I. Definition and classification of evoked potentials In physiology, the measurable potential changes produced in any part of the central nervous system by conscious stimulation of sensory organs, sensory nerves and any point on the sensory pathway are called evoked potentials. Also, the potential changes in the spinal cord and brain produced by electrical stimulation of the anterior roots of the spinal cord and the cerebral cortex, as well as the potential changes in the cerebral cortex caused by random movements of the limbs, are also called evoked potentials. According to the method of stimulation, the evoked potentials can be divided into the following categories: 1. Somatosensory evoked potentials: referred to as somatosensory evoked potentials (SEPs). Generally, it refers to a series of potential changes at different levels of the nervous system caused by stimulating nerve trunks or nerve endings with electrical impulses. 2. Visual evoked potentials (VEPs): VEPs refer to the potential changes in the cerebral cortex induced by stimulating the retina with light. 3, Auditory evoked potential (AEP): AEP refers to the potential changes in the auditory conduction pathway and cerebral cortex caused by stimulation of auditory receptors with short sounds or pure tones. 4.Event-related potentials: mainly reflect the potential changes of higher brain function activities, including P300, N400, MMN, CNV and acoustic shadow matching. 5.Motor evoked potentials: refers to the potential activity recorded by placing recording electrodes on the subject’s limbs according to certain requirements and giving stimulation from the scalp and spinal cord. 6. Olfactory and gustatory evoked potentials. In addition, there are other methods of classification. In this section, only the first few evoked potentials are described, because they have emerged from the laboratory and are widely used in clinical examination. The discovery and development of evoked potentials The evoked potentials were discovered by Richard Caton, a professor of physiology at the University of Liverpool in England, at the same time as the discovery of brain waves Caton’s discovery was used by neurophysiologists to study sensory physiology. They used evoked potentials to determine the central localization of sensory functions, neural connections and projection relationships. However, early evoked potentials could only be recorded in the laboratory from the surface of the brain or spinal cord of animals, and could not be used clinically. Dawson further investigated the method of recording evoked potentials from the human surface and improved the signal-to-noise electro-mechanical iteration technique, which opened up a new era for the recording of evoked potentials. 1958 Ciark of Lincoln Laboratory, USA, designed the first average In 1958, Ciark designed the first averaging computer at Lincoln Laboratory in the United States, which better solved the problem of extracting evoked potentials from spontaneous EEG activity, so that evoked potentials could be clearly recorded from the body surface. The application of electronic computer averaging and iteration technology solved the problem of recording methods of evoked potentials and created the necessary conditions for studying human evoked potentials. From around 1960, many neurophysiologists and neurologists became interested in evoked potentials, and they studied the interrelationship between evoked potentials and neural function and structure with the same curiosity as they did the intrinsic mechanism of brain electrical activity. In the 1960s, research on evoked potentials focused on analyzing the waveforms and influencing factors of evoked potentials and exploring the relationship between evoked potentials and conduction pathways and the sites of origin; Jewett et al. (1970) were the first to record and study auditory brainstem evoked potentials; Halliday (1972) reported that VEP with graphic flip stimulation had greater utility than VEP with flash stimulation; Cracco (1976) reported that VEP with flash stimulation had greater utility than VEP with flash stimulation. Cracco (1976) was the first to record far-field potentials from electrically stimulated median nerves, and Desmedt followed up with a fruitful study of such short latency SEPs. It was in the early seventies that evoked potentials began to enter the clinical application phase and developed rapidly. In conclusion, evoked potentials have been around for nearly a century since their introduction. It is only in the last three decades that evoked potentials have made the transition from the laboratory to the clinic after long and extensive research. In general, evoked potentials are still at the stage of applied research in clinical practice. At present, there is no unified standard for examination and judgment, and there are many problems that need further research. Because the amplitude of evoked potentials is very small, only one tenth to one hundredth of the EEG amplitude, so to extract the evoked potentials from the EEG activity, it is necessary to use the average iteration technique. The basic method of this technique is to amplify the EEG signal for a period of time after stimulation, convert it to digital and store it in an analog-to-digital converter, iterate and average the signal after repeated stimulation, and present the result on a monitor with a certain waveform after digital-to-analog conversion. In this way, signals with a fixed temporal relationship to the stimulus become clearer and clearer after iteration, while signals without a fixed relationship to the stimulus, positive and negative potentials cancel each other out. Evoked potentials are signals with a certain latency and polarity, while noisy signals such as EEG and EMG appear randomly. Therefore, after averaging iterative processing, evoked potentials are highlighted. In order to improve the effect of averaging, the newer averagers are equipped with a filtering function, which can exclude the excessive interference signals and prevent them from entering the averaging system. The various evoked potentials recorded by stimulating different sensory systems have some common characteristics. In summary, there are several aspects: (1) latency period, there is a certain time relationship between the appearance of evoked potentials and the given stimulus. The time from the start of the stimulus to the appearance of the potential is the initial latency period, and the time from the start of the stimulus to the peak of the potential wave is the peak