• VEPs (visual evoked potentials) are visually
evoked electrophysiological signals extracted
from the electroencephalographic activity in the
visual cortex recorded from the overlying scalp.
• As visual cortex is activated primarily by the
central visual field, VEPs depend on functional
integrity of central vision at any level of the
visual pathway including the eye, retina, the
optic nerve, optic radiations and occipital cortex
• International Federation of Clinical
• International Society for Clinical
Electrophysiology of Vision (ISCEV
• A visual stimulus is presented to the subject for
a selected number of times, and the cerebral
responses are amplified, averaged by a
computer, and displayed on an oscilloscope
screen or printed out on paper
• The visual system processes information along
multiple parallel channels
• The magno cellular system is involved primarily
with motion analysis.
• The parvo cellular system is associated with
color selectivity and shows a preference for high
• The two major processing systems, directed
toward posterior parietal and inferior temporal
cortex, exist in all primates
• Steady-state VEP: waveform of a VEP
depends upon the temporal frequency of
the stimulus. At rapid rates of stimulation,
the waveform becomes approximately
• Transient VEP: At low temporal
frequencies, the waveform consists of a
number of discrete deflections and .
• All ISCEV standard VEPs are transient
• Sintered silver-silver chloride, standard silver-silver
chloride, or gold disc electrodes
• skin preparation to ensure good, stable electrical
• The electrode impedances should be below 5 k
measured between 10 and 100 Hz and, to reduce
electrical interference, they should not differ by more
than 20% between electrode sites.
• Electrode placement- International 10/20 system The
active electrode at Oz with the reference electrode at Fz
• Ground at the forehead, vertex (Cz), mastoid, earlobe
(A1 or A2) or linked earlobes.
• The ISCEV standard VEP protocols are defined
for a single recording channel with a midline
occipital active electrode. These protocols are
intended for assessment of prechiasmal
• If chiasmal or retrochiasmal disease is
suspected, a three channel montage, using the
midline and two lateral active electrodes, is
recommended in addition to the basic standard
• The two most commonly used patterns are checks and
• The pattern should be achromatic (black and white).
• The size of the individual checks should be expressed in
terms of visual angle.
• where B is the visual angle in minutes of are, W is the
width of the checks in millimeters, and D is the distance
of the pattern from the corneal surface in millimeters
• Although checks are customarily reported in minutes of
arc ('), gratings are usually reported in cycles per degree.
• Measurements in cycles per degree (abbreviated as
c/deg) define the spatial frequency of the stimulus.
• Measurements of the visual angle in minutes of arc can
be converted to cycles per degree by the formula
• where W is the width of the grating in minutes. In the
case of checks, however, W represents the diagonal
measure of the checks in minutes of arc.
• a high contrast black and white checkerboard
• two check element sizes should be used: 1° ±
20% and 0.25° ± 20% of arc per side.
• All checks should be square and equal number
of light and dark checks.
• Field size- ratio between width and height should
not exceed 4:3 and at least 15 deg in its
• Rectangular or circular with fixation point in
• subdued room lighting with no bright sources visible to
• The luminance of the white checks should be 100 ± 20
candelas per meter squared (cd·m-2).
• The luminance of the black checks should be low
enough to achieve a Michelson contrast of greater than
• mean luminance of the checkerboard will be between
40 and 67 cd·m-2.
• The luminance and contrast of the stimulus should be
uniform between the center and the periphery of the
• black and white checks change phase abruptly (i.e.,
black to white and white to black) and repeatedly at a
specified number of reversals per second.
• No overall change in the luminance of the screen,
• The large check (1°) and small check (0.25°) stimuli are
specified by the check width (visual angle), the stimulus
rate (in reversals per second) number of reversals, the
mean luminance, the pattern contrast and the field size.
• A reversal rate of 2 reversals per second (+/- 10%)
should be used to elicit the standard pattern reversal
Pattern onset/offset stimuli
• the checkerboard pattern is abruptly exchanged
with a diffuse gray background.
• The mean luminance of the diffuse background
and the checkerboard must be identical.
• Pattern onset duration should be 200 ms
separated by 400 ms of diffuse background.
• The ISCEV standard onset/offset response is
the onset response.
• At least two pattern element sizes should be
used: checks of 60 min and 15 min per side
• The flash VEP should be elicited by a brief flash
that subtends a visual field of at least 20 deg,
presented in a dimly illuminated room.
• The strength (time-integrated luminance) of the
flash stimulus should be 3 (2.7-3.3) photopic
candelas seconds per meter squared (cd·s·m-2).
• A hand held stroboscopic light or by positioning
an integrating bowl (ganzfeld) .
• The flash rate should be 1 per second (1.0 Hz
• Pattern-reversal is the preferred stimulus for
most clinical purposes. Pattern-reversal VEPs
are less variable in waveform and timing than
the VEPs elicited by other stimuli.
• The pattern onset/offset stimulus is best suited
for the detection of malingering and for use in
patients with nystagmus.
• Flash VEPs are useful when poor optics, poor
cooperation or poor vision makes the use of
pattern stimulation inappropriate
• Amplification- 20,000-50,000 times, input impedance of
100 Mohm and the common mode rejection ratio should
• Filters-Analogue high pass and low pass filters should
be set at <1Hz ( time constant 0.16 s or more) and at
>100 Hz .
• Averaging-minimum number of sweeps per average
should be 64.
At least two averages should be performed to verify the
reproducibility of each VEP
• Analysis Time: The minimum analysis time (sweep
duration) for all adult transient flash and pattern reversal
VEPs is 250 ms and 500 ms for pattern onset/offset
• VEPs to a pattern-reversing checkerboard (the most
commonly used stimulus in clinical laboratories) consist
of a set of sequential waveforms .
