Vertical divergence and counterroll eye movements evoked by whole-body position steps about the roll axis of the head in humans

In healthy human subjects, a head tilt about its roll axis evokes a dynamic counterroll that is mediated by both semicircular canal and otolith stimulation, and a static counterroll that is mediated by otolith stimulation only. The vertical ocular divergence associated with the static counterroll too is otolith-mediated. A previous study has shown that, in humans, there is also a vertical divergence during dynamic head roll, but this report was not conclusive on whether this response was mediated by the semicircular canals only or whether the otoliths made a significant contribution. To clarify this issue, we applied torsional whole-body position steps (amplitude 10 degrees, peak acceleration of 90 degrees /s(2), duration 650 ms) about the earth-vertical (supine body position) and earth-horizontal (upright body position) axis to healthy human subjects who were monocularly fixating a straight-ahead target. Eye movements were recorded binocularly with dual search coils in three dimensions. The dynamic parameters were determined 120 ms after the beginning of the turntable movement, i.e., before the first fast phase of nystagmus. The static parameters were measured 4 s after the beginning of the turntable movement. The dynamic gain of the counterroll was larger in upright (average gain: 0.48 +/- 0.10 SD) than in supine (0.36 +/- 0.10) position. The static gain of the counterroll in the upright position (0.21 +/- 0.06) was smaller than the dynamic gain. Divergent eye movements (intorting eye hypertropic) evoked during the dynamic phase were not significantly different between supine (average vergence velocity: 0.87 +/- 0.51 degrees /s) and upright (0.84 +/- 0.64 degrees /s) positions. The static vertical divergence in upright position was 0.32 +/- 0.14 degrees. The results indicate that the dynamic vertical divergence in contrast to the dynamic ocular counterroll is not enhanced by otolith input. These results can be explained through the different patterns of connectivity between semicircular canals and utricles to the eye muscles. Alternatively, we hypothesize that the small dynamic vertical divergence represents the remaining vertical error necessary to drive an adaptive control mechanism that normally maintains a vertical eye alignment.

Vergence adaptation in children and its clinical significance

BACKGROUND AND PURPOSE

Vergence adaptation has not been well investigated in children even though it may contribute to binocular dysfunction and near work induced asthenopia.

METHODS

We compared vergence adaptation in 18 children and 18 young adult subjects by assessing tonic vergence (TV) before and immediately after a period of sustained near fixation, by measuring heterophoria with a synoptophore through 0.5 mm binocular pinholes. Adaptation was induced by a reading task at 15 cm for a continuous 5 minute period.

RESULTS

Mean pre-task TV values of 0.70 MA (Meter Angles) and 0.20 MA were observed for the children and young adults, respectively (p = 0.08). The initial mean vergence adaptation for children and adults was +0.45 MA and +0.11 MA, respectively (p = 0.001).

CONCLUSION

The greater vergence adaptation observed in children may impact upon the clinical assessment of their binocular vision, especially heterophoria measurement which may require longer periods of dissociation than previously recommended, and might also ultimately be partly responsible for the predominant development of esodeviations during childhood

Yes-no staircases with fixed step sizes: psychometric properties and optimal setup

Interest in the use of adaptive staircase methods in clinical practice is increasing, but time limitations require that they be based on yes-no trials. The psychometric properties of yes-no staircases with fixed step sizes (FSS staircases) in small-sample situations have never been studied in depth. As a result, information is lacking as to what is the optimal setup for an FSS staircase. To determine this optimal setup, we used simulation techniques to study the asymptotic and small-sample convergence of yes-no FSS staircases as a function of the up/down rule, the size of the steps up or down, the starting stimulus level, the spread of the psychometric function, and the lapsing rate. Our results indicate that yes-no FSS staircases with steps up and down of the same size are unstable because with these settings, the staircases yield different results across variations in irrelevant parameters such as the spread of the psychometric function or the starting level. Our study also identified settings with which the properties of estimates are unaffected by these factors. With these optimal settings, yes-no FSS staircases can provide very quick and accurate estimates in 7 to 8 trials. Practical recommendations are given to get the best out of yes-no FSS staircases.

