2. INTRODUCTION
• Various electrical potentials are recorded from a human or
animal following presentation of a stimulus.
- A reflex activity from stimulus to response e.g. SD curve
• Various techniques detects spontaneous potentials, where a
pre-determined stimulus is not needed.
- EEG: electroencephalogram (brain potentials)
- ECG: electrocardiogram (heart potentials)
- EMG: electromyography (muscle potentials)
3. INTRODUCTION
• Amplitudes of various potentials:
EPs : <1-few microvolts
EEG : *10 of microvolts
EMG : millivolts
ECG: volts
4. DEFINITION
• Evoked potentials are electrical activities that
occur in the neural pathways and structures as
a response to various external stimulations
induced by light, sound or electric.
• Evoked potentials are polyphasic waves that
oftenly present with an amplitude between 0.1-
20 μA which are formed within 2-500 ms.
5. DEFINITION
• The source of these activities is probably the
summation of the action potentials generated by the
afferent tracts and the electrical fields or activities
of the synaptic discharges or post-synaptic
potentials on those tracts.
• Understanding evoked potentials bears importance in
terms of controlling the entire pathway from
stimulation point to the cortical areas, in other
words, to the primary cortex.
6. CLINICAL UTILITY
• Demonstrate abnormal sensory system conduction, when the
history and/or neurological examination is equivocal.
• Reveal subclinical involvement of a sensory system (silent
lesions), particularly when demyelination is suggested by
symptoms and/or signs in another area of CNS.
• Help define the anatomic distribution and insight into
pathophysiology of a disease process.
• Motor changes in a patients’ neurological status.
7. CLINICAL INDICATIONS
• Persistence of symptoms despite decided and ongoing
treatment
• Presence of subjective complaints without supportive
objective findings
• Negative X-ray, CT, MRI, or EMG results despite existing
complaints
• Presence of radicular complaints
• Existing nerve irritation or damage requiring definition
• Need for advanced diagnosis/treatment
8. ANSWERS TO THE QUESTIONS
• Does the response against the stimulus reach intended
destinations on time?
• Does the response show any loss of intensity?
• If there is a problem in the neural pathways, what is its
exact location?
9. ADVANTAGES
• Although modern imaging methods other than PET can
depict pathological localizations in detail, they can not
provide data on functional/physiological structures .Thus,
evoked potential tests compensate for this shortcoming.
• Non-invasive character
• Objective measurement
• Subcortical components of auditory and somatosensory
evoked potentials are not influenced by general anesthesia,
sleep, and states of consciousness.
• Significantly low cost compared with the modern imaging
modalities.
10. DISADVANTAGES
• There is still no standard in technical regard. Even the
fundamental terminological standards have not been
completed yet, let alone being a “gold standard”. However,
currently; SSEP, BAEP, and VEP techniques have been
almost standardized.
• Tests are long and tedious for patients
• The risk of technical error, therefore the likelihood of
repeating the test, is high
• While the characteristics of the acquired recordings are
mostly known, their sources and mechanisms have not yet
been completely or clearly understood (due to the
complexity of the brain anatomy and nonlinear nature of the
brain physiology).
13. ELECTRODES
• Ag/AgCl or gold-plated surface electrodes with a
hole are used.
• Needle electrodes should not be employed because
they produce many artifacts and have high
electrode-skin impedance (5000–7000 Ω).
• The electrode-skin impedance in surface electrodes
is lower than 5000 Ω, within a range of 2000–3000
Ω.
14. ELECTRODES
• Prior to the placement of the electrodes, the related skin
surface should be cleaned with alcohol or acetone in order
to adequately reduce the electrode-skin impedance. In case
of need, abrasive gels can be applied for this purpose.
• During recording, bipolar electrodes are.
• 35mm distance between electrodes is preferred.
17. SIGNAL AVERAGING
• Resolves the potentials that are extremely low and
extremely high.
• The EPs can be quantified by measuring peak amplitudes and
latencies, in milliseconds and they provide numerical data
that are quantitative extensions of the neurological
examinations.
20. FILTER CIRCUIT
• It comprises serial connection of a low-cut filter (high-pass
filter) and a high-cut filter (low pass filter) including passive
and active components, or a band-pass filter bearing the
characteristics of those two filters.
