Target Vergence Research Articles

The interaction between size and accommodation cues in predicting proximity

Purpose: We have shown previously that proximal stimulation using size change can enhance or degrade accommodation step responses in open and closed loop conditions. The aim of the current experiment is to determine the relationship between the perceptual response to proximal stimulation based on size change and blur change for a range of target vergences.Methods: Perceptual thresholds for pure blur change, pure size change, coherent blur and size change and conflicting blur and size change were measured monocularly in 12 visually normal subjects (mean refractive error of ‐0.79D ± 2.41D and mean age 25.17 years ±3.10 years) using a 2‐alternative forced‐choice technique with 20 presentations per condition. Threshold estimates were obtained from psychometric functions using the bootstrap method. All subjects were fully corrected and had normal amplitudes of accommodation for their age. Threshold for proximal perception to blur only stimulation was established using target vergence changes of 0, 1, 2 and 3D. The effect of size change was then assessed at the same four levels of blur change. Stimuli were presented randomly with the position of the near and far vergences also randomised. This was repeated under four conditions: (1) Blur only. (2) Size only. (3) Size change coherent with the vergence change. (4) Size change conflicting with the vergence change.Targets consisted of pairs of photographic slides that contained strong monocular size and depth cues which were viewed successively in a 4D Badal lens system. On each trial the subjects compared the perceived proximity of the targets.Results: Considerable individual variation was found in the subject's perceptual responses to the four experimental conditions presented. The responses of all subjects were predicted by one of the following rules: (1) No effect of size. (2) Blur and size interact constructively. (3) No effect of blur. (4) Blur elicits a proximal response but size is used to determine the direction of the proximal effect.Without any size cue, eight subjects (66%) identified the direction of the vergence change correctly while the remaining subjects consistently perceived the target with lower dioptric vergence to be closer. The threshold required to accurately determine proximity from blur only cues was 2.27D ± 0.31D. In the absence of blur, size cues induced a proximal effect in 33% (n = 4) subjects. The combination of size and blur cues produced a variety of response patterns, some subjects’ responses were dominated by blur cues while others relied heavily on size stimulation.Conclusions: Our ability to perceive a target as close can be dependent upon either size and/or accommodation signals. Interactions between the stimuli of dioptric target vergence and target size were found to be additive for producing a perceived proximity response.

Technical Note: Effect of contact lenses on measurement of the accommodation microfluctuations

Dynamic measurement of accommodation in subjects with myopia usually involves recording through soft contact lenses (CLs) to correct the refractive error. Conversely, dynamic accommodation measurement in emmetropic control subjects is generally undertaken without any corrective lenses. The aim of this experiment was to determine whether CL correction affects the measurement of accommodation microfluctuations using infrared refractometry, and whether this needs to be considered in studies which attempt to compare accommodation responses between the two groups. Ten young emmetropic subjects viewed a high contrast Maltese cross target monocularly using the right eye at a target vergence of 0 D. The subjects viewed the target under two conditions: with CL condition and without CL condition, where the subjects viewed the target with the eye only. Accommodation responses of the right eye were recorded continuously for 2 min at a sampling rate of 52 Hz using the Shin-Nippon SRW-5000 autorefractor. No significant difference (two-tailed paired t-test, t(9) = -1.499, p = 0.168) was found in mean accommodation response between the with CL (mean +/- S.D. = -0.02 +/- 0.24 D) and without CL conditions (mean +/- S.D. = +0.01 +/- 0.25 D). No significant (two-tailed paired t-test, t(9) = 0.151, p = 0.883) difference in the magnitude of the accommodation microfluctuations was found between the with CL (mean +/- S.D. = 0.162 +/- 0.04 D) and without CL condition (mean +/- S.D. = 0.169 +/- 0.04 D). Power spectrum analysis revealed no differences in the characteristics of the microfluctuations waveform between the two conditions. A control experiment carried out on a subgroup of five subjects using a negative (-3 D) CL demonstrated that there was no significant effect of the dioptric power of the CL on the magnitude of the accommodation microfluctuations (anova: F(3,15) = 0.254, p = 0.782). Thin soft CLs do not affect the magnitude or frequency characteristics of accommodation microfluctuations when measured using the Shin-Nippon SRW-5000.

