Pure Vergence Research Articles

Computational models to delineate 3D gaze-shift strategies in Parkinson’s disease

Objective: Parkinson’s disease (PD) frequently affects vergence eye movements interfering with the perception of depth and dimensionality critical for mitigating falls. We examined neural strategies that compensate for abnormal vergence and their mechanistic underpinning in PD. Approach: The a priori hypothesis was that impaired vergence is compensated by incorporating rapid eye movements (saccades) to accomplish gaze shifts at different depths. Our experiments examined the hypothesis by simulating biologically plausible computational models of saccade-vergence interactions in PD and validating predictions in the actual patient data. Main results: We found four strategies to accomplish 3D gaze shift; pure vergence eye movements, pure saccadic eye movements, combinations of vergence followed by a saccade, and combination of saccade followed by vergence. The gaze shifting strategy of the two eyes was incongruent in PD. The latency of vergence was prolonged, and it was more so when the saccades preceded the vergence or when the saccades only made 3D gaze shift. Computational models predicted at least two possible mechanisms triggering saccades along with vergence. One is based on the lack of foveal accuracy when the vergence gain is suboptimal. The second mechanism reflects the noise in the gating mechanism, the omnipause neurons, for vergence and saccades. None of the two model predictions alone were completely supported by the patient data. However, a combined model incorporating both abnormal vergence velocity gain and impaired gating accurately simulated the results from PD patients. Significance: The combined strategy is biologically plausible for two reasons: (a) The basal ganglia that is prominently affected in PD projects to the vergence velocity neurons in the midbrain via the cerebellum. The projection directly affects the vergence velocity gain. (b) The basal ganglia, via superior colliculus, influences the pattern of omnipause neuronal activity. Abnormal basal ganglia activity may introduce noise in the omnipause neurons.

Effects of Pure Vergence Training on Initiation and Binocular Coordination of Saccades.

We hypothesized that saccade eye movement properties, particularly latency and binocular coordination, depend on vergence quality. We studied 11 students clinically diagnosed for vergence disorders versus 8 healthy controls. Rehabilitation of vergence disorders was done with a novel research-based method, using vergence in midsagittal plane. Vergence and saccades were recorded in separate blocks, before and after five weekly rehabilitation sessions. Healthy controls showed higher accuracy and velocity of convergence and divergence relative to the vergence disorders group; then rehabilitation led to significant decrease of latency and increase of gain and peak velocity of vergence. Before rehabilitation of the vergence disorders, saccade parameters did not differ significantly from healthy controls, except the binocular coordination that was significantly deteriorated. Following vergence rehabilitation, saccade properties improved: The latency decreased significantly, the gain increased particularly at far, and the binocular coordination improved significantly. Latency and accuracy improved in a durable way, with values even better than the range of accuracy measured in healthy controls; binocular coordination of saccades, although improved, did not normalize. In healthy controls, binocular coordination was optimal at 40 cm (working distance), and the vergence disorders group showed improvement at 40 cm. Results confirm the hypothesis, which is further corroborated by the correlation between vergence and saccade latency. Results are in line with the hypothesis of permanent interaction between saccades and vergence, even when the task requires only saccades. Relevance of such interaction is emphasized by improvements of binocular saccades through the novel research-based method of vergence rehabilitation.

Error Correction in Vergence Eye Movements: Evidence Supporting Hering’s Law

In pure symmetrical vergence eye movements, a fusion initiating component quickly brings the eyes close to the desired position. A small error usually remains after this response which must be corrected to attain the small final vergence error (i.e., fixation disparity). Error correction will usually involve both version and version movements so possible mechanisms include: small saccades, smooth pursuit, symmetrical vergence, or some combination. Alternatively, an asymmetrical vergence or uniocular slow eye movement could be used to achieve the highly precise final position. Saccade-free late fusion sustaining components during the steady state to a symmetrical vergence step stimulus are analyzed using independent component analysis. Results suggest that fine correction is most likely the product of closely coordinated version and vergence components.

