Fixation Disparity Curve Research Articles

Agreement of Fixation Disparity Curve between Two Different Instruments.

Fixation disparity tests with various characteristics of fusional stimulus are very important for assessment of decompensated heterophoria. The results suggest that there was no reasonable agreement between the fixation disparity curve's parameters of the modified near Mallett unit and the Sheedy disparometer. The purpose of this study was to determine the agreement of the fixation disparity curve parameters between the modified near Mallett unit and the Sheedy disparometer in patients with decompensated near heterophoria. A total of 147 young adults (mean age, 22.7 ± 4.8 years) participated in this cross-sectional study. After applying the exclusion criteria, the statistical analysis was done on the data of 134 individuals. All participants underwent preliminary optometric examinations including the measurement of uncorrected and best-corrected visual acuity, objective and subjective refraction, and unilateral and alternating cover tests. The fixation disparity was evaluated using the modified near Mallett unit and the Sheedy disparometer at 40 cm, and forced-vergence fixation disparity curves were generated. There were statistically significant differences in the fixation disparity curve parameters (except the center of symmetry) between the two devices. The median fixation disparity measured by the Sheedy disparometer was more positive compared with the modified near Mallett unit (toward more esodisparity or less exodisparity). The median associated phoria measured by the Sheedy disparometer was more positive compared with the Mallett unit. Also, the slope of the curve obtained by the Mallett unit was steeper. The wide limits of agreement indicated the poor agreement of all fixation disparity curve parameters between the two instruments.

Evaluation of the presence of a central fusion lock effect on fixation disparity curve parameters in symptomatic and asymptomatic subjects

ABSTRACT Clinical relevance: Several parameters are likely to affect the magnitude of fixation disparity (FD) and FD curve characteristics. Presence of a central fusion lock may have an important effect on clinical testing of FD and interpretation of its results. Background: The aim of this study was to evaluate FD curve parameters using the modified near Mallett unit (with a central fusion lock) and the Sheedy disparometer (without a central fusion lock) in symptomatic and asymptomatic subjects. Methods: This cross-sectional study was conducted in 147 patients with a mean age of 22.5 years who presented to the optometry clinic of Paramedical College of Mashhad University of Medical Sciences. The symptoms were recorded in a questionnaire for each patient. FD was measured using the modified near Mallett unit and Sheedy disparometer and FD curves were generated using the AutoCAD 2005 software. Results: There was a significant difference in the FD, associated phoria, and slope measurements between the two devices (all p values < 0.05). Significant difference was found in the mean FD between symptomatic and asymptomatic subjects using the modified near Mallett unit (p < 0.0001) and Sheey disparometer (p = 0.007). In symptomatic subjects, the mean slope was steeper for the modified near Mallett unit compared to the Sheedy disparometer (p = 0.001). Although the mean centre of symmetry was more negative in the modified near Mallett unit versus the Sheedy disparometer, the difference between the instruments was not significant in symptomatic (p = 0.477) and asymptomatic (p = 0.257) participants. Conclusion: There are differences in the FD curve parameters between the modified near Mallett unit and Sheedy disparometer. Slope is a proper criterion for differentiating asymptomatic subjects from symptomatic individuals. The modified near Mallett unit is a more precise tool for assessment of non-compensated heterophoria compared to the Sheedy disparometer.

Stereopsis and ocular alignment in Parkinson's disease patients with and without freezing of gait symptoms

BackgroundParkinson's disease patients are classically described by having motor disorder symptoms. Freezing of gait is one of these motor symptoms that presents in some of these patients. Even though freezing of gait is classically considered as motor dysfunction, it is now widely accepted that deficits in other sensory systems, for example visual system, may lead or contribute to freezing of gait. The purpose of this study is to characterise some of the binocular vision functions in freezing of gait patients, non‐freezing of gait Parkinson's disease patients and age‐matched healthy controls.MethodsBinocular vision measurements included local and global stereopsis using different clinical stereo tests, along with fixation disparity and fixation disparity curves. The fixation disparity measures were primarily an assessment of the vergence motor system, and the clinical stereo tests assess the combined effects of motor and sensory aspects of binocular vision. Twenty‐two freezing of gait patients, 25 non‐freezing of gait patients, and 25 aged‐matched healthy controls completed all of the measurements in this study.ResultsThe freezing of gait group had worse stereopsis than the non‐freezing of gait group, and the non‐freezing of gait group had worse stereopsis than the healthy controls. The impairment of global stereopsis was more common than local stereopsis in Parkinson's disease patient groups. The reduction in stereopsis among Parkinson's disease patients was not associated with fixation disparity.ConclusionsResults from this study clearly indicate that the freezing of gait patients group has a greater loss in stereopsis than the other two groups, especially for the global tests. Whether these impairments are contributing to the freezing of gait or just associated with freezing of gait is uncertain.

