Vergence Eye Movements: From Basic Science to Clinical Application - Foreword to the Special Issue.

The abstract book of the last European Conference on Eye Movements [1] lists abstracts of 373 presentations, but less than five percent investigate vergence eye movements, i.e. the coordination of the right and left eye. Why then a special issue on this neglected issue? Human vision under natural conditions involves both eyes in coordination controlled by interacting processes subsumed under the concept of vergence.. Further, vergence is important for people in their daily lives since disorders of vergence can have serious consequences: ophthalmologists deal with squinting patients on the basis of heterophoria and heterotropia testing, eye strain or visual complaints can be related to impaired vergence dynamic or less accurate static vergence, remediation by optometrist includes vergence training or prism eye glasses, etc.

Effects of aligning prisms on the objective and subjective fixation disparity in far distance.

Fixation disparity (FD) refers to a suboptimal condition of binocular vision. The oculomotor aspect of FD refers to a misadjustment in the vergence angle between the two visual axes that is measured in research with eye trackers (objective fixation disparity, oFD). The sensory aspect is psychophysically tested using dichoptic nonius lines (subjective fixation disparity, sFD). Some optometrists use nonius tests to determine the prisms for constant wear aiming to align the eyes. However, they do not (yet) use eye trackers. We investigate the effect of aligning prisms on oFD and sFD for 60 sec exposure duration of prisms determined with the clinically established Cross test in far distance vision. Without prisms, both types of FD were correlated with the aligning prism, while with prisms the FD was close to zero (these analyses included all base-in and base-out cases). The effect of base-in prisms on oFD was proportional to the amount of the aligning prism for the present 60 sec exposure, similar as for the 2- 5 sec exposure in Schmid et al. (2018). Thus, within 1 minute of prism exposure, no substantial vergence adaptation seems to occur in the present test conditions. Further studies may investigate intra-individual responses to different exposure times of aligning prisms in both prism directions.

Individual objective versus subjective fixation disparity as a function of forced vergence.

Inaccuracy in the vergence eye position ("fixation disparity") can occur despite a fusion stimulus. When measured with eye trackers, this inaccuracy is referred to as "objective fixation disparity". It is a matter of debate whether objective fixation disparity can be estimated with a technically simple psycho-physical procedure, i.e. the perceived offset of aligned dichoptic nonius targets, referred to as "subjective fixation disparity". To investigate the relation between these two measures, simultaneous tests were made in far vision when placing prisms in front of the eyes (for a few seconds) in order to induce forced vergence, i.e. to vary the absolute disparity (from 1 deg divergent to 3.4 deg convergent). Frequent repeated measurements in 12 observers allowed for individual analyses. Generally, fixation disparity values and the effects of prisms were much smaller in the subjective than in the objective measures. Some observers differed systematically in the characteristics of the two types of prism-induced curves. Individual regressions showed that the subjective vs. objective slope was 8% on the average (with largest individual values of 18%). This suggests that sensory fusion shifts the visual direction of the (peripheral) binocular targets by the full amount of objective fixation disparity (since single vision was achieved); however, for the (central) monocular nonius lines this shift was more or less incomplete so that the dichoptic nonius targets indicated an individual percentage of objective fixation disparity. The subjective-to-objective ratio seems to be an individual characteristic of fixation disparity in terms of the amount and in terms of the effect of prism-induced forced vergence. Therefore, on the group level the subjective measures do not allow for a precise prediction of the objective measures.

Individual Objective and Subjective Fixation Disparity in Near Vision.

Binocular vision refers to the integration of images in the two eyes for improved visual performance and depth perception. One aspect of binocular vision is the fixation disparity, which is a suboptimal condition in individuals with respect to binocular eye movement control and subsequent neural processing. The objective fixation disparity refers to the vergence angle between the visual axes, which is measured with eye trackers. Subjective fixation disparity is tested with two monocular nonius lines which indicate the physical nonius separation required for perceived alignment. Subjective and objective fixation disparity represent the different physiological mechanisms of motor and sensory fusion, but the precise relation between these two is still unclear. This study measures both types of fixation disparity at viewing distances of 40, 30, and 24 cm while observers fixated a central stationary fusion target. 20 young adult subjects with normal binocular vision were tested repeatedly to investigate individual differences. For heterophoria and subjective fixation disparity, this study replicated that the binocular system does not properly adjust to near targets: outward (exo) deviations typically increase as the viewing distance is shortened. This exo proximity effect-however-was not found for objective fixation disparity, which-on the average-was zero. But individuals can have reliable outward (exo) or inward (eso) vergence errors. Cases with eso objective fixation disparity tend to have less exo states of subjective fixation disparity and heterophoria. In summary, the two types of fixation disparity seem to respond in a different way when the viewing distance is shortened. Motor and sensory fusion-as reflected by objective and subjective fixation disparity-exhibit complex interactions that may differ between individuals (eso versus exo) and vary with viewing distance (far versus near vision).