LOW CUT HIGH CUT
VEP 0.2–1 Hz 200-300 Hz
SSEP 30 Hz 3000 Hz
23. GENERAL PRINICPLES - BEFORE TEST
• The patient should wash his/her hair on the evening before
the test or a couple of hours in advance.
• Substances such as perfume, lotion, or cream should not be
applied prior to the test.
• Any accessories such as an auditory device, glasses, or
lenses should be brought to the clinic.
• The patient should dress in a comfortable way.
• Drugs or foods containing caffeine should not be taken.
• The people that will undergo visual evoked potential test,
should not use sedatives.
24. GENERAL PRINICPLES - AFTER TEST
• In somatosensory tests, patient is told that the test could
take a while and that he/she would take a nap during the
test.
• In somatosensory evoked potential test, the patient should
be informed that a tingling can be felt in the stimulation
points and flexion may be observed in the thumbs and toes.
• The patient should be informed that there is no limitation of
drug, food, or activity after the test.
25. GENERAL PRINICPLES - AFTER TEST
• Skin surfaces that will receive the electrodes should be
rubbed with alcohol and acetone, and smoothened if
required to.
• In presence of muscular noise during the auditory evoked
potential test and if the patient can not sleep, then sedation
with chloride hydrate or diazepam should be performed.
• Prior to the visual evoked potential test, the visual acuity
and pupil width should be controlled in both eyes of the
patient.
27. INTRODUCTION
• Visual system evaluation: visual acuity, perimetry and color
vision testing are important (can only be obtained in co-
operative patients).
• Uncooperative patients : difficulty in localization of deficit
within the visual pathways.
• So electrophysiological testing may be necessary.
28. DEFINITION
• VEPs are electrical potentials differences recorded from
scalp in response to visual stimuli.
• It assesses the integrity of the visual pathway from the
optic nerve to the occipital cortex.
• Normal cortical responses are obtained if the entire visual
system is intact and disturbances anywhere in the visual
system can produce abnormal VEPs; therefore, the localizing
value of VEP is limited : LIMITATIONS OF VEP
29. ANATOMICAL BASIS OF VEP
• Optic nerve joins retina with the brain
• Receptors: rods and cones
• Axons of ganglion cells form optic nerve
• Macula fibers : central vision
• Nasal(crossed fibers) : temporal(uncrossed fibers) : 2:1
• Primary visual area 17
• Visual association area 18 & 19
30.
31.
32. P100
• Mainly 3 potentials N75, N145 & P100. *
• Mishra : N70, N135 & P100.
• P100 waveform of VEP is generated in the striate and
peristraite occipital cortex when stimulus is given.
• The regional blood flow increases with stimulation rate up to
8Hz and gradually declines thereafter.
• VEP is primarily a reflection of activity originating in the
central 3-6 degrees of visual field, which is relayed to the
surface of occipital lobe.
• P100 (peak time) in milliseconds when EP travels from retina
to visual cortex.
33.
34. SOURCES OF STIMULUS
• Stroboscopic flash light with regular flashing intervals:
Applied on babies and uncooperative patients.
• Flashing LED: Applied as an intraoperative stimulus source.
However, it requires use of specially designed lenses.
• Alternating checkerboard pattern stimulation: This is the
stimulation method used most commonly and it is a more
sensitive and stable technique. VEP test performed by
applying this stimulation is also called as PSVEP (Pattern-
Shift Visual Evoked Potential).
35.
36. METHODS OF VISUAL EVOKED POTENTIAL
1. Pretest evaluation
2. Running the test
3. Partial field stimulation
4. Normal PSVEP
37. PRETEST EVALUATION
• Full patient cooperation.
• Avoid hairspray or oil after the last hair wash.
• Usual glasses if any should be put on during the test.
• Before the test: visual acuity, pupillary diameter and field
charts testing.
• Avoid any miotic or mydriatic drugs before 12h.
• In patients with field defects, besides the midline
electrodes, lateral placement of electrodes may be
necessary because the field defects alter the potential
field distribution on P100.
38. RUNNING THE TEST
• Standard disc EEG electrodes are used.
• Skin is prepared by abrading and degreasing.
• Conducting jelly or electrode paste is used to stick the
electrodes.
• The electrode impedance should be kept below 5KΩ.
• Amplification ranging between 20K-1L is used to record
PSVEP.
• Low cut filters at 1-3Hz and high cut at 100-300Hz.
