Analysis of human vergence dynamics

Disparity vergence is commonly viewed as being controlled by at least two mechanisms, an open-loop vergence-specific

burst mechanism analogous to the ballistic drive of saccades, and a closed-loop feedback mechanism controlled by the

disparity error. We show that human vergence dynamics for disparity jumps of a large textured field have a typical time

course consistent with predominant control by the open-loop vergence-specific burst mechanism, although various

subgroups of the population show radically different vergence behaviors. Some individuals show markedly slow divergence

responses, others slow convergence responses, others slow responses in both vergence directions, implying that the two

vergence directions have separate control mechanisms. The faster time courses usually had time-symmetric velocity

waveforms implying open-loop burst control, while the slow response waveforms were usually time-asymmetric implying

closed-loop feedback control. A further type of behavior seen in a distinct subpopulation was a compound anomalous

divergence response consisting of an initial convergence movement followed by a large corrective divergence movement

with time courses implying closed-loop feedback control. This analysis of the variety of human vergence responses thus

contributes substantially to the understanding of the oculomotor control mechanisms underlying the generation of vergence

movements.