Monocular Versus Binocular Calibrations in Evaluating Fixation Disparity With a Video-Based Eye-Tracker.

When measuring fixation disparity (an oculomotor vergence error), the question arises as to whether a monocular or binocular calibration is more precise and physiologically more appropriate. In monocular calibrations, a single eye fixates on a calibration target that is taken as having been projected onto the center of the fovea; the corresponding vergence state represents the heterophoria (the resting vergence position), which has no effect on the calibration procedure. In binocular calibrations, a vergence error may be present and may affect the subsequent measurement of the fixation disparity during binocular recordings. This study includes a test of the precision of both monocular and binocular calibrations and an evaluation of the impact of the calibration procedure on the measurement of fixation disparity during a dot scanning task. Our results show that 11 participants (out of 19) each exhibited a significant difference in fixation disparity with the two types of calibration procedures. In addition, the fixation disparity was more strongly affected by heterophoria undergoing monocular calibration, as opposed to binocular calibration. This serves as additional evidence showing that the monocular calibration produces a physiologically more plausible fixation disparity and seems to be more appropriate for studying the full extent of fixation disparity.

Effects of Prism Eyeglasses on Objective and Subjective Fixation Disparity.

In optometry of binocular vision, the question may arise whether prisms should be included in eyeglasses to compensate an oculomotor and/or sensory imbalance between the two eyes. The corresponding measures of objective and subjective fixation disparity may be reduced by the prisms, or the adaptability of the binocular vergence system may diminish effects of the prisms over time. This study investigates effects of wearing prisms constantly for about 5 weeks in daily life. Two groups of 12 participants received eyeglasses with prisms having either a base-in direction or a base-out direction with an amount up to 8 prism diopters. Prisms were prescribed based on clinical fixation disparity test plates at 6 m. Two dependent variables were used: (1) subjective fixation disparity was indicated by a perceived offset of dichoptic nonius lines that were superimposed on the fusion stimuli and (2) objective fixation disparity was measured with a video based eye tracker relative to monocular calibration. Stimuli were presented at 6 m and included either central or more peripheral fusion stimuli. Repeated measurements were made without the prisms and with the prisms after about 5 weeks of wearing these prisms. Objective and subjective fixation disparity were correlated, but the type of fusion stimulus and the direction of the required prism may play a role. The prisms did not reduce the fixation disparity to zero, but induced significant changes in fixation disparity with large effect sizes. Participants receiving base-out prisms showed hypothesized effects, which were concurrent in both types of fixation disparity. In participants receiving base-in prisms, the individual effects of subjective and objective effects were negatively correlated: the larger the subjective (sensory) effect, the smaller the objective (motor) effect. This response pattern was related to the vergence adaptability, i.e. the individual fusional vergence reserves.

The vertical monitor position for presbyopic computer users with progressive lenses: how to reach clear vision and comfortable head posture.

When presbyopic employees are wearing general-purpose progressive lenses, they have clear vision only with a lower gaze inclination to the computer monitor, given the head assumes a comfortable inclination. Therefore, in the present intervention field study the monitor position was lowered, also with the aim to reduce musculoskeletal symptoms. A comparison group comprised users of lenses that do not restrict the field of clear vision. The lower monitor positions led the participants to lower their head inclination, which was linearly associated with a significant reduction in musculoskeletal symptoms. However, for progressive lenses a lower head inclination means a lower zone of clear vision, so that clear vision of the complete monitor was not achieved, rather the monitor should have been placed even lower. The procedures of this study may be useful for optimising the individual monitor position depending on the comfortable head and gaze inclination and the vertical zone of clear vision of progressive lenses. PRACTITIONER SUMMARY: For users of general-purpose progressive lenses, it is suggested that low monitor positions allow for clear vision at the monitor and for a physiologically favourable head inclination. Employees may improve their workplace using a flyer providing ergonomic-optometric information.

Computer vision syndrome in presbyopia and beginning presbyopia: effects of spectacle lens type.

