An evoked potential or evoked response is an
electrical potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
in a specific pattern recorded from a specific part of the
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
, especially the
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
, of a
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
or other
animals
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
following presentation of a
stimulus
A stimulus is something that causes a physiological response. It may refer to:
*Stimulation
**Stimulus (physiology), something external that influences an activity
**Stimulus (psychology), a concept in behaviorism and perception
*Stimulus (economi ...
such as a light flash or a
pure tone
Pure may refer to:
Computing
* A pure function
* A pure virtual function
* PureSystems, a family of computer systems introduced by IBM in 2012
* Pure Software, a company founded in 1991 by Reed Hastings to support the Purify tool
* Pure-FTPd, F ...
. Different types of potentials result from stimuli of different
modalities and types.
Evoked potential is distinct from spontaneous potentials as detected by
electroencephalography
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
(EEG),
electromyography
Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyog ...
(EMG), or other
recording method. Such potentials are useful for
electrodiagnosis
Electrodiagnosis (EDX) is a method of medical diagnosis that obtains information about diseases by passively recording the electrical activity of body parts (that is, their natural electrophysiology) or by measuring their response to external elect ...
and
monitoring
Monitoring may refer to:
Science and technology Biology and healthcare
* Monitoring (medicine), the observation of a disease, condition or one or several medical parameters over time
* Baby monitoring
* Biomonitoring, of toxic chemical compounds, ...
that include detections of disease and drug-related sensory dysfunction and intraoperative monitoring of sensory pathway integrity.
Evoked potential
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
s tend to be low, ranging from less than a
microvolt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
to several microvolts, compared to tens of microvolts for EEG, millivolts for EMG, and often close to 20 millivolts for
ECG
Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
. To resolve these low-amplitude potentials against the background of ongoing EEG, ECG, EMG, and other biological signals and ambient noise, signal
averaging
In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the arithmetic mean). For example, the average of the numbers 2, 3, 4, 7, ...
is usually required. The signal is time-locked to the stimulus and most of the
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
occurs randomly, allowing the noise to be averaged out with averaging of repeated responses.
Signals can be recorded from
cerebral cortex
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of ...
,
brain stem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...
,
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
,
peripheral nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...
s and
muscles
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
. Usually the term "evoked potential" is reserved for responses involving either recording from, or stimulation of, central nervous system structures. Thus evoked compound motor action potentials (CMAP) or sensory nerve action potentials (SNAP) as used in
nerve conduction studies
A nerve conduction study (NCS) is a medical diagnostic test commonly used to evaluate the function, especially the ability of electrical conduction, of the motor and sensory nerves of the human body. These tests may be performed by medical specia ...
(NCS) are generally not thought of as evoked potentials, though they do meet the above definition.
Evoked potential is different from
event-related potential
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sense, sensory, cognition, cognitive, or motor system, motor event. More formally, it is any stereotyped electrophysiology, electrophysiologi ...
(ERP), although the terms are sometimes used synonymously, because ERP has higher latency, and is associated with higher cognitive processing.
Evoked potentials are mainly classified by the type of stimulus: somatosensory, auditory, visual. But they could be also classified according to stimulus frequency, wave latencies, potential origin, location, and derivation.
Steady-state evoked potential
An evoked potential is the electrical response of the brain to a sensory stimulus. Regan constructed an analogue Fourier series analyzer to record harmonics of the evoked potential to flickering (sinusoidally modulated) light. Rather than integrating the sine and cosine products, Regan fed the signals to a two-pen recorder via lowpass filters.
This allowed him to demonstrate that the brain attained a steady-state regime in which the amplitude and phase of the harmonics (frequency components) of the response were approximately constant over time. By analogy with the steady-state response of a resonant circuit that follows the initial transient response he defined an idealized steady-state evoked potential (SSEP) as a form of response to repetitive sensory stimulation in which the constituent frequency components of the response remain constant with time in both amplitude and phase.
Although this definition implies a series of identical temporal waveforms, it is more helpful to define the SSEP in terms of the frequency components that are an alternative description of the time-domain waveform, because different frequency components can have quite different properties.
