Effects of sweep VEP parameters on visual acuity and contrast thresholds in children and adults

Background There are many parameters that may impact the thresholds obtained with sweep visually evoked potentials (sVEP), yet a number of these parameters have not been systematically studied, and there is no recognised standard for sVEP recording. In this study, the effects of electrode placement, temporal frequency, sweep direction, presence of a fixation target, stimulus area, and sweep duration on visual acuity (VA) and contrast thresholds of the sVEP were investigated. Additionally, the effect of these parameters on the number of viable threshold readings obtained from five active electrodes was investigated. Methods Participants were six children (aged 6-8 years) and six adults (aged 17-30 years) with normal vision. Binocular sVEP VA and contrast thresholds were measured for two electrode placements (ISCEV and PowerDiva) of five active electrodes, three temporal frequencies (6, 7.5, and 10 Hz), two sweep directions (low to high and high to low), presence or absence of a fixation target, three stimulus areas, and three sweep durations. Results There were differences between adults and children with respect to visual acuity, the adults having better VA than the children (p=0.033 in experiment 2). Overall, there were more viable readings at 7.5 Hz than at either 10 or 6 Hz (p=0.0014 for VA and 0.001 for contrast thresholds). The adults performed better (in terms of viable readings) with the fixation target than without it (p=0.04). The smallest stimulus size used gave rise to fewer viable readings in both adults and children (p=0.022 for VA and 0.0001 for contrast thresholds). The other parameters (electrode placement, sweep direction and sweep duration) did not give rise to significant differences. Conclusions A temporal frequency of 7.5 Hz, a stimulus area of 4° or larger for VA and 10° or larger for contrast thresholds, and the use of a fixation target gave more viable readings, and may be indicated for future application. Consideration of the number of viable readings showed more differences between parameters than the actual thresholds, and it is suggested that more readings presumably would yield more reliable threshold measurements.