PURPOSE: This office field study investigated the effects of different types of spectacle lenses habitually worn by computer users with presbyopia and in the beginning stages of presbyopia. Computer vision syndrome was assessed through reported complaints and ergonomic conditions. METHODS: A questionnaire regarding the type of habitually worn near-vision lenses at the workplace, visual conditions and the levels of different types of complaints was administered to 175 participants aged 35 years and older (mean ± SD: 52.0 ± 6.7 years). Statistical factor analysis identified five specific aspects of the complaints. Workplace conditions were analysed based on photographs taken in typical working conditions. RESULTS: In the subgroup of 25 users between the ages of 36 and 57 years (mean 44 ± 5 years), who wore distance-vision lenses and performed more demanding occupational tasks, the reported extents of 'ocular strain', 'musculoskeletal strain' and 'headache' increased with the daily duration of computer work and explained up to 44 per cent of the variance (rs = 0.66). In the other subgroups, this effect was smaller, while in the complete sample (n = 175), this correlation was approximately rs = 0.2. The subgroup of 85 general-purpose progressive lens users (mean age 54 years) adopted head inclinations that were approximately seven degrees more elevated than those of the subgroups with single vision lenses. CONCLUSIONS: The present questionnaire was able to assess the complaints of computer users depending on the type of spectacle lenses worn. A missing near-vision addition among participants in the early stages of presbyopia was identified as a risk factor for complaints among those with longer daily durations of demanding computer work.

Comparison of progressive addition lenses for general purpose and for computer vision: an office field study.

BACKGROUND: Two types of progressive addition lenses (PALs) were compared in an office field study: 1. General purpose PALs with continuous clear vision between infinity and near reading distances and 2. Computer vision PALs with a wider zone of clear vision at the monitor and in near vision but no clear distance vision. METHODS: Twenty-three presbyopic participants wore each type of lens for two weeks in a double-masked four-week quasi-experimental procedure that included an adaptation phase (Weeks 1 and 2) and a test phase (Weeks 3 and 4). Questionnaires on visual and musculoskeletal conditions as well as preferences regarding the type of lenses were administered. After eight more weeks of free use of the spectacles, the preferences were assessed again. The ergonomic conditions were analysed from photographs. RESULTS: Head inclination when looking at the monitor was significantly lower by 2.3 degrees with the computer vision PALs than with the general purpose PALs. Vision at the monitor was judged significantly better with computer PALs, while distance vision was judged better with general purpose PALs; however, the reported advantage of computer vision PALs differed in extent between participants. Accordingly, 61 per cent of the participants preferred the computer vision PALs, when asked without information about lens design. After full information about lens characteristics and additional eight weeks of free spectacle use, 44 per cent preferred the computer vision PALs. CONCLUSION: On average, computer vision PALs were rated significantly better with respect to vision at the monitor during the experimental part of the study. In the final forced-choice ratings, approximately half of the participants preferred either the computer vision PAL or the general purpose PAL. Individual factors seem to play a role in this preference and in the rated advantage of computer vision PALs.

How to place the computer monitor: measurements of vertical zones of clear vision with presbyopic corrections.

BACKGROUND: This study intended to measure the near and far points of clear vision as a function of the inclination of the line of sight with comfortable head posture. Measurements with different lenses for presbyopic correction were made to suggest comfortable positions of the monitor for computer work. METHOD: An 'inclined optometer' was built, including a concave mirror to shift a visual target of constant angular size from near to infinity (proposed by Reiner). The optometer could be inclined vertically from horizontal to 50 degrees downward to vary the inclination of the line of sight. Measurements were made with a comfortable head position adjusted on a headrest. RESULTS: The near and far points were plotted both in the unit one/metre as a function of eye inclination (optometric diagram) and also as positions from the eyes in workspace co-ordinates (workplace diagram). First, individual examples of plots of the vertical zones of clear vision at the workplace are shown. Second, the group mean data of 22 observers with newly prescribed lenses showed that the vertical zones of clear vision for general purpose progressive addition lenses (PALs) reach infinity and are flatter, while computer vision PALs lead to more steep vertical zones ending at intermediate distances. Third, the mean results of three samples from our laboratory were compared with respect to general purpose PALs, which are most frequently used by presbyopic people. CONCLUSIONS: The diagrams of the vertical zones of clear vision for different spectacles provide information on the ergonomic vertical position of computer monitors for clear vision with a comfortable head position. The grand mean of general purpose PALs suggests that the upper edge of the monitor should be at least approximately 15 cm below eye level at a typical viewing distance of approximately 75 cm. Higher monitor positions are possible with computer vision PALs.

