Objective evaluation of binocular function with pattern reversal VER. III. Effect of stimulus size and localization.

The effect of central and peripheral stimulus field on monocular and binocular amplitude and binocular summation of the pattern reversal VER were investigated. When the central stimulus field size was smaller than 2.0 degrees X 2.0 degrees, there was no significant difference between the amplitudes of the monocular and binocular VER, but when it was equal to or larger than 2.4 degrees, the binocular VER amplitude was significantly larger than the monocular. The value of the binocular summation was highest at the central stimulus field size of 4.0 degrees X 4.0 degrees; at larger sizes, there were no significant changes in the value. Use of a central stimulus field size larger than 2.4 degrees X 2.4 degrees was therefore considered a prerequisite for the effective assessment of visual function, especially binocular function using the pattern reversal VER. Regarding the effect of peripheral stimulus field, both the monocular and binocular VER, but particularly the latter, were found to be sensitive to the central scotoma produced by covering the center of the full-field stimulus. The value of the binocular summation showed a significant reduction with the small central scotoma. We concluded that the pattern reversal VER is very sensitive to a central scotoma and that binocular function is mediated mainly through the central part of the stimulus field.