Closed-loop and open-loop accommodative vergence eye movements

We examined accommodative vergence eye movements under conditions in which feedback about retinal-image slip was and was not present (i.e. closed-loop and open-loop conditions). We found that (a) in both conditions the two eyes started to move at the same time; (b) in the open-loop condition, vergence continued in both eyes; and (c) in the closed-loop condition, vergence continued in the occluded eye only, and the magnitude and velocity exceeded that of the occluded eye in the open-loop condition by a factor of two.

Vertical fixation disparity curve and the effects of vergence training in a normal young adult population

BACKGROUND

The vertical forced vergence fixation disparity (VFD) curve represents the amount of vertical fixation disparity, the steady-state vertical bifixation error of the eyes, at various levels of vertical vergence demand. The main aim of the present study was to examine the effects of vertical vergence training on the slope of the VFD curve in a normal, young adult population.

METHODS

Forty-five subjects with normal vision and binocular function underwent vertical vergence training for 1 week. The training was done using a vertical prism bar, and the vertical fixation disparity was measured using the Disparometer.

RESULTS

The mean slope of the VFD curve in a normal, young adult population was 1.103 min arc/delta. The slope of the VFD curve decreased significantly after the training and remained flattened for at least 3 months. There was no evidence to support the idea that the decrease in the VFD slope was related to the increase of vertical fusional amplitude.

CONCLUSIONS

Vertical prism bar training provided a long-term effect, both increasing the vertical fusional amplitude and flattening the slope of the VFD curve. The decrease in the slope of the VFD curve was thought to be independent of the increase of vertical fusional amplitude.

Disconjugate vertical memory-guided saccades to disparate targets

We studied the binocular coordination of normal memory-guided saccades and the possibility of inducing memory-based disconjugate learning. First, we report that normal vertical memory-guided saccades to non-disparate targets are yoked vertically in the two eyes as well as visually guided vertical saccades. To induce disconjugate vertical learning, at each trial we flashed a target that was disparate (i.e. 8% more elevated or more depressed for one eye); the memory delay was 1 s. Memory-guided vertical saccades developed a vertical disconjugacy that was appropriate for the disparity of the remembered target. After 15 min of repetition, this vertical disconjugacy persisted even when the target to be remembered was no longer disparate; this indicates disconjugate vertical learning based on short-term memory. However, this was observed only for a few individual cases and its amplitude was small. This contrasts with prior findings on horizontal saccades associated with horizontal disparities. We conclude that vertical memory-based disconjugate learning is possible but very limited. Together with other studies, this study suggests that the natural vertical conjugacy of vertical saccades relies little on rapid learning mechanisms. Rather it seems to be built-in, and this is consistent with earlier electrophysiological findings.

Stability analysis of two linear accommodation and convergence models

PURPOSE

To perform time domain stability analysis on two widely accepted linear models of accommodation and convergence.

METHODS

For each model, the governing differential equations were used to compute the system matrix. The system matrices were used to calculate the respective trace and determinant from which eigen-vectors and eigenvalues were quantified. These characteristic numbers fully identified and classified the fixed points of each model and, thus, their stability.

RESULTS

Controller gains, time constants, and accommodation and convergence cross-links determined model stability. Accommodation and convergence cross-links have the greatest influence on stability. A model-specific transition between stabilities was identified as the product of these cross-links, AC.CA(CRITICAL). For each model, three types of fixed points are described and displayed graphically: stable node, line of nonisolated fixed points, and saddle.

CONCLUSION

We demonstrated the stability analysis of a two-dimensional linear system, working only in the time domain. The benefit of time-domain stability analysis is that it can be extended to nonlinear systems when frequency domain analysis techniques fail. Given that we live in a fundamentally nonlinear world and that the use and application of computational models is extensive, this is a valuable and important tool.

Perceptual learning in visual search generalizes over tasks, locations, and eyes

In a visual search task, targets containing elementary features are detected in parallel, while a serial search is necessary for the detection of a target without a feature, or for targets containing conjunctions of features. In this study, we re-investigated the role of practice in visual search tasks, using an uncued visual search paradigm. Under some circumstances, initially serial tasks can become parallel with practice. Perceptual learning of feature search tasks is rapid (a few hundreds of trials are sufficient to transform serial into parallel search), long-lasting (a learned task is retained over several months), but far less specific than learning of other visual tasks (see also Sireteanu & Rettenbach, 1995a [Vision Research, 35, 2037-2043]). Learning transfers from one task to another, from one location in the visual field to another, and between the two eyes of a given subject, even if the subject has reduced stereopsis. Search for a conjunction of orientation and colour becomes more efficient, suggesting that a different search strategy emerges after prolonged practice. These results suggest that learning of visual search tasks modifies neural structures located at a high level in the visual pathway, involving different, presumably more central neural circuits, than the learning of visual discriminations and hyperacuity.

