Fixation disparity and accommodation as a function of viewing distance and prism load

Fixation disparity was measured with dichoptically presented nonius lines at viewing distances of 20, 30, 40, 60, and 100cm, so that both vergence and accommodation were stimulated adequately as in normal vision. As the viewing distance was shortened, mean fixation disparity changed monotonically from 1 min arc eso (i.e., the eyes converged in front of the target) to 3 min arc exo. The average standard deviation of the psychometric function of fixation disparity, which is a measure of the temporal variability of vergence, was smallest at 100cm (when fixation disparity was eso) and increased as viewing distance decreased. Fixation disparity itself and the change of fixation disparity with distance differed reliably among subjects with normal binocular vision, but neither was related to the momentary degree of accommodation. Fixation disparity was also measured at a constant distance of 40 cm, but with prisms in front of the eyes that induced the same vergence angles as viewing distances between 20 and 100cm. The slope of these conventional fixation disparity curves as a function of prism load was generally larger than the slope of fixation disparity as a function of viewing distance (which can be explained by accommodative vergence), but the slopes of the two types of curves were correlated ( r = 0.39, P = 0.02, n = 25).     

Objective vs subjective measurements of dark vergence

Dark vergence is a resting position of vergence (tonic vergence), measured in a dark visual field to eliminate fusional, accommodative, and proximal stimuli. The vergence resting position is relevant for measures of phoria and fixation disparity. Dark vergence differs reliably among subjects: the average subject converges at a viewing distance of about 1 m, while the inter-individual range is from infinity to about 40 cm. In previous research, dark vergence was measured subjectively, i.e. observers adjusted the horizontal offset of dichoptically presented nonius targets to perceived alignment. Results of such subjective vergence tests do not necessarily agree with those of the objective measurements of eye position with eye trackers. Therefore, we made simultaneous subjective and objective measurements of dark vergence and found similar results with both methods in repeated tests in two sessions. Thus, the nonius test is sufficient for a subjective estimation of dark vergence.     

Fixation disparity,accommodation, dark vergence and dark focus during inclined gaze*

We investigated several oculomotor functions at different angles of vertical inclination of the gaze direction from 15 deg upwards to 45 deg downwards. The mean accommodative resting state (measured in a dark visual field) increased when the eyes or the head were declined from 0 to 45 deg. Fixation disparity (the vergence error in minutes of arc relative to the principle visual directions) became more eso when a fusion target at a viewing distance of 40 cm was lowered: declining the gaze by 45 deg changed mean fixation disparity by 1.8 min arc with eye inclination (keeping the head upright), and by 0.9 min arc with head inclination (with eye position unchanged relative to the head). When the eyes were lowered, the individual rate of eso change in fixation disparity was correlated with the amount of the subjects’ near shifts in the resting position of vergence, measured in darkness. Significant test-retest correlations between repeated measurements showed that the effects of eye inclination on vergence varied in a reproducible way among individuals with good binocular vision.     

Fixation disparity at different viewing distances and the preferred viewing distance in a laboratory near-vision task

Previous research has shown that subjects with normal binocular vision differ reliably in the extent to which their fixation disparity changes in the exo direction when the viewing distance is shortened from 100 to 20 cm. Since fixation disparity can lead to asthenopic complaints, the present study investigates whether an exo fixation disparity induced by proximity may cause subjects to move to a longer viewing distance during a near-vision task in order to reduce exo fixation disparity. In two optometric sessions, fixation disparity and accommodation were tested at 60, 40, and 30 cm viewing distance. In a further session, subjects were required to begin a one-hour near-vision task at about 40 cm viewing distance, at which the text characters subtended a comfortable visual angle of 21 min arc. Later, the subjects were free to adopt any viewing distance. In the initial phase of the task, subjects moved back from the screen to a greater or lesser extent that was correlated with the amount of proximal exo fixation disparity: the more a subject's fixation disparity changed to exo when the viewing distance had been shortened from 60 to 30 cm the more he or she moved to longer viewing distances in the course of the near-vision task. Further, the more distant the resting position of vergence (dark vergence), the more visual complaints the subjects indicated after the task relative to before.     

Influence of proximal,accommodative and disparity stimuli upon the vergence system*

The most important cues to the normal vergence response are retinal disparity, accommodative effort and perceived distance (proximal cue). In the past, proximal cues have been thought to contribute little to the total vergence response but more recent studies have suggested that they play an important role. The relative contributions of the proximal, accommodative and disparity cues to the vergence response were assessed by measuring the effect upon the vergence system when each of these cues were altered. The effect upon the vergence system was measured by the change in ‘phoria, the change of fixation disparity and the amount of prism adaptation after 210 s of binocular viewing. The results indicate that the greatest effect of cue disharmony occurred when the proximal and disparity cues were altered. A finding which signifies the importance of the proximal cue to the overall vergence response.     

