Effect of check size and stimulation rate on blue-yellow multifocal visual evoked potentials.

Abstract

To determine the effect of different stimulus frame rates and check sizes on blue-yellow multifocal visual evoked potentials (mVEP). Subjects were examined at the Save Sight Institute at the University Sydney. Experiment 1 involved five adult subjects who underwent binocular stimulation by the Accumap multifocal objective perimeter. The eyes were stimulated with a cortically scaled dartboard pattern consisting of isoluminant blue and yellow checks. These were arranged in three concentric rings extending to an eccentricity of 26 degrees in the visual field. The stimulus pattern was driven by binary sequences resulting in pseudorandom binary exchange of two opposite checkerboard patterns at each of the 32 sites in the visual field. The mVEP were recorded at two different rates of display of the pattern stimulus. In experiment 2, mVEP were tested on 10 normal subjects. Each of the 36 stimulation sites contained a checkerboard pattern of 20, 30, 42 or 56 checks/site, the stimulation pattern was displayed at the optimum rate found in experiment 1. The size of the checks was inversely proportional to the number of checks per site. In experiment 1, the slow frame rate significantly increased the average amplitude throughout the field tested by 50 +/- 10.1% (P = 0.001). Latency was significantly shortened by 6.3% (P < 0.01). In experiment 2, the average amplitude peaked at 30 checks per segment; however, this was only calculated to be significantly different from the smallest check size (F(crit range 4,27) = 0.09 P < 0.05, anova, Tukey's T method). A similar difference was found in ring 1 (F(crit range 4,27) = 0.09, P < 0.05, anova, Tukey's T method). In ring 2, however, there was also a significant difference between 56 checks and 20, 30 and 42 (F(crit range 4,27) = 0.09, anova, P < 0.05). Altering the check sizes did not significantly affect the amplitudes in ring 3. The latencies were not significantly modified by altering check size at any eccentricity. These findings suggest that slowing the stimulation rate and displaying 30 checks per stimulation segment optimizes the blue-yellow mVEP stimulus.