Visual evoked potentials are biological potentials that appear in the cerebral cortex in response to exposure to light on the retina.
A bit of history
They were first described by E. D. Adrian in 1941, but they were firmly fixed after Davis and Galambos put forward the technique of potential summation in 1943. Then the VEP registration method was widely used in the clinic, where the functional position of the visual pathway was studied in patients of the ophthalmological field. To register VEP, specialized standard electrophysiological systems based on modern computers are used.
A metal plate, that is, an active electrode, is placed on the patient's head two centimeters above the occiput in the midline above the area where the visual striate cortex is projected onto the cranial vault. An indifferent second electrode is placed on the earlobe or mastoid process. A ground electrode is fixed on the lobe of the other ear or on the skin in the middle of the forehead. How is a computer vision test performed? How the stimulant is used orlight flash (flash VEP), or reverse patterns from the monitor (VEP pattern). The stimulation field of view is about fifteen degrees. Studies are carried out without pupil enlargement. The age of the person undergoing the procedure also plays a role. Let's figure out how a person sees.
More about the concept
VEPs are the bioelectrical response of visual areas located on the cerebral cortex and thalamocortical pathways and subcortical nuclei. Wave generation of VEP is also related to the generalized mechanisms of spontaneous brain activity, which is recorded on the EEG. Responding to the effect of light on the eyes, VSTs show the bioelectrical activity mainly of the macular sphere of the retina, which is due to its greater representation in the visual cortical centers in comparison with the retinal regions located on the periphery.
How does registration work?
Registration of evoked visual potentials is carried out in the form of oscillations of the electric potential of a consistent nature or components that differ in polarity: the negative potential, or N, is directed upwards, the positive potential, that is, P, is directed downwards. The characteristic of the VIZ contains a form and two quantitative indicators. VEP potentials are normally much smaller (up to about 40 µV) in comparison with electroencephalogram waves (up to 100 µV). Latency is determined using the time period from the moment the light stimulus is turned on until reachingmaximum indicator of the potential of the cerebral cortex. Most often, the potential reaches its maximum value after 100 ms. If there are various pathologies of the visual pathway, then the shape of the VEP changes, the amplitude of the components decreases, the latency lengthens, that is, the time during which the impulse travels to the cerebral cortex along the visual pathway increases.
In what lobe is the visual area? It is located in the occipital lobe of the brain.
Varieties
The nature of the components in the VEP and their sequence is quite stable, but at the same time, the temporal characteristics and amplitude normally have variations. This is determined by the conditions in which the study is carried out, the specifics of the light stimulus, and the application of electrodes. During stimulation of the visual fields and a reverse frequency from one to four times per second, a phasic transient-VEP is recorded, in which three components are sequentially distinguished - N 70, P 100 and N 150. The frequency of reversion with an increase of more than four times per second causes the appearance of a rhythmic the total response in the cerebral cortex in the form of a sinusoid, which is called the VEP of the steady-state stability state. These potentials differ from phasic ones in that they do not have serial components. They look like a rhythmic curve with alternating drops and rises in potential.
Normal evoked potentials
The analysis of the VEP is carried out by the amplitude of the potentials, measured in microvolts, by the form of the record and the time periodfrom exposure to light to the appearance of peaks of SPM waves (calculation in milliseconds). They also pay attention to the difference in the amplitude of the potential and the magnitude of the latency during light stimulation in the right and left eyes in turn.
In VEP (what is it in ophthalmology, many people are interested) of the phasic type, during reversion with a low frequency of a checkerboard pattern or in response to a light flash, P 100, a positive component, is released with special constancy. The duration of the latent period of this component ranges normally from ninety-five to one hundred and twenty milliseconds (cortical time). The preceding component, that is, N 70, is from sixty to eighty milliseconds, and N 150 is from one hundred and fifty to two hundred. Late P 200 is not registered in all cases. This is how a computer vision test works.
Because the amplitude of the VEP differs in its variability, when taking into account the results of the study, it has a relative value. Normally, the values of its magnitude in relation to P 100 range in an adult from fifteen to twenty-five microvolts, higher potential values \u200b\u200bin children - up to forty microvolts. On pattern stimulation, the amplitude value of the VEP is slightly lower and is determined by the magnitude of the pattern. If the value of the squares is larger, then the potential is higher, and vice versa.
Thus, evoked visual potentials are a reflection of the functional state of the visual pathways and allow obtaining quantitative information in the course of the study. The results allow diagnosing pathologies of the visual pathway in patients with neuro-ophthalmicarea.
This is how a person sees.
Topographic mapping of head brain biopotentials by VEP
Topographic mapping of head brain biopotentials by VEP multichannel records biopotentials from different areas of the brain: parietal, frontal, temporal and occipital. The results of the study are transmitted to the monitor screen as topographic maps in color that varies from red to blue. Thanks to topographic mapping, the amplitude value of the VEP potential in ophthalmology is shown. What is it, we explained.
A special helmet with sixteen electrodes (the same as for EEG) is put on the patient's head. Electrodes are installed on the scalp at specific projection points: parietal, frontal over the left and right hemispheres, temporal and occipital. Processing and registration of biopotentials is carried out using specialized electrophysiological systems, for example, "Neurocartograph" from the company "MBN". Through this technique, it becomes possible to conduct an electrophysiological differential diagnosis in patients. With acute retrobulbar neuritis, on the contrary, there is bioelectrical activity, which is expressed in the back of the head, and the almost complete absence of excited areas in the frontal lobe of the brain.