[Regan, D. (1989). Human brain electrophysiology: Evoked potentials and evoked magnetic fields in science and medicine. New York: Elsevier, 672 pp.] For example, the properties of the high-frequency flicker SSEP (whose peak amplitude is near 40–50 Hz) correspond to the properties of the subsequently discovered magnocellular neurons in the retina of the macaque monkey, while the properties of the medium-frequency flicker SSEP ( whose amplitude peak is near 15–20 Hz) correspond to the properties of parvocellular neurons. Since a SSEP can be completely described in terms of the amplitude and phase of each frequency component it can be quantified more unequivocally than an averaged transient evoked potential.
It is sometimes said that SSEPs are elicited only by stimuli of high repetition frequency, but this is not generally correct. In principle, a sinusoidally modulated stimulus can elicit a SSEP even when its repetition frequency is low. Because of the high-frequency
rolloff
Roll-off is the steepness of a transfer function with frequency, particularly in electrical network analysis, and most especially in connection with filter circuits in the transition between a passband and a stopband. It is most typically app ...
of the SSEP, high frequency stimulation can produce a near-sinusoidal SSEP waveform, but this is not germane to the definition of a SSEP.
By using zoom-FFT to record SSEPs at the theoretical limit of spectral resolution ΔF (where ΔF in Hz is the reciprocal of the recording duration in seconds) Regan and Regan discovered that the amplitude and phase variability of the SSEP can be sufficiently small that the bandwidth of the SSEP's constituent frequency components can be at the theoretical limit of spectral resolution up to at least a 500-second recording duration (0.002 Hz in this case).
Repetitive sensory stimulation elicits a steady-state magnetic brain response that can be analysed in the same way as the SSEP.
The "simultaneous stimulation" technique
This technique allows several (e.g., four) SSEPs to be recorded simultaneously from any given location on the scalp.
Different sites of stimulation or different stimuli can be tagged with slightly different frequencies that are virtually identical to the brain, but easily separated by Fourier series analyzers.
For example, when two unpatterned lights are modulated at slightly different frequencies (F1 and F2) and superimposed, multiple nonlinear cross-modulation components of frequency (mF1 ± nF2) are created in the SSEP, where m and n are integers.
These components allow nonlinear processing in the brain to be investigated. By frequency-tagging two superimposed gratings, spatial frequency and orientation tuning properties of the brain mechanisms that process spatial form can be isolated and studied. Stimuli of different sensory modalities can also be tagged. For example, a visual stimulus was flickered at Fv Hz and a simultaneously presented auditory tone was amplitude modulated at Fa Hz. The existence of a (2Fv + 2Fa) component in the evoked magnetic brain response demonstrated an audio-visual convergence area in the human brain, and the distribution of this response over the head allowed this brain area to be localized. More recently, frequency tagging has been extended from studies of sensory processing to studies of selective attention
and of consciousness.
The "sweep" technique
The sweep technique is a hybrid frequency domain/time domain technique.
A plot of, for example, response amplitude versus the check size of a stimulus checkerboard pattern plot can be obtained in 10 seconds, far faster than when time-domain averaging is used to record an evoked potential for each of several check sizes.
In the original demonstration of the technique the sine and cosine products were fed through lowpass filters (as when recording a SSEP ) while viewing a pattern of fine checks whose black and white squares exchanged place six times per second. Then the size of the squares was progressively increased so as to give a plot of evoked potential amplitude versus check size (hence "sweep"). Subsequent authors have implemented the sweep technique by using computer software to increment the spatial frequency of a grating in a series of small steps and to compute a time-domain average for each discrete spatial frequency.
A single sweep may be adequate or it may be necessary to average the graphs obtained in several sweeps with the averager triggered by the sweep cycle.
Averaging 16 sweeps can improve the signal-to-noise ratio of the graph by a factor of four.
The sweep technique has proved useful in measuring rapidly adapting visual processes and also for recording from babies, where recording duration is necessarily short. Norcia and Tyler have used the technique to document the development of visual acuity
and contrast sensitivity
through the first years of life. They have emphasized that, in diagnosing abnormal visual development, the more precise the developmental norms, the more sharply can the abnormal be distinguished from the normal, and to that end have documented normal visual development in a large group of infants.
For many years the sweep technique has been used in paediatric ophthalmology (
electrodiagnosis
Electrodiagnosis (EDX) is a method of medical diagnosis that obtains information about diseases by passively recording the electrical activity of body parts (that is, their natural electrophysiology) or by measuring their response to external elect ...
) clinics worldwide.
