The evoked potential amplitude tends to be low, ranging from less than one microvolt to a few, compared to tens of microvolts for electroencephalography (EEG), millivolts for electromyography (EMG), and often close to 20 millivolts for electrocardiogram (ECG). Signal averaging is usually required to resolve these low amplitude potentials in the face of ongoing EEG, ECG, EMG and other biological signals and ambient noise. The signal is stimulus timed and most of the noise is random, allowing noise to be averaged over repeated responses.
Impulses and signals
Signals can be recorded from the cerebral cortex, brainstem, spinal cord and peripheral nerves. Usually the term "evoked potential" is reserved for responses involving recording or stimulation of structures in the central nervous system.systems. Thus, complex motor or sensory nerve evoked potentials used in nerve conduction studies are not usually considered evoked potentials, although they fit the definition above.
Sensory evoked potentials
These are recorded from the central nervous system after sensory stimulation, such as visually evoked potentials due to a flashing light or a changing pattern on a monitor, auditory potentials evoked by a click or tone stimulus presented through headphones, or tactile or somatosensory potential evoked 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, as well as in intraoperative neurophysiological monitoring, known as surgical neurophysiology. It was thanks to her that the method of evoked potentials became a reality.
Views
There are two kinds of evoked potentials in widespread clinical use:
- Auditory evoked potentials, usually recorded on the scalp, but occurring at the level of the brainstem.
- Visually evoked potentials and somatosensory evoked potentials that result from electrical stimulation of a peripheral nerve.
Anomalies
Long and Allen reported anomalousbrain potentials (BAEP) evoked by auditory potentials in an alcoholic woman recovering from acquired central hypoventilation syndrome. These researchers hypothesized that their patient's brainstem was poisoned but not destroyed by her chronic alcoholism. The method of evoked potentials of the brain makes it easy to diagnose such things.
General definition
An evoked potential is the electrical response of the brain to a sensory stimulus. Regan built an analog Fourier series analyzer to record evoked potential harmonics into flickering (sinusoidally modulated) light. Instead of integrating sine and cosine products, Regan fed signals to a dual-processor recorder through low-pass filters. This allowed him to demonstrate that the brain had reached a steady state, 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 the idealized steady state evoked potential as a form of response to repetitive sensory stimulation in which the frequency components of the response remain constant over time in amplitude and phase.
Although this definition implies a series of identical time waveforms, it is more useful to define the evoked potential method (SSEP) in terms of frequency components, which are an alternative description of the waveform in the time domain,since different frequency components can have completely different properties. For example, the properties of the high-frequency SSEP flicker (which peaks at about 40–50 Hz) correspond to those of subsequently discovered magnocellular neurons in the macaque monkey retina, while the properties of the mid-frequency SSEP flicker (which peaks at about 15–20 Hz) correspond to those of parvocellular neurons. Since SSEP can be fully described in terms of the amplitude and phase of each frequency component, it is quantified more uniquely than the average transient evoked potential.
Neurophysiological aspect
Sometimes it is said that SSEPs are obtained through high repetition rate stimuli, but this is not always correct. In principle, a sinusoidally modulated stimulus can induce SSEP even if its repetition rate is low. Due to the high frequency rolloff of SSEP, high frequency pacing can result in an almost sinusoidal SSEP waveform, but this is not the definition of SSEP. Using zoom-FFT to record SSEP with a theoretical spectral resolution limit of ΔF (where ΔF in Hz is the reciprocal of the recording duration in seconds), Regan found that the amplitude-phase variability of SSEP can be quite small. The bandwidth of the SSEP frequency components can be at the theoretical limit of spectral resolution up to at least 500 seconds of the recording duration (in this case 0.002 Hz). This is all part of the evoked potential method.
Meaning and application
This method allows multiple (eg four) SSEPs to be recorded simultaneously from any given location on the scalp. Different stimulation sites or different stimuli can be marked with slightly different frequencies, which are almost identical to brain frequencies (calculated using the brain evoked potential method), but are easily separated by Fourier series analyzers.
For example, when two non-proprietary light sources are modulated at several different frequencies (F1 and F2) and superimposed on each other, multiple non-linear frequency cross-modulation components (mF1 ± nF2) are created in SSEP, where m and n are integers. These components allow you to explore non-linear processing in the brain. By marking the frequency of the two superimposed grids, the spatial frequency and orientation adjustment properties of the brain mechanisms that process spatial form can be isolated and studied.
Stimuli of various sensory modalities can also be labelled. For example, a visual stimulus 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 brain magnetic response demonstrated an area of audiovisual convergence in the human brain, and the distribution of the response over the head made it possible to localize this area of the brain. Recently, frequency tagging has expanded from sensory processing research to selective attention and consciousness research.