A dual visual-local feedback model of the vergence eye movement system

Pure vergence movements are the eye movements that we make when we change our binocular fixation between targets differing in distance but not in direction relative to the head. Pure vergence is slow and controlled by visual feedback. Saccades are the rapid eye movements that we make between targets differing in direction. Saccades are extremely fast and controlled by a local, non-visual feedback loop. Usually, we change our fixation between targets that differ in both distance and direction. Then, vergence eye movements are combined with saccades. A number of models have been proposed to explain the dynamics of saccade-related vergence movements. The models have in common that visual input is ignored for the duration of the responses. This type of control is realistic for saccades but not for vergence. Here, I present computations performed to investigate if a model using dual visual and local feedback can replace the current models. Simulations and stability analysis lead to a model that computes an estimate of target vergence instead of retinal disparity and uses this signal as the main drive. Further analysis shows that the model describes the dynamics of pure vergence responses over the full physiological range, saccade-related vergence movements, and vergence adaptation. The structure of the model leads to new hypotheses about the control of vergence.

Correction of Saccade-Induced Midline Errors in Responses to Pure Disparity Vergence Stimuli.

Purely symmetrical vergence stimuli aligned along the midline (cyclopean axis) require only a pure vergence response. Yet, in most responses saccades are observed and these saccades must either produce an error in the desired midline response or correct an error produced by asymmetry in the vergence response. A previous study (Semmlow, et al. 2008) has shown that the first saccade to appear in a response to a pure vergence stimulus usually increased the deviation from the midline, although all subjects (N = 12) had some responses where the initial saccade corrected a vergence induced midline error. This study focuses on those responses where the initial saccade produces an increased midline deviation and the resultant compensation that ultimately brings the eyes to the correct binocular position. This correction is accomplished by a higher level compensatory mechanism that uses offsetting asymmetrical vergence and/or corrective saccades. While responses consist of a mixture of the two compensatory mechanisms, the dominant mechanism is subject-dependent. Since fixation errors are quite small (minutes of arc), some feedback controlled physiological process involving smooth eye movements, and possibly saccades, must move the eyes to reduce binocular error to fixation disparity levels.

Speed and accuracy of saccades, vergence and combined eye movements in subjects with strabismus before and after eye surgery

The purpose of the study was to examine spatio-temporal characteristics of horizontal eye movements in the natural space (saccade, vergence and combined movements) in young subjects with early onset convergent or divergent strabismus. Nine young subjects (8–20 years old) were tested: three with divergent strabismus, six with convergent strabismus. A standard paradigm was used to elicit pure horizontal saccades at far and at close viewing distance, pure vergence along the median plane (convergence and divergence) and saccades combined with vergence movements. Horizontal eye movements from both eyes were recorded by a photoelectric device. Eye movements were recorded before surgery, and, for the majority of the subjects, two times after surgery. Before surgery the accuracy of convergence and divergence movements in their pure or combined form was poor with respect to normal values. The mean velocity of convergence was also abnormally slow. Strabismus surgery improved significantly the accuracy of these types of eye movements. The speed of pure convergence and of divergence combined movements increased significantly after surgery. We concluded that poor vergence eye movement’s performance, particularly those found for convergence in strabismic subjects could be due to impairment in the central structures related to sensory disparity inputs. Adaptive mechanisms promoted by the realignment of the eyes could be at the origin of the improvement in the vergence performances observed in our subjects after strabismus eye surgery.

Latency of saccades and vergence eye movements in dyslexic children

The goal of this study was to explore the latency of eye movements both in direction and in depth in dyslexic children. Sixteen dyslexic (mean age: 11.12 +/- 1.08 years) and 14 non-dyslexic children (mean age: 12.08 +/- 0.99 years) were tested. Two different paradigms (simultaneous and gap) were used to elicit pure saccades at far and at near distance, pure vergence (convergence and divergence) and combined saccade-vergence movements. Horizontal eye movements from both eyes were recorded simultaneously by a photoelectric device (Oculometer, Dr. Bouis). The mean latency of saccades at far distance (pure or combined) is significantly longer in dyslexics, regardless of the paradigm used. For both dyslexic and non-dyslexic children, latencies in the gap paradigm are shorter. Moreover, the occurrence of express latencies for divergence (pure or combined) is significantly higher for dyslexics. Difficulties in both voluntary and reflexive control shifts of visual attention from near to far distance in dyslexics could be at the origin of these findings.

