Dependence of the amplitude of fusional convergence movements on the velocity of the eliciting stimulus.

Foveal stimuli were used to elicit fusional convergence movements of the left eye in 5 subjects icith normal binocular vision. The amplitude of fusional convergence was measured by recording eye position. The eyes were light adapted, and the stimulus to the left eye was driven at angular velocities ranging from 1/8 to 6 degrees per second. Although large individual differences were found in the maximum amplitude of fusional convergence, this maximum was elicited in all subjects with velocities of less than 1 degree per second. As the angular velocity of the stimulus increases, the amplitude of fusional convergence first rises abruptly and then descends. The range of approximately 10 to 1 between the lower and upper velocity values which give rise to 75 per cent maximum fusional convergence probably accounts for why the velocity of eliciting fusional convergence is not crucial in determining the clinical result.