Changes In Vergence Research Articles

Assessing the Role of Vergence Changes in the Perception of Random-Dot Stereograms by Using Open-Loop Control of Vergence

The fact that the 3-D shape of surfaces depicted by random dot stereograms can take several seconds or even tens of seconds to appear has been attributed to the failure to make appropriate vergence changes [B Julesz, 1971 Foundations of Cyclopean Perception (Chicago, IL: University of Chicago Press)]. Alternatively, the long latencies could be a consequence of the processing time needed to match the disparate images. To distinguish between these possibilities we measured perceptual latencies in a situation in which vergence changes had no effect on retinal disparities. To do this, horizontal eye movements were recorded with the aid of close-fitting scleral search coils in both eyes and the difference signal used to shift horizontally the two halves of a random-dot stereogram by equal and opposite amounts. When the amount of shift was equal to the magnitude of the vergence change, changes of vergence had no effect on the pattern of disparities—open-loop vergence. Three observers were presented with a sequence of stereograms depicting both ‘simple’ surfaces (a single square lying in front of the surround) and ‘complex’ surfaces, including spirals, ‘wedding cakes’, and saddle shapes under both normal and open-loop conditions. Under open-loop conditions, the complete 3-D shape was never perceived when the disparity range of the stereogram was large (>40 min arc), demonstrating the necessity of vergence changes, but the 3-D structure of ‘complex’ surfaces did build up over a period of several seconds indicating a separate disparity processing limitation.

Gain changes in the accommodative convergence cross-link

The output of the accommodative vergence cross-link, the ACA ratio, was measured before and after subjects viewed through a telestereoscope which increased their interocular separation (pd) approximately fourfold. The initial paradigm was designed to force an increase in the ACA ratio in that subjects alternately fixated targets set at differing distances. In this paradigm, the varying vergence demands could not be met by a constant increase in phasic or tonic vergence responses. In a second paradigm, only one target was viewed at a fixed position from the eye. Now the constant vergence demand could be met by a set increase in phasic and/or tonic vergence. A significant increase in the ACA ratio was found in the two-target study but not in the single-target study. Increases in the accommodative vergence gain did show considerable individual differences. Tonic vergence changes were interpolated from the data. This parameter increased significantly in both cases but more so when two targets were alternately fixated. This finding is consistent with current near triad models which predict that accommodative convergence input acts along with disparity vergence input to increase tonic levels of convergence. When ACA ratios were calculated using clinical measures of the phoria at 6 and 0.4 m, increases were found in the two-target study and to a lesser degree in the single-target study. This clinical method of ACA measure appears to have been confounded by changes in tonic vergence.

Gain changes in the accommodative convergence cross‐link

The output of the accommodative vergence cross-link, the AC/A ratio, was measured before and after subjects viewed through a telestereoscope which increased their interocular separation (pd) approximately fourfold. The initial paradigm was designed to force an increase in the AC/A ratio in that subjects alternately fixated targets set at differing distances. In this paradigm, the varying vergence demands could not be met by a constant increase in phasic or tonic vergence responses. In a second paradigm, only one target was viewed at a fixed position from the eye. Now the constant vergence demand could be met by a set increase in phasic and/or tonic vergence. A significant increase in the AC/A ratio was found in the two-target study but not in the single-target study. Increases in the accommodative vergence gain did show considerable individual differences. Tonic vergence changes were interpolated from the data. This parameter increased significantly in both cases but more so when two targets were alternately fixated. This finding is consistent with current near triad models which predict that accommodative convergence input acts along with disparity vergence input to increase tonic levels of convergence. When AC/A ratios were calculated using clinical measures of the phoria at 6 and 0.4 m, increases were found in the two-target study and to a lesser degree in the single-target study. This clinical method of AC/A measure appears to have been confounded by changes in tonic vergence.

Map restoration of an early Pliocene horizon along the Hosgri-Purisima-Lompoc fault system, central California margin: ABSTRACT

The Hosgri fault is the southern part of the San Gregorio-Sur-San Simeon-Hosgri fault system, an important element of the Neogene California transform system. The Hosgri fault proper (top 1-2 km) is located in the hanging-wall of a 10 to 20 km wide zone of mostly E-dipping faults, the Hosgri-Purisima-Lompoc fault system. These faults have undergone Miocene extension, and have been reactivated by post-Miocene contraction or transpression. This wider fault system is truncated against or merges with the E-striking faults in northwestern Santa Barbara Channel. From this starting point of zero displacement, right-lateral displacement on a N- S fault can not be more than the sum of N-S shortening east of it, the N-S extension west of it, and the right-lateral slip fed into it by other faults. We are using a 3-D map restoration technique to quantify the displacements along this fault system. Depth-contoured folded surfaces are flattened using the software UNFOLD, and the restored surfaces are fit back together across faults using an interactive graphic program. Displacements are calculated by comparing the restored surface to the present state with respect to a relatively-fixed block. Our mapping of the early Pliocene top Sisquoc horizon indicates that broad, gentle folds characterizemore » the Point Arguello oil field. Folds located between Point Arguello and Point Sal, are characterized by abrupt changes in amplitude, symmetry, and vergence along strike. The folded surface is unfaulted over wide areas, despite dips up to 45[degrees]. These folds are the result of basin inversion along former rift border faults. Folds are amplified where the basin fill is transpressed against NW-striking restraining bends in a regional NNW-striking system of right-reverse-oblique faults.« less

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.