Depth selectivity of vertical fusional mechanisms

We measured the ability to fuse dichoptic images of a horizontal line alone or in the presence of a textured background with different vertical disparity. Nonius-line measurements of vertical vergence were also obtained. Diplopia thresholds and vertical vergence gains were much higher in response to an isolated vertically disparate line than to one with a zero vertical-disparity background. The effect of the background was maximum when it was coplanar with the target and decreased with increasing relative horizontal disparity. We conclude that vertical disparities are integrated over a restricted range of horizontal disparities to drive vertical vergence.

Transformation of the visual-line value in binocular vision: stimuli on corresponding points can be seen in two different directions

We examined Wheatstone's (1838 Philosophical Transactions of the Royal Society of London 128 371-394) claim that images falling on retinally corresponding points can be seen in two different directions, in violation of Hering's law of identical visual direction. Our analyses showed that random-dot stereograms contain stimulus elements that are conceptually equivalent to the line stimuli in the stereogram from which Wheatstone made his claim. Our experiment demonstrated that two lines embedded in a random-dot stereogram appeared in two different directions when they stimulated retinally corresponding points, if the disparity gradient value of the lines was infinity relative to adjacent elements. To ensure that the two lines stimulated corresponding points, observers made vergence eye movements while maintaining the perception of the two lines in two different directions.

Characteristics of saccades and vergence in two kinds of sequential looking tasks

We determined how saccades were used in the experiments described in Epelboim, Steinman, Kowler, Edwards, Pizlo, Erkelens and Collewijn (1995) [Vision Research, 35, 3401-3422], where unrestrained subjects looked at or tapped nearby targets. We report: (i) the size of binocular saccades; (ii) how well saccade size matched in the two eyes; and (iii) saccadic vergence. A representative sample (3375 saccades) was measured: 83% were <15 degrees, 53% were <5 degrees. Only two were 'microsaccades'. Saccade sizes were very similar in the two eyes. These results imply that subjects prefer avoiding large saccades. They can do this simply by re-orienting the head appropriately. Subjects under-verged by 25-35% and preformed well. None experienced diplopia.

Neither occlusion constraint nor binocular disparity accounts for the perceived depth in the 'sieve effect'

Current notions of binocular depth perception include (1) neural computations that solve the correspondence problem and calculate retinal positional disparity, and (2) recovery of ecologically valid occlusion relationships. The former framework works well for stimuli with unambiguous interocular correspondence, but less so for stimuli without well-defined disparity cues. The latter framework has been proposed to account for the phenomenon of perceived depth in stimuli without interocular correspondence, but its mechanism remains unclear. In order to obtain more insight into the mechanism, we studied the depth percept elicited by a family of stereograms - 'sieve' stimuli, adapted from Howard (1995) [Perception, 24, 67-74] - with interocular differences but no well-defined positional disparity cue. The perceived depth was measured by comparison to references at various depths established by standard retinal disparity and was consistently found to lie behind the fixation plane. Moreover, the magnitude of the depth percept depended on both the horizontal and vertical spatial characteristics of the stimulus in ways that were at odds with constraints of occlusion geometry. In comparison to the depth percept elicited by stimuli with well-defined disparity cues, the precision of the percept from the sieve stimuli was 10-20 times worse, suggesting that a different underlying computation was involved. Thus, neither of the above frameworks accounts for the depth percept arising from these stimuli. We discuss implications of our results for physiologically based computations underlying binocular depth perception.

