Lecture 3c- VEP (Visual Evoked Potential) Flashcards
What is the VEP
a recording of the electrical activity that occurs in the brain in response to visual stimulation
by time-variant diffuse or structured stimuli
What type of stimulus is used for VEP
diffuse or structured stimuli
What types of patient is the flash stimulus best for
infants or adults with very poor co op or vision
What is the flash stimulus good for and what can’t it estimate
great for detecting misrouting but cannot estimate cortical acuity to better than ‘rudimentary’ as patients can still see a
bright flash even when almost totally blind!
What can the VEP not do
estimate cortical acuity
What type of chequerboard is used
a counter-phasing or ‘reversing’ chequerboard. (Similar to that used for the PERG.)
What is the benefit of using a reversing chequerboard
it elicits the most consistent and repeatable VEPs
What type of patients is the VEP unsuitable for
motion blur from nystagmus, which reduces contrast
When is an onset chequerboard presented
When patients have nystagmus and motion blur
an onset chequerboard where a 100% contrast chequerboard pattern appears from a 50% grey background and then disappear
What can the onset chequerboard do
elicit a response to both onset and offset
What type of patients is the onset chequerboard best for
this is better in the case of
nystagmus, but responses are more variable than for reversal stimuli in normals.
What remains constant for both the reversal and onset stimulus
mean luminance
What does the chequerboard stimulus consist of
1° chequers for macular stimulation
OR
15′ chequers for foveal stimulation
How many reversals per second are presented with a stimulus field >15 degrees
2 reversals per second
What is necessary for VEP
S, C, F
steady fixation, co operation and focus
Should the patient be dilated and refracted
no dilation and adequate refraction is needed
What is the order of the set up for VEP
Patient- electrodes- amplifier- filter- analog to digital converter- computer- stroboscope or pattern stimulator
Where are the electrodes placed for VEP (A, B, C)
3 behind the head and a reference electrode on the forehead
sometimes only the mid occipital electrode is recorded from B
What are the components for the VEP (3)
N70, P100, N135
What is the N70
Foveal component
= e.g. negative peak at 70 milliseconds
What is the P100
Macular component
= e.g. positive peak at 100 milliseconds
What is the N135
Paramacular component
What convention is followed in VEP recording graph
Cartesian convention with +ve at y- axis
Does the shape of the waveform vary
yes according to chequer size
What do smaller chequers stimulate
the foveal representation in the visual cortex, leading to a bigger N70
What do larger chequers stimulate
the paramacular representation leading to a bigger N135
What does myelinaton of optic nerves in infancy result in
a considerable increase in conduction velocity
over the first 6 months. Cortical maturation results in greater response amplitude and complexity.
At what age do infants have adult size ERG responses and morphology
the ERG matures and all responses approximate adult size and morphology by 6 months of age
What are some applications of the VEP (5- detail in notes)
DCOFO
Demyelination
Compression of optic nerve
Optic neuropathy
Functional integrity of the visual pathway
Objective cortical VA measurement
How is the VEP recorded
using pattern stimuli with many different element sizes down to the limit of visual acuity
When do infants approach adult levels of VEP acuity
at 6 months of age
What are the 2 methods of VEP recording
Minimum VEP Acuity
Sweep VEP
What is the minimum VEP likely to do
underestimate actual acuity. if responses are only recordable to flash then VA is rudimentary.
Blind patients and VEP
A patient may not be completely blind even if no VEPs recordable at all as only a small number of neurones are required for a patient to see a ‘flash’ but we need thousands to fire for a recordable VEP.
What is the calculation of minimum VEP acuity
6/6 x spatial element size (in mins of arc)
How does the sweep VEP work
Theres a rapid presentation of different chequer sizes. Some variants of this test sweep through a large range of different sizes regardless of what is recorded
Some determine the next size of chequer based on the response to the previous chequers, in real-time
What do good paradigms ensure
robust and objective measurements in as little as 10 seconds
How does the VEP amplitude reduce
it reduces linearly with decreasing chequer size as the limit of visual acuity is approached.
Basic how sweep VEP works
In its simplest form, amplitudes are
plotted and a straight-line, leastsquares fit, is extrapolated to 0 µV
from peak of function.
Whats on the sweep VEP x and y axis
A and S
Amplitude and Spatial frequency
What is half field stimulation like in normal patients
this stimulus presented to either eye will activate the left hemisphere
What is the projection like with half field stimuli
Temporal projection for the left eye & Nasal
projection for the right
Nasal projection for the left eye & Temporal
projection for the right
What does full field stimulation activate
both hemispheres equally whether monocular or binocular
How many hemispheres does half field stimulation activate
one hemisphere only
What does half field stimulation along with three occipital recording electrodes allow for
Locating x and x
- locating lesions to R/L side of the R/L optic nerves, chiasm and R/L optic radiation/hemisphere
- locating misrouting as seen in ocular albinism (excessive decussation at the chiasm) and achiasmia (no decussation)
Where is the VEP P100 paradoxically recorded from
the side of scalp ipsilateral to stimulated half-field
Where is the P100 produced
by dipole generations in the calcarine sulcus
What is the best electrode placemenet to detect P100
Electrode on scalp ipsilateral to stimulated half-field
What does half field stimulation activate
one hemisphere which is the sane for either eye in normals
What does full field stimulation in normals cause
cancellation in lateral electrodes, but not at the midline
Which hemisphere is activated in albinism
the contralateral hemisphere to the stimulated eye
What is activated in achiasmia
the ipsilateral side to the stimulated eye
What does crossed asymmetry in flash VEPs suggest
excessive decussation in ocular albinism
