Binocular VEP summation in infants and adults with abnormal binocular histories.

Visual evoked potentials (VEPs) were recorded under binocular and monocular viewing conditions in 19 2- to 10-month-old infants, 19 stereodeficient adults, and 12 normal adults. VEPs were elicited by medium contrast, phase-alternated checkerboards with check sizes ranging from 10-52 min of arc. Binocular VEP summation was defined as the percentage by which the binocular VEP amplitude exceeded the mean of the two monocular VEP amplitudes. In the stereonormal adults, binocular VEP summation was significantly greater than zero and significantly less than 100%. In the stereodeficient adults, binocular VEP summation was not significantly greater than zero. These findings support the hypothesis that the magnitude of binocular VEP summation reflects the activation of binocular cortical neurons. However, binocular VEP summation in the infants averaged approximately 95%. Moreover, infants under 5 months of age, many younger than the reported age of onset for stereopsis, showed binocular VEP summation of nearly 145%. The significantly higher level of binocular VEP summation in the infants was the result of much larger binocular VEP amplitudes, while monocular VEP amplitudes were equivalent to those of stereonormal and stereodeficient adults. These results support the hypothesis that VEP amplitude is mediated by two independent pools of monocular cortical neurons and that binocular VEP amplitude in stereonormal and stereodeficient adults saturates at a lower level than in infants. Thus, it is hypothesized that binocular VEP summation is not representative of the activation of binocular cortical neurons.