Evoked potential feedback
This technique allows the SSEP to directly control the stimulus that elicits the SSEP without the conscious intervention of the experimental subject.
For example, the running average of the SSEP can be arranged to increase the luminance of a checkerboard stimulus if the amplitude of the SSEP falls below some predetermined value, and to decrease luminance if it rises above this value. The amplitude of the SSEP then hovers about this predetermined value. Now the wavelength (colour) of the stimulus is progressively changed. The resulting plot of stimulus luminance versus wavelength is a plot of the spectral sensitivity of the visual system.
Sensory evoked potentials
Sensory evoked potentials (SEP) are recorded from the
central nervous system
The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...
following stimulation of
sense organ
A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...
s, for example,
visual
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the ...
evoked potentials elicited by a flashing light or changing pattern on a monitor,
[O'Shea, R. P., Roeber, U., & Bach, M. (2010). Evoked potentials: Vision. In E. B. Goldstein (Ed.), Encyclopedia of Perception (Vol. 1, pp. 399-400, xli). Los Angeles: Sage. ] auditory evoked potentials by a click or tone stimulus presented through earphones), or tactile or
somatosensory
In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
evoked potential (SSEP) elicited by tactile or electrical stimulation of a sensory or mixed nerve in the
periphery
Periphery or peripheral may refer to:
Music
*Periphery (band), American progressive metal band
* ''Periphery'' (album), released in 2010 by Periphery
* "Periphery", a song from Fiona Apple's album '' The Idler Wheel...''
Gaming and entertainm ...
. Sensory evoked potentials have been widely used in
clinical diagnostic medicine since the 1970s, and also in intraoperative neurophysiology monitoring (IONM), also known as surgical neurophysiology.
There are three kinds of evoked potentials in widespread clinical use: auditory evoked potentials, usually recorded from the scalp but originating at
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
level; visual evoked potentials, and
somatosensory evoked potentials
An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light fla ...
, which are elicited by electrical stimulation of peripheral nerve. Examples of SEP usage include:
* SSEP can be used to locate lesions such as peripheral nerve or spinal cord.
* VEP and BAEP can supplement
neuroimaging
Neuroimaging is the use of quantitative (computational) techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Incre ...
as part of workups to diagnose diseases such as
multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
.
* Short latency EPs such as SSEP, VEP, and BAEP can be used to indicate prognosis for traumatic and anoxic brain injury. Early after anoxic brain injury, no response indicates mortality accurately. In traumatic brain injury, abnormal responses indicates failure to recover from coma. In both types of injury, normal responses may indicate good outcome. Moreover, recovery in responses often indicates clinical recovery.
Long and Allen were the first investigators to report the abnormal brainstem auditory evoked potentials (BAEPs) in an alcoholic woman who recovered from
acquired central hypoventilation syndrome
Central hypoventilation syndrome (CHS) is a sleep-related breathing disorder that causes ineffective breathing, apnea, or respiratory arrest during sleep (and during wakefulness in severe cases). CHS can either be congenital (CCHS) or acquired (A ...
. These investigators hypothesized that their patient's
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
was poisoned, but not destroyed, by her chronic alcoholism.
Visual evoked potential
Visual evoked potential (VEP) is an evoked potential elicited by presenting light flash or pattern stimulus which can be used to confirm damage to visual pathway
including
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
,
optic nerve
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...
,
optic chiasm
In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrate ...
,
optic radiations, and
occipital cortex
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
.
One application is in measuring infant's visual acuity. Electrodes are placed on infant's head over
visual cortex
The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus and ...
and a gray field is presented alternately with a checkerboard or grating pattern. If the checker's boxes or stripes are large enough to be detected, VEP is generated; otherwise, none is generated. It's an objective way to measure infant's visual acuity.
VEP can be sensitive to visual dysfunctions that may not be found with just physical examinations or MRI, even if it cannot indicate etiologies.
VEP may be abnormal in
optic neuritis
Optic neuritis describes any condition that causes inflammation of the optic nerve; it may be associated with demyelinating diseases, or infectious or inflammatory processes.
It is also known as optic papillitis (when the head of the optic nerv ...
,
optic neuropathy Optic neuropathy is damage to the optic nerve from any cause. The optic nerve is a bundle of millions of fibers in the retina that sends visual signals to the brain.
Damage and death of these nerve cells, or neurons, leads to characteristic featu ...