Diagnostic value of visual evoked potentials in various pathologies
In physiological and clinical studies, if the visual acuity is high enough, it is best to use the method of registration of the physical VEPfor reversion.
In clinical and physiological studies with sufficiently high visual acuity, it is preferable to use the method of registering physical VEP on reverse chess patterns. These potentials are quite stable in terms of amplitude and temporal properties, are well reproducible and are sensitive to various pathologies in the visual pathways.
On the flash, VEPs are more variable and less sensitive to changes. This method is used in case of a serious decrease in visual acuity in a patient, the lack of fixation of his gaze, with an impressive clouding of the eye optical means, pronounced nystagmus, and in young children.
The following criteria are involved in the vision test:
- no response or large drop in amplitude;
- longer latency of all potential climaxes.
When recording visual evoked potentials, it is necessary to take into account the norm by age, especially for the study of children. When interpreting VEP registration data in early childhood with pathologies of the visual pathways, one should take into account the characteristic features of the electrocortical reaction.
There are two phases in the development of VEP, which are registered in response to pattern reversion:
- fast - from birth to six months;
- slow - from six months to puberty.
Already in the first days of life, VEPs are registered in children.
Topicaldiagnosis of brain pathologies
What does the EEG show? At the chiasmatic level, the pathology of the visual pathways (tumors, injuries, optochiasmal arachnoiditis, demyelinating processes, aneurysms) shows a decrease in the amplitude of the potentials, the latency increases, and individual elements of the VEP fall out. There is an increase in changes in the VEP simultaneously with the progression of the lesion. The prechiasmatic region of the optic nerve is involved in the pathological process, which is confirmed ophthalmoscopically.
Retrochiasmal pathologies are distinguished by interhemispheric asymmetry of visual potentials and are better seen with a multichannel type of recording, topocraphic mapping.
Chiasmal lesions are characterized by a crossover VEP asymmetry, expressed in significant changes in biopotentials in the brain on the opposite side of the eye, which has reduced visual functions.
During the analysis of VEP, hemianopic visual field loss should also be taken into account. In this regard, in chiasmal pathologies, light stimulation of half of the visual field increases the sensitivity of the method, which makes it possible to identify distinguishing features between dysfunction in the fibers of vision that come from the nasal and temporal parts of both retinas.
At the retrochiasmatic level of defects in the visual pathways (Graziole's fasciculus, optic tract, visual area of the cerebral cortex of the head), a unilateral dysfunction is observed, manifested in the form of non-crossed asymmetry, which is expressed in pathological VEP, which have the same indicators forstimulating each eye.
The reason why the bioelectrical activity of neurons in the central regions of the visual pathways decreases is homonymous defects in the visual field. If they capture the macular region, then during stimulation, half of the field changes and acquires a shape that is characteristic of central scotomas. If the primary visual centers are preserved, then the VEP may have normal values. What else does the EEG show?
Pathologies of the optic nerve
If there are pathological processes in the optic nerve, then their most characteristic manifestation is an increase in the latency of the main component of VEP R 100.
Neuritis of the optic nerve from the side of the affected eye, along with an increase in latency, is characterized by a decrease in the amplitude of the potentials and a change in the components. That is, the central vision is impaired.
Often, a W-shaped component of P 100 is registered, associated with a decrease in the functioning of the axial bundle of nerve fibers in the optic nerve. The disease progresses along with an increase in latency of thirty to thirty-five percent, a decrease in amplitude, and formal changes in the components of the VEP. If the inflammatory process subsides in the optic nerve, and visual functions increase, then the shape of the VEP and the amplitude indicators are normalized. The timing characteristics of the VEP remain increased for two to three years.
Optical neuritis, which develops against the background of multiple sclerosis, is determined even beforedetection of clinical symptoms of the disease by changes occurring in the VEP, which indicates the early involvement of the visual pathways in the pathological process.
One-sided optic nerve lesion, however, has very significant differences in the latency of the P 100 component (twenty-one milliseconds).
Anterior and posterior ischemia of the optic nerve due to an acute defect of arterial circulation in those vessels that feed it, are accompanied by a noticeable decrease in the amplitude of the VEP and a not too high (by three milliseconds) increase in the latency of P 100. In this case, the VEP values of the he althy eye usually remain normal.
A congestive disc at the initial stage is characterized by a decrease in the amplitude of visual evoked potentials (VEP) of a moderate nature and a slight increase in latency. If the disease progresses, then the violations get even more tangible expression, which is fully consistent with the ophthalmoscopic picture.
With atrophy of the optic nerve of the secondary type after suffering ischemia, neuritis, congestive disc and other pathological processes, a decrease in the amplitude of the VEP and an increase in the latency time P 100 are also observed. Such changes can be characterized by varying degrees of expression and appear independently of each other.
Pathological processes in the retina and choroid (serous central choriopathy, numerous forms of maculopathy, macular degeneration) contribute to an increase in the latency period and a decrease in amplitudepotentials.
There is often no correlation between a decrease in amplitude and an increase in the latency length of potentials.
Conclusion
So, we can conclude that although the VEP analysis method is not specific in determining any pathological process of the visual pathway, it is used for early diagnosis in the clinic of various kinds of eye diseases and clarifying the degree and level of damage. Of particular importance is the vision test and in ophthalmic surgery.
