Evoked potential

In neurophysiology, an evoked potential (or "evoked response") is an electrical potential recorded from a human or animal following presentation of a stimulus, as distinct from spontaneous potentials as detected by electroencephalograms or electromyograms. 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 EKG. To resolve these low-amplitude potentials against the background of ongoing EEG, EKG, 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.^[1]

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 CMAP (compound motor action potentials) or SNAP (sensory nerve action potentials) as used in NCV (nerve conduction studies) are generally not thought of as evoked potentials, though they do meet the above definition.

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Sensory evoked potentials

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 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 since the 1970s: auditory evoked potentials, usually recorded from the scalp but originating at brainstem level (ABR, BAER, BSER, BAEP, BSEP); visual evoked potentials, and somatosensory evoked potentials, which are elicited by electrical stimulation of peripheral nerve. See the articles on each of these modalities.

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 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 are located in the ventral spinal cord, specifically the lateral corticospinal tract. 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.

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 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).

EXAMPLES: Contingent Negative Variations (CNV)

TMS-induced MEPs have been used in experiments regarding mirror neurons.^[2]

Evoked Potentials Procedure

Electrodes need to be attached to various points of on your scalp. Your head is measured using a standardized EEG measurement technique to determine the right spots (each spot corresponding to a type of EP that will be measured - e.g. the two locations on the back of the skull for the visual cortex, etc.), which are marked with a writing implement akin to a very thick pencil. Each of these spots is rubbed with an oil-removing scrub to get rid of the skin oil, then an electrode dipped in a liberal quantity of conductive gel (approximately the consistency of soft butter) is applied and pressed to each spot, and affixed with a strip of adhesive tape.

For visual evoked potential (VEP), you are placed in front of a computer screen, which shows a pattern of white and black squares like a chessboard, and a red dot in the middle that you are supposed to focus your eyes on with minimal movement. The procedure is done one eye at a time, with the eye that is not being tested blocked off with an eye patch. During the actual procedure, these squares alternate (white ones become black, black ones become white) at a rate of several times a second, which produces responses in the visual cortex, which is picked up by your skull electrodes. Since the computer controls the exact timing of the changes of the square colors, and receives the exact timing of the electric response in the corresponding electrodes, it is able to determine precisely the amount of time it takes for the visual stimulus to reach the visual cortex. For the somatosensory evoked potentials (SEP), additional electrodes are applied, in the same manner as described earlier.

For the upper SEP (arms), two stimulus electrodes are attached on the inside wrist, closer to the thumb. These electrodes will receive timed electric pulses that will produce an involuntary twitch of the thumb. An additional sensor electrode is applied on the back of your shoulder, close to the attachment point of the clavicle. Similar to the VEP, the computer times the electric pulses (which come at a rate of several times a second) and gets the responses from the appropriate skull electrode, thus determining the exact time it takes for the stimulus to reach the intermediate point on your shoulder, and then the brain. The same is then repeated on the other arm. For the lower SEP (legs), two stimulus electrodes are attached to the inside of your ankle, in such a way as to produce an involuntary twitch of the big toe. Additional sensor electrodes are placed at the back of the knee (closer to the outside), on the spine of the lower back, and on the spine of the upper back. Electric pulses are then sent at a rate of several times a second, and the responses are recorded in the same manner as above.

For the brain auditory evoked potential (BAEP), the stimulus is supplied through headphones. The ear that is being tested receives a clicking sound, at a rate of several times a second, while the other ear receives static. Additional sensor electrodes are placed on the backs of your earlobes. The timing is determined as above.

Evoked potential signal determination

There are many things going on at once in the brain, so it is difficult to determine when the evoked potential from a particular stimulus arrives from just one stimulus. The technique used to amplify the signal is called signal averaging. The stimulus in each evoked potential test is applied many times (one or two thousand times), and since everything else besides the evoked potential is not related to the signal, it happens at various random times relative to the stimulus, whereas the potential that is evoked by the stimulus always occurs at the same time relative to the stimulus. This allows the computer to pick out and amplify the one consistent peak or series of peaks, that are caused by the applied stimulus.

In order to improve the efficacy of this technique, you are advised to relax and not move, so as to reduce the noisiness of the signal and make the averaging technique more effective with fewer iterations of the stimulus.

See also

• Steady-state evoked potentials
• Visual evoked potentials
• Auditory evoked potentials
• Somatosensory evoked potentials
• Event-related potential
• Evoked field
• Induced activity
• Ongoing activity

References