Fixation Disparity Curve Research Articles

The prevalence of vergence accommodation disorders in a school‐age population

There is a substantial void in the literature of studies that examine the prevalence of non-strabismic binocular disorders in the population. Using contemporary vergence accommodation criteria this study set out to classify the binocular status of school-age patients presenting for initial examination in an optometric practice. Seventy-seven per cent of those presenting had a significant vergence or accommodation disorder on the basis of the stated criteria, and disorders at near made up 90 per cent of the disorders detected. Disorders of accommodation effected 57 per cent of the sample and vergence disorders 58 per cent. Analysis of presenting complaint was not of predictive value in determining whether an anomaly was present, nor was it helpful in predicting which type of anomaly. Hyperopia, astigmatism and anisometropia were however more likely to be associated with a vergence accommodation disorder. Distribution of fixation disparity (FDC) curve types for this clinical population is presented together with some evidence to suggest a distinct sub- group within Type 1 responses.

Quantitative analysis of associated and disassociated phorias: linear and nonlinear static models.

Ogle proposed two measures of oculomotor balance, called associated and disassociated phorias, which he assumed were equivalent. However, experimentally determined values of these phorias do not show a close correspondence. To analyze the rationale behind Ogle's assumption of equality, a linear static model was evaluated. It was found that indeed the linear model predicts an exact correspondence between associated and disassociated phorias. Thus, his assumption depended on the presence of a linear model. To account for the discrepancy between these two measures, a nonlinear static model, containing the dead space operators depth of field and Panum's fusional area, was evaluated. Four equations for fixation disparity were derived corresponding to the four combinations of deadspace operator outputs. It was found that only one of these four equally possible solutions for associated phoria corresponded to the disassociated phoria. This suggests that the variability in the four solutions may account for the scatter in the experimental data. The nonlinear model was analyzed further to determine its sensitivity to parameter changes and to show how such a model could generate the classical shape of the fixation disparity curve.

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 Disparometer Fixation Disparity Curves as Measured With and Without the Phoropter

Traditionally, Disparometer fixation disparity curve (FDC) measurement has been performed using a phoropter, although more recently some clinicians have suggested that the Disparometer be used outside the phoropter. Whereas vertical fixation disparity (FD) measurements have been found to correlate well when obtained with and without a phoropter, there is currently no evidence that horizontal FDC's are similar in the two testing situations. A FDC was plotted for 30 visually asymptomatic young adults both in and out of the phoropter. Statistical analysis revealed no significant differences between results on the basis of curve type, slope, y- or x-intercept; however, noteworthy individual variations did occur. Therefore, we conclude that consistency of testing method is important when serial curves are plotted for a given patient. Although each method has some advantages and disadvantages, neither is clearly superior based on this study.

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.

Forced vergence fixation disparity parameters with and without a central fusion lock

The effect of a central fusion lock on forced vergence fixation disparity curve parameters, that is, magnitudes of fixation disparity and associated phoria, curve type and the central slope of the curve, was studied in 84 subjects because of ambiguity in the previous studies. It was found that the magnitudes of exo fixation disparity and exo associated phoria decreased significantly when they were measured with the central fusion lock. The central lock had no noticeable effect on the type of the curve, although, the central slope of the curve was flatter in the presence of the central lock.

Binocular vision, age and symptoms.

Changes with age in heterophoria, associated heterophoria, fixation disparity and stereopsis for near vision were investigated in a sample of 187 subjects (age range from 10 to 65 years) divided into six age groups. With increasing age, heterophoria, associated heterophoria and fixation disparity increased in the exo-direction. No change in stereopsis was found. A comparison with visual symptoms for near vision indicated a relationship between symptoms and the magnitude of fixation disparity and associated heterophoria for all age groups, but no relationship was found between the presence of symptoms and heterophoria measurement. No relationship between the presence of symptoms and the central slope of the forced vergence fixation disparity curve was found. These findings suggested that neither the measurement of heterophoria nor the forced vergence disparity curve are appropriate tests in the assessment of binocular vision anomalies. However, fixation disparity and associated heterophoria are related to binocular problems for near vision in all age groups.

Lens-induced fixation disparity curves.