Saccadic Behavior during the Response to Pure Vergence Stimuli I: General Properties

If two targets are carefully aligned so that they fall along the cyclopean axis, the required eye movement will be symmetrical with the two eyes turning equally inward or outward. When such “pure vergence stimuli” are used only a “pure vergence movement” is required, yet almost all responses include saccadic eye movements, a rapid tandem movement of the eyes. When saccades occur, they must either produce an error in the desired symmetrical response or correct an error from an asymmetrical vergence response. A series of eye movement responses to pure convergence stimuli (4.0 deg step stimuli) were measured in 12 subjects and the occurrence, timing and amplitude of saccades was measured. Early saccades (within 400 msec of the stimulus onset) appeared in 80% to 100% of the responses. In most subjects, the first saccade increased the asymmetry of the response, taking the eyes away from the midline position. In three subjects, these asymmetry-inducing saccades brought one eye, the preferred or dominant eye, close to the target, but in the other subjects these asymmetry-inducing saccades were probably due to the distraction caused by the transient diplopic image generated by a pure vergence stimulus. While many of these asymmetry-inducing saccades showed saccade-like enhancements of vergence, they were, with the exception of two subjects, primarily divergent and did not facilitate the ongoing convergence movement. All subjects had some responses where the first saccade improved response symmetry, correcting an asymmetry brought about by unequal vergence movements in the two eyes. In five subjects, large symmetry-inducing saccades corrected an asymmetrical vergence response, bringing the eyes back to the midline (to within a few tenths of a degree).

Eye-Movement Responses to Disparity Vergence Stimuli with Artificial Monocular Scotomas

The effects of artificial monocular scotomas on eye-movement responses to horizontal disparity vergence stimuli were studied in six subjects with normal binocular vision. Subjects viewed stereoscopic 1.5° horizontal step disparity vergence stimuli through liquid crystal shutter glasses. The central portion of the stimulus presented to the right eye was removed to simulate monocular artificial scotomas of variable diameters (2° to 10°). Eye movements were recorded with a binocular head-mounted eye tracker. Responses included pure vergence, vergence followed by saccades, and pure saccadic eye movements. The rate of responses with saccadic eye movements increased with the diameter of the artificial scotoma (p < 0.0001); there was an increase in the rate of responses starting with saccades (p < 0.0001), as well as an increase in the rate of saccades after initial vergence responses (p < 0.01). The probability of saccades after initial vergence responses was affected by the open-loop gain of the vergence response (p < 0.001). The open-loop gain decreased with increased diameters of the artificial scotomas (p < 0.0001). As the diameter of the artificial scotomas increased, the amplitude of the initial vergence eye-movement responses decreased, and the prevalence of saccadic eye movements and asymmetric vergence increased. The effects of the diameter of artificial monocular scotomas on eye-movement responses in subjects with normal binocular vision are consistent with the effects of diameter of suppression scotomas on eye-movement responses to disparity vergence stimuli in patients with infantile esotropia.

Latency of saccades, vergence, and combined movements in children with early onset convergent or divergent strabismus

The goal of this study was to examine latency of horizontal eye movements in the natural space (saccades, vergence, and combined saccade–vergence movements) in children with early onset convergent or divergent strabismus. Ten children were tested (8–11 years old): three with divergent strabismus, seven with convergent strabismus. A paradigm was used to elicit pure lateral saccades at far and near distance, pure vergence (convergence and divergence) and saccades combined with vergence movements. Horizontal eye movements from both eyes were recorded simultaneously by a photoelectric device (Oculometer, Dr. Bouis). The latency of saccades (at far and near distance), of vergence (convergence and divergence), and of combined movements greatly varies among subjects and has tendency to be longer than that observed in normal children of matched age, however, these differences reach significance in only a few cases. Children with divergent strabismus and residual gross binocular vision show abnormally longer vergence latencies than children with convergent strabismus without binocular vision. The initiation of combined movements does not show a dominant pattern, such as preceding vergence, as is found in normal children. Finally, strabismus surgery has no major effect on latencies. We conclude that there is no overall deficiency in latencies of eye movements in 3D space in children with early onset strabismus. Most likely, monocular visual input can be efficient as normal binocular vision for vergence movements. In a few subjects with divergent strabismus and fragile, intermittent binocular vision, latencies can be abnormally long, just because of the fragile binocular input and/or attention effort needs to use it. The absence of a pattern of initiation similar to normal children could be due to attention and fixation capabilities.

