The amplitude of the evoked potential tends to be low, ranging from less than one microvolt to several, compared to tens of microvolts for electroencephalography (EEG), millivolts for electromyography (EMG) and often close to 20 millivolts for an electrocardiogram (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 tied to the stimulus time, and most of the noise occurs randomly, which makes it possible to average the noise by averaging the repeated responses.
Impulses and signals
Signals can be recorded from the cerebral cortex, brain stem, spinal cord, and peripheral nerves. Typically, the term “evoked potential” is reserved for responses involving recording or stimulating the structures of the central nervous system. Thus, evoked complex motor potentials of action or potentials of the sensory nerves used in studies of nerve conduction are usually not considered as evoked potentials, although they correspond to the above definition.
Sensory evoked potentials
They are recorded from the central nervous system after stimulation of the senses, for example, visually evoked potentials that appear due to a blinking light or a changing pattern on the monitor, auditory potentials caused by a click or tonal stimulus presented through the headphones, or tactile or somatosensory potential caused by tactile or electrical stimulation of the sensory or mixed nerve in the periphery. Sensory evoked potentials have been widely used in clinical diagnostic medicine since the 70s of the last century, as well as in intraoperative neurophysiological monitoring, known as surgical neurophysiology. It is thanks to her that the method of evoked potentials has become a reality.
Kinds
There are two types of evoked potentials in widespread clinical use:
- Auditory evoked potentials, usually recorded on the scalp, but occurring at the level of the brain stem.
- Visually evoked potentials and somatosensory evoked potentials that occur as a result of electrical stimulation of the peripheral nerve.
Anomalies
Long and Allen reported abnormal brain potentials (BAEP) caused by the auditory potentials of an alcoholic woman who recovered from acquired central hypoventilation syndrome. These researchers hypothesized that their patient’s brain stem was poisoned but not destroyed by her chronic alcoholism. The method of evoked brain potentials makes it easy to diagnose such things.
General definition
Evoked potential is the electrical response of the brain to a sensory stimulus. Regan built an analog Fourier series analyzer to record the harmonics of the evoked potential 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 reached a stationary mode in which the amplitude and phase of the harmonics (frequency components) of the response were approximately constant in time. By analogy with the steady-state response of the resonant circuit, which follows the initial transient response, he defined the idealized steady-state evoked potential as a response form to repeated 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 signals, 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 high-frequency scintillation SSEP (whose peak amplitude is about 40-50 Hz) correspond to the properties of subsequently detected magnocellular neurons in the macaque monkey's retina, while the properties of mid-frequency scintillation SSEP (whose peak amplitude is about 15–20 Hz) correspond to the properties 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
It is sometimes said that SSEPs are obtained through stimuli with a high repetition rate, but this is not always correct. In principle, a sinusoidally modulated stimulus can cause SSEP, even if its repetition rate is low. Due to the high-frequency decline of SSEP, high-frequency stimulation can lead to an almost sinusoidal waveform of SSEP, but this does not apply to 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 time in seconds), Regan found that the amplitude-phase variability of SSEP can be quite small. The bandwidth of the components of the frequency component of SSEP can be at the theoretical limit of spectral resolution, at least up to 500 seconds of recording time (in this case, 0.002 Hz). All this is part of the evoked potentials method.
Meaning and Application
This method allows you to simultaneously register several (for example, four) SSEPs from any given location on the scalp. Different stimulation sites or different stimuli can be labeled with slightly different frequencies, which are almost identical to brain frequencies (calculated using the method of evoked brain potentials), but are easily separated by Fourier analyzers.
For example, when two generic light sources are modulated at slightly different frequencies (F1 and F2) and superimposed on each other, multiple nonlinear 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, two superimposed lattices, the spatial frequency and orientation properties of the brain mechanisms that process the spatial form can be isolated and studied.
Incentives of various sensory modalities can also be labeled. For example, a visual stimulus flickered at Fv Hz, and simultaneously the auditory tone presented was modulated in amplitude Fa Hz. The existence of the (2Fv + 2Fa) component in the evoked magnetic response of the brain demonstrated the region of audiovisual convergence in the human brain, and the distribution of the response over the head made it possible to localize this region of the brain. Recently, frequency marking has been expanded from studies of sensory processing to studies of selective attention and consciousness.