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.     

Horizontal fixation disparity measures with nonius lines.

PURPOSE: It has been reported that changes in visual direction and retinal correspondence may limit the validity of nonius lines as a subjective measure of vergence, at least in particular viewing conditions, e.g., dynamic or forced vergence. Nonius lines may be valid at larger spatial separation between fusion contour and nonius lines. Therefore, we measured fixation disparity varying the amount of a vertical gap between nonius lines. METHODS: A static central fusion stimulus was given at a 100-cm viewing distance. The nonius lines were presented with various vertical gaps either continuously (and adjusted to subjective alignment) or flashed in a series of trials (in adaptive testing). Twelve subjects with normal binocular vision were tested repeatedly to identify individual vergence characteristics. RESULTS: When the vertical gap between nonius lines was increased up to 7.9 deg, the amount of subjective fixation disparity tended to increase in part of the subjects, predominantly in those with an exo fixation disparity. Most subjective fixation disparity measures were correlated with each other and with tonic vergence (i.e., dark vergence tested subjectively without fusion stimulus); however, flashed nonius lines at larger nonius gaps were an exception. CONCLUSIONS: We found physiologically plausible measures of idiosyncratic fixation disparity with continuous nonius lines at any amount of nonius gap or with flashed nonius lines at small gaps. In these conditions, the intersubject variability of fixation disparity was much larger than effects of the spatial separation between fusion stimulus and nonius lines.     

Resting positions of vergence and their relation to asthenopic complaints

Dark vergence is a resting position of vergence, measured in a dark visual field to eliminate fusional, accommodative, and proximal stimuli. Dark vergence differs among subjects: the average subject converges to a viewing distance of about 1 m, while the inter-individual range is from infinity to about 40 cm. This paper reviews previous ergonomic studies on dark vergence in relation to asthenopic complaints that occur when working at near computer screens: subjects with distant dark vergence exert more convergence and experience stronger asthenopic complaints than subjects with near dark vergence. The dissociated phoria, a common clinical measure of resting vergence, is only weakly correlated with asthenopic complaints, probably because of accommodative influences that cause the difference between dark vergence and dissociated phoria.     

Accommodative and fusional components of fixation disparity.

Traditional measurements of fixation disparity, like other binocular measurements, confound influences from both blur-driven and disparity-driven components. Measuring fixation disparity with accommodation open-loop eliminates accommodative interactions, and results show fixation disparity is reduced to about half the value observed normally for high levels of forced vergence. Thus fixation disparity curves are shaped by both accommodative and fusional vergence processes.     

Lateral phoria at distance: contributions of accommodation.

Lateral heterophoria at distance often is cited as an index of tonic vergence, yet recent research has shown that vergence in darkness is more convergent than most measures of phoria. Two experiments were conducted to investigate the possible role of accommodation in this discrepancy. Experiment I compared measures of distance phoria of 19 young adults with measures of vergence and accommodation of the same subjects. Dark vergence was found to be correlated with but significantly more convergent than distance phoria. Hierarchical regression analysis showed that individual measures of phoria were related to subjects' negative accommodation (from the dark focus to the distant target) as well as to dark vergence. Experiment II tested 13 young adults to evaluate three simple models of the influence of accommodation on distance phoria. Distance phoria was predicted best by a model that included the subjects' negative accommodation, accommodative vergence, and dark vergence. These findings support the hypothesis that distance phoria is influenced by accommodation for the fixation target. Dark vergence is a simpler index of tonic vergence.     

The accommodative contribution to binocular vergence eye movements

Forced vergence fixation disparity curves were generated for both with and without an accommodative stimulus (accommodative closed and open loop conditions respectively) for fifteen asymptomatic subjects. All subjects exhibited an exophoric shift in fixation disparity for the accommodative open loop condition at the forced convergence side of the curve. No corresponding bias was found at the forced divergence side of the curve. The findings are discussed in terms of the results of previous studies and the existing models of vergence eye movements.     