Binocular Coordination in Fore/Aft Motion

Stabilization of images on the fovea during either fore/aft translation of a subject or fore/aft movement of a visual target in front of a stationary observer imposes complex geometrical requirements that depend upon the eccentricity of the object of interest with respect to the eyes. Each eye needs to be rotated independently with varying proportions of conjugate (version) and disconjugate (vergence) eye movements to maintain fixation of the target. Here, we describe binocular coordination in the early response to translational movements of normal subjects along their naso-occipital axis. We recorded the responses evoked by small (about 4 cm), abrupt (about 0.7 g), fore/aft translations in four normal subjects while they viewed a near target. In the forward and backward starting positions the target was 15 or 10.5 cm away, respectively. Each subject was tested with the target centered between the eyes, aligned on the right eye, and placed to the right of the right eye by approximately 3 cm. The three conditions differed only in the lateral eccentricity of the target, yet the geometrical requirements for image stabilization are very different: pure vergence, one eye still, or mostly version. We found that the eye-movement responses closely matched what was needed for visual stabilization of the target, though responses to stimuli calling for divergence were less accurate than those for convergence. The latency of these responses ranged from 40 to 65 ms and achieved about 80% of the ideal response by 90 to 100 ms after the onset of the stimulus. Next, we asked whether these eye movements were generated by the vestibular system or by high-level strategies for image stabilization, such as pursuit. Thus, in a second set of experiments we used the mean profile of fore\aft body motion computed for each subject to drive a small visual target across the same distances and in the same eccentricities used during body translations. We found that visually driven responses had longer latencies (by at least 80 ms, ranging from 144 to 155 ms) and slower dynamics (with significantly lower peak eye velocities), highlighting the different subsystems producing the two types of responses. Saccades were also an important component of the response to both visual and vestibular stimuli, less frequent during the centered-target configuration and more frequent during viewing of eccentric targets. Visual stimuli evoked saccadic corrections more often and at shorter latencies than did vestibular stimuli. Both smooth and saccadic eye movements were appropriately disconjugate and their pattern depended on whether the eyes were converging or diverging.

Ocular oscillations generated by coupling of brainstem excitatory and inhibitory saccadic burst neurons.

The human saccadic system is potentially unstable and may oscillate if the burst neurons, which generate saccades, are not inhibited by omnipause neurons. A previous study showed that combined saccade vergence movements can evoke oscillations in normal subjects. We set out to determine: 1) whether similar oscillations can be recorded during other paradigms associated with inhibition of omnipause neurons; 2) whether lesions of the fastigial nuclei disrupt such oscillations; and 3) whether such oscillations can be reproduced using a model based on the coupling of excitatory and inhibitory burst neurons. We recorded saccadic oscillations during vergence movements, combined saccade-vergence movements, vertical saccades, pure vergence and blinks in three normal subjects, and in a patient with saccadic hypermetria due to a surgical lesion affecting both fastigial nuclei. During combined saccade-vergence, normal subjects and the cerebellar patient developed small-amplitude (0.1 - 0.5 degrees), high-frequency (27-35 Hz), conjugate horizontal saccadic oscillations. Oscillations of a similar amplitude and frequency occurred during blinks, pure vergence and vertical saccades. One normal subject could generate saccadic oscillations voluntarily (approximately 0.7 degrees amplitude, 25 Hz) during sustained convergence. Previous models proposed that high-frequency eye oscillations produced by the saccadic system (saccadic oscillations), occur because of a delay in a negative feedback loop around high-gain, excitatory burst neurons in the brainstem. The feedback included the cerebellar fastigial nuclei. We propose another model that accounts for saccadic oscillations based on 1) coupling of excitatory and inhibitory burst neurons in the brainstem and 2) the hypothesis that burst neurons show post-inhibitory rebound discharge. When omnipause neurons are inhibited (as during saccades, saccade-vergence movements and blinks), this new model simulates oscillations with amplitudes and frequencies comparable to those in normal human subjects. The finding of saccadic oscillations in the cerebellar patient is compatible with the new model but not with the recent models including the fastigial nuclei in the classic negative-feedback loop model. Our model proposes a novel mechanism for generating oscillations in the oculomotor system and perhaps in other motor systems too.

