Saccadic behavior during the response to pure vergence stimuli.

If two targets are carefully aligned, the required eye movement will be symmetrical with the two eyes turning equally inward or outward: a pure vergence movement. Yet almost all responses contain saccades, a rapid tandem movement of the eyes. When saccades occur, they must either be producing an error in the desired symmetrical response or correcting an error produced by an asymmetrical vergence response. Results show that in most subjects, error-producing saccades predominate. The symmetry error produced by these saccades is then corrected by either a compensatory asymmetrical vergence, offsetting saccades (including saccadic asymmetry), or a mixture of the two. The compensatory mechanism is subject-dependent with individual subjects favoring one of the three strategies in the majority of their responses. The factors that lead to symmetry errors and subsequent compensatory mechanisms are described, quantified, and discussed.