• The waveforms are alternately positive and negative and
are designated in accordance with their polarity and
• Positive waves are designated P, followed by a number
indicating the peak latency in milliseconds (e.g., P60 and
• negative waves are designated N, followed by a number
indicating the peak latency N70 and NI45
• ISCEV-recommend that VEP traces be
presented as positive upwards.
• P-100:pattern size, pattern contrast, mean
luminance, signal filtering, patient age, refractive
error, poor fixation and miosis.
• Pattern onset: typically consists of three main
peaks in adults; C1 (positive approximately 75
ms), C2 (negative approximately 125 ms) and
C3 (positive, approximately 150ms)
• Flash:N2 and P2 peaks,90 ms and120 ms.
• considerable variation in the morphology of normal
VEPs, but the dominant wave is the PIOO component.
• some subjects the initial negativity (N70) is absent,
whereas in other subjects N70 is as large as PIOO.
• 0.5 percent of normal cases, PlOO has a W-shaped
configuration (i.e., PIOO is subdivided into two peaks).
In these normal subjects, both peaks have latencies
within the boundaries of normality.
The best way to determine which peak corresponds to
PIOO is to obtain VEPs to patterns of three different
sizes. Usually the larger checks will yield only one PIOO
• Pupil constriction affects the amplitude and the
latency of N70 and PIOO in the same manner as
decreased luminance of the stimulus does.
• Check size also influences the latency and
amplitude of the responses. A decrease in check
size is usually associated with a prolongation of
N70 and PIOO latency.
• However, the relationship between pattern size
and PIOO latency is not linear
• certain studies suggest that as check size
increases above 30', the latency of P lOO also
• Age is another important variable that influences
• PIOO latency increases with age. The effect of
aging was more prominent when small checks
were used to elicit the responses
• The age-related increase in latency of VEP was
related to changes in the visual pathways or
• Ganglion cell loss, dysmyelination, axonal
swelling, and nerve fiber loss have been
described in the optic nerve
• Shorter-latency and larger-amplitude VEPs than in
males have been described in females.
• Uncorrected refractory errors may affect the amplitude
and latency of the VEP, especially for patterns of small
• Blurring of the pattern stimulus not only prolongs PlOO
latency but often drastically changes VEP morphology,
with elimination of N70 and broadening of the PlOO
• Two diopters of blur reduces Snellen visual acuity from
20/20 to 20/120
• Patients to be tested not infrequently have 1 or 2
diopters of uncorrected myopia
PVEP – NORMAL DATA
• P 100 LATENCY ( m sec ) = 102 ± 5
• R-L difference ( msec) = 1.3 ± 2.0
• Amplitude (μV) =10 ± 4.2
• Duration = 63 ± 8.7
FULL FIELD PVEP- CRITERIA
• PROLONGATION > 3 SD
• INTEROCULAR LATENCY OF P100>10 msec,
LONGER LATENCY ABNORMAL
• INTEROCULAR AMPLITUDE RATIO>2
• ABNOMALLY LOW OR HIGH AMPLITUDE
• ABSENCE OF IDENTIFIABLE VEP FROM MIDLINE
AND LATERAL OCCIPITAL SITES.
FULL FIELD PVEP-
• Monocular abnormal full field stimulation
suggest anterior visual pathway defect.
• Bilateral abnormal full field VEP is seen
with bilateral optic nerve, tracts or
PVEP IN DEMYELINATION
• Acute attack-decrease amplitude or
absent VEP, prolonged latency
• P100 prolongation persists for many years
even after clinical recovery
• Abnormal VEP is seen in 75-85% of
• In infants the sweep duration should be at least 500 ms
• By six months of age, the peak time of the main positive
peak of the pattern reversal VEP for large checks (1°) is
usually within 10% of adult values.
• improved if a recording trial can be paused or interrupted
when fixation wanders
• Binocular pattern stimulation, flash VEP is easy
• Monocular testing to at least one stimulus is desirable to
assess the function of each eye.
• the degree of cooperation and arousal of the child
Multi-channel VEP recording
• Intracranial visual pathway dysfunction
• asymmetrical distribution of the VEP over the posterior
• Chiasmal dysfunction gives a “crossed” asymmetry
whereby the lateral asymmetry obtained on stimulation
of one eye is reversed when the other eye is stimulated.
• Retrochiasmal dysfunction gives an “uncrossed”
asymmetry such that the VEPs obtained on stimulation
of each eye show a similar asymmetrical distribution
across the hemispheres
• field of 30 degrees ,minimum of three active electrodes,
two lateral electrodes placed at O1 and O2, and a third
midline active electrode at Oz. All three active electrodes
should be referenced to Fz
Hemi field stimulation
• Chiasmatic or retro chiasmatic lesion
• Marked inter hemispheric asymmetry on
• Additional electrodes and channels are
used -LT,LO,MO,RO,AND RT.
• Paradoxical localization.
Multimodal VEP• The visual system analyzes spatial, temporal, and chromatic
aspects of objects via multiple, parallel channels.
• Isoluminant chromatic sinusoidal gratings (red green) at 2 cycled
deg (cpd) appeared for 200 msec and were replaced by a yellowish
background for 800 msec in recording chromatic pattern
• High contrast (90%) achromatic sinusoidal gratings (black white) at
5.3 cpd also appeared for 200 msec and were replaced by a
grayish-white background for 800 msec for recording achromatic
• Motion VEPs. Two squares (60 minutes of arc) at opposite comers
of a hypothetical square were presented together for a duration of
500 msec and then switched off, followed by two squares appearing
simultaneously on the remaining two corners.
• Steady-state VEP- high-contrast (90%) achromatic gratings at 2 cpd
were reversed at a rate of 4 Hz (8 reversal/sec) for recording