Speed-accuracy of saccades, vergence and combined eye movements in children with vertigo.

Vergence abnormalities could lead to inappropriate vestibulo-ocular reflex (VOR), causing vertigo and imbalance (Brandt 1999). Indeed, a recent study by Anoh-Tanon et al. (2000) reported the existence of a population of children with symptoms of vertigo in the absence of vestibular dysfunction but with abnormal vergence findings in orthoptic tests. The purpose of this study was to examine in such children the accuracy, duration and mean velocity of vergence and saccades; additionally, for a few subjects, the effect of orthoptic vergence training on these parameters was also investigated. LEDs were used to stimulate saccades, pure vergence along the median plane and combined saccade-vergence movements. Movements from both eyes were recorded with a photoelectric device (Bouis). The results show that children with vertigo perform saccades as normal subjects of comparable age. In contrast, vergence, particularly convergence, shows abnormalities: poor accuracy, long duration and low speed. During combined movements, the well known reciprocal interaction between the saccade and the vergence is present only for saccades combined with divergence; for saccades combined with convergence such interaction is abnormal: the saccade is slowed down by the convergence but the convergence is not accelerated by the saccade. Orthoptic training improves significantly the accuracy of all eye movements; such improvement was significant for all types of eye movements except for divergence (pure and combined). Furthermore, convergence remains abnormal and the lack of acceleration by the saccade persists. These specific convergence deficits could be of both subcortical and cortical origin. Orthoptic training improves the accuracy presumably via visual attentional mechanisms, but cannot completely override deficits related to subcortical deficiencies.

Latency of saccades, vergence and combined saccade vergence movements in a subject with manifest latent nystagmus

purpose To study eye movements in 3D space – saccades, vergence and combined eye movements – in a subject with Manifest Latent Nystagmus (MLN).methods A 13-year-old girl diagnosed with MLN participated in this study. Saccades, pure vergence along the median plane and combined saccade-vergence movements were recorded under both binocular and monocular viewing. Horizontal eye movements from both eyes were recorded simultaneously with a photoelectric device (Bouis Oculometer).results The recordings of saccades, vergence or both components of combined movements show that such movements have a staircase trajectory. The consequence of this staircase behavior is that the time for approaching the target is prolonged. The latency of all types of eye movements is extended when the subject is viewing binocularly, while latency values are normal when viewing monocularly; the difference between the viewing conditions is significant for vergence.conclusion The normal latency values under monocular viewing are attributed to facilitation of eye movement initiation by the increased nystagmus.

Saccades, vergence and combined eye movements in a young subject with Congenital Central Hypoventilation Syndrome (CCHS)

Purpose To study saccades, vergence and combined eye movements in a case of Congenital Central Hypoventilation Syndrome. Methods A 16-year-old girl with congenital partial third cranial nerve palsy, with ptosis and divergent strabismus dependent on viewing distance, participated in this study.A first operation for her ptosis was done seven years ago and an operation for her strabismus was done four years ago. The day of our oculomotor test, the patient had a residual exotropia of 12 prism diopters, mild amblyopia of the left eye and no binocular vision. LEDs on a table placed at eye level were used to stimulate saccades, pure vergence along the median plane and combined saccade-vergence movements. Horizontal eye movements from both eyes were recorded simultaneously with a photoelectric device (Bouis Oculometer). Results The binocular coordination of saccades in this subject was unstable and more variable than normal, and there was divergent post-saccadic drift, i.e., in the direction of her residual deviation. The patient had difficulty making movements in space, particularly vergence eye movements. Pure saccades and combined movements showed abnormally long latencies and marked hypometrias. The velocity of pure saccades was normal. In contrast, the velocity of saccades in the combined movements was abnormally slow. Conclusion The long latency and the low accuracy of the eye movements in natural space indicate a general deficit in the central circuitry that controls the initiation and programming of all these types of eye movements. The absence of pure vergence and the slowness of the saccades in the combined movements could be due to a brainstem deficit specific to the vergence oculomotor system.