Effect of induced anisometropia on binocular through-focus contrast sensitivity.

The aim of this study was to characterize binocular through-focus function in white light and to investigate the effect of induced anisometropia on binocular depth of focus. The subjects viewed sine-wave gratings generated on a monitor through a modified Badal system that produced gradual changes in target vergence ranging from -4.00 to +2.00 D. Binocular through-focus contrast sensitivity curves were obtained at a spatial frequency of 11 cpd and for different levels of induced anisometropia. Subjective depths of focus were derived from the through-focus curves. An induced anisometropia lower than 1.00 D led to a monomodal through-focus curve involving a single depth of focus, whereas with higher anisometropia, the curves became bimodal indicating a lack of performance at intermediate distance. Binocular thresholds predicted by the quadratic summation model from our monocular measurements were well correlated to our binocular measurements. Predictions allowed us to estimate optimum levels of induced anisometropia.

Dynamics of the accommodation response to abrupt changes in target vergence as a function of age

The dynamic accommodation responses to small, abrupt changes in an accommodation stimulus were studied in two experiments. In the first, responses of 19 subjects with ages distributed between 18 and 49 years were measured for step stimuli of ±1.05 D. In the second, responses to small step stimuli (±0.75, ±1.75 D) were recorded for a group of six ‘young’ (mean age 22, range 16–26 years) and six ‘old’ (mean age 42, range 36–48 years) subjects. In both experiments, the low target vergences always lay within the accommodation range of all subjects. Whether the data were analysed in terms of simple reaction and response times, or in terms of the frequency response, no marked systematic changes with age were found in the dynamics of the response in either of the experiments. It is concluded that, for small stimuli within the amplitude of accommodation, the response dynamics over the adult age range studied remain remarkably constant, even though the amplitude progressively reduces with age as complete presbyopia is approached.

The calibration of a 2.5× Galilean focusable telescope as an optometer for refraction

A 2.5x Selsi achromatic Galilean focusable telescope was calibrated for refraction at 6 m. In its calibration, minus power trial lens was placed at the objective of the telescope to simulate vergence of a target at a finite distance, before the back vertex power (BVP) of the telescope at each setting (telescope length) was measured by a focimeter. By using a graphical presentation of the results, the BVP of the telescope at each setting could be determined at different selected target distances. For a target vergence of -0.167 D or a distance of 6 m, the common testing distance in clinical practice, this calibrated telescope had a BVP or refraction measuring range of -7.27(-)+7.52 D. When this telescope was used to measure simulated manifest refractive errors at 6 m, it yielded a mean error of +0.13 D with a 95% confidence limit of agreement of -0.38(-)+0.64 D. These results indicated that the accuracy and precision of telescopic refraction were comparable to that of retinoscopy. Therefore, the calibrated telescope could be considered as a reliable and inexpensive instrument for determining spherical refractive errors. Telescopic refraction is applicable in refracting economically disadvantaged population in underserved areas where modern equipment and electricity are not available. In addition, it provides an alternative subjective refraction method for low vision population because the magnification of this calibrated telescope has the advantage of allowing low vision patients to be refracted at the common 6 m testing distance in clinical practice.

The calibration of a 2.5× Galilean focusable telescope as an optometer for refraction

A 2.5× Selsi achromatic Galilean focusable telescope was calibrated for refraction at 6 m. In its calibration, minus power trial lens was placed at the objective of the telescope to simulate vergence of a target at a finite distance, before the back vertex power (BVP) of the telescope at each setting (telescope length) was measured by a focimeter. By using a graphical presentation of the results, the BVP of the telescope at each setting could be determined at different selected target distances. For a target vergence of −0.167 D or a distance of 6 m, the common testing distance in clinical practice, this calibrated telescope had a BVP or refraction measuring range of −7.27–+7.52 D. When this telescope was used to measure simulated manifest refractive errors at 6 m, it yielded a mean error of +0.13 D with a 95% confidence limit of agreement of −0.38–+0.64 D. These results indicated that the accuracy and precision of telescopic refraction were comparable to that of retinoscopy. Therefore, the calibrated telescope could be considered as a reliable and inexpensive instrument for determining spherical refractive errors. Telescopic refraction is applicable in refracting economically disadvantaged population in under-served areas where modern equipment and electricity are not available. In addition, it provides an alternative subjective refraction method for low vision population because the magnification of this calibrated telescope has the advantage of allowing low vision patients to be refracted at the common 6 m testing distance in clinical practice.

