The effect of mean luminosity change on pattern reversal visual evoked response (VER), especially monocular VER, binocular VER and binocular summation, was studied. The mean luminosity level was changed with the use of neutral density filters. When mean luminosity level was decreased binocularly, binocular and monocular VER amplitudes decreased and binocular summation increased, reaching its peak at a luminosity of -0.3 log foot-lambert. When mean luminosity was changed monocularly and the interocular luminosity difference was larger than 0.6 log unit, the binocular VER became almost equal to or smaller than the monocular VER. In dim luminosity, the binocular system functioned better than the monocular system. Even a slight luminosity difference between the two eyes could disturb the binocular system.

The effect of change of contrast on the pattern reversal monocular VER, binocular VER, and binocular summation (binocular VER amplitude/monocular VER amplitude) was studied in normal subjects. The monocular VER amplitudes showed significant increase until the 0.50 contrast and saturation above that contrast. The largest binocular VER amplitude was observed at 0.20 contrast, and the amplitude tended to decrease with increasing contrast. The value of the binocular summation was highest at 0.20 contrast and decreased significantly with increasing contrast. At high contrast, there was no significant binocular summation. The difference of peak time (phase) between the binocular and the monocular VER was greatest at low contrast and decreased with increasing contrast. These results are explained by the difference of saturation level between the binocular and monocular systems. Our findings lead us to advocate strongly the use of low- to midlevel contrast for evaluating binocular function with pattern reversal VER in clinical ophthalmology.