• Sweep duration: 250 and 500ms.
43. TRANSIENT VEP
• Low rates of stimulation with discrete deflections. (<3.5Hz)
• Steady
State:
>3.5Hz
44. PATIENT & SEATING PRE-REQUISITES
• Testing environment should be dark, or at least dim.
• The patient is instructed to fix his gaze at the center of
the screen.
• The patient sits in front of a 70-100 cm distant B/W
monitor with a checkerboard pattern showing constant
luminance and he/she continuously and carefully looks at this
pattern.
• The checks alternate from black to white and from white to
black at every 1 or 2 seconds. Each alternation acts as a
stimulation and generates an evoked potential at the
occipital lobe.
45. PATIENT & SEATING PRE-REQUISITES
• Each eye tested separately.
• Patient seated at a distance of 0.75 to 1.5 meters.
• Eye glasses to be worn.
• The eye not tested should be patched.
• Gaze at the center of the monitor.
46.
47. VARIABLES INFLUENCING VEP
• AGE:
• It influences the latency of P100 at a rate of 2.5ms/decade
after the 5th decade.
• GENDER:
• P100 latency is longer in adult males compared to females.
Attributed to larger head size and lower core body
temperature in males.
• Mean P100 amplitude is greater in female, unknown reasons,
might be because of hormonal differences.
48. VARIABLES INFLUENCING VEP
• EYE DOMINANCE:
• P100: shorter latency and amplitude is greater.
• EYE MOVEMENT:
• Reduces amplitude of P100, but its latency is not affected.
• The patients with nystagmus having a normal visual pathway also
have normal P100 latency.
• VISUAL ACUITY:
• Latency not affected.
• 20/200 (snellen’s chart)- normal latency and amplitude.
• > - amplitude decreases.
49. VARIABLES INFLUENCING VEP
• DRUGS:
• Pupillary constriction drugs (pilocaprine) increases P100
latency.
• Dilatation drugs reduced latency.
• REPRODUCIBILITY AND VARIABILITY:
• Problem solving: latency decreases and amplitude increases.
• Affected by closing the eye, gazing off the screen,
converging in front of target or even his/her nose.
• Intraocular variability 1 week= 0-8ms and 6 months= 11ms.
• Intereye 1 week= 0-6ms and 6 months= 9ms.
50. BASIS OF VEP ABNORMALITIES
1. Latency prolongation: demyelination in the optic
pathways, A=normal
2. Amplitude reduction: Interindividual variability.
Interocular amplitude ratio is used for defining
abnormalities. Ischemic optic neuropathy- normal latency
and decreased A.
3. Combined latency and amplitude abnormalities: Optic
nerve compression.
4. Shape abnormalities: W-shaped complex (bifid), two
peaks are separated by 10-50ms.
51. CLINICAL IMPLICATIONS
• Demyelinating diseases:
• Multiple Sclerosis:
• P100 latency is prolonged with or without attenuation of
amplitude.
• Cross examine with clinical symptoms and MRI.
• Serial VEP studies have been carried out to monitor the
progress of disease and therapeutic response to treatment.
• Neuromyelitis optica:
• Transverse myelitis +optic neuritis
• Unrecordable P100.
52. CLINICAL IMPLICATIONS
• Optic neuritis:
• Monocular loss of vision in patient between 20-50 years,
may or may not associated with pain or movement of eyeball.
• VEPs are often deformed and incompressible.
• Ischemic optic neuropathy:
• Attenuation of amplitude before its latency is affected.
• HIV infection:
• Nutritional and toxic optic neuropathy:
• Prolonged latency.
53. CLINICAL IMPLICATIONS
• VEP in cortical blindness:
• Not useful.
• Malingering and hysteria:
• Hysterical blindness: temporary functional loss of vision.
• Normal VEP.
• Malingering: voluntary suppression can occur of VEP so
pattern VEP should be used.
54. CLINICAL IMPLICATIONS
• Intraoperative monitoring:
• Different intraoperative neurophysiological monitoring
techniques assess the function of the brain, brainstem,
spinal cord, cranial nerves, and peripheral nerves during the
procedure.
• They are immensely valuable in the detection and prevention
of neurological insult.
• Intraoperative monitoring is now becoming part of standard
medical practices and routinely used during many surgical
procedures, including the risk of neurological injury.
