Comparison of two fixation disparity determinantions

Fixation disparity values derived from horopter measurements are compared with those determined using the disparometer on seven subjects wearing lateral prisms. Although the two measures are intrinsically different, five subjects showed similarity in the form and position of the two fixation disparity-forced vergence functions. Two subjects manifested fixation disparities that indicated the use of proximal or voluntary control of relative eye position for both base-out and base-in forms of lateral prism. Another subject showed a paradoxical response to base-out prism that could be attributed to recovery from previous application of base-in prism, adaptation to base-out prism, or both. An additional subject showed irregular alterations in vergence posture during nonius horopter measurements. The clinical implications of vergence responses other than those associated with fusion and accommodation are discussed and the effect of this form or eye movement control is related to the change in position and shape of the equidistance horopter when it is measured through lateral prism.