Saccade-vergence interactions in humans

Abstract

1. We recorded eye movements in four normal human subjects during refixations between targets calling for various combinations of saccades and vergence. We confirmed and extended prior observations of 1) transient changes in horizontal ocular alignment during both pure horizontal saccades (relative divergence followed by relative convergence) and pure vertical saccades (usually divergence for upward and convergence for downward saccades); 2) occasional, high-frequency (20-25 Hz), conjugate oscillations along the axis orthogonal to the main saccade; and 3) the speeding up of horizontal vergence by both horizontal and vertical saccades. 2. To interpret these findings, we developed a hypothesis for the generation of vergence to step changes in target depth, both with and without associated saccades. The essential features of this hypothesis are 1) the transient changes in horizontal ocular alignment during pure horizontal saccades reflect asymmetries in the mechanical properties of the lateral and medial rectus muscles causing adduction to lag abduction; 2) pure vergence movements in response to step changes in target depth are generated by a neural network that uses a desired change in vergence position as its input command and instantaneous vergence motor error (the difference between the desired change and the actual change in vergence) to drive vergence premoter neurons; and 3) the facilitation of horizontal vergence by saccades arises from nonlinear interactions in central premotor circuits. 3. The hypothetical network for generating pure vergence to step changes in target depth is analogous in structure to the local feedback model for the generation of saccades and has the same conceptual appeal. With the assumption of a single nonlinearity describing the relationship between a vergence motor error signal and the output of the neurons that generate promoter vergence velocity commands, this model generates pure vergence movements with peak velocity-amplitude relationships and trajectories that closely match those of experimental data. 4. Several types of models are proposed for the central, nonlinear interaction that occurs when saccades and vergence are combined. Common to all models is the idea that omnidirectional pause neurons (OPN), which are thought to gate activity for saccade burst neurons, also gate activity for saccade-related vergence. In one model we hypothesize the existence of a separate class of saccade-related vergence burst neurons, which generate premotor horizontal vergence commands but only during saccades. In a second model we hypothesize separate right eye and left eye saccadic burst neurons that receive not only conjugate, but also equal but oppositely directed vergence error signals.(ABSTRACT TRUNCATED AT 400 WORDS)

Vergence eye movements during fixations in reading

“…For almost all of the present subjects, the majority of fixations ended in a more converged eye position, a finding more consistent with the early observations of vergence in reading by Schmidt (1917), than those by Clark (1935) and Taylor (1966) who reported observing only divergence during fixations. The present finding of convergence during fixations in reading also A .\ \ \ Hendriks / Acm Psychologicu 92 I ¡996) corresponds with the results of more recent studies in which LEDs were used as stimuli (Collevvijn et al, 1988a;Zee et al, 1992).…”

“…Previous research with LEDs has shown that vergence velocity during the initial part of a fixation period decreases as a function of the length of the preceding saccade (Collewijn et al, 1988a;Zee et al, 1992). To investigate whether this relationship exists in reading and whether vergence velocity could also be affected by the experimental manipulations directly, an additional analysis was carried out on the vergence velocity data of the Text Type and the Reading Objective experiment (to control simultaneously for the effect of differences in fixation duration, vergence velocities of the initial 80 ms of the fixations were used).…”

Pontine Omnipause Activity During Conjugate and Disconjugate Eye Movements in Macaques

“…Although the effects of saccades on vergence velocity have been extensively reported (Enright 1984;Kenyon et al 1980;Ono and Nakamizo 1978;Zee et al 1992), the possibility that vergence might have some effect on saccades has attracted much less attention. In studies with humans, horizontal saccades during vergence often showed slower velocities and resulted in longer durations than conjugate saccades of comparable size (Collewijn et al 1995;Erkelens et al 1989) but the effects of vergence on vertical saccades were less clear (van Leeuwen et al 1998).…”

Dynamic properties of eye movements in mitochondrial chronic progressive external ophthalmoplegia

“…In studies underway, we are testing this hypothesis using mathematical models that describe the normal behaviour of the eye muscles and fascia (ocular motor plant) during saccades. 19 Do any prior reports support our hypothesis?…”