Study On The Different Stimulative Fileds P-VEP

IntroductionVisual(vision)mainly includes light perception,color and form,it is important for people to feel outside of the system.In the forensic identification,visual function mainly based on form(visual acuity and vision)to determine the function of.The assessment of visual field is usually involved in clinical forensic practice.The more subjective result recorded by the subjective measure of visual field often becomes unreliable 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).P100 can be recorded in any spatial frequencies in all of healthy persons,and as it's little variation,stable and reliable,this study select it for the mainly observed values.The aplication of P-VEP is an important method to evaluate the visual function,and Previous studies show that there are difference among different stimulating field in P—VEP.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 visual field orderly of 30 subjects(60 eyes)with normal visual function.A multi-channel recording electrode were placed in Oz,O1,T5,T3,O2,T6,T4,Pz,Oz,Iz and set up a recorded point X to record.To study each stimulation field's P—VEP in the normal latency and amplitude and its characteristics of P100 on the Oz points and their mutual relationship.And the characteristics of each stimulation field P—VEP of different parts of the occipital scalp distribution of the electric field.Hope to provide basic information to explore the objective evaluation method of perspective.Subjects and MethodsSubject of the test:We studied 30 subjects(16 men and 14 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 HaiShen multifunctional electrophysiological recorder.The screen of the stimulator is 17 inch and pattern were reversed at a frequency of 3 times per second,the visual angle of the stimulation is 30＇with the contrast and the luminance is fixed at 80%.The band-pass from 1-100Hz,the scanning time is 250ms and summation time is 100.Scalp potentials were recorded from four Ag-AgCl surface electrodes.The record point are Oz(5cm above the inion);O₁(2 cm on the lift of Oz);T₅(5cm on the lift of Oz); T₃(10cm on the lift of Oz);O₂(2cm on the right of Oz);T₆(5cm on the right of Oz); T₄(10cm on the right of Oz);Pz(5cm above Oz);Iz(at the inion);X(2cm below the inion).All recording channels were referred to a single reference electrode at the Fz point(12cm above the root of the nose),and the safety electrode is deposited on the wrist.The visual field VEP was recorded in four recording channels according to the international 10-2 ECG recording system.The tested monocular eye was kept with natural pupil and 1.15 meters away from the stimulator.The resistance of each electrodes was less than 5kΩ,and the differences between the electrodes were less than 3kΩ.During testing the subject was asked to gaze at a targetwith the other eye covered. The VEP was evoked in the condition of each stimulate field and checkerboard stimulation respe- ctively.Each test lasted about 40s,and take a rest for 2-3 minutes between tests into eliminate the tiredness.We Recorded the P—VEP results under the vision correction.Result 1.The P—VEP among Above/Middle/below stimulated field.The latency of P100 among each stimulated field:below＜middle＜above.The latency of P100 among each stimulated field:above＜below＜middle.The amplitude of P100 among each record poin:X＜Iz＜Pz＜OzThe amplitude of P100 among each record poin:X＜Iz＜Pz＜OzThe VEP of middle and below stimulated field are better than the above one,as them have large amplitude and easy to distinguish,and about 15.83%above stimulated field VEP appear phase reversed.Although the VEP record On the Pz have small amplitude than on the Oz especially the above stimulation,it was little interferenced. v2.the distribution of electric fieldStimulated by full-central-field in normal individuals,the VEP of bilateral record position are symmetric,the distance between record position and Oz and the latency/amplitude of P₁₀₀are Negative correlation.It means that stimulated by the same field the amplitude of VEP became lower and lower while the distances between record and Oz are longer and longer,and the latency of VEP are gradually shorter.3.P—VEP by left/right-helf-field:Half-field stimulated a lesser amplitude VEP,in the head,it was significantly asymmetric distribution,on the same side of stimulating field can record constant wave VEP:N-P-N composite wave.On the contralateral side of stimulating field can record VEP more complex waveforms,variation,which may be similar to the potential of zero-line,and can also be reversed wave(P-N-P wave).There are four main types VEP in normal:1,stimulating field ipsilateral N-P-N wave,the contralateral non-identifiable VEP ingredients;2,stimulating field ipsilateral N-P-N wave,the contralateral phase reversed P-N-P wave;3,stimulating field ipsilateral N-P-N wave,the contralateral part of phase reversed VEP;4,stimulating field ipsilateral and contralateral are all N-P-N wave.In the half-field stimulation:stimulating field ipsilateral electrodes can record N-P-N compound wave,the Central electrodes can also record NPN composite wave, in the contralateral electrodes about 7.1%can't record identifiable VEP,about 12.92% can record reversed waves or parts of reversed one,about 79.98%can record NPN compound wave.In the half-peiheral-field stimulation:stimulating field ipsilateral electrodes about 11.94%can't record identifiable VEP,about 88.06%can record N-P-N compound wave, the VEP of Central electrodes are likely to the ipsilateral ones or the contralateral ones, in the contralateral electrodes about 14.58%can record N-P-N wave.43.06%can record P-N-P wave,42.36%can't record identifiable VEP.4.The VEP of nasal/temporal-field stimulation:Central-field stimulating VEP is likely to the nasal-field one(compare with temporal one).The result of comparing P₁₀₀of nasal-field and temporal-field exists significant difference,the latency of nasal-field stimulate is longer than the temporal-field one about 3.9±1.2ms.There are no significant difference between eyes of the same stimulation.5.The VEP of full- and central-field stimulationThere are no significant difference between full-field 16°stimulation and the central-field 7.5°stimulation in latency of P₁₀₀,the amplitiude of full-screen stimulate is larger than the central-field.6.The VEP of half-field;half-central-field and half- peiheral-field.P₁₀₀latency:semi-centre-field≈semi-full-field＜semi-peiheral-field.P₁₀₀amplitude:semi-peiheral-field＜semi-centre-field≈semi-full-field.Conclusion1.There are difference among up-field-stimulation;mid-field-stimulation and down-field-stimulation both in latency and amplitude.There are also difference among Pz;Oz;Iz and X(set up by myself)by the same stimulation.Iz and X can record more steable P—VEP.2.The VEP of full-field stimulation recorded by different point are difference,the latency and amplitude of P₁₀₀get shorter and smaller among the increasing distance to the Oz both in horizontal and vertical direction. 3.The VEP of different semi-field stimulation recorded by different horizontal points are difference.In stimulating field ipsilateral points can stably record VEP,and in the contralateral points can record specific VEP.4.In the all kinds of semi-field Stimulation the one which contain central-field can evoke VEP easily.In the semi various semi-wild,the center includes stimulation induced by the high rate of VEP.In the semi-peiheral-field stimulation,increasing the stimulating area and reducing spatial frequency properly can enhance the detection rate of VEP.