Judged Distance to Virtual Objects in the near Visual Field

Errors in judged depth of nearby virtual objects presented via see-through, helmet mounted displays are examined as a function of monocular, biocular and stereoscopic viewing conditions, accommodative demand and subjects’ age. These errors are argued to be related to changes in binocular vergence. Suggestions for improved control of the judged distance to virtual objects and the cause of the judgment errors are briefly discussed.

Tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks: an example from the Ardennes Massif (France-Belgium)

The discrimination of the respective contribution of strain and mineralogy of the bulk Anisotropy of low field Magnetic Susceptibility (AMS) of rocks is addressed by a study of the Ardennes Variscan Thrust Belt. Unmetamorphosed to low-medium-grade rocks in the study area define a succession of magnetic parageneses: hematite + maghemite (?), hematite + maghemite (?) + magnetite, magnetite + pyrrhotite. A direct control of the magnetic mineralogy on the magnitude of AMS axes is shown. In addition, in all cases, the interpretation of the directions of AMS axes as kinematic indicators also applies to these rocks. We show a progressive change in shearing direction from the south of the Rocroi massif, where the nappe transport is N-directed; in the north, the vergence changes to northwestward.

The Relationship of Binocular Convergence and Errors in Judged Distance to Virtual Objects

Errors in judged depth of nearby virtual objects presented via see-through helmet mounted displays are shown to be linked to changes in binocular vergence. This effect varies measurably across the subjects examined and correlates with the magnitude of the subjects’ individual depth judgment errors. The relationship is demonstrated by visually superimposing computer-generated virtual objects on physical backgrounds in a manner similar to that suggested for some practical applications. Suggestions for improved control of virtual objects under these viewing conditions are briefly discussed.

Dark focus of accommodation and vergence posture.

We evaluated the dark focus of accommodation and vergence posture in 20 boys and girls, 10 with and 10 without accommodative esotropia, before and after they wore glasses for at least 1 month. Refractive error was measured by Nidek Autorefractometer AR1600. We used two definitions of the dark focus, DFcus (Non-Cyclo R) and DFcus (Cyclo R). DFcus (Non-Cyclo R) = Dark R - Non-Cyclo R. DFcus (Cyclo R) = Dark R - Cyclo R. (Dark R: refractive state in the dark; Non-Cyclo R: non-cycloplegic refractive error; Cyclo R: cycloplegic refractive error.) Vergence posture was measured by prism cover test during distance fixation while uncorrected. DFcus (Cyclo R) was significantly greater in esotropic subjects than in non-esotropic subjects, although DFcus (Non-Cyclo R) did not differ significantly. A significant hyperopic shift in Dark R was observed after wearing glasses. DFcus (Cyclo R) was significantly decreased after wearing glasses, suggesting that wearing glasses is an important variable of the dark focus. Unlike the dark focus, the vergence posture did not change in either group after wearing glasses. The dissociation of the dark focus from vergence posture seems to be inconsistent with previous findings. It is postulated that change in dark vergence after wearing glasses is responsible for the results.

THE STABILITY OF BINOCULAR FIXATION DURING READING IN ADULTS AND CHILDREN

The authors recorded the binocular eye-movements of children and adults while they read single words appropriate for their reading ability. Normal data were obtained from nine- to 11-year-old primary-school children and adults, because very little is known about changes in binocular vergence during reading. These normal data were compared with those from poor readers of the same age who had passed or failed the Dunlop Test. On average, normal children made larger vergence errors while they fixated words than did adults. There were no differences between the groups of children, therefore the authors conclude that poor vergence control during reading fixations is not the immediate cause of the non-word error effect found among children who fail the Dunlop Test.

The relationship between absolute disparity and ocular vergence.