EEG cortical potentials preceding vergence and combined saccade-vergence eye movements.

Previous event-related potential (ERP) studies of eye movements were restricted to saccades, the rapid conjugate movements used to change direction of fixation. Our study explored ERP activity for vergence eye movements used for the adjustment of the angle of the visual axes to the distance of the object. These movements are essential when exploring the natural 3D world. Subjects made the following visually guided movements to LED targets: pure convergence or divergence along the median plane, pure lateral saccades within the same distance, and combined movements (lateral and divergent or convergent). ERPs were recorded from 14 surface electrodes and were analyzed for 200 ms prior to the onset of the eye movement. The highest activation was observed prior to vergence. Activation was diffused and peaked at 50 ms prior to the onset of vergence. Combined vergence and sacccadic movements revealed a composite pattern of ERP activity from both saccades and pure vergence. These findings suggest a cortical substrate for vergence eye movements in humans that is engaged in the natural exploration of visual space.

Stimulation in the rostral pole of monkey superior colliculus: effects on vergence eye movements.

Recent studies have indicated that the superior colliculus (SC), traditionally considered to be saccade-related, may play a role in the coding of eye movements in both direction and depth. Similarly, it has been suggested that omnidirectional pause neurons are not only involved in the initiation of saccades, but can also modulate vergence eye movements. These new developments provide a challenge for current oculomotor models that attempt to describe saccade-vergence coordination and the neural mechanisms that may be involved. In this paper, we have attempted to study these aspects further by investigating the role of the rostral pole of the SC in the control of vergence eye movements. It is well-known that, by applying long-duration electrical stimulation to rostral sites in the monkey SC, saccadic responses can be prevented and interrupted. We have made use of these properties to extend this paradigm to eye movements that contain a substantial depth component. We found that electrical intervention in the rostral region also has a clear effect on vergence. For an eye movement to a near target, stimulation leads to a significant suppression and change in dynamics of the pure vergence response during the period of stimulation, but the depth component cannot be prevented entirely. When these paradigms are implemented for 3D refixations, the saccade is inactivated, as expected, while the vergence component is often suppressed more than in the case of the pure vergence. The data lead us to conclude that the rostral SC, presumably indirectly via connections with the pause neurons, can affect vergence control for both pure vergence and combined 3D responses. Suppression of the depth component is incomplete, in contrast to the directional movement, and is often different in magnitude for 3D refixations and pure vergence responses. The results are discussed in connection with current models for saccade-vergence interaction.

Gaze-shift dynamics in subjects with and without symptoms of convergence insufficiency: influence of monocular preference and the effect of training

We studied gaze-shift dynamics during several gaze-shift tasks and during reading, in five subjects with convergence insufficiency (C.I., a diminished ability to converge), and in ten subjects without C.I. Furthermore, we studied the effect of vergence training in order to verify previous claims that orthoptic exercises can improve vergence performance. We recorded binocular eye movements with the scleral coil technique. Subjects switched fixation between nearby and distant light emitting diodes (LEDs) arranged in isovergence arrays (distances 35 and 130 cm) in a dimly lit room. In both the C.I. and non-C.I. group, two classes of subjects occurred: vergence responders and saccadic responders. During pure vergence tasks, saccadic responders made saccades with no or little vergence; vergence responders made vergence movements with no or small saccadic components. In saccadic responders, fixation of nearby targets was monocular. Subjects with a preferred eye, according to our determination, used the preferred eye. The five C.I. subjects showed idiosyncratic responses with insufficient vergence during most trials. They all had a tendency to alternate fixation between the left and right eye. Vergence–version tasks always elicited larger vergence components than pure vergence tasks. During a reading task, vergence angles were more accurate than during gaze-shifts between LEDs. After the pre-training sessions, nine subjects (one of which had C.I.) practised a pure vergence task three times a day for at least 2 weeks. Vergence amplitudes of four of these subjects were larger after training. We conclude that vergence training can change oculomotor performance. Although C.I. is often associated with abnormal vergence dynamics, there are no typical C.I. vergence dynamics. Unstable monocular preferences may play a role in the aetiology of C.I.