The dependence of motion repulsion and rivalry on the distance between moving elements

We investigated the extent to which motion repulsion and binocular motion rivalry depend on the distance between moving elements. The stimuli consisted of two sets of spatially intermingled, finite-life random dots that moved across each other. The distance between the dots moving in different directions was manipulated by spatially pairing the dot trajectories with various precisions. Data from experiment 1 indicated that motion repulsion occurred reliably only when the average distance between orthogonally moving elements was at least 21.0 arc min. When the dots were precisely paired, a single global direction intermediate to the two actual directions was perceived. This result suggests that, at a relatively small spatial scale, interaction between different directions favors motion attraction or coherence, while interaction at a somewhat larger scale generates motion repulsion. Similarly, data from experiment 2 indicated that binocular motion rivalry was significantly diminished by spatially pairing the dots, which moved in opposite directions in the two eyes. This supports the recent proposal that rivalry occurs at or after the stage of binocular convergence, since monocular cells could not have directly responded to our interocular pairing manipulation. Together, these findings suggest that the neural mechanisms underlying motion perception are highly sensitive to the fine spatial relationship between moving elements.

Does convergence, not accommodation, cause axial-length elongation at near? A biometric study in teens

To determine whether convergence rather than accommodation has a primary effect on the changes in axial length and other biometric components during near fixation, we measured the anterior chamber depth, lens thickness, vitreous length and axial length in the right eyes of 124 young male subjects while their left eyes focused at distance (6 m) and near (20 cm). The measurements were performed before and after cycloplegia in the right eye, so we aimed to study biometric components of the eye in the states of accommodation and nonaccommodation, but converging at near. While the left eye focused at near, the axial length increased significantly with and without cycloplegia (p < 0.0005 and p < 0.0005). The vitreous length was the main increasing ocular biometric component at near both with and without cycloplegia (p < 0.044 and p = 0.001, respectively). At near, there was no difference between two mean axial length and two vitreous length measurements both with and without cycloplegia (p = 0.672 and p = 0.595, respectively). Under cycloplegia, anterior chamber depth also increased significantly at near fixation (p = 0. 012). Axial elongation at near fixation, mainly due to an increase in vitreous length, may result from the effect of accommodative convergence rather than accommodation itself. Much use of convergence, not accommodation, may be one of the contributing factors in adult onset and adult progression of myopia.

Comparing extra-retinal information about distance and direction

The idea that extra-retinal information about the orientation of the eyes could be used to judge an object's distance has a long history, and has been the issue of considerable debate throughout this century. We here show that the poor performance in comparison with judgements of direction has geometrical rather than physiological reasons, and discuss why previous studies have misled us into believing that information about distance is even poorer than the geometry predicts.

AC/A ratio, age, and refractive error in children

PURPOSE

To examine how the response AC/A ratio (the amount of accommodative convergence per unit of accommodative response) varies as a function of refractive error and age, to determine whether it is a risk factor for the onset of myopia, and to examine the relation between ocular structural features and the AC/A ratio.

METHODS

Accommodation was stimulated by a letter target presented in a Badal system at 0.00, 2.25, and 4.37 D to 828 children aged 6 through 14 years in 1996. Of these, 726 had no myopia in 1996 and were available for examination the following year. Accommodative response and cycloplegic refractive error were measured by autorefraction and convergence by monitoring the relative movement of Purkinje images I and IV. Lens radii of curvature were measured by video phakometry, corneal radius of curvature by topography, and ocular axial dimensions by A-scan ultrasonography.

RESULTS

Adjusted for age, the response AC/A ratio was highest in myopes (6.39 delta/D), intermediate in emmetropes (3.94 delta/D), and lowest in hyperopes (3.40 delta/D; P < 0.0001; two-way analysis of variance [ANOVA]). The stimulus AC/A ratio did not vary with refractive error. Adjusted for refractive error, the response AC/A ratio did not change as a function of age. In non-myopic children, having a response AC/A ratio of 5.84 delta/D or more elevated the risk of development of myopia within 1 year by 22.5 times (95% CI = 7.12-71.1). In a subsample of children without myopia who had refractive errors less than +0.75 D, having a response AC/A ratio of 5.84 delta/D or more elevated the risk of development of myopia within 1 year by 3.21 times (95% CI = 1.14-9.07). The AC/A ratio was associated with all measured ocular features except lens spherical volume. Only the negative correlations with refractive error and the shape of the crystalline lens (Gullstrand lens power) were significant in a multiple regression model (adjusted R2 = 0.16).