55. CLINICAL IMPLICATIONS
• Intraoperative monitoring:
• Limited value because of technical problems such as
difficulty in giving controlled stimuli and providing controlled
illumination.
• VEP has been used to monitor surgery of pituitary and
cavernous sinus tumor and aneurysm surgery.
• In IOM, there is significant variability; therefore for
defining abnormality experience and standardization are
needed.
• If VEP shows considerable variation from the baseline
values, the surgeon should be informed.
57. EVIDENCES
• Relatively major postoperative visual impairment can be
detected by intraoperative decreases in the flash VEP
amplitude.
• In the future, flash VEPs may be used in clinical settings as
part of routine monitoring in various procedures that could
cause postoperative visual impairment, such as surgeries in
the prone position or with the head tilted downward, as well
as in neurosurgical procedures.
(Hayashi, H., & Kawaguchi, M. (2017). Intraoperative monitoring of flash visual evoked potential under general
anesthesia. Korean journal of anesthesiology, 70(2), 127–135.)
58. EVIDENCES
• In this series of intraaxial brain procedures, reliable
intraoperative VEP monitoring was achieved.
• The standardization of this technique appears to be a
valuable effort in regard to the functional risks of
homonymous hemianopia.
(Gutzwiller EM, Cabrilo I, Radovanovic I, Schaller K, Boëx C. Intraoperative monitoring with visual evoked
potentials for brain surgeries. J Neurosurg. 2018 Mar 30;130(2):654-660.)
59. EVIDENCES
• Intraoperative VEPs were sensitive enough to detect vascular damage
during aneurysm clipping and mechanical manipulation of the anterior
visual pathway in an early reversible stage.
• Intraoperative VEP monitoring influenced surgical decisions in
selected patients and proved to be a useful supplement to the toolbox
of intraoperative neurophysiological monitoring.
(Luo Y, Regli L, Bozinov O, Sarnthein J. Clinical utility and limitations of intraoperative monitoring
of visual evoked potentials. PLoS One. 2015 Mar 24;10(3):e0120525.)
62. INTRODUCTION
• SEPs are particularly useful for evaluation of function of
the spinal cord.
• Lesions of the cord may be invisible to routine imaging,
including MRI and myelography yet may have devastating
effects on cord function.
• Transverse myelitis, MS and cord infection are only three of
the potential causes that can be missed on structural
studies.
• Brief electric pulses are delivered to the peripheral nerve
stimulation selectively and so a small muscle twitch is seen.
63. INTRODUCTION
• 3 types of SEPs:
1. Short latency (<50ms)
2. Middle latency (50-100ms) little clinical utility
3. Long latency (100-300ms)
64. INTRODUCTION
• Short latency SEPs are the electrical potentials
generated mainly by the large diameter peripheral
and central sensory pathways in response to sensory
stimulus.
• The major advantage of SEP lies in evaluating the
relatively long sensory pathway from peripheral
nerve to spinal cord and cerebral cortex.
65. DEFINITION
• SEP is the response to electrical stimulation of peripheral
nerves.
• They are used for evaluating the synaptic terminals
extending towards cortex, by stimulating the peripheral
sensory pathways via delivery of an electric current.
• In short, it is aimed to acquire a response that can be
recorded electrically in the central nervous system against a
stimulation applied on vibration, position, or tactile senses.
• Stimulation of almost any nerve is possible, by commonly
used is; median nerve (upper limb) and posterior tibial
nerve (lower limb).
66. ANATOMICAL AND PHYSIOLOGICAL BASIS OF SEP
• SEP studies evaluate the proprioceptive pathways.
• Peripheral sense organs dealing with proprioceptive
information are situated in the muscles, tendons and joints.
• The important areas are the neuromuscular and
neurotendious spindles with pacinian and possible the golgi
mazzoni corpuscles, which respond to pressure, tension,
stretching of muscle fibers and related stimuli.
• The impulses travel along type A fibers, which are
myelinated.
67.
68. METHODS
• PRETEST INSTRUCTIONS:
• Supine and proper head support to relax the neck muscles.
• Important problem in SEP recording is excessive muscle
artifacts.
• Besides a comfortable position, uncomfortable temperature,
urge to pass urine or difficulty in lying supine may interfere
with the patients’ relaxation.
