VEPs are on order of _____ microvolts
2-20
How can signal-related activity be extracted?
It is time-locked to the presentation of the stimulus and signal averaging then extracts the signal
What are 3 assumptions of averaging methods?
1) Signal and noise (in each “epoch”) sum linearly to produce the recorded waveform for each epoch (not some peculiar interaction)
2) The evoked signal wave shape attributable solely to the stimulus is the same for each presentation (latency jitter)
3) The noise contributions can be considered to constitute statistically independent samples of a random process
What kind of electrodes, amplification, and filter do you need for a VEP test?
- Gold cup electrodes
- 10,000x amplification
- 1-100 Hz bandpass and 60 Hz notch
Where are the typical placements of a VEP test?
The ground electrode is placed on the earlobe
The other 2 are placed on back of head and on top of head.
Where is the patient placed from the monitor?
At a distance from the monitor that will allow the monitor to subtend about 10 degrees at the patient’s eye. Also, you don’t want the patient to use a lot of accommodation.
If the image on the monitor is blurry for the patient, can this affect results?
Yes, it will give inaccurate results.
True or False:
VEP’s generally do not habituate.
True
How do you create an event-related potential for a set of stimuli?
You average together the Epochs for like stimuli.
What are VEP components?
Components that make up part of the waveform that is received. By changing certain components, it will change certain parts of the waveform.
What are 3 ways components are defined?
1) Locate the positive and negative peaks.
2) Identify aspects of VEP that co-vary across subjects, manipulations and locations (PCA approach- most common).
3) Identify the neural structures that generate the parts of the VEP (source modeling approach)
Using the peak approach to identify components, what do you look for?
Polarity
Latency
Placement of electrode on scalp
Sensitivity to experimental manipulations
using the peaks approach to components, components are labeled by ________ and the normal ________ at active recording site.
polarity; latency
How do you quantify components using peaks approach?
- Minimum and maximum amplitude relative to baseline or from the peak of previous component
- Area underneath the component
- Latency
Components can be defined by their ______ or ________ sources.
functional or structural
True or False:
Same brain structure may contribute to more than one portion of the VEP.
True
“Peak latency” or “peak time” is measured from stimulus onset to peak of ___.
N70
“Amplitude” generally refers to the _______ trough to peak voltage change.
N70(trough) P100(peak)
Why do we focus on changes in the parameters from the stimulus onset to peak of N70?
The VEP is quite variable from one patient to the next. The only thing that is the same between any two patients is this latency value.
Describe the stimuli in transient mode.
less than 2 stimuli per second
Response completed before onset of next flash; waveform characteristics preserved with averaging
Describe the stimuli in steady-state mode.
- greater than 8 stimuli per second.
- Visual system “follows” the stimulus train. Each response is affected by previous one.
- Responses very sensitive to changes in pattern size and reversal.
Can be used to predict VA.
What is a swept parameter?
One stimulus parameter (e.g., spatial frequency, contrast, wavelength, etc.) is changed progressively during steady-state presentation.
What are 3 VECP stimuli? Describe them.
1) Flash
- Transient
- Steady-State
2) Pattern Reversal
- Transient
- Steady-State: can use whenever
brain responds the same to each
stimulus change
- Swept-Parameter
3) Appear /Disappapear
- Transient only
What was the sweep VEP developed for?
rapidly assess visual acuity
What is plotted on the sweep VEP to find VA?
Spatial frequency versus the amplitude of the response is plotted to determine VA
What thresholds can be determined by Sweep VEP?
- Threshold for contrast
- Thresholds for differentiating between wavelengths
- Temporal frequency thresholds, etc.
What is flash VEP clinically used for? Where might a problem be located if the patient has an abnormal latency or absence of p100?
Clinically useful for demonstrating the presence or absence of gross light perception within central visual field (e.g., severe damage to eye, cataract, corneal edema).
If abnormal or absent P100, the patient may have problem in optic nerve, chiasm, tract, LGN, radiation, or striate cortex.
True or False:
Flash VEPs are much more variable across subjects than pattern responses but show a lot of interocular asymmetry.
False; They show little asymmetry.
Peaks are designated as negative and positive in a ___________ sequence.
numerical
What is pattern reversal VECP clinically used for?
Clinically useful for swept-size spatial frequency, measurements of visual acuity and disorders that could negatively influence visual spatial resolution and/or motion sensitivity in the central portion of the visual field.
People that have unstable fixation will get _____________ with pattern reversal VECP stimuli. What would this result in?
-motion artifacts
Results in invalid results
What neural pathways does a pattern reversal VECP send a strong signal to?
Pattern specific and Motion specific.
What 3 things do the peak latency of P100 show?
- relatively little variation between subjects,
- minimal within subject interocular difference
- minimal variation with repeated measurements over time.
What is P100 peak latency affected by? Give 6 examples.
Affected by Non-pathologic parameters
Examples: pattern size pattern contrast pattern mean luminance refractive error poor fixation miosis
What is frequency doubling in transient pattern reversal?
Since there are two changes per cycle, the temporal frequency of the response is twice the temporal frequency of the stimulus.
What is the steady state pattern reversal format sensitive to?
changes in pattern size and pattern contrast.
What is PatternAppearance/Disappearance VECP clinically used for?
Evaluating the integrity of pattern-specific visual procesessing within the central portion of the visual field and for differential diagnosis of ocular albinism.
What are the 3 main peaks for the response to pattern onset/offset in the Pattern appearance/disappearance VECP?
C1 (positive approximately 75 ms)
C2 (negative approximately 125 ms)
C3 (positive, approximately 150 ms)
How are amplitudes measured in the PatternAppearance/Disappearance VECP?
Amplitudes are measured from the preceding negative peak.
What mode should Appearance/Disappearance stimulation be presented in?
ONLY in transient mode
True or False:
Cortical responses to the appearance and to the disappearance of a pattern are different.
True
What does the multifocal VEP typically sample?
Samples V1 cortical responses representing the central 50 degrees of visual field.
With the same stimuli, does the activity of the fovea contribute more to the VEP or the ERG?
It contributes more to the VEP
When is the phase lag of the response close to zero?
When a spatial frequency generates the largest VEP amplitude.