Trajectories of the human binocular fixation point during conjugate and non-conjugate gaze-shifts.

This paper describes the spatial trajectories of the binocular fixation point (the intersection point of the two lines of sight) during gaze-shifts within a horizontal plane of regard. Gaze was voluntarily shifted between pairs of real, continuously visible LED targets that were either iso-vergent at 5-25 deg convergence (conjugate version saccades) or differed in vergence angle (by 5-20 deg) as well as in direction (by 5-60 deg; combined version and vergence). Orientations of both eyes were recorded by phase detection in a homogeneous magnetic field with scleral sensor coils. "Conjugate" saccades showed an outward-looping, curved trajectory as a result of transient divergence, typically associated with horizontal saccades. These outward loops were disproportionately larger for far than for near targets, due to the non-linear relation between vergence and distance. Transient divergence increased moderately in magnitude and duration when basic vergence increased from 5 to 25 deg. As a result, transient saccadic disparities increased in angular magnitude as targets got close. Increasing tonic vergence did not, however, slow down conjugate saccades, in contrast to the previously described dynamic slowing effects of vergence on version during gaze-shift involving simultaneous vergence and version changes. Convergent and divergent non-conjugate gaze-shifts each had characteristic trajectories; outward loops were much reduced in convergent and virtually absent in divergent movements. The saccadic component of non-conjugate gaze-shifts was preceded by a pre-saccadic vergence component in the direction of the imminent gaze-shift; its magnitude increased systematically with the increase in vergence demand and with the decrease in version demand. For both pre-saccadic convergence and divergence, this pre-saccadic part of the trajectory tended to follow an iso-direction line through the target of origin; directional change did not start until the saccade began. This suggests that for targets that differ in direction as well as distance, control of the vergence and version components of the gaze-shift can be dissociated to some degree. This seems to argue against models of binocular oculomotor control which assume that each eye responds primarily to its own target, and suggests rather that target vergence and target direction may be processed and responded to separately by ocular vergence and version, with a strong interaction between the two oculomotor activities whenever they occur at the same time.

A Note on Some Phase Differencing Algorithms for Disparity Estimation

Disparity can be estimated from the local phase differences between a pair of stereo images after Gabor filterings. In this paper, some phase differencing algorithms are discussed from the point of view of numerical computation. They are all categorised as certain types of Newton iterations, hence are restricted by the preconditions of Newton iterations. Furthermore, a numerical sensitivity analysis is made for phase computation near its singularity points without referring to any specific filtering method. It makes clear that the numerical instability has its roots in the singularities of the phase functions themselves rather than in their derivatives, therefore severe errors cannot be properly reduced by Newton iterations. Hence, the singularity problem must be carefully treated for any disparity estimation using phase differencing techniques. In the case of foveal vision, such as target detection or vergence control, the central parts of scenes are of interest. A shift-trials algorithm is proposed to contain the effects of singularities instead of relying on detection and removal of singularity neighbourhoods through local computation. This algorithm directly tackles the singularity problem by posing the local disparity estimations as a minimisation of the phase differences of the left and right views in terms of a global matching residual norm. By a direct search for solution, no numerical derivative is needed, phase singularity can be tolerated and robust results are thus produced.

