From Wikipedia, the free encyclopedia
An evoked potential (or "evoked response") is an electrical
potential recorded from the nervous
system of a human or other animal following presentation of a stimulus,
as distinct from spontaneous potentials as detected by electroencephalography (EEG) or electromyography (EMG).
Evoked potential amplitudes
tend to be low, ranging from less than a microvolt to several microvolts, compared to tens of
microvolts for EEG, millivolts for EMG, and often close to a volt for ECG. To resolve these low-amplitude
potentials against the background of ongoing EEG, ECG, EMG and other biological
signals and ambient noise, signal averaging is usually required. The signal is
time-locked to the stimulus and most of the noise
occurs randomly, allowing the noise to be averaged out with averaging of
Signals can be recorded from cerebral
cortex, brain stem, spinal
cord and peripheral nerves. 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 (NCS) are generally not thought of as evoked potentials,
though they do meet the above definition.
Sensory evoked potentials (SEP) are recorded from the central nervous system following
stimulation of sense organs (for example, visual evoked potentials elicited by a
flashing light or changing pattern on a monitor;auditory evoked
potentials by a click or tone stimulus presented through earphones) or by
tactile or somatosensory evoked potential (SSEP) elicited by
tactile or electrical stimulation of a sensory or mixed nerve in the periphery. They have been widely used
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 level; visual evoked potentials, and somatosensory evoked
potentials, which are elicited by electrical stimulation of peripheral
nerve. See below.
Long and Allen reported the
abnormal BAEPs in an alcoholic woman who recovered from Ondine's curse. These investigators
hypothesized that their patient's brainstem was
poisoned, but not destroyed, by her chronic alcoholism.
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
but, rather than integrating the sine and cosine products, fed them 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
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 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
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 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 analyzed
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
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
[Top of the Page]
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
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) clinics Worldwide.
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.
Visual evoked potentials (VEPs) are described by O'Shea et al. (2009). They are
caused by sensory stimulation of a subject's visual
field and are observed using electroencephalography. Commonly used
visual stimuli are flashing lights, or checkerboards on a video screen that
flicker between black on white to white on black (invert
contrast). The resulting waveform includes the C1 and
P1 followed by the visual N1.
Visual evoked potentials are very useful in detecting blindness in patients that cannot communicate, such
as babies or animals. If repeated stimulation of the visual field causes no
changes in EEG potentials, then the subject's brain
is probably not receiving any signals from his/her eyes. Other applications
include the diagnosis of optic neuritis,
which causes the signal to be delayed. Such a delay is also a classic finding in
Multiple Sclerosis. Visual evoked potentials
are furthermore used in the investigation of basic functions of visual perception. VEPs are also sometimes
used to determine if someone is fraudulently alleging blindness.
The term "visual evoked potential" is used interchangeably with "visually
evoked potential". It usually refers to responses recorded from the occipital cortex. Sometimes, the
term "visual evoked cortical potential" (VECP) is used to distinguish the VEP
from retinal or subcortical
The multifocal VEP is used to record
separate responses for visual field locations.
Some specific VEPs are:
Auditory evoked potential 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, through the cochlear nucleus, superior olivary complex, lateral lemniscus, to the inferior colliculus in the midbrain, on to
the medial geniculate body, and finally to
Auditory evoked potentials (AEPs) are a subclass of event-related potentials
(ERP)s. 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.
Somatosensory Evoked Potentials
(SSEPs) are used in neuromonitoring to assess the function of a
patient's spinal cord during surgery. They are recorded by stimulating peripheral
nerves, most commonly the tibial nerve, median nerve or ulnar
nerve, typically with an electrical stimulus. The response is then recorded
from the patient's scalp.
Because of the low amplitude of the signal once
it reaches the patient's scalp and the relatively high amount of electrical
noise caused by background EEG, scalp muscle EMG or electrical
devices in the room, the signal must be averaged. The use of averaging improves
the signal-to-noise ratio. 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. 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 dysfuncion.
During surgery, the large amounts of anesthetic gases used can affect the amplitude and
latencies of SSEPs. Any of the halogenated agents or nitrous oxide 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.
[Top of the Page]
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 electrical 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
Motor evoked potentials (MEP) are recorded from muscles following direct
stimulation of exposed motor cortex, or transcranial stimulation of motor
cortex, either magnetic 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
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).
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