The Convergence Insufficiency Neuro-mechanism in Adult Population Study (CINAPS) Randomized Clinical Trial: Design, Methods, and Clinical Data

ABSTRACTPurpose: To describe the design and methodology of the Convergence Insufficiency Neuro-mechanism in Adult Population Study (CINAPS), the first randomized clinical trial (RCT) studying young adults with symptomatic convergence insufficiency (CI) using a combination of traditional clinical tests, objective eye movement recordings, and functional brain activities as outcome measures.Methods: In this double-masked RCT, binocularly normal controls (BNC) (N = 50) and CI patients (N = 50) are randomized into office-based vergence/accommodative therapy (OBVAT) or office-based placebo therapy (OBPT). Outcome measures included clinical signs and symptoms, phoria adaptation, forced fixation disparity curves, binocular rivalry, vergence and saccadic objective eye movements, and task-induced functional brain activities. This study is registered on ClinicalTrials.gov NCT03593031.Results: No significant baseline differences are observed between the BNC (p > .4) or CI (p > .3) participants assigned to OBVAT or OBPT for age, near point of convergence (NPC), positive fusional vergence (PFV), phoria at distance and near, amplitude of accommodation, or the Convergence Insufficiency Symptom Survey (CISS). Significant differences are observed between the CI and BNC cohorts at baseline measurements for NPC, PFV, difference in phoria from far to near, amplitude of accommodation, and CISS (p < .001). For the CI patients, 26% had a comorbidity of accommodation insufficiency, and 16% self-reported ADHD.Conclusion: Features of the study design include the following: standardized diagnostic and office-based therapeutic intervention, placebo treatment arm, masked clinical outcome examinations, objective eye movement recordings, functional imaging, phoria adaptation, fixation disparity curves and binocular rivalry measurements.

An Optimal Measurement of Fixation Disparity Using Ogle’s Apparatus

ABSTRACTPurpose: Fixation disparity (FD) is a small misalignment of the eyes within the normal alignment when viewing under binocular condition. Ogle’s apparatus measures FD. Standards of procedures vary, which may lead to different outcomes.Methods: Students with normal ocular alignment, stereopsis ≤60 seconds of arc and visual acuity <0.1 logMAR, were included in this prospective comparative study. Four procedures (P1-P4) of measuring FD with Ogle’s apparatus were performed with divergent placement of the line (P1 and P3), or the line moving from subjective zero (P1 and P2: prisms of ascending strength; P3 and P4: prisms alternating base in base out; combined and P4). Differences in the FD curve were determined by looking at point zero, motor fusion amplitude, and the degree of FD.Results: Twenty-six participants were examined by these 4 procedures. Point zero showed a significant difference between P1-P2 (P=0.006) and P3-P4 (P=0.001). P1 and P3 indicated the highest point zero: median of -1 and -1.5 minutes of arc exodisparity. Motor fusion amplitude showed a significant difference between P1-P2 (P=0.037), P1-P3 (P=0.004), and P2-P4 (P=0.002). P1 revealed the highest motor fusion amplitude (median of 34Δ) and P4 the lowest amplitude (median of 28Δ). No significant differences were found in esodisparity. In exodisparity there was a significant difference comparing P1-P2 (P=0.000), P3-P4 (P=0.000), and P1-P3 (P=0.021). P1 gave the highest exodisparity (median 22 minutes of arc) and P4 the lowest (median 10 minutes of arc).Conclusion: Clinically relevant differences were found in exodisparity, mainly caused by difference in line shifting. Exodisparity was significantly lower, moving the line from subjective zero. The most accurate procedure is using prisms of ascending strength combined with divergent placement of the line (P1). These findings standardize a reliable procedure of measuring the FD curve for clinical use. Patients will not be misdiagnosed with reduced FD.

강제vergence 주시시차곡선의 시기능훈련 후의 변화

Purpose: This research was designed to figure out if there are changes based on types of vergence fixation disparity curve after optical correction and vision training to for people with steep slope of forced vergence fixation disparity curve and symptoms. Methods: For 30 subjects out of 91 subjects, who have steep slopes of fixation disparity curve examined with Wesson Fixation Disparity Card and symptoms about steep slope of fixation disparity curve, proper vision training was offered for 5 weeks. Results: After Vision training for 30 people with the symptoms of type I, II, III, IV for 5 weeks, slope of fixation disparity curve was significantly changed (p=0.013), and the results of binocular vision test and subjective symptoms were also significantly improved. Conclusions: Vision training can not only affect the change of fixation disparity curve, but also relieve the symptoms. Fixation disparity curve has lots of visual function information of the subject and can be suggested as a guideline for prescribing.

