Vertical fixation disparity correction: effect on the horizontal forced-vergence fixation disparity curve

Following a suggestion made by Percival in regard to dissociated phorias, we corrected vertical associated phoria in several patients who had disparities in both vertical and horizontal fixation. The principal objective result of this correction was a flattening of the slope of the type I horizontal forced-vergence curve. This result may be significant particularly because the slope has been identified as being a good prognosticator of patients likely to be symptomatic. Attention to a concurrent vertical component may offer a convenient way to normalize a steep slope on horizontal fixation disparity curves.     

Effect of nearpoint stress on the horizontal forced vergence fixation disparity curve

The effect of nearpoint stress on the forced vergence fixation disparity curve was investigated directly. Data gathered from asymptomatic subjects with normal binocular vision showed that fixation disparity was not influenced by 25 min of continuous reading. Further investigation using symptomatic subjects with large fixation disparities or steep forced vergence curves showed that the nearpoint reading task had a significant effect on the curves of these subjects. Clinical implications and a possible physiological substrate are presented.     

Monocular components of the fixation disparity curve

Experiments were conducted to characterize the distribution of the monocular components of fixation disparity, i.e., ascertain whether or not the fixation disparity measured by nonius alignments is equally divided between the two eyes or has an unequal distribution. Computer generated nonius lines were used to measure the monocular components of the forced vergence fixation disparity curve (FDC) for subjects with normal binocularity. Thirty-three percent of all subjects tested showed unequal distribution of their monocular components. The composite FDC's for subjects with unequal distribution of the monocular components tended to have steeper slopes than those with equally distributed components. It was concluded that equally divided monocular components are not a prerequisite for good binocularity.     

Comparison of fixation disparity curve parameters obtained with the Wesson and Saladin fixation disparity cards.

PURPOSE: This study compared fixation curve parameters with two commercially available fixation disparity cards, one that has been available for several years, the Wesson card, and a new one, the Saladin card. METHODS: Fixation disparity curves were measured on 50 subjects with the Wesson fixation disparity card and the Saladin fixation disparity card. RESULTS: The x intercepts were on average more in the base-in direction with the Wesson card than with the Saladin card. The y intercepts were shifted in the exo direction with the Wesson card compared with the Saladin card. The slope with the Wesson card was steeper than the slope obtained with the Saladin card. The distribution of curve types was also different with the two different instruments. CONCLUSION: Fixation disparity curves measured with these two instruments are different, and separate norms should be used for each fixation disparity measurement method.     

Use of horizontal prism to correct vertical fixation disparity

By noting the direction of gaze in which the vertical phoria is largest, one can determine where binocular discomfort is likely to be greatest. Treatment of the discomfort is then based upon prescribing horizontal prism for the horizontal phoria. The recommended amount of horizontal prism is that amount which reduces the vertical fixation disparity to zero. Although the fixation disparity can change in different directions of gaze, in most instances, the prescribed amount of horizontal prism is determined in the primary position of gaze.     

The effect on fixation disparity and associated heterophoria of reading at an abnormally close distance

It is well established that prism stress on binocular vision produces fixation disparity. It is therefore suggested that fixation disparity is a sign of stress on binocular vision. The authors have previously looked at the stress caused by reading in reduced levels of illumination, and found that the average fixation disparity and associated heterophoria become more exophoric. In this paper the effect on fixation disparity and associated heterophoria of reading at an abnormally close distance is reported. It was found that in 50 young adults fixation disparity and associated heterophoria increased to a greater degree of exo-disparity. An analysis of the visual symptoms indicated that there was also an increase in the visual discomfort and other symptoms.     

Differentiation of asymptomatic patients from symptomatic patients by the slope of the forced vergence fixation disparity curve

Eighty-nine optometry students were divided into an asymptomatic group and a symptomatic group on the basis of a case history. A forced vergence fixation disparity (FD) curve was generated for each subject using a Disparometer (Vision Analysis, Columbus, Ohio). Slopes were calculated for each curve. In addition, each curve was labeled steep or flat based on a previously determined cutoff point of -0.96 min/delta (Sheedy, 1980). Steep curves did not correlate well with symptomatic patients, whereas flat curves did not correlate well with asymptomatic patients. An independent t-test found no significant difference between the two groups.     

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.     

Comparison of two fixation disparity determinantions

Fixation disparity values derived from horopter measurements are compared with those determined using the disparometer on seven subjects wearing lateral prisms. Although the two measures are intrinsically different, five subjects showed similarity in the form and position of the two fixation disparity-forced vergence functions. Two subjects manifested fixation disparities that indicated the use of proximal or voluntary control of relative eye position for both base-out and base-in forms of lateral prism. Another subject showed a paradoxical response to base-out prism that could be attributed to recovery from previous application of base-in prism, adaptation to base-out prism, or both. An additional subject showed irregular alterations in vergence posture during nonius horopter measurements. The clinical implications of vergence responses other than those associated with fusion and accommodation are discussed and the effect of this form or eye movement control is related to the change in position and shape of the equidistance horopter when it is measured through lateral prism.     

