Closely Spaced, Fast Dynamic Movements in Disparity Vergence

Tara L. Alvarez 1 , John L. Semmlow 1 , 2 , and Weihong Yuan 1 1 Department of Biomedical Engineering, Rutgers University, Piscataway, 08855; and 2 Department of Surgery (Bioengineering), Robert Wood Johnson Medical, School-University of Medicine and Dentistry of New Jersey, New Brunswick, New Jersey 08901 Alvarez, Tara L., John L. Semmlow, and Weihong Yuan. Closely spaced, fast dynamic movements in disparity vergence. J. Neurophysiol. 79: 37-44, 1998. Conflicting theories exist describing how symmetrical vergence responses, the inward or outward turning of the eyes, are mediated. Classical theories describe vergence control as mediated by visual feedback. Extensive experimental evidence indicates that two distinct control components comprise the vergence response, and a recent theory supports the concept that one of these components is not visually guided. Occasionally, saccadic eye movements will respond to a single step stimulus with two closely spaced saccades, a behavior that is indicative of its switching control structure. If a portion of the vergence response is controlled in a manner analogous to saccades, then occasional double fast dynamic components might be expected. During this study, eye movements were recorded in response to symmetrical vergence stimuli of 2, 4, 6, 8, and 10° steps. The left and right eye movements were subtracted to yield a net vergence response, and only convergent responses were investigated. Double fast dynamic components associated with high-velocity movements were observed in all four subjects studied. In double high-velocity component responses, the average peak velocity of both fast dynamic movements was always considerably less than the average peak velocity found when the response to the same stimulus was made with only a single component. Response amplitudes of single and double movements showed that if the primary component of a double response did not reach 80% of the stimulus amplitude, a secondary component would be generated to aid the movement. Plots of peak velocity as a function of response amplitude for both double and single movements followed the main sequence for vergence eye movements demonstrating that the first-order dynamic characteristics of all high-velocity components were the same. The time at which maximum velocity occurred (relative to stimulus onset) was also the same for both single component responses and the first component of double responses. The similarity in dynamics and timing imp