The first and second order dynamics of accommodative convergence and disparity convergence

Main sequences, the function describing the relationship between eye movement amplitude and velocity, have been used extensively in oculomotor research as an indicator of first-order dynamics yet it is difficult to find main sequence analyses for accommodative vergence or for disparity vergence in isolation when all mitigating factors have been well controlled and there are no studies in which accommodative vergence and disparity vergence main sequences have been generated for the same group of subjects. The present study measured main sequences in: (1) accommodative vergence with disparity vergence open loop, (2) disparity vergence with accommodation open loop, and (3) combinations of accommodative and disparity vergence. A dynamic AC/A ratio was defined and was found to be similar to the traditional static AC/A ratio. Vergence acceleration was measured for all conditions. A pulse-step model of accommodation and convergence was constructed to interpret the dynamics of the crosslinked interactions between the two systems. The model supports cross-coupling of both the pulse and step components and simulates the primary empirical findings that: (1) disparity vergence has a higher main sequence slope than accommodative vergence, (2) both accommodative and disparity vergence acceleration increase with response amplitude whereas accommodation acceleration does not.     

Static vergence and accommodation: population norms and orthoptics effects

The steady-state characteristics of the accommodation and vergence systems can be described by a model with six major oculomotor parameters. These include the system biases (tonic vergence and accommodation) and forward-loop gains (vergence and accommodative gains), as well as the interactive system gains (AC/A and CA/C ratios). We investigated these parameters in two populations: (1) 22 visually-normal asymptomatic individuals, and (2) 21 visually-abnormal symptomatic individuals before and after conventional orthoptic therapy. Two parameters related to system gain differentiated between the symptomatic and asymptomatic individuals: the slope of the fixation disparity curve with accommodation open-looped and the slope of the accommodative response/stimulus curve. Following orthoptic therapy, 4 static model parameters and 1 dynamic clinical parameter showed changes toward the normal mean; this included tonic accommodation, slope of the fixation disparity curve with accommodation closed-loop (2.5D), slope of the accommodative response/stimulus curve, the CA/C ratio, and the ± 2D monocular accommodative flipper rate.     

Fixation disparity: clinical implications and utilization.

Research suggests that fixation disparity data is extremely useful in the assessment of the binocular and accommodative systems. However, clinicians have been slow to integrate this method of analysis into their practices. Fixation disparity measurement, under different fusional and accommodative demands, aids the practitioner in prescribing ophthalmic prism and/or plus lenses. In addition, it helps determine when vision therapy is appropriate and provides an objective assessment of the effectiveness of a vision therapy program. This paper discusses significant aspects of horizontal and vertical fixation disparity measurement, with particular emphasis on its clinical usefulness in understanding and resolving asthenopic symptoms attributable to vergence and/or accommodative dysfunction. The goal of this paper is to provide the practitioner with information so that fixation disparity testing can be incorporated within the clinical regimen.     

The proximity-fixation-disparity curve and the preferred viewing distance at a visual display as an indicator of near vision fatigue.

PURPOSE: This laboratory study investigates the relation between measures of fixation disparity (FD) (and other optometric measures) and near vision fatigue at a computer workstation. METHODS: Young adult subjects with normal binocular vision performed three blocks of a visual task of 30 min each. In Block A, the viewing distance was 100 cm, as a reference without near vision. In Block B, the viewing distance of 50 cm induced a defined near vision load. In Block C, subjects were free to choose a comfortable viewing distance. This preferred viewing distance was used as an indicator of near vision fatigue because subjects adopting longer viewing distances in Block C had more near vision fatigue at 50 cm in Block B. RESULTS: Subjects with preferred viewing distances longer than average (63 cm) had steeper slopes of FD as a function of viewing distance (100-30 cm), as shown by discriminant analyses. CONCLUSIONS: Thus, this steep proximity-FD curve indicates a weak disparity vergence system that may cause near vision fatigue. This may explain why some young adults prefer longer viewing distances at the computer workstation.     

Near work, visual fatigue, and variations of oculomotor tonus

One hour of near work produced adaptive changes in the resting states of accommodation and binocular vergence, which may be responsible for different aspects of visual fatigue. Two groups of college students read from either hard copy or a video display terminal (VDT). Immediately before and after reading, the subjects' distance acuity, dark vergence, dark focus, and accommodative response accuracy for a monocular stimulus were measured. After reading, subjects also rated their subjective feelings of visual fatigue. Reading produced significant changes in both accommodation and vergence, which did not differ for the hard copy and VDT modes of presentation. Dark focus and accommodative responses shifted in the myopic direction by an average of 0.6 D and at least 0.35 D, respectively; dark vergence distance shifted in the convergent direction by an average of 11.4 cm. These changes were greatest for subjects whose initial resting postures corresponded to a far distance. After reading, one third of the subjects exhibited lower visual acuity at distance. This change was significantly correlated with changes in dark focus (r = 0.35) but not with changes in dark vergence (r = -0.12). In contrast, subjective ratings of visual fatigue were not correlated with changes in the dark focus (rho = 0.13), but they were significantly correlated with changes in dark vergence (rho = 0.58). These findings indicate that ordinary near work can produce significant changes in the resting states of accommodation and vergence, whose magnitude depend on the subject's initial oculomotor resting tonus.(ABSTRACT TRUNCATED AT 250 WORDS)     