CONCLUSIONS

An elevated response AC/A ratio was associated with myopia and was an important risk factor for its rapid onset. The association between higher AC/A ratios and flatter crystalline lens shapes, as well as other reported features of accommodation in myopia, may be explained by "pseudocycloplegia," which the authors define as tension on the crystalline lens that increases the level of effort needed to accommodate. Accommodative deficits in myopia may be the functional consequences of the underlying anatomy of the enlarged eye.

Disjunctive optokinetic nystagmus in a naturally esotropic macaque monkey: interaction between nasotemporal asymmetries of versional eye movement and convergence

OBJECTIVE

To assess the interaction between versional and vergence eye movements in normal and strabismic monkeys.

METHODS

Horizontal optokinetic nystagmus (OKN) and vergence were measured using the magnetic scleral search coil technique in a normal adult monkey and a strabismic monkey who had naturally occurring early-onset esotropia. Mean eye velocity and vergence angles were calculated during the slow phases of OKN.

RESULTS

The strabismic monkey had a nasotemporal asymmetry of OKN favoring nasally directed motion in each eye. During monocular optokinetic stimulation, mean eye velocities were substantially greater for the adducting as compared to the abducting eye. The velocity of the abducting eye was between 55 and 80% of the velocity of the adducting eye (p < 0.01). As a consequence of the disjunctive movements, the eyes converged an average of 4 +/- 2.8 degrees during OKN. Saccadic analysis documented normal lateral rectus function in each eye. Neither an OKN asymmetry nor disjunctive OKN was observed in the normal monkey.

CONCLUSION

Disjunctive OKN in the esotropic monkey suggests that the cerebral maldevelopment responsible for nasally biased OKN also contributes to nasal biases in vergence pathways.

Prism induced accommodation in infants 3 to 6 months of age

Convergence-accommodation, one of several cross-linkages in the oculomotor system is manifested by opening the accommodative feedback loop and increasing the vergence input. We elicited this response in human infants aged 3-6 months by placing a 15 delta prism (base-out) before one eye while they viewed a diffuse patch of light. Accommodation was measured and ocular alignment was confirmed with a video photorefractor. The convergence-accommodation response is therefore present during a time when blur driven accommodation and disparity vergence are maturing. The gain of convergence-accommodation (expressed as the stimulus CA/C ratio) appeared to be greater for infants than adults.

Getting the measure of vergence weight in nearness perception

Combining multiple sources of information allows the human nervous system to construct an approximately Euclidean representation of near (personal) space. Within this space, binocular vergence is an important source of egocentric distance information. We investigated how the nervous system determines the significance (weight) accorded to vergence information when other (retinal) distance cues are present. We found that weight decreases with (1) increasing discrepancy between vergence information and other cues and (2) reduced vergence demand. The results also provided evidence that the nervous system represents vergence related distance information in units of nearness (the reciprocal of distance).

Effects of nonius line and fusion lock parameters on fixation disparity

PURPOSE

Clinical devices for fixation disparity vary in the size and position of the nonius test-lines and in the location of the binocular fusion lock, and published results of studies comparing fixation disparity devices also vary. This study examined how varying stimulus parameters affects the magnitude of horizontal fixation disparity and the precision of nonius alignment in normal observers.

METHODS

Targets were bright vertical lines presented for 150 ms on an oscilloscope and viewed in the dark from 3.95 m through a mirror haploscope. The fusion stimulus was either central, peripheral, or central-plus-peripheral. Fixation disparity was measured for nonius lines with a fixed vertical separation and variable lengths, using the method of constant stimuli. In a second experiment, fixation disparity was assessed for vergence demands of 6delta base-in to 12delta base-out using nonius lines of fixed length and variable vertical separations. For comparison, binocular Vernier thresholds and constant errors were also assessed.

RESULTS

Mean values of fixation disparity are invariant with the length and separation of the nonius lines and proximity to the fusion lock, over the range of values tested. When measured with a central fusion lock, the precision of binocular nonius alignment (vergence variability) worsens if the separation between the nonius lines increases beyond approximately 20 min arc. Vergence variability is larger with a peripheral fusion lock for small nonius-line separations and approximately the same as with the central lock for large line separations. Fixation disparity and precision were smallest with a central-plus-peripheral fusion-lock combination. Similar effects of line length and separation were found for binocular Vernier.

