3. Introduction
• When light falls on the retina, a series of nerve impulses in the
form of electrical changes are generated and passed onto the
visual cortex via the visual pathway.
• The electric potential changes produced in the visual cortex by
these impluses can be recorded by electroencephalography
(EEG) technique; and is also known as visual evoked potential
(VEP).
• Thus VEP is the EEG records taken from the occipital lope (visual
cortex). In other words VEP is an averaged and amplified record
of action potential in the visual cortex.
4. • The terms visually evoked potential (VEP), visually evoked
response (VER) and visually evoked cortical potential (VECP)
are equivalent.
• VEPs are used primarily to measure the functional integrity of
the visual pathways from retina via the optic nerves to the
visual cortex of the brain.
• VEPs better quantify functional integrity of the optic pathways
than scanning techniques such as magnetic resonance
imaging (MRI).
• VEPs are most useful for testing optic nerve function and less
useful for assessing postchiasmatic disorders. In patients with
retrochiasmatic lesions, MRI is a more useful test.
5. History
• VEPs initiated by strobe flash were noticed in the early years
of clinical encephalography (EEG) in the 1930s.
• Evoked potentials, whether auditory, visual or somatosensory,
are extracted from the EEG by a simple program. This
technique of extracting a signal from random noise is one of
the oldest applications of computer technology. This process
is similar to programs used to extract radar signals from
jamming nearly 70 years ago.
6. • Adding the electrical activity for set time periods is called “signal
averaging”.
• Dawson first demonstrated a signal-averaging device in 1951 and signal-
averaging computers have been available since the early 1960s.
• The computer programs save a defined time period of EEG activity
following a visual stimulus, which is repeated over and over adding the
signals together.
• The random EEG activity averages away, leaving the visually evoked
potential.
• Depending on the signal to noise ratio, an evoked potential can be seen
forming following only a few stimuli such as flashes of light.
7. Fig: EEG tracing showing the time periods following flash
presentations (blue and yellow areas) that are typically
the waveforms of VEP.
8. Neurophysiology
• The visual evoked potential (VEP) is primarily a relatively
large, positive polarity wave generated in the occipital cortex
in response to visual stimulation.
• It measures the conduction time of neuronal activity from the
retina to the occipital cortex and is used clinically as a
measure of the integrity and function of that pathway. The
optic nerve is the primary structure examined.
9. • The VEP is of large enough voltage that it can be seen
occasionally on a routine EEG as an occipital waveform within
the first 150 ms after a single photic stimulus.
• The standard VEP averages many such waveforms,
time-locked to the stimulus of primary interest is the latency
of the positive wave at a midline occipital EEG electrode,
usually at approx 100 ms after stimulation, called the P100.
This P100 peak is usually easy to recognize and measure.
10. • Any abnormality that affects the visual pathways or
visual cortex in the brain can affect the VEP.
• Examples are cortical blindness due to meningitis or
anoxia, optic neuritis as a consequence of demyelination,
optic atrophy, stroke, and compression of the optic
pathways by tumors, amblyopia, and neurofibromatosis.
• In general, myelin plaques common in multiple sclerosis
slow the speed of VEP wave peaks.
• Compression of the optic pathways such as from
hydrocephalus or a tumor also reduces amplitude of
wave peaks.
11. Electrode locations on the scalp
• Visually evoked potentials elicited by flash stimuli can be
recorded from many scalp locations in humans.
• Visual stimuli stimulate both primary visual cortices and
secondary areas.
• Clinical VEPs are usually recorded from occipital scalp
overlying the calcarine fissure. This is the closest location
to primary visual cortex (Brodmann’s area 17).
12. • A common system for
placing electrodes is the
“10-20 International
System” which is based on
measurements of head size.
(Jasper, 1958)
• The mid-occipital electrode
location (OZ) is on the
midline. The distance above
the inion calculated as 10 %
of the distance between the
inion and nasion, which is 3-
4 cm in most adults.
13. • Another set of locations is the
“Queen Square system” in which
the mid-occipital electrode is
placed 5 cm above the inion on
the midline and 5 cm lateral from
that location for lateral occipital
electrodes (Blumhardt et al,
1977).
• The Queen Square locations,
further off the midline, are better
able to lateralize anomalies such
as when using hemi-field
stimulation.
14. Electrodes
• Skin electrodes such as sintered silver–silver chloride,
standard silver–silver chloride or gold cup electrodes are
recommended for recording VEPs.
• The skin should be prepared by cleaning, and a suitable paste
or gel used to ensure good, stable electrical connection.