Web-based office ergonomics intervention on work-related complaints: a field study.

The aim of this study was a proof of concept to examine the effects of a Web-based office ergonomics intervention on subjects' individual workplace adjustments. An intervention study was conducted with 24 office workers lasting 6 weeks with three consecutive phases (before, 1 and 5 weeks after the intervention). Employees used a purpose-made website for adjusting their computer workplaces without any personal support of ergonomics experts. Workplace measurements were taken directly on site and by analysing photos taken of the employee. Self-reported complaints were assessed by filling in a questionnaire. It was found that 96% of the employees changed their workplaces on their own and retained them mostly unchanged after the intervention. Furthermore, self-reported musculoskeletal complaints and headache symptoms decreased significantly after the intervention. These findings suggest an improvement of workplace conditions so that cost-effective ergonomic Web-based interventions appear promising in further research and application.

Accommodation to uncomfortable patterns.

Grating patterns can cause discomfort and perceptual distortions. Individuals who experience discomfort and are susceptible to these distortions generally show weaker accommodation than those who are less susceptible. We measured the accommodative response to grating patterns known to differ in the discomfort they evoke because of differences in their colour, motion, or spatial frequency. The parameters known to affect discomfort and distortion had no influence on the mean or variance in the accommodative response, even when accommodative demand was manipulated systematically and the accommodative response varied as expected.

Individual differences in binocular coordination are uncovered by directly comparing monocular and binocular reading conditions.

PURPOSE: We evaluated systematically binocular coordination during a reading task by comparing binocular and monocular reading, and considering the potential effects of individual heterophoria and eye dominance. METHODS: A total of 13 participants (aged 19-29 years, refractive errors -0.5 to 0.125 diopters [D]) read single sentences in a haploscope while eye movements were measured with an EyeLinkII eyetracker. RESULTS: When reading monocularly, saccade amplitudes increased by 0.04 degrees and first fixation durations became longer by approximately 10 ms. Furthermore, saccade disconjugacies increased, and compensatory vergence drifts during fixation turned into a divergent drift relative to the viewing distance. The vergence angle adjusted for the actual viewing distance became less convergent during monocular reading by 0.5 degrees. Moreover, in participants who were almost orthophoric, only the first fixation duration became longer (by 20 ms) when the reading conditions changed from binocular to monocular. For exophoric participants, all parameters of binocular coordination changed, and first fixation duration decreased by 20 ms. When reading monocularly, no differences between the dominant right eye and the nondominant left eye were found. CONCLUSIONS: Because of obvious differences in binocular coordination between monocular and binocular reading, some vergence adjustments are driven actively by fusional processes. Furthermore, higher demands on these binocular fusional processes can be uncovered only by a detailed evaluation of monocular reading conditions.

Binocular coordination during reading of blurred and nonblurred text.

PURPOSE: Reading a text requires vergence angle adjustments, so that the images in the two eyes fall on corresponding retinal areas. Vergence adjustments bring the two retinal images into Panum's fusional area and therefore, small remaining errors or regulations do not lead to double vision. The present study evaluated dynamic and static aspects of the binocular coordination when upcoming text was blurred. METHODS: Binocular eye movements and accommodation responses were simultaneously measured for 20 participants while reading single, nonblurred sentences and while the text was blurred as if it were seen by a person in whom the combination of refraction and accommodation deviated from the stimulus plane by 0.5 D. RESULTS: Text comprehension did not change, even though fixation times increased for reading blurred sentences. The disconjugacy during saccades was also not affected by blurred text presentations, but the vergence adjustment during fixations was reduced. Further, for blurred text, the overall vergence angle shifted in the exo direction, and this shift correlated with the individual heterophoria. Accommodation measures showed that the lag of accommodation was slightly larger for reading blurred sentences and that the shift in vergence angle was larger when the individual lag of accommodation was also larger. CONCLUSIONS: The results suggest that reading comprehension is robust against changes in binocular coordination that result from moderate text degradation; nevertheless, these changes are likely to be linked to the development of fatigue and visual strain in near reading conditions.

Subjective fixation disparity affected by dynamic asymmetry, resting vergence, and nonius bias.