,
demyelinating disease
A demyelinating disease is any disease of the nervous system in which the myelin sheath of neurons is damaged. This damage impairs the conduction of signals in the affected nerves. In turn, the reduction in conduction ability causes deficiency i ...
,
multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
,
Friedreich’s ataxia
Friedreich's ataxia (FRDA or FA) is an autosomal-recessive genetic disease that causes difficulty walking, a loss of sensation in the arms and legs, and impaired speech that worsens over time. Symptoms generally start between 5 and 20 year ...
,
vitamin B12 deficiency,
neurosyphilis
Neurosyphilis refers to infection of the central nervous system in a patient with syphilis. In the era of modern antibiotics the majority of neurosyphilis cases have been reported in HIV-infected patients. Meningitis is the most common neurologic ...
,
migraine
Migraine (, ) is a common neurological disorder characterized by recurrent headaches. Typically, the associated headache affects one side of the head, is pulsating in nature, may be moderate to severe in intensity, and could last from a few hou ...
, ischemic disease, tumor compressing the optic nerve,
ocular hypertension
Ocular hypertension is the presence of elevated fluid pressure inside the eye (intraocular pressure), usually with no optic nerve damage or visual field loss.
For most individuals, the normal range of intraocular pressure is between 10 mmHg and 2 ...
,
glaucoma
Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...
,
diabetes
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...
,
toxic amblyopia Toxic amblyopia, or nutritional optic neuropathy, is a condition where a toxic reaction in the optic nerve results in visual loss. Various poisonous substances may cause the condition as well as nutritional factors.
Tobacco amblyopia is a form of t ...
, aluminum neurotoxicity,
manganese intoxication,
retrobulbar neuritis
Optic neuritis describes any condition that causes inflammation of the optic nerve; it may be associated with Demyelinating disease, demyelinating diseases, or infectious or inflammatory processes.
It is also known as optic papillitis (when the ...
, and
brain injury
An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, or o ...
.
It can be used to examine infant's visual impairment for abnormal visual pathways which may be due to delayed maturation.
The P100 component of VEP response, which is the positive peak with the delay about 100 ms, has a major clinical importance. The visual pathway dysfunction anterior to the optic chiasm maybe where VEPs are most useful. For example, patients with acute severe optic neuritis often lose the P100 response or have highly attenuated responses. Clinical recovery and visual improvement come with P100 restoration but with an abnormal increased latency that continues indefinitely, and hence, it maybe useful as an indicator of previous or subclinical optic neuritis.
In 1934, Adrian and Matthew noticed potential changes of the occipital EEG can be observed under stimulation of light. Ciganek developed the first nomenclature for occipital EEG components in 1961. During that same year, Hirsch and colleagues recorded a visual evoked potential (VEP) on the occipital lobe (externally and internally), and they discovered amplitudes recorded along the
calcarine fissure
The calcarine sulcus (or calcarine fissure) is an anatomical landmark located at the caudal end of the medial surface of the brain of humans and other primates. Its name comes from the Latin "calcar" meaning "spur". It is very deep, and known a ...
were the largest. In 1965, Spehlmann used a checkerboard stimulation to describe human VEPs. An attempt to localize structures in the primary visual pathway was completed by Szikla and colleagues. Halliday and colleagues completed the first clinical investigations using VEP by recording delayed VEPs in a patient with retrobulbar neuritis in 1972. A wide variety of extensive research to improve procedures and theories has been conducted from the 1970s to today and the method has also been described in animals.
VEP Stimuli
The diffuse-light flash stimulus is rarely used nowadays due to the high variability within and across subjects. However, it is beneficial to use this type of stimulus when testing infants, animals or individuals with poor visual acuity. The checkerboard and grating patterns use light and dark squares and stripes, respectively. These squares and stripes are equal in size and are presented, one image at a time, via a computer screen.
VEP Electrode Placement
Electrode placement is extremely important to elicit a good VEP response free of artifact. In a typical (one channel) setup, one electrode is placed 2.5 cm above the
inion
Near the middle of the squamous part of occipital bone is the external occipital protuberance, the highest point of which is referred to as the inion. The inion is the most prominent projection of the protuberance which is located at the posterioin ...
and a reference electrode is placed at Fz. For a more detailed response, two additional electrodes can be placed 2.5 cm to the right and left of Oz.
