Evoked potential
[3] Signals can be recorded from cerebral cortex, brain stem, spinal cord, peripheral nerves and muscles.
Usually the term "evoked potential" is reserved for responses involving either recording from, or stimulation of, central nervous system structures.
Regan constructed an analogue Fourier series analyzer to record harmonics of the evoked potential of flickering (sinusoidally modulated) light.
Rather than integrating the sine and cosine products, Regan fed the signals to a two-pen recorder via lowpass filters.
[5] 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.
[8] 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.
[10] 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.
[16] 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.
Examples of SEP usage include:[4] Long and Allen[24] were the first investigators to report the abnormal brainstem auditory evoked potentials (BAEPs) in an alcoholic woman who recovered from acquired central hypoventilation syndrome.
Electrodes are placed on infant's head over visual cortex and a gray field is presented alternately with a checkerboard or grating pattern.
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.
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.
Normal values are depending on used stimulation hardware (flash stimulus vs. cathode-ray tube or liquid crystal display, checkerboard field size, etc.).
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.
[32] 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.
[33] Somatosensory evoked potentials (SSEPs) are EP recorded from the brain or spinal cord when stimulating peripheral nerve repeatedly.
[35] 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, increased sedation and other CNS injuries such as the spinal cord can affect SEP.[34] 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.
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.
In the acute stage after a traumatic spinal injury or brain trauma, the absence of SEP responses do not correlate with prognosis.
SEPs can be helpful to evaluate subcortical and cortical function in comatose patients and are less sensitive to sedative drugs than EEG.
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, lateral brain stem, and fibers carrying pain and temperature signals from the thalamus to the cortex.
Transcranial electrical MEP (TCeMEP) has been in widespread use for several years for intraoperative monitoring of pyramidal tract functional integrity.
TMS-induced MEPs may thus serve as an index of covert motor preparation or facilitation, e.g., induced by the mirror neuron system when seeing someone's else actions.
[39] 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.
This technique effectively evaluates the motor pathways in the central nervous system during surgeries which place these structures at risk.
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.