The influence of rapid prism adaptation upon fixation disparity.

Small errors of fusional vergence (fixation disparity) were examined as a function of the magnitude of horizontal prism stimulating convergence or divergence for a short (30sec) duration. Marked differences were observed between the amplitude of fixation disparity resulting from convergent and divergent stimulus disparities. In another experiment, subjects wore a horizontal prism for 30 sec after which time one eye was occluded for 40 sec. Measurements of vergence eye movements revealed an incomplete relaxation of fusional vergence (prism adaptation) after 40 sec of occlusion. Marked differences were observed between the amplitude of prism adaptation resulting from convergent and divergent stimuli. Maximum prism adaptationand minimum fixation disparity occurred with the same direction of prism, suggesting that a slow fusional vergence mechanism minimizes errors of binocular vergence.     

Effect of induced fixation disparity on binocular visual acuity

Fixation disparities were artificially created for distance vision by prisms, and the monocular and binocular visual acuities were measured. The normal approximate 10% improvement in binocular visual acuity compared to monocular visual acuity, deteriorated in proportion to the amount of fixation disparity created by the prisms. This was true in both eso- and exo-disparity, although not to the same extent.     

Effect of fixation disparity on distance binocular visual acuity

Subjects with appreciable values (2 Δ and greater) of associated' phoria, as measured by a Mallett unit, had their distance monocular and binocular visual acuities measured. In the latter case measurements were taken both with the associated' phoria corrected by prisms and without such a correction. The improvement in binocular acuity compared to monocular acuity was less than would occur in normal subjects with minimal associated' phorias although the improvement differed accordingly as to whether the readings were eso- or exo-in nature. When the associated' phorias were corrected with prisms the improvement in binocular over monocular visual acuity was similar to that found in normal subjects.     

Effect of reading in low illumination on fixation disparity

Fixation disparity has been shown to be produced by prism stress, and it has therefore been proposed that its presence constitutes a sign that binocular vision is under stress (decompensated heterophoria). Fixation disparity can be present in the absence of any other indication of stress, and therefore may have a physiological basis. This study investigates the effect on fixation disparity and visual discomfort of placing the visual system under a different type of stress, that of requiring the subject to read under conditions of inadequate illumination. In 50 young adults, it was found that fixation disparity increased under inadequate illumination and that this was accompanied by symptoms in the form of visual discomfort. After a period of rest from any near-vision task, the fixation disparity decreased and the symptoms were reduced.     

Effect of a central fusion stimulus on fixation disparity

Fixation disparity (FD) measurements were made with a 1.5 degree diameter fusional stimulus both with and without a central fusion stimulus. Measurements were made at 40 cm with no prism, 5 delta base-in (BI) and base-out (BO), and 10 delta BI and BO, on a group of asymptomatic subjects. The central fusion stimulus results in less variable measurements and a flatter slope of the fixation disparity curve (FDC). Implications of the use of the central stimulus for clinical measurements are discussed.     

Gain of human torsional optokinetic nystagmus depends on horizontal disparity

PURPOSE: To investigate the effects of horizontal disparity on torsional optokinetic nystagmus (tOKN) in humans. METHODS: Ten healthy human subjects were selected for this experiment. Monocular eye movements were recorded three-dimensionally using dual-search coil methods. Torsional OKN was induced by a rotating random-dot pattern (22 degrees in diameter, constant angular velocity: +/-54 deg/s) projected on the virtual screen of the optical see-through, head-mounted display (HMD). The optical distance of the HMD's virtual screen was 2 m. A red LED that could be fixated through the virtual screen of the HMD was located near the center of the rotating random-dot pattern. Horizontal disparity was induced by changing the distance between the fixated target and the subject systematically (1, 1.5, 2, or 3 m; five subjects) or by the prism (+1.5, +0.5, 0, or -0.5 prism-diopter [PD] in each eye; five subjects) in front of the HMD. RESULTS: The average gain with zero horizontal disparity (0.022 +/- 0.008/0.025 +/- 0.014, fixated target at 2 m/fixated target with the plain glass) was significantly higher than the gain with crossed disparity (0.017 +/- 0.003/0.019 +/- 0.008, target at 3 m/with the prism of -0.5 PD) and uncrossed disparity (0.017 +/- 0.002, target at 1 m/with the prism of +1.5 PD; one-way ANOVA, P <0.05). CONCLUSIONS: The horizontal disparity of optokinetic stimulus affects tOKN. Nonzero horizontal disparity decreases the gain of tOKN.