Ocular vergence under natural conditions. I. Continuous changes of target distance along the median plane

Abstract

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil - revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: ( a ) the target was moved manually by the experimenter; ( b ) the target was moved manually by the subject; ( c ) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from ‘slow’ to ‘very fast’, corresponding to changes in target vergence from about 10° s -1 to about 100° s -1 . The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When ‘very fast’ changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100° s -1 . Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i. e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i. e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the ‘slow’ and ‘medium’ conditions (stimulus speeds about 10-40° s -1 ) tracking was accurate to within 1-2°, irrespective of the way in which the target was moved. In the ‘fast’ and ‘very fast’ conditions (stimulus speeds about 40-100° s -1 ), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects. The changes they could make were unreliable and poorly correlated with changes in the distance of the imagined target. Vergence changes did not occur when the distance to the target, imagined in darkness, was varied by keeping the target stationary and moving the torso back and forth. In conclusion, when ocular vergence was studied under relatively natural conditions in which there were many cues to the distance of the target, oculomotor vergence was both much faster and much more accurate than could have been anticipated from previous studies done under more restricted stimulating conditions.

Visuomotor Coordination Is Different for Different Directions in Three-Dimensional Space

“…system to track. This follows from results by Erkelens et al (1989), who showed that errors between gaze and a moving target are Ͻ1°in the depth direction for movement velocities in the range between 10 and 40°/s. Therefore, we conclude that the different lead time of gaze relative to the finger for the frontal plane and for depth reflects differences in the dynamics of visuomotor control for version and vergence.…”

The Brain Stem Saccadic Burst Generator Encodes Gaze in Three-Dimensional Space

“…However, numerous studies have provided results that argue against this view. Vergence velocities are greater than what would be predicted by a linear summation of a conjugate saccade with a saccade-free vergence movement, while conjugate velocities are decreased Mays 2003, 2005a;Collewijn et al 1995Collewijn et al , 1997Enright 1984Enright , 1992Erkelens et al 1989; Kenyon et al 1980;Kumar et al 2005;Maxwell and King 1992;Ono et al 1978;Oohira 1993;Zee et al 1992). Moreover, the amount of vergence facilitation is dependent on peak saccadic velocity (Busettini and Mays 2005a).…”

Saccades during symmetrical vergence

“…Such omnipresence of saccades during symmetrical vergence in our study is compatible with previous observations, even though this had not been quantified in detail. Erkelens et al [15] observed that symmetrical vergence was primarily smooth but often accompanied by small saccades. In the study by Zee et al [62], many pure vergence trials were contaminated by Other notations as in Table 3.…”

“…Such disconjugacy during horizontal saccades appears to be an epiphenomenon that may be due to different mechanical properties of the lateral and medial rectus muscles [9], or to a timing difference of premotor signals due to the presence of the abducens internuclear neurons intercalated between burst neurons and medial rectus motoneurons [31,62]. Reciprocally, when a symmetrical vergence is required along the median plane (where no change in version is required), small saccades occur in most cases [11,15,62]. The function of these saccades is still under debate.…”