• The electrode–skin contact impedances should be below 5
k.om as measured between 20 and 40 Hz. To reduce electrical
interference, electrode–skin contact impedances should differ
by no more than 1 k.om between electrodes.
15. Fig: Electrode locations. A. Location of active and reference electrodes
for standard responses. The active electrode is located along the midline
at Oz. The reference electrode is located at location Fz. The subscript z
indicates a midline position.
B. Locations of the lateral active electrodes, O1, O2, PO7 and PO8 are
indicated along with the midline active electrode location, OZ
16. • The scalp electrodes should be placed relative to bony
landmarks, in proportion to the size of the head, according to
the International 10/20 system.
• The anterior/posterior midline measurements are based on
the distance between the nasion and the inion over the
vertex.
• The active electrode is placed on the occipital scalp over the
visual cortex at Oz with the reference electrode at Fz.
• A separate electrode should be attached and connected to the
ground. Commonly used ground electrode positions include
the forehead, vertex (Cz), mastoid, earlobe (A1 or A2) or
linked earlobes.
17. VEP stimulation
• There are two main classes of standard VEP stimulation,
pattern and flash.
• All stimulus parameters should be calibrated either locally or
by the manufacturer, and regular recalibration is advised.
18. Pattern stimulation
• All standard pattern stimuli are high-contrast, black and white
checkerboards consisting of squares with equal sides whose corners
meet.
• The stimuli may be generated on a screen, with the viewing
distance typically between 50 and 150 cm, adjusted to obtain the
required check sizes and a suitable field size for any physical size of
display screen.
• Optical systems may be used to produce checkerboard dimensions
that are equivalent to those described for a freely viewed display
screen. Stimulus changes, whether pattern reversal or onset/offset,
must be achieved without a change in the average luminance of the
stimulus.
• Both transient and step changes in luminance (luminance artifacts)
can evoke VEPs associated with the luminance artifact in those who
cannot resolve the pattern stimulation.
19. Field and check size
• Patterned stimuli are defined by the visual angle subtended by the side of
a single check in degrees (°) or minutes of arc (min) subtended at the eye
(1° = 60 min).
• For standard pattern VEPs, two check element sizes should be used: 1°
(with an acceptable range of 0.8° to 1.2°) and 0.25° (0.2° to 0.3°) of arc per
side.
• All checks should be square, and there should be an equal number of light
and dark checks.
• It is not necessary to use a square field, but the aspect ratio between
width and height should not exceed 4:3 and the field size should be at
least 15° in its narrowest dimension.
• A fixation point, when used, should be positioned at a corner of the four
checks located at the center of the field.
20. Luminance and contrast
• A mean photopic luminance of 50 cd.m-2 (with an acceptable
range of 40–60 cd.m-2) is required.
• Contrast between black and white squares should be high (as
defined by Michelson CONTRAST ≥ 80 %).
• The mean luminance of the stimulus screen must be constant
during checkerboard reversals (i.e., no transient luminance
change). This is easily achieved with classical CRT (cathode ray
tube) stimulators. Typical current liquid crystal display (LCD)
screens present a brief luminance artifact during pattern
reversal, rendering them unsuitable for VEP recording unless
special precautions are taken.
• The luminance and contrast of the stimulus should be uniform
between the center and the periphery of the field. However,
variation from center to periphery of up to 30 % is acceptable.
21. Background illumination
• The luminance of the background beyond the stimulus field is
not critical when using standard VEP techniques, provided
dim or ordinary room lighting is used. Ambient lighting should
be the same for all recordings. Care should be taken to keep
bright lights out of the subjects’ direct view.
25. Flash VEP
Flash stimulus
• The Standard flash VEP is elicited by a brief flash (≤ 5 ms) that
subtends a visual field of at least 20°, presented in a dimly
illuminated room.
• The strength (time-integrated luminance) of the flash stimulus
should be 3 photopic candelas seconds per meter square (cd.s.m-2).
The acceptable range for the standard flash strength is 2.7 to 3.4
cd.s.m-2, which matches the ISCEV standard flash for full-field ERG
testing.
• For VEPs, the standard flash may be presented on a flashing screen,
by a handheld stroboscopic light or by positioning an integrating
bowl (ganzfeld), such as that used for ERG tests, in front of the
patient. The flash rate should be 1 per second (1.0 Hz, range 0.9 to
1.1 Hz).
26. Flash VEP contd…
• Cruder response than pattern VEP.
• Merely indicates that light has been perceived by cortex.