Sweep
Sweep methodis a subspecies of the evoked potential method vp. For example, a plot of response amplitude versus stimulus checkerboard pattern size can be obtained in 10 seconds, which is much faster than averaging over the time domain to record the evoked potential for each of several control sizes.
Schematic
In the original demonstration of this technique, the sine and cosine products were fed through low-pass filters (as in SSEP recording) while viewing a fine test circuit whose black and white squares swapped six times per second. The size of the squares was then gradually increased to obtain a plot of evoked potential amplitude versus control size (hence the word "sweep"). Subsequent authors implemented a sweep technique using computer software to increase the spatial frequency of the grating in a series of small steps and calculate the time domain average for each discrete spatial frequency.
One sweep may be enough, or it may be necessary to average the graphs over several sweeps. Averaging 16 sweeps can improve the signal-to-noise ratio of the graph by a factor of four. The sweep technique has proven useful for measuring rapidly adapting visual processes, as well as for recording children, where the duration is necessarily short. Norcia and Tyler used the technique to document the development of visual acuity andcontrast sensitivity during the first years of life. They emphasized that in diagnosing abnormal visual development, the more accurate the developmental norms, the more clearly one can distinguish between abnormal and normal, and to this end, normal visual development has been documented in a large group of children. For many years, the sweep technique has been used in pediatric ophthalmology clinics (in the form of electrodiagnostics) around the world.
Method benefits
We have already talked about the essence of the evoked potential method, now it's worth talking about its advantages. This method allows SSEP to directly control the stimulus that elicits SSEP without the conscious intervention of the experimental subject. For example, a moving average of SSEP can be arranged to increase the brightness of the checkerboard stimulus if the SSEP amplitude falls below some predetermined value, and decrease the brightness if it rises above that value. The amplitude of the SSEP then oscillates around this setpoint. Now the wavelength (color) of the stimulus changes gradually. The obtained plot of the dependence of the stimulus brightness on the wavelength is a graph of the spectral sensitivity of the visual system. The essence of the method of evoked potentials (VP) is inseparable from graphs and diagrams.
Electroencephalograms
In 1934, Adrian and Matthew noticed that potential changes in the occipital EEG could be observed with light stimulation. Dr. Cyganek developed the first nomenclature for occipital EEG components in 1961. During the same year Hirsch andhis colleagues recorded visually evoked potential (VEP) on the occipital lobe (outside and inside). In 1965, Spelmann used chessboard stimulation to describe human WEP. Shikla and colleagues completed an attempt to localize structures in the primary visual pathway. Halliday and colleagues completed the first clinical studies by recording delayed VEPs in a patient with retrobulbar neuritis in 1972. From the 1970s until today, a large amount of extensive research has been done to improve procedures and theories, and this method has also been tested on animals.
Flaws
Scattered light stimulus is rarely used these days due to high variability both within and between subjects. However, this type is advantageous when testing infants, animals, or people with poor visual acuity. The checkerboard and lattice patterns use light and dark squares and stripes, respectively. These squares and stripes are equal in size and are presented one by one on the computer screen (as part of the evoked potential method).
Electrode placement is extremely important to obtain a good VEP response without artifacts. In a typical (single channel) setup, one electrode is located 2.5 cm above the ion and the reference electrode is located at Fz. For a more detailed answer, two additional electrodes can be placed 2.5 cm to the right and left of the ounce.
Auditory method of evoked potentials of the brain
He canused to track the signal generated by sound through the ascending auditory pathway. The evoked potential is generated in the cochlea, passes through the cochlear nerve, through the cochlear nucleus, the superior olive complex, the lateral lemniscus, to the inferior colliculus in the midbrain, to the medial geniculate body, and finally to the cerebral cortex. This is how the method of evoked potentials of the central nervous system, carried out with the help of sound, works.
Auditory evoked potentials (AEPs) are a subclass of event-related potentials (ERPs). ERPs are brain responses that are time-bound to an event such as a sensory stimulus, a mental event (recognition of a target stimulus), or skipping a stimulus. For AEP, an "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 various tones, speech sounds, etc.
Auditory brainstem evoked potentials are small AEPs that are recorded in response to an auditory stimulus from electrodes placed on the scalp.
AEP are used to assess auditory function and neuroplasticity. They can be used to diagnose learning disabilities in children, helping to develop specialized educational programs for people with hearing or cognition problems. Within the framework of clinical psychology, the method of evoked potentials is used quite often.
