SAMJ VOLUME 67 16 FEBRUARY 1985 243 assessment of visual an ethnically population Standards for the evoked potentials in heterogeneous adult A. S. DE GRAAF, C. J. LOMBARD, G. L. HART Summary Visual evoked potentials (VEPs) were recorded by chequerboard pattern-reversal stimulation in 276 normal adult subjects aged 15 - 73 years. The sample comprised comparable white, coloured and black groups of both sexes. Significantly shorter latencies of the major positive component were found in both male and female black subjects compared with those in whites. No significant differences were found between the amplitudes in the different population groups, but females in each population group showed significantly higher amplitudes of the major positive component than males. There was a close relation- ship between latency and amplitude for both right and left eyes. Except in coloured females, a non- linear relationship of the major positive component with age was demonstrated, the shortest latency being at about 35 years of age. It is suggested that the following criteria be used in the evaluation of VEP recordings: latency as well as the difference between latencies and amplitudes of the two sides of the major positive component (P1) as compared with standardized values for ethnic, sex and age groups. S AI! Med J 1985; 67: 243 - 245. Published results reveal differences in normative values for VEP latencies 'and amplitudes related to age and sex.6-12 In some studies the numbers of subjects are too small to allow statistical evaluation. Since most papers on this subject originate from the Western world, normative values for large non-white population groups have yet to be determined. This article is concerned with the analysis of VEP para- meters by age and sex in three different ethnic groups in the Cape Peninsula. Subjects and methods A group of 276 normal adult subjects was studied. White, coloured and black males and females were adequately repre- sented (Table I). Thirty-one per cent of the subjects were between IS and 29 years old, 32% between 30 and 44 years old, 28% between 45 and 59 years old and 9% older than 60 years. The mean age for females was 37 years and that for males 42 years, with a range of IS - 73 years. TABLE I. SAMPLE SIZES Ethnic group White Coloured Black Total Male 49 42 40 131 Female 49 45 51 145 Total 98 87 91 276 During the last decade the usefulness of the evoked potentials in clinical neurology has been proved. By applying the tech- nique of signal averaging to stimulus-coupled responses, ex- traction of the signal from background noise makes it possible to quantify its latency and amplitude. The reversing chequer- board pattern, in which dark and white squares are reversed, has become the standard stimulus for eliciting the visual evoked potential (VEP). Rotating mirrors,1.2 television tubes 3 and light-emitting electrodes 4 have been used to produce the pattern reversal, each method having its own distinctive charac- teristics. Stimulus characteristics and laboratory methods vary widely and markedly influence the parameters of the normal response. 5.6' It is therefore necessary to adhere to a standardized procedure with well-established normal values derived from a sufficient number of healthy subjects. Department of Neurology, Tygerberg Hospital, Parowvallei, CP A. S. DE GRAAF, M.D. (LEIDEN), Professor G. L. HART, Senior Technologisc Institute for Biostatistics of the South African Medical Research Council, Parowvallei, CP C. J. LOMBARD, PHD, Chief Research Officer Subjects with refractive errors were accepted when their visual acuity with spectacles could be corrected to 6/6 or better (Snellen chart), and colour-blindness was excluded by means of the Ishihara colour-chart test. The subjects were adapted to a dimly lit room and placed in a comfortable, semi-recumbent position. If they used spectacles they were requested to keep them on. They were tested with one eye fixing on a central target dot; the other eye was occluded with a patch. Silver-silver chloride electrodes were applied at the midline 5 cm above the inion (inverting), the reference electrode was placed at FZ and an electrode attached to the wrist was grounded. Electrode impedance was kept below 5K ohms. The electrodes were connected to the input of a pre-amplifier (type AA6M) of a Medelec MS6 EMG machine. A band width of between 1,6 and 32 Hz and a gain of 20 }l V per division was maintained. Pattern stimulation with a chequerboard array was per- formed by a Medelec visual stimulus pattern grader. The pattern was displayed on a 625-line 50 Hz television screen. The stimulus was delivered at a rate of 1 per second and the contrast ratio was kept at 100%. The pattern on the television measured 53 cm horizontally and 39,5 cm vertically and was placed 1,50 m in front of the stimulated eye resulting in subtended arcs of approximately 20° and 15° respectively. The individual squares subtended 27,5 min arc horizontally and 25 min arc vertically.