69. METHODS
• Mild hypnotics may be used for obtaining an optimal
recording.
• Sleep-deprived study may also be used in difficult cases by
asking the patient to sleep 2h late and avoid any stimulant in
the breakfast.
• The room should be quiet and comfortable.
• For ensuring full relaxation and cooperation, the test
procedure should be explained to the patient.
70. MEDIAN SEP
• Disc surface electrodes (1cm) : filled with conducting jelly
or electrode paste.
• Impedance <5kHz.
• Recording electrodes:
Erb’s point (2-3 cm above midclavicular point)
Spinous process of 5th cervical vertebra and 2 cm posterior
to C3 or C4 .
Left and right erb’s point as EP1 and EP2, EPc (contralateral
to stimulation) and Epi (ipsilateral to stimulation).
71. MEDIAN SEP
• Spinal electrode is
designated as C5S or C5Sp.
• Scalp electrode as C3’ and
C4’ or Cc (contralateral to
stimulus side) and Ci.
• Fz : reference electrode.
• Channel 1: Cc-Fz(cephalic
ref.)
• Channel 2: Cc-
Epc(noncephalic)
• Channel 3:C5Sp-EPc
• Channel 4:Epi-EPc
72. MEDIAN SEP- IMP POINTS
• 2 important potentials are N13 and P14.
• Instead of Fz , ear lobe can also be preferred as reference
electrode point.
• Even scalp recording or cephalic reference can be changes
as Cc-Ci.
• For identification of N13 potential, an anterior or
supraglottal reference may be useful.
• Difference montage:
Channel 1: Cc-Fz
Channel 2:Cc-EPc
Channel 3:Ci-EPc
Channel 4:Fc-EPc
73. PARAMETERS
• SEPs are best recorded at amplification of potentials
between 10K and 5L.
• Filter setting at 20-30Hz for low filters and at 3000 Hz for
high.
• Analysis time should be 50-60ms, which maybe extended to
60-100ms.
• The number of channels is determined by the potential field
distribution of different waveforms of interest and nature
of problem under investigation.
• The electrode closest to the generator site gives the
optimal recording.
74. PLACEMENT
• Stimulus is applied to median nerve at wrist keeping the
cathode 2cm proximal to the wrist crease.
• The ground is placed between stimulating and recording site.
• A 200microvolt square wave pulse sufficient to produce a
painless twitch of thumb.
• Current ranging from 5-15mA with 200 microseconds
duration is sufficient.
• Frequency between 3-8Hz.
• More produces abnormal waves and painful.
75. PARAMETERS
• Since SEPs are very small, 1000 or more epochs need to be
averaged.
• Methods:
Multiple trails.
Increase the averaging to 2000 trials.
Increase stimulus intensity.
Use of sedative in children and in adults if excessive muscle
artifacts are present.
78. MEASUREMENT
• Latency
• Amplitude
• Interpeak latency (IPL)
• The measurement of arm length from cathode to Erb’s point
can give an estimate of peripheral conduction.
• IPLs:
1. Brachial plexus to spinal cord (N9-N13)
2. Central sensory conduction time (CSCT) (N13-N20)
82. TIBIAL SEPs
• Stimulus: posterior tibial nerve at the ankle.
• Cathode is placed at midpoint between achillis tendon and
medial malleolus, keeping the anode 3cm distal to the
cathode.
• 200-300 microseconds square wave electrical pulse with
intensity sufficient to produce a painless twitch of toes at
3-8Hz.
83. TIBIAL SEPs
• Recording electrodes should be placed at the popliteal fossa
(PF) 4-6 cm above the crease midway between ST & BF.
• Reference electrode on medial surface of knee (K).
• Spinal electrodes are placed over the first lumbar vertebra,
L1 to L3 & T10 to T12 or iliac crest.
• Scalp recording electrode is placed 2cm posterior to Cz’
referred to Fz.
87. PATIENT RELATED FACTORS
• AGE:
Young children N9 and N13 potentials of median SEP occur
quite early while central conduction is relatively low.
Elderly, latency limits are longer by 5-10% (55y) and
increase in N13-N19 IPL after 50-60y.
Tibial delays of lumbar potential and prolongation of CSCT
above 60y.
88. PATIENT RELATED FACTORS
• GENDER: Females have a shorter CSCT compared to males,
unknown reason.