Perceived motion in depth

We examine how various sources of information contribute to the percept of motion in depth. Subjects were presented with targets moving in depth, and were asked to judge their velocities and final positions. On each presentation, the target's position relative to the two eyes (target vergence), the size of the target's retinal image and the difference in this image's position relative to that of the background in the two eyes (relative disparity), each either changed as they normally would for a target moving at a fixed speed towards the observer, or did not change at all. Subjects' judgements for various such combinations show that all three sources of information influence both the perceived velocity of motion in depth and the final perceived position, but in different ways. This is not too surprising, because the assumptions that the use of each source of information are based on, are different for the two tasks. We propose that the way the different sources are combined is governed by the likelihood of the assumptions, that are required to use that information, being true under the given circumstances.

Excess cyclovergence in patients with intermittent exotropia

Recently, we developed a model of binocular fixation. This model predicts the amount of cyclovergence as a function of target elevation and horizontal target vergence. The prediction derives from the assumption that version and vergence add linearly and that the eye positions are constrained in three respects: (1) the foveae of the two eyes are directed towards the target, (2) the version component follows Listing's law, i.e. cycloversion, and horizontal and vertical version are not independent, (3) the vergence component is restricted to a plane approximately perpendicular to Listing's plane, i.e. horizontal, vertical and torsional vergence are not independent. The version and the vergence components are characterized by a common primary direction for the two eyes. We applied this model to data of patients with intermittent exotropia. In two patients with an amblyopic eye we found that the common primary direction rotates towards the amblyopic eye. In the third patient, not suffering from amblyopia, the common primary direction was practically straight ahead. In all three patients, cyclovergence angles were larger than those found in normal subjects. We found that the increased cyclovergence was compatible with our model for normal subjects if an offset on the horizontal vergence was given. This offset represents the additional convergence effort required in these patients to overcome the exodeviation of the eyes. According to our model the increased horizontal vergence effort results in excess cyclovergence. The relation between horizontal vergence and cyclovergence offers a new method for measuring the angle of exotropia.

Comparison of the depths of focus with the naked eye and with three types of presbyopic contact lens correction

Experiments are described in which monocular, high-contrast, letter acuity was measured as a function of target vergence (effective distance) for six simulated presbyopic subjects, who were placed under cycloplegia with a 4mm diameter artificial pupil. Measurements were made with the naked eye or single-vision spectacle lens corrected eye, and with three types of simultaneous-vision (simultaneous image) ‡ presbyopic contact lens correction: a centre-near biofocal; a centre-distance varifocal or progressive power lens; and a centre-mear varifocal. Results show that all designs usefully improve the range of distances over which an adequate standard of vision is achieved, the depth of focus typically being increased by about 1–2 dioptres, depending upon the criterion used to define depth of focus. Peak visual acuity at optimal focus is, however, slightly worse with the presbyopic contact lens corrections than with the naked or spectacle-lens corrected eye. Intersubject differences suggest that individual lens characteristics, the aberrations of the individual eye and lens centration may all play a role in determining the optical effectiveness of any particular design.

Neural Representations for Sensorimotor Control. III. Learning a Body-Centered Representation of a Three-Dimensional Target Position

Abstract A neural model is described of how the brain may autonomously learn a body-centered representation of a three-dimensional (3-D) target position by combining information about retinal target position, eye position, and head position in real time. Such a body-centered spatial representation enables accurate movement commands to the limbs to be generated despite changes in the spatial relationships between the eyes, head, body, and limbs through time. The model learns a vector representation-otherwise known as a parcellated distributed representation-of target vergence with respect to the two eyes, and of the horizontal and vertical spherical angles of the target with respect to a cyclopean egocenter. Such a vergence-spherical representation has been reported in the caudal midbrain and medulla of the frog, as well as in psychophysical movement studies in humans. A head-centered vergence-spherical representation of foveated target position can be generated by two stages of opponent processing that combine corollary discharges of outflow movement signals to the two eyes. Sums and differences of opponent signals define angular and vergence coordinates, respectively. The head-centered representation interacts with a binocular visual representation of nonfoveated target position to learn a visuomotor representation of both foveated and nonfoveated target position that is capable of commanding yoked eye movements. This head-centered vector representation also interacts with representations of neck movement commands to learn a body-centered estimate of target position that is capable of Commanding coordinated arm movements. Learning occurs during head movements made while gaze remains fixed on a foveated target. An initial estimate is stored and a VOR-mediated gating signal prevents the stored estimate from being reset during a gaze-maintaining head movement. As the head moves, new estimates are compared with the stored estimate to compute difference vectors which act as error signals that drive the learning process, as well as control the on-line merging of multimodal information.