Change in fixation disparity was measured as lens sphere power was changed at distance and near on a young adult population. The results were graphed to yield a lens-induced fixation disparity curve. The lens curves found were grouped into four basic types (A through D). Two types (A and B) had increasing eso fixation disparity with increasing minus lens power. Type C had little change in fixation disparity with any change in lens power, whereas type D had an initial increase and then no further change with increasing minus power. In addition, approximately 74% of the 38-subject sample had different curve types at distance and near. Possible clinical uses of the lens-induced fixation disparity curve include the prescribing of near additions for pre-presbyopes based on measures of the binocular vergence response to various lens sphere combinations.

Comparison of Vertical Fixation Disparities between Essential Hypertensives and Matched Normotensives

Using American Optical Vectographs and a Disparometer, vertical fixation disparities and associated phorias were measured on 19 essential hypertensives and 19 matched normotensives. Subjects were matched on the basis of race, sex, age, use of tobacco, and family history of hypertension. Correlated variance comparisons of the measured associated phorias and fixation disparities, as well as calculated x and y intercepts derived from forced-duction vertical fixation disparity curves were made and a t-test for related measures was used to compare slopes. The x and y intercepts and slope data showed no significant differences between the groups. However, statistically larger measured associated phorias for the hypertensive group were found on all but one test. This suggests a relation between the magnitude of vertical associated phorias and the occurrence of essential hypertension, but the relation found was not of sufficient strength to warrant clinical application.

Horizontal and vertical vergence training and its effect on vergences and fixation disparity curves: I. Horizontal data.

The purpose of this study was to assess the effects of horizontal and vertical vergence training on fusional vergences and the fixation disparity (FD) curve. Thirty-four subjects were divided into three groups. One-third served as controls and the other two-thirds underwent 5 h of supervised horizontal and vertical vergence training, respectively. Before and after the 4 week training period, vergences and FD curves were measured by a single individual who was intentionally uninformed of each subject's group. We hypothesized that the subjects in the horizontal training group would have increased vergence amplitudes and flatter FD slopes. Our analysis revealed that the positive vergences increased significantly for those in the horizontal group. No evidence was found to suggest changes in any variables related to the FD curve. Because type II errors were likely to be unacceptably large, further work is necessary to determine the relation between fusional vergence training and variables derived from the FD curve.

Horizontal and vertical vergence training and its effect on vergences, fixation disparity curves, and prism adaptation: II. Vertical data.

The purpose of this study was to assess the effects of horizontal and vertical vergence training on vertical fusional amplitudes, the vertical fixation disparity (VFD) curve, and prism adaptation. Thirty-four subjects were divided into three groups. One-third served as controls and the other two-thirds underwent 5 h of supervised horizontal and vertical vergence training, respectively. We hypothesized that subjects in the vertical group would manifest increased vertical vergence amplitude and coefficients of adaptation in concert with flatter VFD slopes. Before and after the 4-week training period, vertical vergences, fixation disparity (FD) curves, and coefficients of vertical prism adaptation were measured by a single individual who was intentionally uninformed of each subject's group. Analysis of the data suggests that changes in the vertical fusional amplitudes increased slightly. Although changes in the VFD slope and coefficient of prism adaptation were not statistically significant, the changes were much greater in the vertical group and in the hypothesized direction. We suggest that the results offer preliminary support for our hypothesis.

Comparison of associated heterophoria measurements using the Mallett test for near vision and the Sheedy Disparometer

The Mallett test for near vision and the Sheedy Disparometer are compared in terms of their ability to measure associated heterophoria. Twenty-two asymptomatic subjects were used. With the Disparometer, ten of these subjects, familiar with making prism vergence measurements, exhibited stable forced vergence fixation disparity curves, and the associated heterophoria values using both instruments were not significantly different. However, the twelve subjects not familiar with making such measurements generated fluctuating, unstable fixation disparity curves, and the associated heterophoria values were significantly different. Repeated trials with the randomly selected naïve subjects led to flatter, more stable fixation disparity curves and lower values for the x- and y-intercepts when using the Disparometer. The Mallett test, however, produced identical values for associated heterophoria in each successive trial. Therefore, the validity of measurements using the Disparometer in a clinical setting is questioned.

The effect of volition on the horizontal forced-vergence fixation disparity curve.

An anomalous horizontal forced-vergence fixation disparity curve in the shape of the letters V or U is presented. This phenomenon is associated with an increase in eso fixation disparity with base-out prism. This response is shown to be related to instructional set. Intrusion of voluntary vergence is suggested as the most likely basis for this effect.