VEP Waves
![VEP-normal](https://upload.wikimedia.org/wikipedia/commons/b/b4/VEP-normal.gif)
The VEP nomenclature is determined by using capital letters stating whether the peak is positive (P) or negative (N) followed by a number which indicates the average peak latency for that particular wave. For example, P100 is a wave with a positive peak at approximately 100 ms following stimulus onset. The average amplitude for VEP waves usually falls between 5 and 20 microvolts.
Normal values are depending on used stimulation hardware (flash stimulus vs.
cathode ray tube
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), pictu ...
or
liquid crystal display
A liquid-crystal display (LCD) is a flat panel display, flat-panel display or other Electro-optic modulator, electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liqui ...
, checkerboard field size, etc.).
Types of VEP
Some specific VEPs are:
* Monocular pattern reversal (most common)
* Sweep visual evoked potential
* Binocular visual evoked potential
* Chromatic visual evoked potential
* Hemi-field visual evoked potential
* Flash visual evoked potential
* LED Goggle visual evoked potential
* Motion visual evoked potential
*
Multifocal visual evoked potential
* Multi-channel visual evoked potential
* Multi-frequency visual evoked potential
* Stereo-elicited visual evoked potential
*
Steady state visually evoked potential
Auditory evoked potential
Auditory evoked potentials (AEP) can be used to trace the signal generated by a sound through the ascending auditory pathway. The evoked potential is generated in the cochlea, goes through the
cochlear nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
, through the
cochlear nucleus
The cochlear nuclear (CN) complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN).
The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus ...
,
superior olivary complex
The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system. The SO ...
,
lateral lemniscus
The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain. Three distinct, primarily in ...
, to the
inferior colliculus
The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The ...
in the midbrain, on to the
medial geniculate body
The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei that ...
, and finally to the
cortex
Cortex or cortical may refer to:
Biology
* Cortex (anatomy), the outermost layer of an organ
** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain''
*** Motor cortex, the regions of the cerebral cortex i ...
.
Auditory evoked potentials (AEPs) are a subclass of
event-related potentials
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event. More formally, it is any stereotyped electrophysiological response to a stimulus. The study of the brain ...
(ERPs). ERPs are brain responses that are time-locked to some "event", such as a sensory stimulus, a mental event (such as recognition of a target stimulus), or the omission of a stimulus. For AEPs, the "event" is a sound. AEPs (and ERPs) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc.
Brainstem auditory evoked potentials are small AEPs that are recorded in response to an auditory stimulus from electrodes placed on the scalp.
AEPs serve for assessment of the functioning of the
auditory system
The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system.
System overview
The outer ear funnels sound vibrations to the eardrum, increasin ...
and
neuroplasticity
Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of Neural circuit, neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that diffe ...
.
They can be used to diagnose learning disabilities in children, aiding in the development of tailored educational programs for those with hearing and or cognition problems.
Somatosensory evoked potential
Somatosensory evoked potentials
An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light fla ...
(SSEPs) are EP recorded from the brain or spinal cord when stimulating peripheral nerve repeatedly.
SSEPs are used in
neuromonitoring
Intraoperative neurophysiological monitoring (IONM) or intraoperative neuromonitoring is the use of electrophysiological methods such as electroencephalography (EEG), electromyography (EMG), and evoked potentials to monitor the functional integ ...
to assess the function of a patient's
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
during
surgery
Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...
. They are recorded by stimulating peripheral nerves, most commonly the
tibial nerve
The tibial nerve is a branch of the sciatic nerve. The tibial nerve passes through the popliteal fossa to pass below the arch of soleus.
Structure Popliteal fossa
The tibial nerve is the larger terminal branch of the sciatic nerve with root val ...
,
median nerve
The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus.
The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...
or
ulnar nerve
In human anatomy, the ulnar nerve is a nerve that runs near the ulna bone. The ulnar collateral ligament of elbow joint is in relation with the ulnar nerve. The nerve is the largest in the human body unprotected by muscle or bone, so injury is ...
, typically with an
electrical
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
stimulus. The response is then recorded from the patient's
scalp
The scalp is the anatomical area bordered by the human face at the front, and by the neck at the sides and back.
Structure
The scalp is usually described as having five layers, which can conveniently be remembered as a mnemonic:
* S: The ski ...
.
Although stimuli such as touch, vibration, and pain can be used for SSEP, electrical stimuli are most common because of ease and reliability.