Evaluation of Fixation Disparity Curve Parameters With the Modified Near Mallett Unit in Symptomatic and Asymptomatic University Students

BackgroundResearch suggests that fixation disparity data are extremely useful in the assessment of the binocular and accommodative systems.ObjectivesThe purpose of this study was to evaluate fixation disparity curve (FDC) parameters with a modified near Mallett unit in symptomatic and asymptomatic students of Paramedical Sciences School of Mashhad University of Medical Sciences in 2007.Patients and MethodsIn this analytical-descriptive study, 100 students were selected randomly and divided into symptomatic and asymptomatic groups. Fixation disparity curve parameters were determined for each subject and compared in symptomatic and asymptomatic groups.ResultsThere were more subjects with exo fixation disparity than eso fixation disparity in the study sample. The means for fixation disparity, fixation disparity curve x-intercept, and slope with the modified Mallett unit were each significantly different by Mann-Whitney U test in the symptomatic and asymptomatic groups. Also there was a significant difference in the distributions of fixation disparity curve types in the two symptom groups by Chi-square test.ConclusionsThe X-intercepts (point at which the FDC crosses the X-axis) were on average more in the base-in direction, Y-intercepts (point at which the FDC crosses the Y-axis) were shifted in the exo direction, and slopes were steeper in the symptomatic group.

근거리에서 수직주시시차와 입체시와의 관계

목적: 수직주시시차의 유형별 분포를 분석하고 입체시와의 상관관계를 알아보고자 한다. 방법: 입체시 검사표는 부분적으로 random dot을 이용하는 RANDOT(R) stereotest를 사용하였다. 근거리에서 수직주시시차는 환자와의 거리를 25 cm 유지한 상태에서 Wesson fixation disparity card를 이용해, 한쪽 눈에 하나씩 보이는 두 개의 가는 선이 프리즘을 가입하여 한 줄로 일치될 때의 측정결과를 바탕으로 주시시차곡선을 6가지 유형으로 분류하였다. 결과: 43명을 대상으로 근거리 수직주시시차 곡선의 유형은 제 1유형이 55.82%로 가장 많이 분포하였고, 제 2유형이 23.25%로 가장 적게 분포하였으며, 제 3유형이 4.65%, 제 4유형은 4.65%, 제 5유형은 6.98%, 제 6유형은 4.65%로 분포하였다. 두 변수들 사이의 선형적 상관관계의 정도를 보는 상관분석의 결과를 보면 입체시는 수직주시시차를 나타내는 Y-intercept(r = -0.07), 일부융합제거시 사위량(r = -0.03), 완전융합제거시 사위량(r = -0.00)과 상관관계가 없는 것으로 나타났고, 통계적으로 유의하지 않았다(p>0.05). 수직주시시차 곡선의 기울기와는 r = 0.36으로 양의 상관관계를 가지며 그 관계는 낮은 편으로 통계적으로 유의하였다(p<0.05). 수직주시시차를 나타내는 Y-intercept는 일부융합 제거시 사위량(r = 0.89)과는 높은 상관관계를 보였으며 통계적으로 유의하였다(p<0.05). 수직주시시차 곡선의 기울기는 r = -0.33으로 음의 상관관계를 보였고, 그 관계는 낮았으며 통계적으로 유의하게 나타났다(p<0.05). 회귀분석(regresson analysis)결과, 입체시와 수직주시시차의 변화에 따른 관계는 수직주시시차가 1분( ' ) 변하면 입체시는 7.631초( ' ) 만큼 변화하며, 입체시가 1초( ' ) 변하면 수직주시시차는 0.017분( ' ) 만큼 변화하였고 통계적으로 유의하였다(p<0.05). 결론: 입체시와 수직주시시차는 낮은 상관관계를 보였으며, 수직주시시차 만으로 입체시를 판단하기 힘들며 입체시에 영향을 미치는 인자는 수직주시시차 이외의 다른 요인이 더 클 것으로 사료된다. Purpose: To investigate a correlation between vertical fixation disparity and stereopsis by analyzing distribution of types of vertical fixation disparity. Methods: In this study, RANDOT(R) stereotest partly using random dot was used in stereopsis tests. The vertical fixation disparity in close proximity categorized fixation disparity slope into six types on the basis of the result of measurement in when two thin lines, one for one eye looks, become overlapped as a line by adding into prism with Wesson fixation disparity card while maintaining a distance of 25 cm from patients. Results: In the types of near vertical fixation disparity curve, targeting 43 people, the first type 55.82% was the most distributed, the second type is the least 23.25%, the third type of 4.65%, the fourth type of 4.65%, the fifth type of 6.98%, the sixths types of 4.65% were distributed. The result of a correlation analysis, which shows the degree of linear correlation between two variables, represented that stereopsis is not correlated with Y-intercept (r = -0.07) which show vertical fixation disparity, associated phoria (r = -0.03) and dissociated phoria (r = -0.00), but it was not statistically significant (p>0.05). Fixation disparity slope of vertical fixation disparity (r = 0.36) was shown to have a positive correlation with stereopsis. It has a low positive correlation and a meaningful statistics (p<0.05). The Y-intercept which indicates vertical fixation disparity was also not associated with stereopsis (r = -0.07) and dissociated phoria (r = -0.03), and this was not statistically significant (p>0.05), while it had a high correlation as well as a statistically significant with associated phoria (r = 0.89). There was a negative correlation between Y-intercept and fixation disparity slope of vertical fixation disparity (r = -0.33). It showed a low relationship but statistically valuable (p>0.05). As a result of regression analysis, the stereopsis was changed as 7.631" if vertical fixation disparity changes 1' and the vertical fixation disparity changed as 0.017' if stereopsis changes 1', and the change was statistically significant (p<0.05). Conclusions: The study shows that there is a low correlation between stereopsis and vertical fixation disparity, and it is difficult to determine stereopsis only using vertical fixation disparity. Therefore, it suggests other factors above vertical fixation disparity have greater influence on stereopsis.

