Objects at different distances are viewed using vergence eye movements. These eye movements are brought about by a negative feedback vergence controller that monitors the eye and the target position. The vergence controller contains a fast and a slow vergence component. The fast component has an initial open-loop portion that elicits the vergence movement followed by a closed-loop component that completes the movement. Sustained vergence posture is maintained by the slow component, the neural innervation of which results in vergence adaptation. Control system models predict that for sustained viewing, the slow controller relieves the fast controller to respond to novel stimuli. The purpose of the current dissertation was to experimentally assess interactions between the slow and fast vergence components. Specifically, parameters including vergence latency, amplitude and velocity were studied before and after vergence adaptation.
Twenty subjects were enrolled. Computer generated vergence targets were presented in a haploscopic arrangement. Subjects viewed a 12° vergence target initially for 5s (pre-vergence adaptation) and subsequently for 5 minutes (post-vergence adaptation). Subjects made a divergence or convergence movement of 4° from the 12° vergence position for both the viewing durations. Phoria measures were made at three different time intervals to monitor vergence adaptation. Twenty trials (10 for convergence and 10 for divergence) were measured on different days for each subject.
It was found that the divergence latency increased by 11.5 %, while divergence velocity and amplitude decreased by 43.8 % and 34 % after vergence adaptation. This trend was present after sustained vergence regardless of vergence adaptation. For convergence, the velocity (8.2 %) and amplitude (17.7 %) were found to be significantly higher after a period of sustained convergence only if vergence adaptation occurred.
The change in vergence amplitude and velocity brought about by vergence adaptation followed the main sequence ratio (1:4) established in the literature. These data suggest that there is a decline in the divergent disparity detectors after sustained vergence. Finally, these data suggest that slow vergence innervation is gated during a fast vergence movement.