SSEP can be used for prognosis in patients with severe traumatic head injury.
Because SSEP with latency less than 50 ms is relatively independent of consciousness, if used early in comatose patient, it can predict outcome reliably and efficiently.
For example, comatose patients with no responses bilaterally has 95% chance of not recovering from coma.
But care should be taken analyzing the result. For example, increased sedation and other CNS injuries such as the spinal cord can affect SEP.
Because of the low
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
of the signal once it reaches the patient's scalp and the relatively high amount of electrical noise caused by background
EEG
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
, scalp muscle
EMG or electrical devices in the room, the signal must be averaged. The use of averaging improves the
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
. Typically, in the operating room, over 100 and up to 1,000 averages must be used to adequately resolve the evoked potential.
The two most looked at aspects of an SSEP are the amplitude and latency of the peaks. The most predominant peaks have been studied and named in labs. Each peak is given a letter and a number in its name. For example, N20 refers to a negative peak (N) at 20ms. This peak is recorded from the cortex when the median nerve is stimulated. It most likely corresponds to the signal reaching the
somatosensory cortex
In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch (haptic perception), as well as temperature (thermoception), body position (proprioception), and pain. It is ...
. When used in intraoperative monitoring, the latency and amplitude of the peak relative to the patient's post-intubation baseline is a crucial piece of information. Dramatic increases in latency or decreases in amplitude are indicators of neurological
dysfunction
Dysfunction can refer to:
* Abnormality (behavior)
* Dysfunctional family
* Sexual dysfunction
* ''Dysfunction'' (album), an album by the rock band Staind
* Manifest and latent functions and dysfunctions
Manifest may refer to:
Computing
* Mani ...
.
During surgery, the large amounts of
anesthetic
An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia — in other words, to result in a temporary loss of sensation or awareness. They may be divided into two ...
gases used can affect the amplitude and latencies of SSEPs. Any of the
halogenated
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
agents or
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
will increase latencies and decrease amplitudes of responses, sometimes to the point where a response can no longer be detected. For this reason, an anesthetic utilizing less halogenated agent and more intravenous hypnotic and narcotic is typically used.
Clinical Uses
SEP findings do not by themselves lead to a specific diagnosis, and organic diseases cannot necessarily be excluded with normal SEP findings. Findings must be interpreted in the context of the patient’s clinical presentation. Evaluating the peripheral responses with SEPs could contribute to the diagnosis of peripheral nerve damage.
Furthermore, SEPs could be abnormal in different pathologies such as
multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This d ...
(MS), hereditary spinocerebellar degenerations, hereditary spastic paraplegia, AIDS and vitamin B
12 or vitamin E deficiency. In patients with MS, evoked potential findings often complement findings on MRI.
In the acute stage after a traumatic spinal injury or brain trauma, the absence of SEP responses do not correlate with prognosis. However, an early return to normal or preserved cortical responses in the subacute stage correlate with a positive outcome.
SEPs can be helpful to evaluate subcortical and cortical function in comatose patients and are less sensitive to sedative drugs than EEG. SEP´s and BAEP´s together are the best tools to assist in the confirmation of brain death in comatose patients
Clinical consideration in children
As in the adult, SEP findings in combination with the clinical assessment and EEG findings can contribute to the determination of prognosis in comatose children. In high risk newborns, tracking SEP findings over time can be helpful for outcome prognostication. Several neurodegenerative disorders have abnormal findings in spinal and cortical SEP components. Moreover, compressive lesions on the spine (e.g. Arnold-Chiari malformation or mucopolysaccharidosis) are associated with abnormal SEPs, which may precede abnormalities on MRI.
Laser evoked potential
Conventional SSEPs monitor the functioning of the part of the somatosensory system involved in sensations such as touch and vibration. The part of the somatosensory system that transmits pain and temperature signals is monitored using laser evoked potentials (LEP). LEPs are evoked by applying finely focused, rapidly rising heat to bare skin using a laser. In the central nervous system they can detect damage to the
spinothalamic tract
The spinothalamic tract is a part of the anterolateral system or the ventrolateral system, a sensory pathway to the thalamus. From the ventral posterolateral nucleus in the thalamus, sensory information is relayed upward to the somatosensory cor ...
, lateral
brain stem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...
