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.
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.
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.
Visual evoked potentials are very useful in detecting blindness in patients that cannot communicate, such as babies or non-human 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.
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 potentials. VEPs are also sometimes used to determine if someone is fraudulently alleging blindness.
The multifocal VEP is used to record separate responses for visual field locations.
Some specific VEPs are:
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.
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 agent and more narcotic is typically used.
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.
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)
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.
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 clairfy, not 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.