VEP Flashcards
What does VEP stand for?
(Cortical) Visually Evoked Potential
True or False - VEP waveforms are dependant on mass responses of millions of cortical neurones
True
What is an advantage of using a VEP?
They have a high temporal resolution.
Temporal resolution refers to the time taken to take multiple measurements and reconstruct the image ( ie. for the output).
What is a disdvantage of VEPs?
They have a poor spatial resolution. (Thus difficult to determine whihc particualr area of the occipital lobe is responsible for producing that response).
True or False- VEPs are always in response to binocular stimuli
Flase - VEPs are always in response to monocular stimuli
What are the three different types of stimuli for VEPs?
Patten Reversal- this is basically a checkerboard where white and black spaces flip over
Pattern Offset/Onset - this is basically when the stimuli goes from a grey box to a checkerboard and then back to a grey box
Flash - these are basically strobbing lights (subtending at 20 degrees of the visual field)
What is seen in a typical VEP waveform?
3 potentials/waveforms are seen:
N75 (AKA N1)
P100 (AKA P1)
N135 (AKA N2)
[In VEPs waveforms are names by whether they are positive or negative and their latency e.g. N135]
[The words potentials and waveforms can be used interchangeably].
What is happening at the different waveforms in a VEP?
N75 - Represents Excitatory Synaptic LGN inputs to layer 4C of V1 are occuring
P100 - Represents Mixed Excitatory responses to V1 and the Extra Striate Cortex are occuring
N135 - Represents Excitatory responses in the extrastriate cortex
What factors affect VEP response?
Overall stimulus Size
Check size (acuity) and contrast (sensitivity)
Why may responses be unreliable or too variable from a VEP?
- Refractive error: correct for near (1.5 m presentation)
- Pathology: pupil dilatation (mydriasis), media opacities
- Fixation instability (e.g., nystagmus)
Why is pattern reversal stimuli chosen over Offset/Onset stimuli for VEPs?
As Offset/Onset Stimuli has greater intersubject variability.
[Patten Reversal stimuli is the gold standard]
What is an advantage of Offset/Onset stimuli over Pattern Reversal stimuli for use in VEPs?
It is less affected by fixation problems
What is nystagmus?
It is nicknamed drifting eyes.
“Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement, acquired in infancy or later in life, that may result in reduced or limited vision. Due to the involuntary movement of the eye, it has been called “dancing eyes”
If you increase the stimuli contrast and size (i.e. contrast sensitivity and acuity) what chnage will be seen in the P100 wave?
Increased Amplitude and decreased latency
[If you are using a high contrast check pattern you will be activating both parvo and magno pathways (in a synchronised fashion) which will result in bigger and faster responses]
True or False- The N135 wave is unaffected by increasing the check contrast
True
Which electrophysiology tests can be used to detect pathologies (in general)?
What are some common cortical pathologies that are picked up by VEPs?
(In ocular albinism optic pathways are completely crossed (even temporal pathways) - this is picked up in a VEP as the right side of the brain is activated exclusively by the left eye,
How does a multifocal VEP (mVEP) stimulus work and how is this useful in detecting pathology?
In a whole checkerboard stimulus only a single black square will change to white while everything else remains the same.
Thus you can work out what part of the visual pathway has the defect and how this affects the visual field.
(The VEP is mapped onto the visual field)
[It is also helpful for monitoring disease progression]