Dynamics of horizontal vergence movements: interaction with horizontal and vertical saccades and relation with monocular preferences.

We studied the dynamics of pure vergence shifts and vergence shifts combined with vertical and horizontal saccades. It is known from earlier studies that horizontal saccades accelerate horizontal vergence. We wanted to obtain a more complete picture of the interactions between version and vergence. Therefore we studied pure version (horizontal and vertical), pure vergence (divergence and convergence) and combinations of both in five adult subjects with normal binocular vision and little phoria (< 5 degrees). The visual targets were LED's in isovergence arrays presented at two distances (35 and 130 cm) in a dimly lit room. Two targets were continuously lit during each trial and gaze-shifts were paced by a metronome. The two subjects with a strong monocular preference made vergence eye movements together with small horizontal saccades during pure vergence tasks. The other subjects, who did not have a strong monocular preference, made pure vergence movements (without saccades). These findings, suggest that monocular preferences influence the oculomotor strategy during vergence tasks. Vergence was facilitated by both horizontal and vertical saccades but vergence peak-velocity during horizontal saccades was higher than during vertical saccades.

Voluntary binocular gaze-shifts in the plane of regard: Dynamics of version and vergence

We studied the dynamics of voluntary, horizontal, binocular gaze-shifts between pairs of continuously visible, real three-dimensional targets. Subjects were stabilized on a biteboard to allow full control of target angles, which were made to differ only in distance (pure vergence), only in direction (pure version; conjugate saccades) or in both distance and direction (disjunctive saccades). A wide range of changes in vergence (0–25 deg) and version (0–65 deg) was recorded to study the dynamics of disjunctive saccades, described until now for limited ranges, throughout the horizontal oculomotor range within manual working space, and to study the velocity-duration-amplitude relations (“main sequence”) of disjunctive vs conjugate saccades. Pure vergence was almost never observed; divergence, especially, was always associated with saccades. Likewise, horizontal saccades were never strictly conjugate, they always contained a transient divergence-convergence sequence. The amplitude and velocity of these transient components varied systematically with saccadic size. In combined version-vergence movements, vergence was, in general, accelerated and shortened as a function of increasing version. This effect was fairly uniform for divergence, which appeared to increase in velocity by about as much as the transient peak divergent velocity of the version saccade. The intrasaccadic fraction of divergence increased from about 50% to close to 100% as a function of increasing version. For convergence, saccades up to about 20 deg were also accelerating; in this case it appeared as if the transient peak convergent velocity of the version saccade was added to the basic convergence velocity. For larger saccades this effect was partly counteracted by the penetration of an initial divergence associated with the saccade. This initial divergence delayed and slowed down convergence. The intrasaccadic fraction of convergence varied between about 40% and 70%. In disjunctive saccades the individual eyes did not follow the main-sequence parameters of conjugate saccades of comparable sizes, except for the eye that moved with the combination “abduction and divergence”. For all other combinations of vergence and version, disjunctive saccades had lower peak velocities and longer durations than conjugate saccades. As a consequence, disjunctive version was also slower than conjugate version. Thus, while version accelerates vergence, vergence slows down version: in the generalized case of three-dimensional gaze-shifts, peak velocities and durations are in between those of the limiting cases of pure version and pure vergence. We conclude that, within manual working space, binocular gaze-shifts are effected by the highly integrated action of conjugate and disjunctive mechanisms, both of which are expressed preferentially in fast, saccadic movements.