Indications –
media haze,
infants,
poor patient cooperation.
27. Pattern onset/offset VEP
Pattern onset/offset stimuli
• For pattern onset/offset, the checkerboard pattern is
abruptly exchanged with a diffuse gray background.
• The mean luminance of the diffuse background and the
checkerboard must be identical with no change of
luminance during the transition from pattern to diffuse
blank screen.
• This is difficult to achieve with cathode ray tube (CRT)
displays, and it is not possible with unmodified liquid
crystal displays (LCDs).
28. Pattern onset/offset VEP contd…
• Pattern onset duration should be 200 ms separated by
400 ms of diffuse background.
• This temporal pattern ensures that the VEP waveform to
pattern onset is not contaminated by the pattern offset
response.
• More intersubject variability than pattern reversal VEP.
• Useful in detection of patients with malingering , patients
with nystagmus.
29. Pattern reversal VEP
Pattern-reversal stimuli
• For the pattern-reversal protocol, the black and white checks change
phase (reverse) abruptly (i.e., black to white and white to black) with no
overall change in the luminance of the screen.
• To meet this requirement, there must be equal numbers of light and dark
checks in the display. Displays used for standard VEP testing must be
synchronized with the averager and designed to avoid transient luminance
artifacts.
• Standard pattern- reversal VEPs should be obtained using a reversal rate
of 2.0 ± 0.2 reversals per second (rps) (this corresponds to 1.0 ± 0.1 Hz, as
a full cycle includes two reversals). Reversal rate must be reported in rps,
not in Hz.
30. • For a specific standard pattern-reversal VEP test, users
should specify check width (for both large and small
checks), stimulus rate (in reversals per second), number
of reversals averaged, mean luminance, Michelson
contrast and field size.
• For check pattern visual angle subtended by a single
check is used.
• Preferred technique for most clinical purposes, gives an
estimate of form sense and thus visual acuity.
31.
32. VEP stimuli
• In the past two decades, new forms of
visual stimuli have been applied to clinical
testing.
• Most notable of these are:
Motion VEP,
Sweep VEP,
Binocular beat response and
Multifocal VEP.
33. Motion VEP
• The motion VEP is generated by the onset, reversal, or drift of a
grating or dot pattern.
• The response generated by the motion stimulus has different
characteristics from other pattern VEPs and appears to be
generated primarily by the magnocellular visual pathway.
• Asymmetries in the motion VEP nasally and temporally
have been found in strabismic patients and correlate with
perceptual asymmetries.
• Thus, the motion VEP may prove useful in the early detection of
strabismic amblyopia and in the monitoring of its treatment.
34. (A) Illustration of the mVEP
stimulus. The grating is shifted 90°
left and right every 42 ms.
(B) Example showing a rightward
moving RDP on the left side. Direction
of motion (leftward or rightward) and
location (right or left side) were
randomized across trials. This pattern
contains N motion when viewed with
the left eye or T motion when viewed
with the right eye.
35. Sweep VEP
• A relatively new technique for measuring the visual acuity and
contrast sensitivity electrophysiologically is the sweep visual
evoked potential (sVEP).
• Studies have demonstrated that the sVEP is a potentially
important tool for assessing visual acuity and contrast
sensitivity in non-verbal individuals with disorders affecting
their visual system.
• Visual stimuli were displayed on a high-resolution video
display at a frame rate of 100 Hz.
• Sinusoidal grating was generated using a personal computer–
based pattern generator.
36. • Horizontal grating bars were shown to patients during testing.
• A sweep consisted of a 10.24-second period, during which the
spatial frequency of the temporally modulated grating
increased linearly.
• The range of spatial frequency was determined by clinical
experience and guided by published normative data.
• Patients were tested with the appropriate refraction at a
viewing distance of 0.5 to 2.0 m to ensure that sufficiently
high spatial frequencies could be used.
• Testing was first performed under binocular viewing,
Monocular testing then followed.
37.
38. Binocular beat response
• The binocular beat response can be used to measure
binocularity of the visual cortex in amblyopic patients.
• The beat response is a nonlinear difference frequency (for
example, 2 Hz), which is seen when different
frequencies of stimulation (for example,16 and 18 Hz) are
presented to each eye dichoptically.
• Patients who lack binocular cells in the visual cortex because
of untreated childhood amblyopia do not generate a beat
frequency in their VEPs.
39. Multifocal VEP
• The multifocal VEP is a technique developed to obtain
independent responses from multiple areas of the visual field
simultaneously.