latency period. Since the starting point of the evoked potential is often less clear than the wave crest, the peak latency is usually measured. The length of the latency is related to the peripheral nerve conduction velocity, the distance between the stimulation site and the recording site, the number of transitions on the conduction path, and the length of the synaptic delay. In addition, the latency of evoked potentials is also affected by age, gender, stimulus intensity, and skin temperature. Therefore, the latency of evoked potentials is not a fixed number, but has some individual differences. (2) Evoked potentials are distinguished by positive and negative potentials, i.e., they have polarity. Positive and negative waves are distinguished by the fact that when the recording electrode is connected to the negative pole of the preamplifier, the upward wave is negative and the downward wave is positive; conversely, the upward wave is positive and the downward wave is negative. The evoked potential recorded on the body surface has a small wave amplitude, usually below 5 μν. The range of variability of evoked potential amplitude is relatively large. Not only can they vary exponentially from person to person, but the amplitudes of the corresponding components on both sides of the same person can also be significantly different. The reasons for the large variation in amplitude may be related to factors such as stimulus intensity, electrode impedance, signal interference and excitability of peripheral nerves and cortex. Since the variability of wave amplitude is too large, it is difficult to judge whether the evoked potentials are normal only based on the change of wave amplitude, so the change of wave amplitude is generally only used as a reference when distinguishing evoked potentials. (3) Evoked potentials have certain waveforms, and due to the different structures of different sensory systems, the evoked potentials of different sensory systems have their own special patterns. However, in the same system, the pattern of evoked potentials caused by stimulating different parts of the body is the same except that the latency period varies according to the distance between the stimulation point and the recording point, and the waveform composition is the same. Since the waveforms of evoked potentials are reproducible from person to person, the normalcy of evoked potentials can be judged according to whether each wave of evoked potentials is recorded or not. (4) Evoked potentials have a certain distribution range on the scalp, and can be divided into early and late components according to the early and late appearance of each component of the evoked potentials, with the early component having a short latency, being more fixed and having a more limited distribution on the scalp. The early component has a short latency, is relatively fixed, and has a limited distribution on the scalp. It is usually recorded only in the corresponding cortical sensory area, and the wave amplitude can be seen to be significantly smaller when the electrode position is moved. Late component latencies are longer and more widely distributed across the scalp. When one side of the sensory pathway is stimulated, it can be recorded on both sides of the scalp. Because evoked potentials have a certain distribution range on the scalp, the electrodes should be placed at the site where the maximum amplitude can be recorded when recording evoked potentials. (5) Evoked potentials have a certain origin, and each component of evoked potentials is generated by the sensory conduction pathway and the electrical activity of a certain part of the cerebral cortex. At present, the origin of some components of evoked potentials is relatively well understood, while the origin of some other components has not been elucidated and needs further study. In view of the relationship between evoked potentials and neural structures, the site of the lesion can be determined with the help of evoked potential examination. V. Mechanism of evoked potentials The human body is a volume conductor. The neural electrical activity caused by stimulating a sensory organ or a part of the pathway can be transmitted to the surface of the body and recorded by the instrument, which is the evoked potential. Therefore, the evoked potentials on the body surface are essentially the field potentials generated by the electrical activity of the nerve, and the recorded potentials can be positive or negative, or large or small, depending on the position of the electrode in the electric field. Larger positive potentials are recorded when the lead electrode is close to the power source, and larger negative potentials are recorded when it is close to an electrical point. It is generally accepted that cortical evoked potentials are formed by localized postsynaptic potentials. When the nerve impulse wave caused by stimulating a part of the sensory system is transmitted to the cortex, the deep cell bodies and parietal dendrites of the cerebral cortex are depolarized and repolarized successively, resulting in the formation of postsynaptic potentials that can be recorded from the body surface. As to the question of whether evoked potentials come from cone cells or stellate cells, it is generally believed that the electric field formed by cone cells is open and can be recorded from the surface, whereas the electric field formed by stellate cells is closed and cannot be recorded from the surface. Thus, evoked potentials may be generated by cone cell activity. Subcortical evoked potentials may have two sources. Some potentials may be postsynaptic potentials from the nucleus accumbens, and others may be electrical activity from the presynaptic nerve conduction bundle. One of the more commonly used methods of naming evoked potentials is to name them according to their polarity and latency. A positive potential is denoted by “P”, a negative potential by “N”, and the peak latency of the potential is written in the lower right corner of “N” or “P in the lower right corner of the “N” or “P”. Another method is to name the potentials in the order of their polarity and appearance, such as N1, P1, N2, P3 …… The advantage of this method is that the names of the evoked potential components are consistent and easy to compare; the disadvantage is that if a new component appears between these two components, it is not easy to name it. The more common method of naming brainstem auditory evoked potentials is to use the Roman letters I-VII according to the order of appearance of each component.