Question

Describe the signal averaging technique for measuring weak evoked
electrical responses from the brain with scalp electrodes

Answer 1

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 flash or a pure tone. Different types of potentials result from stimuli of different modalities and types. EP is distinct from spontaneous potentials as detected by electroencephalography (EEG), electromyography (EMG), or other electrophysiologic recording method. Such potentials are useful for electrodiagnosis and monitoring that include detections of disease and drug-related sensory dysfunction and intraoperative monitoring of sensory pathway integrity

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 20 millivolts 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 repeated responses.

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.

Evoked potential is different from event-related potential (ERP), although the terms are sometimes used synonymously, because ERP has higher latency, and is associated with higher cognitive processing.

Sensory evoked potentialsEdit

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 tactile or somatosensory evoked potential (SSEP) elicited by tactile or electrical stimulation of a sensory or mixed nerve in the periphery. 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 level; visual evoked potentials, and somatosensory evoked potentials, 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 as part of workups to diagnose diseases such as multiple sclerosis.

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. These investigators hypothesized that their patient's brainstem was poisoned, but not destroyed, by her chronic alcoholism.

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.[7][8] 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.[8][9] 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 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" techniqueEdit

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.[12] 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 attentionand 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 sensitivitythrough 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 infant

.For many years the sweep technique has been used in paediatric ophthalmology (electrodiagnosis) 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.

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, optic nerve, optic chiasm, optic radiations, and occipital cortex. One application is in measuring infant's visual acuity. Electrodes are placed on infant's head over visual cortex 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 neuropathy, demyelinating disease, multiple sclerosis, Friedreich’s ataxia, vitamin B12 deficiency, neurosyphilis, migraine, ischemic disease, tumor compressing the optic nerve, ocular hypertension, glaucoma, diabetes, toxic amblyopia, aluminum neurotoxicity, manganese intoxication, retrobulbar neuritis, and brain injury.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