Using a synoptophore to test Listing's law during vergence in normal subjects and strabismic patients.

The synoptophore was used to measure torsional interocular disparity. This, in turn, was used to compute how much the angle between the Listing's plane (LP) of the two eyes changes as a function of the vergence angle. The ratio of these two angles was defined as G. We measured G in normals and in patients suffering from intermittent horizontal strabismus. Consistent with previous search-coil experiments and with our previous visual test measures, the results using the synoptophore suggest that, for normals, G is less than 1. In the patient group the mean G was similar in magnitude but more variable. The variations in G did not appear to be related to the patient's measurement of ocular deviation. This result suggests that the vergence-related rotation of LP in these patients may be related to other factors besides the effort required to fuse the lines of sight.