Vertical Fixation Disparity Correction

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.

FIXATION DISPARITY IN BINOCULAR STRESS

Fixation disparity has been taken as a sign of stress on binocular vision because it is established that prism stress creates fixation disparity. This paper looks at the effect on fixation disparity of the stress caused by requiring subjects to read in inadequate illumination. It is found that the reduction in illumination does not in itself immediately change the magnitude of the fixation disparity. There is, however, an increase in the mean slope of the central part of the fixation disparity curve which suggests that when the effect of reduced illumination is added to prism stress, fixation disparity is increased. The stress created by asking subjects to read in reduced illumination for half an hour resulted in the mean associated heterophoria being increased, and over half the subjects reported symptoms of stress. It is concluded that fixation disparity is changed by this type of visual stress in some subjects, and in near vision is increased to a more marked degree of exo-disparity. Most of this increase occurs in the first ten minutes.

Nonlinear forced vertical vergence fixation disparity curves and their clinical significance.

Two examples of nonlinear, forced vertical vergence, fixation disparity curves are presented. These curves have a flattened portion similar to the sigmoid forced horizontal vergence fixation disparity curve described as "type I". The existence of sigmoid, forced vertical vergence, fixation disparity curves suggests that the horizontal and vertical vergence mechanisms may operate in similar fashion. Possible clinical implications of this finding relative to prismatic prescriptions for vertical vergence anomalies are also discussed.

Computerized Fixation Disparity Measurement

Fixation disparity (FD) curves have many clinical implications. A new method for evaluating binocular function using FD curves is described. This method is shown to compare very well with traditional methods of measurement. During clinical applications, it has proven to be very useful in diagnosis of problems and monitoring of training. The operation of the program as well as clinical data are presented.

Clinical evaluation of vertical fixation disparity. Part IV. Slope and adaptation to vertical prism of vertical heterophoria patients.

The purpose of this study was to determine the characteristics of the clinically measured vertical fixation disparity (VFD) curve and the ability to adapt to vertical prism for patients who have a vertical heterophoria and are comfortably wearing vertical prism, and to relate these data to similar findings for nonvertical heterophoria patients. The correlation between the amount of vertical prism that was being worn comfortably and the amount of vertical prism that reduced the VFD to zero was also evaluated. The results indicate that patients with vertical heterophoria who are wearing vertical prismatic corrections comfortably have VFD curves with shapes and slopes similar to nonvertical heterophoria patients; have lower coefficients of adaptability to vertical prism than nonvertical heterophoria patients; and the amount of the vertical prismatic correction to produce visual comfort can be determined by the vertical prism that reduces the VFD to zero.

Combined effects of spatial frequency and retinal eccentricity upon fixation disparity.

The variations in fixation disparity (FD) with spatial frequency and retinal locus of fusion locks were examined in two groups of subjects having either steep or flat forced-duction FD curves. FD for subjects with flat curves was unaffected by either spatial frequency or diameter of fusion locks. FD of observers with steep forced-duction curves increased with both the coarseness and eccentricity of fusion locks. Panum's horizontal fusion range increased markedly with target spatial period but only modestly with retinal eccentricity. These results indicate that factors other than the dimensions of Panum's fusional area (PFA) result in the increase of FD found in some observers with peripheral fusion locks.

Disparity vergence dynamics and fixation disparity.

The initial velocity of the vergence response to step changes in disparity was examined in two subjects as a function of the retinal locus and spatial frequency of fusion stimuli. Vergence velocity decreased for both subjects as retinal eccentricity increased and it was unaffected by spatial frequency. The same subjects had very different amounts of fixation disparity (FD). The flat forced duction FD curve for one subject who had rapid vergence adaptation remained unaffected by these two stimulus variables. In contrast, the steep forced duction FD curve for the other subject became steeper as retinal eccentricity and target coarseness increased. These results suggest that prism adaptation overshadows the effects of disparity vergence dynamics upon FD, but in the absence of adaptation, two dynamic properties of vergence (velocity and decay during occlusion) determine the slope of the peripheral limits of the forced duction FD curves. The results also suggest that the vergence system is more sensitive to small disparities subtended by high than low spatial frequency detail. We recommend that the slope of the extreme limits of the forced duction curve be used clinically to discriminate between normal and abnormal disparity vergence dynamics.