Directional discrimination of motion in depth based on changing target vergence

Difference limens for the apparent direction of motion in depth were measured in eight observers. Two points of tight were presented dichoptically in an otherwise dark field. Their lateral motion (changing target-vergence stimulus) mimicked the oscillation of an object in depth along different paths. All seven paths of 100 cm length had a common distant position in the median plane 150 cm from the observer. The directions of the paths were equidistant; the second and sixth paths were directed toward the left and right eye of the observer, respectively. Discrimination of direction was best for motion along the median plane and deteriorated toward the periphery. There was no indication of local minima of die difference limen for motions toward the eyes as has been observed with changing-disparity stimuli.

Subjective and Objective Assessment of Soft Bifocal Contact Lens Performance

Subjective and objective techniques were used to assess the on-eye performance of soft bifocal contact lenses. In the subjective technique a young observer whose accommodation had been paralyzed with a cycloplegic agent was fitted with the contact lens type under investigation and aligned with a Maxwellian view Badal optometer using a bite-bar. Visual acuity was measured as a function of both target vergence (0.00 to -4.00 D) and pupil size (1 to 5 mm). Aspheric, concentric, and diffractive soft bifocal contact lens designs were investigated. Diffractive and concentric bifocal designs with +2.00 D near additions showed "twin peaks" of visual acuity with one peak at 0.00 D target vergence (equivalent to distance viewing) and the second at -2.00 D target vergence (equivalent to a target at 50 cm). Some aspheric designs produced a relatively constant visual acuity across this vergence range, whereas others gave poor acuity at near. Visual acuity with the Echelon diffractive lens was relatively unaffected by pupil size, confirming theoretical predictions. Visual acuity with concentric designs was also relatively free of pupil size effects. In the objective technique, the variation in surface power across the lens was assessed using video-keratography. This technique provided an elegant means of visualizing the power profile of the lens. The location of the zones of increased power and the magnitude of power variations allowed an accurate prediction of the visual performance measured subjectively. Temporal displacement of lenses may explain the occasions where visual performance did not vary with pupil size.

Estimates of time to contact based on changing size and changing target vergence.

Apparent constant-velocity approaches of an outline circle were generated by changing the size and/or the lateral separation of dichoptically presented circles at a physically constant distance from subjects. The lateral separation of the circles defines target vergence which corresponds proximally to combinations of ocular vergence and (absolute) retinal disparity. In the first experiment, estimates of time to contact were found to depend both on changing size and on changing target vergence; in the case of conflicting information, changing size turned out to be the more powerful stimulus. In the second experiment, the size of the stimulus object was varied. The relatively stronger influence of changing size on estimates of time to contact was again found for the larger stimulus objects, but for the smaller stimulus objects changing target vergence became more powerful. The results of both experiments show that estimates of time to contact are not uniquely determined by changing size, as seems to be implied by some tests of the so-called "tau hypothesis', but that they are based on multiple sources of information that have situationally specific effects on the estimates.

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)

Influence of target color and vergence of light on ocular accommodation during binocular fixation

In an earlier study we reported large discrepancies between ocular refraction and perception of blur for black/white letters. In the present study we report on the influence of target color and vergence of light on the human focusing system. Twenty visually normal volunteers between 14 and 25 years of age participated in this experiment. Targets were brightness-matched red, yellow, green, blue or white 21 minarc letters displayed on the black background of a high resolution RGB monitor, at a 40 cm observation distance. Changes in the vergence of light were effected via increasing amounts of positive and negative power spherical lenses placed binocularly in front of the subject's eyes. Our results showed that: 1) the accommodative level varied as a function of the color of a target, 2) the vergence of light was not an infallible cue for accurate accommodation and 3) the inter-subject variability seen in response to the vergence of light was not linked with the chromaticity of test targets. All subjects showed a decreasing ability to fully relax accommodation with increasing plus power lenses. For minus power lenses, accommodative response profiles were divided into 3 categories, category 1 having accurate accommodation for the test target, category 2 showing a low level lag of accommodation and category 3 being totally unresponsive to increasing divergence of light. While subjects across response categories showed differential endpoints in clinical estimates of positive relative accommodation, other clinical measurements related to refractive error, accommodative amplitude and fusional ability failed to predict the accommodative behavior of our subjects as did measurements of the resting focus of accommodation.