• A pseudo-random sequence of stimuli, called a maximum-
Iength sequence (or m-sequence), is used to derive linear and
nonlinear components of the VEP.
• This technique can potentially be used to obtain objective
measurement of dysfunction in localized areas of the visual
field.
40.
41. Clinical protocol
Preparation of the patient:
• Pattern stimuli for VEPs should be presented when the pupil of
the eyes are unaltered by mydriatic or miotic drugs.
• Pupils need not be dilated for the flash VEP. Extreme pupil sizes
(miosis or dilation) and any anisocoria should be noted for all
tests.
• For pattern stimulation, the visual acuity of the patient should be
recorded and the patient must be optimally refracted for the
viewing distance of the stimulus.
• With standard electrodes and any additional electrode channels
attached, the patient should view the center of the pattern field
from the calibrated viewing distance.
42. • Monocular stimulation is standard, this may not be practical in
infants or other special populations; in such cases binocular
stimulation may be used to assess visual pathway function
from both eyes.
• When a flash stimulus is used with monocular stimulation,
care should be taken to ensure that no light enters the
unstimulated eye. There should be no distracting sound or
light waves.
• Usually this requires a light-tight opaque patch to be placed
over the unstimulated eye. Care must be taken to have the
patient in a comfortable, well-supported position to minimize
muscle and other artifacts.
43. Equipment required
• Visual stimulus producing
device.
• Scalp electrode.
• Amplifier.
• Computer and read out
systems.
44.
45. Factors affecting VEP
1. Stimulus: the character of the VEP depends upon the type of
stimulus used. In patterned stimulus, the transit response
increases in amplitude with the decrease in the size of check,
reaching a peak when the check subtends about 15° arc at the
eye.
2. Check Size: An individual checkerboard square usually has a visual
angle of 30′.
• Smaller checks are more sensitive in detecting visual system
defects, but visual acuity can be a problem.
• Peripheral vision is stimulated better by larger checks. Large checks
produce more variable responses, perhaps because the subject
focuses on areas of different luminance rather than on the edges.
• Checks larger than 50′ may help to compensate for poor visual
acuity, but sensitivity to contrast in central vision is better with
smaller checks.
46. 3. Contrast:
• Contrast is the difference in luminance (or brightness) of the dark
and light areas divided by the sum of their luminance.
• Low contrast increases the latency and decreases the P100
amplitude. The patient’s pupils should not be dilated
pharmacologically because this will alter luminance at the retina.
4. Repetition Frequency :
• The pattern reversal rate is usually approx two per second. Some
components of the VEP last hundreds of milliseconds after the
stimulus. Thus, repetition faster than four per second can produce
overlap and distort the waveform; it also may increase the P100
latency.
• Slower repetition rates prolong testing and might produce a varied
response because of diminished attention.
47. 5. Averaging:
• The time-locked voltage signals are averaged over 100 to 200 trials,
usually with a duration of 500 ms each. A signal sampling rate of
1000 samples in 500 ms (2000 samples per second) is high enough
to avoid distortion of the waveform. VEPs are usually amplified by a
factor of 50,000.
• Averaging the waveforms eliminates the variation unrelated to the
stimulus, , distinguishing the VEP from the EEG background.
6. Filters:
• The low-frequency filter is usually set at 1 Hz and the high-
frequency filter at 100 to 300 Hz (the shape of the standard P100
has a frequency of approx 15–20 Hz). A lower high-frequency filter
may cause an apparent increase in P100 latency.
48. 7. Patient Factors :
• The patient should be alert and comfortable. No noise should
accompany the stimulus; this could cause artifact.
• It is important to be sure that the stimulus can be seen clearly.
Visual acuity must be tested. Usual eyeglasses are used to optimize
visual acuity.
• There should be no pharmacological pupillary dilation. One eye is
tested at a time. The technologist should ascertain that the patient
is actually focusing on the center of the target throughout the test.
• This is particularly difficult for children and infants (note that
fixation is not required with flash stimuli; they can be used even
with comatose patients).
49. References
• Electrophysiology testing in disorders of the Retina, Optic
Nerve, and Visual Pathway;2nd edition, Gerald allen fishmann,
David G. Brich, Graham D. Holder,mitchell G. Brigell.
• Visual Evoked Potentials Frank W. Drislane.
• ISCEV standard for clinical visual evoked potentials: (2016
update). J. Vernon Odom . Michael Bach . Mitchell Brigell .
Graham E. Holder . Daphne L. McCulloch . Atsushi Mizota .
Alma Patrizia Tormene . International Society for Clinical
Electrophysiology of Vision