, and fibers carrying pain and temperature signals from the
thalamus
The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...
to the
cortex
Cortex or cortical may refer to:
Biology
* Cortex (anatomy), the outermost layer of an organ
** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain''
*** Motor cortex, the regions of the cerebral cortex i ...
. In the peripheral nervous system pain and heat signals are carried along thin (
C and
A delta) fibers to the spinal cord, and LEPs can be used to determine whether a
neuropathy
Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or o ...
is located in these small fibers as opposed to larger (touch, vibration) fibers.
Motor evoked potentials
Motor evoked potentials (MEP) are recorded from muscles following direct stimulation of exposed motor cortex, or transcranial stimulation of motor cortex, either
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
or electrical. Transcranial magnetic MEP (TCmMEP) potentially offer clinical diagnostic applications. Transcranial electrical MEP (TCeMEP) has been in widespread use for several years for intraoperative monitoring of pyramidal tract functional integrity.
During the 1990s, there were attempts to monitor "motor evoked potentials", including "neurogenic motor evoked potentials" recorded from peripheral nerves, following direct electrical stimulation of the spinal cord. It has become clear that these "motor" potentials were almost entirely elicited by antidromic stimulation of sensory tracts—even when the recording was from muscles (antidromic sensory tract stimulation triggers myogenic responses through synapses at the root entry level). TCMEP, whether electrical or magnetic, is the most practical way to ensure pure motor responses, since stimulation of sensory cortex cannot result in descending impulses beyond the first synapse (synapses cannot be backfired).
TMS-induced MEPs have been used in many experiments in
cognitive neuroscience
Cognitive neuroscience is the scientific field that is concerned with the study of the biological processes and aspects that underlie cognition, with a specific focus on the neural connections in the brain which are involved in mental proces ...
. Because MEP amplitude is correlated with motor excitability, they offer a quantitative way to test the role of various types of intervention on the motor system (pharmacological, behavioral, lesion, etc.). TMS-induced MEPs may thus serve as an index of covert
motor preparation or facilitation, e.g., induced by the
mirror neuron
A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons hav ...
system when seeing someone's else actions.
In addition, MEPs are used as a reference to adjust the intensity of stimulation that needs to be delivered by TMS when targeting cortical regions whose response might not be as easily measurable, e.g., in the context of TMS-based therapy.
Intraoperative monitoring
Somatosensory evoked potentials provide monitoring for the dorsal columns of the spinal cord. Sensory evoked potentials may also be used during surgeries which place brain structures at risk. They are effectively used to determine cortical ischemia during carotid endarterectomy surgeries and for mapping the sensory areas of the brain during brain surgery.
Electrical stimulation of the scalp can produce an electric current within the brain that activates the motor pathways of the pyramidal tracts. This technique is known as transcranial electrical motor potential (TcMEP) monitoring. This technique effectively evaluates the motor pathways in the central nervous system during surgeries which place these structures at risk. These motor pathways, including the lateral corticospinal tract, are located in the lateral and ventral funiculi of the spinal cord. Since the ventral and dorsal spinal cord have separate blood supply with very limited collateral flow, an anterior cord syndrome (paralysis or paresis with some preserved sensory function) is a possible surgical sequela, so it is important to have monitoring specific to the motor tracts as well as dorsal column monitoring.
Transcranial magnetic stimulation versus electrical stimulation is generally regarded as unsuitable for intraoperative monitoring because it is more sensitive to anesthesia. Electrical stimulation is too painful for clinical use in awake patients. The two modalities are thus complementary, electrical stimulation being the choice for intraoperative monitoring, and magnetic for clinical applications.
See also
*
*
Contingent negative variation The contingent negative variation (CNV) is a negative slow surface potential, as measured by electroencephalography (EEG), that occurs during the period between a warning stimulus or signal and an imperative ("go") stimulus. The CNV was one of the ...
*
Difference due to memory Difference due to memory (Dm) indexes differences in neural activity during the study phase of an experiment for items that subsequently are remembered compared to items that are later forgotten. It is mainly discussed as an event-related potential ...
*
Early left anterior negativity
The early left anterior negativity (commonly referred to as ELAN) is an event-related potential in electroencephalography (EEG), or component of brain activity that occurs in response to a certain kind of stimulus. It is characterized by a negativ ...
