Visual test of Listing's law during vergence.

A simple visual test was used to measure how much Listing's plane rotates as a function of the vergence angle. This test measured the elevation-dependent torsional disparity of horizontal and vertical lines during three tasks: vergence on a near target, vergence through prisms that remained fixed, and through prisms that rotated with eye elevation. Consistent with our previous search-coil measurements, the results here suggest that the angle between the Listing's planes of the two eyes is somewhat less than the vergence angle.