• Absolute peak latencies correlate with height, whereas IPLs
are less significant to it.
• SEPs are resistant to various drugs, sedation can be used
for improving recording in uncooperative patients.
89. PATIENT RELATED FACTORS
• Sleep may change the apparent N20 latency by increasing
the amplitude of peak component compared to wake state.
• Peripheral nerve conduction since is slowed by lowering the
limb temperature, this may lead to prolongation of absolute
SEP latencies.
• It is therefore, recommended to maintain the normal limb
temperature during SEP study.
• Temperature change affects peripheral conduction more
than central.
90. CLINICAL APPLICATIONS OF SEP
• SEPs have a good correlation with impairment of joint
position and vibration sensations but not with pinprick and
touch.
• The correlation is better in spinal cord compared to
cerebral lesions.
• Demylinating : latency problem
• Compressive lesions: latency and amplitude both
• Only amplitude in ischemic lesions
91. CLINICAL APPLICATIONS OF SEP
• Demyelinating diseases : multiple sclerosis – conduction
delay + abnormal IPL with normal peripheral potentials.
• Trauma : thoracic outlet syndrome and brachial plexus.
• Vascular : stroke
• Infections : acute transverse myelitis and acute
disseminated encephalopathy.
• HIV myelopathy.
• Pott’s paraplegia : normal until 6 months.
• Degenerative diseases: cervical and lumbar spondylosis
92. SURGICAL MONITORING
• Helpful in identifying and confirming the functional
continuity of nerve roots in spinal cord surgery.
• SEPs are mainly used in scoliosis surgery , neurosurgical and
cardiovascular surgery.
• Monitoring provides reassurance to the surgeon about
integrity of sensory pathways.
• It has also reduced the neurologic deficit by 50%.
• Both invasive and noninvasive SEP monitoring is performed
during surgery.
93. SURGICAL MONITORING
• Preferred stimulation site based on the surgery but mainly
L1, C2 and cortex Cz’.
• Preferred stimulation rate is 5 Hz.
• Filter setting is kept at 30 Hz- 3KHz.
• For invasive monitoring, the recording electrodes are places
near the spinal cord within the surgical site.
94. SURGICAL MONITORING
• Reduction in ulnar and tibial potentials was reported shortly
after induction of anesthesia : considered normal.
• During surgery core body temperature decrease by 1
degree, causes prolonged latency.
• In surgical monitoring, a 50% drop in amplitude or latency
increase of more than 5% compared to baseline is used for
raising alarm.
• Sensitivity : 92% and specificity : 98.8% (scoliotic surgery)
95. SURGICAL MONITORING
• Spine and spinal cord surgery including scoliosis and
Kyphosis correction with instrumentation, spinal cord
decompression/stabilization, anterior and posterior spinal
fusions (cervical, thoracic, and thoracolumbar), the release
of tethered cord, correction of spina bifida, resection of
the tumor, cyst, aneurysm or arteriovenous malformation of
the spinal cord
• Brain and brain stem surgeries including craniotomy for
tumor removal, craniotomy for aneurysm repair,
arteriovenous malformation repair, localization of cortex
during craniotomy, thalamotomy
• Cerebrovascular surgery, including clipping of intracranial
aneurysms, interventional neuroradiology
96. SURGICAL MONITORING
• Stereotactic surgery on the brain stem, thalamus, and
cerebral cortex
• Pelvic fracture surgery
• Thoracoabdominal aortic aneurysm repair
• Repair of coarctation of the aorta
• Brachial plexus and lumbosacral plexus surgery
• Peripheral nerve repair
• Carotid endarterectomy
• Thyroid surgery
97. REFERENCES
• Walsh P, Kane N, Butler S. The clinical role of evoked
potentials. Journal of neurology, neurosurgery & psychiatry.
2005 Jun 1.
• Akay, Ahmet. (2012). Evoked Potentials.
• Misra UK, Kalita J. Clinical neurophysiology: nerve
conduction, electromyography, evoked potentials. Elsevier
Health Sciences; 2019 Aug 30.
Editor's Notes
A1/A1 M1/M2 E: MASTOID PROCESS, EAR
Even numbers : right side
Odd numbers : left side
Fpz : pre-frontal
Cz : Cephalic
Z : midline or reference electrodes
W and V PATTERNS WERE ALSO FOUND WITH NORMAL SEPs.
