| IntroductionVisual field is the range that is the eyes of peopled fixation seen , it is the important constitution of visual function. In these days, there are many measure to visual field including the automatism examine of visual field and so on. The assessment of visual field is usually involved in clinical forensic practice. The more subjective result recorded by the subjective measure of visual field is often applied in clinical forensic practice difficultly due to such special phychological action of the patient as disguising the exact condition. How to assess the state of visual field function objectively, therefore, becomes an important study of clinical forensics.Visual evoked potential (VEP) is the electrical activity evoked at the region of occipital cortex through the visual pathway by stimulating the retina, which indicates the function of the pathway from the retinal ganglion cell to the visual cortex. Among all kinds of VEP, the pattern visual evoked potential (P -VEP) is widely applicated which is mainly composed of negative response 75 ( N75 ) and positive response 100(P100) as well as negative response 145(N145). The aplication of P - VEP is an important method to evaluate the visual function. So it is meaningful in forensic practice to apply P - VEP to evaluate different visual field and the function of different retinal cells objectively.The study applied P - VEP to stimulate the full - field , central field and the peripheral field orderly of 30 subjects (60 eyes ) with normal visual function as to study normal response threshold and the relationship between them as well as the different visual fields' distributing character by three channels track record. The study was designed to investigate the effect of different visual field andspecial locations of the retina on the visual acuity normal response threshold and the interaction between them.Subjects and methodsSubject of the test: We studied 30 subjects (15 men and 15 women) with normal visual function who can cooperate in the subjective test. They aged meanly 23 years old and the rage is 21 to 26, all of whom come from the China Medical University. The criterion of normal visual function is that the corrective vision is or more than 1.0 and the subjective visual field is normal.Method: The visual stimulus viewed by each subject on the television monitor is the black — white pattern reversal checkerboard and the record is taken by the Nicolet Compact Four multifunctional electrophysiological recorder. The screen of the stimulator is 12 inch and pattern were reversed at a frequency of 1. 9 times per second, the visual angle of the stimulation is 30'with the contrast and the luminance is fixed when the subject feel best. The band - pass from 1 -100Hz, the scanning time is 250ms and summation time is 100.Scalp potentials were recorded from three Ag - AgCl surface electrodes ( one applied 2cm above the inion, at Oz point, the other two symmetrically on either side of the midline, the Oj and O2 point). All recording channels were referred to a single reference electrode at the FPz point (4 cm above the root of the nose) , the safety electrode is deposited at Cz point (the centre of the line connecting the two ears) the visual field VEP was recorded in three recording channels according to the international 10-2 ECG recording system. The tested monocular eye was kept with natural pupil and 1.4 meters away from the stimulator. The resistance between the electrodes was less than 5kH, the differences between the electrodes were less than 3kft. During testing the subject was asked to gaze at a target (the centre of the screen) with the other eye covered. The VEP was evoked in the condition of full - field stimulation ( 8 ° visual field) , centre stimulation(2° visual field) , peripheral stimulation(8° visual field except 2° visual field) and checkerboard stimulation respe - ctively. We asked the subject to count the reversal times to themselves in order to kee them gazing. A5 minutes'rest was introduced after each time so as to eliminate the tiredness. We recorded the Nl ,P1 and N2 of the P - VEP with the corrected vision under the condition of full -field, center field and peripheral field.Result1. The full - field and the peripheral field as well as the central field PRVEP are composed of Nl, PI and N2 in normal - vision subjects, but it iden-tificates hardly about Nl compare d with PI and N2 in the central field PRVEP.2. there is not marked diversity througt the paired test of t at full - field and peripheral field as well as the central field PRVEP in the same case of normal - vision subjects ( P > 0.05 ) .3. there is not obvious Nl in the 38 cases of 60 eyes at peripheral field PRVEP, and the wave of Nl s appear probability is 36. 77% . the amplitude of Nl is distinct small in the 22 cases. But the wave of Nl' s appear probability is 100% at full - field and peripheral field PRVEP. And the wave of PI' s appear probability is 100% at full - field, peripheral field and central field PRVEP as well as the wave of N2s appear probability.4. The delitescence of Nl is remarkable difference (P <0. 05) , but the amplitude of Nl is not obvious difference (P >0.05) at full -field and peripheral field PRVEP.The delitescence and the amplitude of PI is obvious difference(P <0.001) through one way analysis of variance and S - N - K test at full - field and peripheral field as well as central field PRVEP.The delitescence and the amplitude of N2 is obvious difference ( P <0. 001) through one way analysis of variance and S - N - K test at full - field and peripheral field as well as central field PRVEP.5. The each wave of three different stimulate visual -fields PRVEP at Ox 02 points is basal accord with recorded at 0, point, and there is not statistical significance( P > 0.05 ) . But the amplitude of each wave at three different stimulate visual - fields PRVEP of the Oj 02 record points is smaller than the Oz point. As well as the amplitude of each wave at the 0, point is smaller than the02 point.Conclusion1. The latency of the P - VEP recorded from full - field, peiheral - field and central - field in normal individuals exists diffrenences, so does the amplitude of PI andNl.2. It can appraise orientation to the function of the different area of visual field and the different vision cell, through the form^delitescence and amplitude of different stimulate field PR VEP' s Nl and pi. And especially, Nl, which is the specific assessment index of peripheral - field and rod cells — photoreceptor pathway.3. it is feasible that P - VEP evoked by right and left sides of the occipital cortex under different stimulating fields is applied to assess objectively the function of different fields. |
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