Evaluation of Fixation Disparity Curve with the Modified Near Mallett Unit and the Wesson Fixation Disparity Card in Symptomatic and Asymptomatic Subjects

Purpose: The purpose of this study is the evaluation of fixation disparity curve (FDC) parameters with an instrument that includes a central fusion lock (the modified near Mallett unit) and another without one (the Wesson fixation disparity card) to determine which is useful for diagnosis of symptomatic from asymptomatic subjects.Materials and Methods: In this analytical-descriptive study, 100 students were selected randomly and divided into symptomatic and asymptomatic groups. FDC parameters were determined with the Wesson card and the modified near Mallett unit for each subject and compared in symptomatic and asymptomatic groups. Data were analyzed by Wilcoxon, Mann-Whitney U, Spearman correlation coefficient and Chi-square tests.Results: The mean slope, y-intercept, and x-intercept with the Mallett unit and the Wesson card were significantly different in the symptomatic and asymptomatic groups (p < 0.001). Significant correlations were obtained between the Mallett unit and the Wesson card in y-intercept, x-intercept and slope (p < 0.001). There was a significant difference in the distributions of fixation disparity curve types in the two symptom groups with the Mallett unit (p = 0.01) and the Wesson card (p = 0.002) by Chi-square test.Conclusions: Among symptomatic participants in this study, both Type I and Type III FD are more common than previously thought, but depend upon the method used to measure it. The x-intercepts were on average displaced in the base-in direction, y-intercepts were shifted in the exo direction, and the slopes were steeper with the Wesson FD card compared with the modified near Mallett unit. This may be related to the design of two devices. These differences were more significant in the symptomatic group.

Test-retest reliability of the Saladin card

Test-retest reliability is a measure of the confidence that results will be identical when the same patient is measured with an instrument in the same manner on more than one occasion. Using the Saladin Near Point Balance Card--an instrument designed to test various near visual findings, including visual acuity, phorias, AC/A ratios, dynamic retinoscopy, fixation disparity, associated phorias, fixation disparity curves, as well as accommodative and vergence facilities--28 first- and second-year optometry students were evaluated on two occasions by one clinician, with the tests separated by approximately two weeks. Subjects were required to demonstrate 20/20 distance visual acuity and at least 100 sec of arc stereopsis. A total of 38 findings were compared, which included near horizontal and vertical phorias, associated phoria, and fixation disparity curves. Twenty-two findings were performed through the subjects' habitual prescription and the remaining findings were taken through the lens indicated by MEM retinoscopy. The purpose of this study was to investigate whether the Saladin Near Point Balance Card had acceptable test-retest reliability. If reliability can be demonstrated, this instrument could be used in clinical situations to diagnose visual departures from normal. All but two of the 38 tests performed demonstrated acceptable test-retest reliability. Coefficients of repeatability and 95% limits of agreement were calculated. The Saladin Near Point Balance Card demonstrated acceptable test-retest reliability. Since it is light-weight, portable, easily and quickly administered, and reliable, the Saladin card can be used by clinicians who are performing screenings or examinations in non-clinical situations, such as nursing homes or schools.