The relationship between disparity and ocular vergence was investigated under closed-loop as well as under open-loop viewing conditions. First we examined whether vergence responded similarly to disparity presented under open-loop and closed-loop conditions. Similar response were observed in both conditions. The direct relationship between disparity and vergence was examined by presenting constant disparities between 0.2 degrees and 4 degrees under open-loop viewing conditions. Such vergence responses are described as the outputs of first-order low-pass filters with different filter characteristics for each amplitude of disparity. By analyzing the latency of vergence responses induced by constant disparities with help of the transfer function of disparity-controlled vergence, the time delay of disparity processing in the vergence loop was estimated. We suggested that the time delay was approximately between 80 and 120 ms instead of 160 ms as is generally assumed. The relationship between the rate of disparity change and vergence was examined by comparing responses to ramp and stepwise changes in target vergence. From the similar responses to ramp and staircase changes in disparity we concluded that vergence is not sensitive to the velocity of target vergence as such. On the basis of these findings we developed a model of disparity-controlled vergence. In this model disparity is processed through several parallel, imperfect integrators with slightly different low-pass filter characteristics, each of them susceptible to a limited range of disparities. Gains as well as phase lags of vergence responses to sinusoidal disparities are accurately simulated by this model.(ABSTRACT TRUNCATED AT 250 WORDS)

The remarkable saccades of asymmetrical vergence

The saccades that usually arise near the onset of asymmetrical changes in vergence, when one eye is aligned with both targets, are remarkably different from ordinary saccades: (1) the excursions of the two eyes are typically very unequal, often differing by several fold from each other; (2) mean excursion (version) is extremely variable across replicate tests with identical targets; (3) at the end of the saccades, eye orientation is usually not even briefly stable: the aligned eye immediately reverses its movement, indicating that the pulse in muscular forces is apparently not followed by a corresponding step; and (4) a second saccade in the opposite direction can immediately follow the initial saccade of asymmetrical divergence, with no sign of refractoriness. These phenomena suggest that the pulse and step components of saccadic motoneuron activity may be generated by largely independent processes; that the step component for each eye depends only on that eye's visual input; and that the pulse components generated for each eye depend on weighted averaging of visual stimuli that impinge on both eyes. This interpretation is incompatible with most current models of saccade generation, but was anticipated in its essentials by Ditchburn [(1973) Eye movements and visual perception. Oxford: Clarendon Press]. A corollary of this hypothesis is that disparity-evoked vergence changes can be viewed as the general-case output from that system which produces fully conjugate saccades as a special case.

Saccade-vergence interactions in humans.

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)

Disparity sensitivity of neurons in monkey extrastriate area MST

We tested the disparity sensitivity of neurons from the medial superior temporal area (MST) in awake behaving monkeys. While the monkey looked at a fixation spot on a screen in front of it, random dot stimuli moved in the preferred direction of the cell under study, and the disparity of the dots made the stimuli appear to move in a frontoparallel plane in front of, on, or behind the screen. Over 90% of the 272 neurons studied were sensitive to the disparity of the visual stimulus. Of those disparity-sensitive cells, 95% were most responsive either to near stimuli (stimuli with crossed disparities appearing to move in front of the screen) or to far stimuli (stimuli with uncrossed disparities appearing to move behind the screen). In a smaller sample of the disparity-sensitive cells, we found cells whose preferred direction of stimulus motion reversed as the disparity of the stimulus reversed. For example, a cell that responded best to rightward motion for near stimuli responded best to leftward motion for far stimuli. We found that 40% of the disparity-sensitive cells had this disparity-dependent direction selectivity. This disparity-dependent direction selectivity was maintained over the entire range of speeds tested (6-56 degrees/sec). We tested whether the disparity sensitivity of the neurons indicated the distance of the stimulus from the screen where the monkey was fixating (relative depth) or the distance of the stimulus from the monkey (absolute depth) by having the monkey fixate at different depths in front of or behind the screen. For most MST neurons, the changes in vergence did not alter the disparity response, indicating that the disparity sensitivity of these neurons conveyed information on depth relative to the plane of fixation. We conclude that the disparity characteristics of cells in the dorsomedial MST are those expected of a system serving primarily coarse rather than fine stereopsis. The correlation between disparity selectivity and direction selectivity in these neurons, as well as their other properties, suggests a role in signaling the direction of self-motion of the observer through the environment.

Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas (1)

Structural cross sections and a regional tectonic map show that the Central Basin uplift consists of upthrust blocks of alternating vergence. The uplift is separated from the adjacent Delaware basin by a major fault that changes throw and structural style along strike, accompanying changes in block vergence found along the uplift. The west-facing basement blocks were thrust westward over the Delaware basin, whereas the adjacent blocks face eastward. The distribution and style of structures suggest that small amounts of left-lateral movement occurred along west-trending and northwest-trending faults that separated rising blocks of the uplift. Transfer of lateral movement from vertical, west-trending faults on the Central Basin uplift and the Ozona arch to the west-trending Mid-basin fault in the Delaware basin rotated blocks of the uplift in a clockwise sense. Faults and folds within the study area have trends and slip directions that are generally similar to those within the Reagan and Washita Valley fault zones in the Arbuckle uplift adjacent to the Anadarko basin. The difference in the style of deformation between the two uplifts relates to varied angular relationships between preexisting structure and the direction of late Paleozoic stress. The identity of structural trends within these two uplifts suggests that other late Paleozoic basins and uplifts of the Ancestral Rocky Mountains may have formed in a similar stress field and that crustal rotation was pervasive along the southwestern margin of the continent during the late Paleozoic Ouachita-Maratho deformation.