The use of video refraction to measure the dynamic properties of the near triad in observers of a 3-D display

OBJECTIVE: Recording of the dynamic behaviour of the near triad during viewing of a three-dimensional (3-D) display is attempted. This may then be used to identify the possible origin of asthenopia and other ocular problems which occur whilst viewing such displays. METHODS: Subjects viewed a 3-D display through an image splitter. Video recordings of ocular responses (pupil diameter, vergence and accommodation) were made with a commercially available video refraction unit at a distance of 1.2 m from the subject. Continuous recordings were made whilst a step-wise disparity of up to 2.6 degrees was introduced between the targets presented to the two eyes, but the accommodative stimulus remained unchanged. Image analysis from the recordings was carried out on a personal computer. RESULTS: Results showed various complex patterns of response, with the accommodation and vergence recorded reflecting the effect of stimulus conflict. Difficulty in fusional convergence was recorded when disparity was large, and the recordings reflected the subjects' comments. CONCLUSION: The video refraction method is a useful tool for studying the dynamic behaviour of the near triad.     

Relation between static and dynamic aspects of vergence, estimated with a subjective test using flashed dichoptic nonius lines

The psychophysical technique for measuring vergence with dichoptic nonius lines was used to investigate dynamic responses to step vergence stimuli. Liquid crystal shutter glasses were operated with a cathode ray tube monitor to present convergent or divergent step stimuli of 32 min arc at t = 0. The resulting initial vergence response was estimated with nonius lines that appeared for 80 ms at a fixed delay of 400 ms after the vergence step stimulus. The vergence state reached 400 ms after the step depended on the vergence accuracy assumed before the step, i.e. baseline fixation disparity (FD). The following physiologically plausible results were found: (i) the amount of dynamic vergence changes within 400 ms tended to be negatively correlated between the convergent and divergent direction; (ii) subjects with a smaller convergent dynamic change had a steeper proximity-FD curve measured with static fusion targets as a function of viewing distance; (iii) FD was more eso or more exo depending on whether the vergence change was larger in the convergent or divergent direction respectively.     

Effects of persistent viewing of 3 D TV on human visual function

目的:研究持续观看3D电视对视功能的影响,以及探索不同视功能参数之间的关系.方法:本研究录入19位受试者,每位受试者观看90分钟的2 D电视或90分钟的3 D电视,两部电视观看间隔时间为一周,顺序随机.每次观看电视前及观看后15 min内都做相关视功能检查,检查包括以下四种视功能参数:调节功能(调节反应、调节微波动、调节灵敏度、正相对调节、负相对调节、AC/A),融像性聚散功能(远距负融像性聚散功能、远距正融像性聚散功能、近距负融像性聚散功能、近距正融像性聚散功能),隐斜量(远距隐斜量、近距隐斜量),立体视.结果:无论是与观看前的基础值比较,还是与观看2D电视后的数据比较,观看3D电视后近距离调节反应增加,汇聚减少,远距隐斜量向外隐斜改变(P<0.05).结论:持续观看3D电视会对人们的视功能有一定影响,主要是打破了调节和聚散系统的平衡.厂家和公众应该引起重视,将调节功能、融像性聚散功能、隐斜量等参数作为评估3D电视安全性的指标.     

Binocular eye movements during accommodative vergence

Binocular eye position was monitored by the photoelectric technique during accommodative vergence. Contrary to previous reports indicating that accommodative vergence was a uniocular phenomenon, without exception, binocular accommodative vergence movements were recorded. The total vergence amplitude in the viewing eye was reduced, on the average, by approximately 88% with respect to the vergence movement measured in the covered eye. Some saccadic eye movements that occurred during vergence movements were likewise reduced in amplitude in the viewing eye by up to 20%. Smooth eye movements were utilized to counteract the vergence movement in the viewing eye. This smooth movement alone, or in conjunction with a late saccade, returned the eye to the target and helped to maintain the retinal image of the target coincident with the foveal center for the duration of the accommodative vergence movement. Thus, there appears to be a fixation-holding mechanism which produced a general attenuation of both vergence and some saccadic movements in the viewing eye. Although this control strategy produced violations of Hering's law with respect to the magnitude of the movements in the eyes but not with respect to the direction of the movement, it was implemented in the interest of retaining the target within the sensitive foveal region.