CONCLUSIONS

The finding of invariance of fixation disparity with line length for nonius targets < 1degrees extends those of earlier findings for larger targets. Because the precision of binocular nonius alignment is poorer for large nonius-line separations or with a peripheral fusion lock, fixation disparity should be most repeatable when measured with a central fusion lock and nonius lines separated by 20 min arc or less. However, a peripheral fusion lock and small nonius-line separation may provide the most effective diagnostic combination for fixation disparity.

Selective loss of vergence control secondary to bilateral paramedian thalamic infarction

The supranuclear pathways for vergence eye movements are poorly understood. The authors report a 57-year-old patient who presented with selective loss of vergence control and dissociation of light and near reaction. MRI showed a symmetric paramedian thalamic infarction without midbrain lesion. The findings suggest that this syndrome is due to an interruption of supranuclear fibers to midbrain vergence neurons.

Vergence adaptation and the clinical AC/A ratio

The accuracy of the gradient technique for measuring the clinical accommodative convergence to accommodation (AC/A) ratio is dependent upon obtaining veridical heterophoria measurements. However, previous studies have demonstrated that the sustained output of slow fusional vergence, which may take several minutes or even hours to decay, can bias heterophoria assessment. In the clinical setting, it is usual to estimate the AC/A ratio after just a few seconds of dissociation. This study investigated whether the slow fusional vergence response alters this crosslink ratio by comparing values of AC/A measured both before and immediately after a 1-hr period of continuous monocular occlusion. Sustained occlusion produced a significant change in near heterophoria in 10 out of the 21 subjects examined, but no significant change in AC/A was observed in either the adapting or non-adapting subgroups. Accordingly, while the sustained output of slow fusional vergence will influence clinical measurements of heterophoria, its presence does not alter the stimulus AC/A ratio significantly.

Modelling adaptation effects in vergence and accommodation after exposure to a simulated virtual reality stimulus

Natural viewing conditions place equal demands on the vergence and accommodation systems. The two responses are co-ordinated via the interactive components which couple the two systems: accommodative vergence and vergence accommodation. The crosslink components are usually quantified by the AC/A and CA/C ratios. Whether these ratios are stable entities, genetically determined, or modifiable by experience has been the subject of some debate. A 'Virtual Reality' stimulus was used to place unequal demands on the vergence and accommodation systems. Pre-task and post-task measures of AC/A and CA/C were objectively determined. The changes in the post-task measures are discussed with reference to a dual interactive model of vergence and accommodation. Model simulations suggest that adaptation of the vergence and accommodative controllers (tonic adaptation) may also play a part by altering the open loop bias. The results therefore support the idea that vergence accommodation and accommodative vergence are capable of adaptation.

Visually evoked cyclovergence and extended listing's law

Cyclovergence is a simultaneously occurring cyclorotation of the two eyes in opposite directions. Cyclovergence can be elicited visually by opposite cyclorotation of the two eyes' images. It also can occur in conjunction with horizontal vergence and vertical version in a stereotyped manner as described by the extended Listing's law (or L2). We manipulated L2-related and visually evoked cyclovergence independently, using stereoscopic images of three-dimensional (3D) scenes. During pursuit in the midsagittal plane, cyclovergence followed L2. The amount of L2-related cyclovergence during pursuit varied between subjects. Each pursuit trial was repeated three times. Two of the three trials had additional image rotation to visually evoke cyclovergence. We could separate the L2-related and visual components of cyclovergence by subtraction of the cyclovergence response in matched trials that differed only in the image rotation that was applied during pursuit. This indicates that visual and L2-related contributions to cyclovergence add linearly, suggesting the presence of two independent systems. Visually evoked cyclovergence gains were characteristic for a given subject, little affected by visual stimulus parameters, and usually low (0.1-0.5) when a static target was fixated. Gain and phase lag of the visually evoked cyclovergence during vertical pursuit was comparable with that during fixation of a static target. The binocular orientations are in better agreement to orientations predicted by L2 then would be predicted by nulling of the cyclodisparities. On the basis of our results, we suggest that visually driven and L2-related cyclovergence are independent of each other and superimpose linearly.