Dynamic Demonstration of Proximal Vergence and Proximal Accommodation

The complex interactions between accommodation and vergence have been described by dual interactive models which include influences of convergence accommodation and accommodative vergence. Using an SRI Eyetracker, we investigated changes in vergence and accommodation stimulated while looking through prisms or lenses, and while looking at real targets located at different distances. Our results suggest that both proximal accommodation and proximal vergence are stimulated when looking from a distant to a near real target. We suggest that models of convergence and accommodation interactions include proximal accommodation and proximal vergence before the crosslinks.

Apparent size as a function of vertical gaze direction: New tests of an old hypothesis.

The hypothesis was tested that the decline of apparent size with elevated gaze results from a latent tendency of the eyes to diverge and thus increased vergence effort. Through the use of a method of category estimation, the decline of apparent size on elevation or depression of gaze was found not to be different between subjects with larger or smaller changes of dark vergence and thus vergence effort. In a 2nd experiment, vergence effort was varied by varying gaze elevation and the angle of convergence. With vergence effort constant, apparent size was dependent on the angle of convergence rather than being constant. It is concluded that apparent size does not depend on vergence effort and that the effect of gaze elevation on apparent size cannot be attributed to its concomitant effect on dark vergence.

The Relation of Vergence Effort to Reports of Visual Fatigue Following Prolonged Near Work

Two experiments examined the link between vergence effort and subjective reports of visual fatigue following prolonged near work. In Experiment 1, dark vergence and reports of visual fatigue symptoms associated with computer use were measured in 104 persons. Dark vergence was significantly correlated with four of six symptoms of visual fatigue. In Experiment 2, 16 subjects who exhibited clear "Heuer effects" (systematic gaze-related changes in dark vergence) read from a video display terminal (VDT) at a distance of 20 cm for 60 min under two gaze elevation conditions: 20 deg above and 20 deg below a baseline gaze elevation. This manipulation was designed to vary the amount of vergence effort required to fixate the VDT. Results indicate a positive relation between vergence effort and visual fatigue both within and between subjects, and that prolonged near work leads to decreased vergence accuracy at far distances. Implications for VDT workstation design are discussed.

The Effect of Stimulus Duration on Tonic Accommodation and Tonic Vergence

Changes in tonic accommodation and tonic vergence were studied as a function of stimulus duration and time after near fixation. Thirteen visually normal young adults each participated in experimental sessions comprising each of 12 combinations of stimulus type (accommodative or vergence) and stimulus duration (0.25, 0.5, 1, 2, 4, or 8 min). During each session, measurements were made of either tonic accommodation or tonic vergence 30 s before stimulus onset and at 0.5, 2, 4, 6, 8, 10, 12, and 14 min after stimulus offset. The stimulus (a high contrast reduced Snellen visual acuity chart) was viewed monocularly at the accommodative nearpoint and binocularly through pinholes at the fusional nearpoint on the accommodative and vergence sessions, respectively. For tonic accommodation, stimulus duration had no significant effect on either the magnitude of the aftereffect or its rate of decay. For tonic vergence however, the aftereffect was larger and had a slower rate of decay as stimulus duration increased. The results suggest that the adaptive mechanisms within the two systems have different time constants.

Proximal Vergence and Perceived Distance

Proximal vergence was measured haploscopically in response to a change in physical distance of a large screen of blurred horizontal lines. These responses were compared to proximal vergence measurements made using a clinical method. Perceived distance of the horizontal stripes was estimated according to the perceived size of a projected afterimage. For 18 normal observers the change in vergence measured haploscopically (in prism diopters per meter angle change in test distance) averaged 2.59 (SD = 1.44) binocularly and 2.11 (SD = 1.22) monocularly. Clinical measures averaged 2.88 (SD = 1.57) binocularly and 2.10 (SD = 1.27) monocularly. Haploscopic and clinical measures of proximal vergence correlated only when the clinical technique was modified by viewing through pinholes. Taking estimated perceived distance into consideration only partially reduced the differences between proximal vergence measured haploscopically and clinically as well as binocularly and monocularly. Both haploscopic and clinical measures of proximal vergence were sizeable, indicating that this vergence component can contribute significantly to the normal vergence response.