*
Error-related negativity Error-related negativity (ERN), sometimes referred to as the Ne, is a component of an event-related potential (ERP). ERPs are electrical activity in the brain as measured through electroencephalography (EEG) and time-locked to an external event (e.g ...
*
Event-related potential
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sense, sensory, cognition, cognitive, or motor system, motor event. More formally, it is any stereotyped electrophysiology, electrophysiologi ...
*
Evoked field Evoked fields are part of the Magnetoencephalography, magnetoencephalogram. They are brain signals evoked by sensory stimulation, but usually buried by the ongoing brain activity. Repeating the stimulus multiple times and averaging the signals reduc ...
*
Electroencephalography
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
*
Electroretinography
Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors ( rods and cones), inner retinal cells ( bipolar and amacrine cells), and the ganglion cells. Electrodes are placed on the ...
*
Slow vertex response The slow vertex response (also called SVR or V potential) is an electrochemical signal associated with electrophysiological recordings of the auditory system, specifically Auditory evoked potential
An evoked potential or evoked response is an elec ...
*
Event-related potential
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sense, sensory, cognition, cognitive, or motor system, motor event. More formally, it is any stereotyped electrophysiology, electrophysiologi ...
**
N100
In neuroscience, the N100 or N1 is a large, negative-going evoked potential measured by electroencephalography (its equivalent in magnetoencephalography is the M100); it peaks in adults between 80 and 120 milliseconds after the onset of a stimul ...
,
N200,
N2pc N2pc refers to an ERP component linked to selective attention.Luck, S. J. (2005). "The operation of attention—millisecond by millisecond—over the first half second." In H. Ogmen & B. G. Breitmeyer (Eds.), ''The first half second: The microgene ...
,
N170 The N170 is a component of the event-related potential (ERP) that reflects the neural processing of faces, familiar objects or words. Furthermore, the N170 is modulated by prediction error processes.
When potentials evoked by images of faces are co ...
,
N400,
Visual N1
The visual N1 is a visual evoked potential, a type of event-related electrical potential (ERP), that is produced in the brain and recorded on the scalp. The N1 is so named to reflect the polarity and typical timing of the component. The "N" indic ...
**
C1 and P1,
P200
In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential (ERP) measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synapti ...
,
P300,
P3a The P3a, or novelty P3, is a component of time-locked (EEG) signals known as event-related potentials (ERP). The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak l ...
,
P3b
The P3b is a subcomponent of the P300, an event-related potential (ERP) component that can be observed in human scalp recordings of brain electrical activity. The P3b is a positive-going amplitude (usually relative to a reference behind the ear ...
,
P600 (neuroscience)
The P600 is an event-related potential (ERP) component, or peak in electrical brain activity measured by electroencephalography (EEG). It is a language-relevant ERP component and is thought to be elicited by hearing or reading grammatical errors ...
*
International Society for Clinical Electrophysiology of Vision The International Society for Clinical Electrophysiology of Vision (ISCEV) is an association that promotes research and applications of electrophysiology, electrophysiological methods (e.g. electroretinogram, electrooculography, electrooculogram, an ...
*
Late positive component The late positive component or late positive complex (LPC) is a positive-going event-related brain potential (ERP) component that has been important in studies of explicit recognition memory.Munte, T. F., Urbach, T. P., Duzel, E., & Kutas, M., (200 ...
*
Lateralized readiness potential In neuroscience, the lateralized readiness potential (LRP) is an event-related brain potential, or increase in electrical activity at the surface of the brain, that is thought to reflect the preparation of motor activity on a certain side of the bo ...
*
Mismatch negativity
*
Neural oscillation
Neural oscillations, or brainwaves, are rhythmic or repetitive patterns of neural activity in the central nervous system. Neural tissue can generate oscillatory activity in many ways, driven either by mechanisms within individual neurons or by ...
*
Oddball paradigm
The oddball paradigm is an experimental design used within psychology research. Presentations of sequences of repetitive stimuli are infrequently interrupted by a deviant stimulus. The reaction of the participant to this "oddball" stimulus is re ...
*
Somatosensory evoked potential Somatosensory evoked potential (SEP or SSEP) is the electrical activity of the brain that results from the stimulation of touch. SEP tests measure that activity and are a useful, noninvasive means of assessing somatosensory system functioning. By co ...
References
External links
*
{{DEFAULTSORT:Evoked Potential
*
Neuropsychology
Electroencephalography
Medical tests