Nonlinear alteration of transient vergence dynamics after sustained convergence.

BACKGROUND Adaptation models of the horizontal disparity vergence system assume a nonadaptable transient component. They also predict identical postadaptation dynamics during convergence and divergence movements. METHOD To test the adaptation property of the transient component, a set of experiments were performed in which closed-loop vergence dynamics measured before and after sustained convergence were compared, primarily by comparing the peak vergence velocity, occurrence time of peak vergence velocity, and steady-state vergence posture. Vergence dynamics after durations of 30, 60, and 90 s of sustained convergence were compared with those after a control duration of 5 s. RESULTS The peak divergence velocity was reduced by about 25% within 30 s of sustained vergence. However, the peak convergence velocity was unchanged for all the exposure durations. Additionally, for all durations, the peak divergence velocity was significantly higher than peak convergence velocity. In contrast to peak velocities, the occurrence time of peak convergence and divergence velocity did not differ significantly and remained unchanged for all durations. CONCLUSIONS The transient component is adaptable. Furthermore, the adaptation is direction dependent and affects divergence and convergence dynamics differently, thereby suggesting involvement of separate pathways for convergence and divergence in the vergence sensorimotor control.