PURPOSE: This study was undertaken to investigate how subjectively measured fixation disparity can be explained by (1) the convergent-divergent asymmetry of vergence dynamics (called dynamic asymmetry) for a disparity vergence step stimulus of 1° (60 arc min), (2) the dark vergence, and (3) the nonius bias. METHODS: Fixation disparity, dark vergence, and nonius bias were measured subjectively using nonius lines. Dynamic vergence step responses (both convergent and divergent) were measured objectively. RESULTS: In 20 subjects (mean age, 24.5 ± 4.3 years, visual acuity, ≥1.0; all emmetropic except for one with myopia, wearing contact lenses), multiple regression analyses showed that 39% of the variance in subjective fixation disparity was due to the characteristic factors of physiological vergence: dynamic asymmetry (calculated from convergent and divergent velocities), and dark vergence. An additional 23% of variance was due to the subjective nonius bias (i.e., the physical nonius offset required for perceived alignment of binocularly [nondichoptically] presented nonius lines). Together, these factors explained 62% of the interindividual differences in subjectively measured fixation disparity, demonstrating the influence of oculomotor and perceptual factors. CONCLUSIONS: Clinically relevant subjective fixation disparity originates from distinct physiological sources. Dynamic asymmetry in vergence dynamics, resting vergence, and nonius bias were found to affect fixation disparity directly, not only via changes in vergence dynamics.

Periodic letter strokes within a word affect fixation disparity during reading.

We investigated the way in which binocular coordination in reading is affected by the spatial structure of text. Vergence eye movements were measured (EyeLink II) in 32 observers while they read 120 single German sentences (Potsdam Sentence Corpus) silently for comprehension. The similarity in shape between the neighboring strokes of component letters, as measured by the first peak in the horizontal auto-correlation of the images of the words, was found to be associated with (i) a smaller minimum fixation disparity (i.e. vergence error) during fixation; (ii) a longer time to reach this minimum disparity and (iii) a longer overall fixation duration. The results were obtained only for binocular reading: no effects of auto-correlation could be observed for monocular reading. The findings help to explain the longer reading times reported for words and fonts with high auto-correlation and may also begin to provide a causal link between poor binocular control and reading difficulties.

Objective vs subjective measures of fixation disparity for short and long fixation periods.

PURPOSE: Fixation disparity, i.e. the vergence error for stationary fusion stimuli, can be measured objectively with eye trackers and subjectively with nonius lines. Between these two measures, previous studies found differences that tended to be proportional to the amount of forced vergence, i.e. the discrepancy between vergence and accommodative stimulus. We investigate whether objective and subjective fixation disparity might be similar without forced vergence. METHOD: We simultaneously measured fixation disparity with the EyeLink II system and with flashed dichoptic nonius lines in 17 subjects who observed targets at a vergence stimulus of 6 deg in an haploscope with 60 cm viewing distance. RESULTS: We found individual differences in objective fixation disparity in a range of about 20 (eso) to -60 (exo) min arc which was considerably larger than the range of subjective fixation disparity. Results were similar for long fixation periods (about 15 s) and short fixation periods (about 1.5 s) between 5 deg saccadic gaze shifts. Further, objective fixation disparity was correlated with objective heterophoria, i.e. the vergence state without a fusion stimulus (r = 0.8, p < 0.001). CONCLUSION: Subjective fixation disparity explained only about 25% of the inter-individual variability in objective fixation disparity. The discrepancy between these two measures might be explained by sensory shifts in retinal correspondence, also in the present condition without forced vergence.

Binocularity during reading fixations: Properties of the minimum fixation disparity.

The present study was based on the physiologically reasonable assumption that the binocular system aims for a reduction of fixation disparity during fixation and that the minimum amount of fixation disparity reflects the optimal binocular status. We measured eye movements (EyeLink II) of 18 participants, while they read 60 sentences from the Potsdam-Sentence-Corpus (PSC) at a viewing distance of 60cm. The minimum fixation disparity was frequently reached directly after the post-saccadic drift, sometimes at the end of fixation and sometimes somewhere in between. Minimum fixation disparity was strongly influenced only by fixation position (within the sentence) while the amplitude of incoming saccade had a negligible effect. Moreover, the effect of fixation position on minimum fixation disparity was correlated with the individual ability to compensate for binocular disconjugacy (due to saccades) while fixating during reading. Generally, we found fixation disparity to be correlated between conditions of reading and fixating single targets, while the reading fixation disparity tended to be more crossed (eso).