Scan
The sweep method is a subtype of the evoked potentials method vp. For example, a graph of the dependence of the response amplitude on the control size of the pattern of the chessboard of a stimulus can be obtained in 10 seconds, which is much faster than when averaging over the time domain for recording the evoked potential for each of several control sizes.
Schematic image
In an initial demonstration of this technique, the sine and cosine products were fed through low-pass filters (as when recording SSEP) when looking at the exact test scheme whose black and white squares swapped six times per second. Then the size of the squares gradually increased to get a graph of the dependence of the amplitude of the evoked potential on the control size (hence the word “sweep”). Subsequent authors introduced the sweep technique using computer software to increase the spatial frequency of the grating in a series of small steps and calculate the average value in the time domain for each discrete spatial frequency.
A single sweep may be sufficient, or averaging of graphs over several sweeps may be required. Averaging 16 sweeps can improve the signal-to-noise ratio on a graph by four times. The sweep technique has proven to be useful for measuring rapidly adapting visual processes, as well as for recording children where the duration is necessarily short. Norsia and Tyler used the technique to document the development of visual acuity and contrast sensitivity during the first years of life. They emphasized that in the diagnosis of abnormal visual development, the more accurate the development standards, the more sharply one can distinguish between abnormal and normal, and for this purpose, normal visual development in a large group of children is documented. For many years, the scanning technique has been used in pediatric ophthalmology clinics (in the form of electrodiagnostics) around the world.
Method Advantages
We have already talked about the essence of the method of evoked potentials, now it is worth mentioning its advantages. This method allows SSEP to directly control the stimulus that induces SSEP without conscious intervention by the experimental subject. For example, a moving average SSEP value can be arranged to increase the brightness of the checkerboard stimulus if the amplitude of the SSEP falls below a certain predetermined value, and decrease the brightness if it rises above this value. The amplitude of the SSEP then oscillates around this setpoint. Now the wavelength (color) of the stimulus is gradually changing. The resulting drawing 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 can be observed when stimulated by light. Dr. Tsyganek developed the first nomenclature for the components of the occipital EEG in 1961. During the same year, Hirsch and colleagues recorded visually evoked potential (VEP) on the occipital lobe (outside and inside). In 1965, Spelmann used the stimulation of a chessboard to describe human VEP. Scikla and his colleagues completed an attempt to localize the structures in the primary visual pathway. Hallyday and his colleagues completed their first clinical studies, recording the delayed VEP in a patient with retrobulbar neuritis in 1972. From the 1970s to today, a large number of extensive studies have been conducted to improve procedures and theories, and this method has also been tested on animals.
disadvantages
Nowadays, the scattered light stimulus is rarely used due to the high variability both within and between subjects. However, this type is beneficial for testing infants, animals, or people with poor visual acuity. Chess and trellis patterns use light and dark squares and stripes, respectively. These squares and stripes are equal in size and presented in a single image on a computer screen (as part of the procedure called evoked potentials method).
The placement of the electrode is extremely important to obtain a good VEP response without artifacts. In a typical (with one channel) installation, one electrode is located 2.5 cm above the ion, and the reference electrode is located on Fz. For a more detailed answer, two additional electrodes can be placed 2.5 cm to the right and left of the ounce.
The auditory method of evoked brain potentials
It can be used 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 lower colliculus in the midbrain, to the medial crankshaft, and finally to the cerebral cortex. This is how the method of evoked potentials of the central nervous system, implemented using sound, works.
Auditory evoked potentials (AEP) are a subclass of event-related potentials (ERP). ERPs are brain reactions that are time-bound to an event, such as a sensory stimulus, a psychic event (recognition of a target stimulus), or skipping a stimulus. For AEP, an “event” is a sound. AEP (and ERP) are very small potentials of electrical voltage emanating from the brain, recorded from the scalp in response to an auditory stimulus, such as various tones, speech sounds, etc.
Auditory evoked potentials of the brain stem are small AEPs that are recorded in response to an auditory stimulus from electrodes placed in the scalp.
AEPs are used to evaluate the functioning of the auditory system and neuroplasticity. They can be used to diagnose learning disabilities in children, helping to develop specialized educational programs for people with hearing or cognitive problems. In clinical psychology, the method of evoked potentials is used quite often.