Evidence Supporting Open-Loop Control During Early Vergence

John Semmlow,Tara Alvarez,Bérangère Granger-Donetti

doi:10.16910/jemr.6.4.2

Abstract

Disparity vergence eye movements were analyzed to determine if the early component of this response operates under open-loop, or preprogrammed, control. The analysis compares ratios of peak velocity to response amplitude (i.e., main sequence ratios) for the isolated early component and for the entire disparity vergence response. The stimuli were limited a 4 deg step changes in vergence so that any differences in movement dynamics (i.e., peak velocities) were due only to internal noise. Nine binocularly normal subjects were studied. A significant correlation between peak velocity and amplitude was observed during the early portion of the movement (p < 0.002), but not for the overall vergence response. Results support the widely held, but unproven assumption that the early component of symmetrical vergence is guided by open-loop, or preprogrammed, control processes while the overall response is influenced by internal and/or external feedback.

Highlights

Eye movement neural control processes are faced with two major challenges: the need for a quick response despite substantial processing delays, and the need to attain accurate positioning despite errors inherent in the neural and muscular apparatus
If a response is open-loop, the main sequence, the plot of peak velocity versus amplitude, will show a linear dependency. This has been shown in the saccadic response which is known to be preprogrammed or open-loop
It is possible for a feedback system to produce a constant main sequence, but only if the system is

Summary

Introduction

Eye movement neural control processes are faced with two major challenges: the need for a quick response despite substantial processing delays, and the need to attain accurate positioning despite errors inherent in the neural and muscular apparatus. These two challenges are best met with different control strategies: feedback can produce extremely accurate responses, but for stability, response velocity must be reduced when delays are present in the feedback loop. Open-loop (i.e., preprogrammed) control can generate rapid responses, even in systems with long delays, but these responses provide limited accuracy Both version and vergence control systems achieve speed and accuracy by combining the two strategies. Considerable evidence supports a “dual mode” control strategy (Semmlow et al, 1986; Hung et al, 1986) that consists of: a sustained component that is driven by visual and internal feedback to slowly bring the response to the final position, and an open-loop pulse-like component that enhances early movement dynamics. (We favor the term “dual-mode” to describe this general configuration rather than “pulsestep” to emphasize the difference in control strategies: open-loop versus feedback.)

Methods

Results

Discussion

Conclusion