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

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil--revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: (a) the target was moved manually by the experimenter; (b) the target was moved manually by the subject; (c) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from 'slow' to 'very fast', corresponding to changes in target vergence from about 10 degrees s-1 to about 100 degrees s-1. The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When 'very fast' changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100 degrees s-1. Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i.e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i.e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the 'slow' and 'medium' conditions (stimulus speeds about 10-40 degrees s-1) tracking was accurate to within 1-2 degrees, irrespective of the way in which the target was moved. In the 'fast' and 'very fast' conditions (stimulus speeds about 40-100 degrees s-1), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Binocular co-ordination of human vertical saccadic eye movements.

1. The binocular co-ordination of human vertical saccades was analysed systematically over the full oculomotor range, with a precise and accurate scleral sensor coil technique. Effects of amplitude (1.25-70 deg), direction (upward vs. downward and centripetal vs. centrifugal), as well as position (upper or lower sector of vertical oculomotor range), were investigated systematically in three subjects. 2. All saccades were made voluntarily between continuously presented pairs of targets, which subtended equal angles of target vergence. 3. Vertical saccades were less accurate than horizontal saccades (as described by Collewijn, Erkelens & Steinman, 1988). For target distances between 10 and 70 deg, upward saccades undershot the target by about 10%, whereas downward saccades tended to overshoot the target. Downward saccades were about 1.5 deg larger than upward saccades between the same targets. 4. Peak velocities continued to increase monotonically with saccadic amplitude up to 513 +/- 27 (S.D.) deg/s for 70 deg saccades; a distinct asymptotic level was not reached. 5. Velocity profiles of upward and downward saccades, made symmetrically about the primary (straight-ahead) position, were very similar for amplitudes up to 30 deg. At larger amplitudes, velocity profiles of upward saccades remained single peaked, whereas those of downward saccades invariably developed a second velocity peak. 6. Parameters of upward saccades depended heavily on the position of the eye. In the upper oculomotor range such saccades had lower maximum speeds, longer durations, and were more skewed than similar saccades in the lower oculomotor range (below primary). Downward saccades were almost independent of eye position. 7. Vertical eye movements during vertical saccades were virtually identical in the two eyes. In contrast, disjunctive horizontal components were systematically present. Upward saccades, at all amplitudes, were associated with diverging eye movements. Converging eye movements occurred during downward saccades. These systematic effects suggest that the vergence subsystem is not turned off during saccades. 8. These changes in vergence were followed by converging horizontal post-saccadic drift after upward saccades, and in diverging horizontal drift after downward saccades.(ABSTRACT TRUNCATED AT 400 WORDS)

Fluctuations in accommodation: a review.

When a young observer attempts to accommodate steadily on a fixed stimulus, the nominally steady-state response shows small instabilities or fluctuations (sometimes termed microfluctuations or oscillations). These fluctuations typically have an amplitude of a few tenths of a dioptre and a frequency spectrum extending up to a few Hertz. The properties of these fluctuations are described for various viewing conditions: pupil diameter, target vergence, target form, target contrast, and target luminance all influence the frequency spectra of the oscillations, as may anomalies of vision such as amblyopia. The possible roles that the fluctuations might play in the function of the accommodative system are discussed. It is suggested that the higher frequency components around 2 Hz may arise from the mechanical and elastic characteristics of the lens, zonule and ciliary body. Components at lower frequencies (less than 0.5 Hz) may be of more significance in the function of the accommodative control system.