Fusional vergence: sustained and transient components.

There is evidence to suggest that fusional vergence acts as a two-stage process consisting of fusion-initiating and fusion-sustaining components. The interrelationship of these components was examined by comparing the amplitude of disparity-induced vergence over the operating range of sustained fusional vergence. The results showed that as sustained convergence increased, disparity-induced divergence increased and disparity-induced convergence remained essentially constant. These results are interpreted as indicating independence between the components and that the obtained increase in disparity-induced divergence resulted from an influence of accommodation on uncrossed retinal disparities.