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.

Vertical fixation disparity curve and the effects of vergence training in a normal young adult population.

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. 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. 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. 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.

The characteristics of near prism induced fixation disparity curve in Hong Kong Chinese

The characteristics of prism induced fixation disparity curve at near were studied in 56 asymptomatic young adult Hong Kong Chinese (average age 21 years). The percentage distribution of types of fixation disparity curves was found to be similar to some previous studies, with a higher prevalence of Type I curve (64.3%), followed by Type II (28.6%) and Type III (7.1%) curves. No Type IV curve was revealed from the sample. Two modified measurement approaches were applied to study the effect of measurement techniques on the parameters of fixation disparity curve obtained. A thorough description of the techniques was given. Although a significant difference in the x- and y-intercepts was obtained between different methods, the curve type and slope were not affected. The average y-intercepts (fixation disparity) were 2.72min of arc and 3.54min of arc from two methods respectively. Forty-one subjects had x-intercept (associated phoria) found from the curve. The average values were 2.42Δ base-out and 3.54Δ base-out from two methods respectively.

The characteristics of near prism induced fixation disparity curve in Hong Kong Chinese.

The characteristics of prism induced fixation disparity curve at near were studied in 56 asymptomatic young adult Hong Kong Chinese (average age 21 years). The percentage distribution of types of fixation disparity curves was found to be similar to some previous studies, with a higher prevalence of Type I curve (64.3%), followed by Type II (28.6%) and Type III (7.1%) curves. No Type IV curve was revealed from the sample. Two modified measurement approaches were applied to study the effect of measurement techniques on the parameters of fixation disparity curve obtained. A thorough description of the techniques was given. Although a significant difference in the x- and y-intercepts was obtained between different methods, the curve type and slope were not affected. The average y-intercepts (fixation disparity) were 2.72 min of arc and 3.54 min of arc from two methods respectively. Forty-one subjects had x-intercept (associated phoria) found from the curve. The average values were 2.42 delta base-out and 3.54 delta base-out from two methods respectively.

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 100 cm, 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 100 cm (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 100 cm. 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).

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 100 cm, 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 100 cm (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 100 cm. 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).

Fixation Disparity as a Function of Viewing Distance and Prism Load

In binocular vision, fixation disparity is present when a fixation point falls within Panum's area, but not on corresponding retinal points. To investigate the effect of vergence load, fixation disparity was measured at viewing distances of 20, 30, 40, 60, and 100 cm (while the test subtended a constant angular size) by the psychophysical method of dichoptically presented nonius lines with a central fusion stimulus. As the viewing distance was shortened from 100 to 20 cm, mean fixation disparity changed monotonically from 1 min arc esophoria (ie the eyes converged in front of the target) to 3 min arc exophoria. The average standard deviation of the psychometric function, which is a measure of the temporal variability of vergence, was smallest at 100 cm (when fixation disparity was esophoric) and increased at shorter distances. 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 would be induced by viewing distances between 20 and 100 cm. The slope of these conventional ‘fixation disparity curves’ as a function of prism load correlated with the slope of fixation disparity as a function of viewing distance ( r=0.39, p=0.02, n=25). However, testing at different distances, as introduced here, has the advantage of preserving the natural interaction between vergence and accommodation. Since the change of fixation disparity with distance differed reliably among subjects (with a test — retest correlation of 0.65 in 34 subjects with good binocular vision) this measure may be useful for identifying subjects who are prone to near-vision complaints.

Influences of monocular image degradation on the monocular components of fixation disparity

SummaryThe monocular components of the forced vergence fixation disparity curve were evaluated for equal retinal images and for two kinds of monocular retinal image degradation. Computer‐generated nonius lines were used to measure the fixation disparity curve components. Monocular defocus was obtained by placing convex lenses before one eye. In the second instance monocular image degradation was achieved by placing light‐scattering filters before one eye. A small percentage of subjects displayed unequal distribution of the monocular components of fixation disparity with no image degradation. For both conditions of monocular image degradation, the distribution of the monocular components was found to be biased in favor of the clear image. However, defocus, which affects the high spatial frequencies relatively more than scatter, was found to have a greater effect on the distribution of the monocular components of fixation disparity than scatter.

Influences of monocular image degradation on the monocular components of fixation disparity

The monocular components of the forced vergence fixation disparity curve were evaluated for equal retinal images and for two kinds of monocular retinal image degradation. Computer-generated nonius lines were used to measure the fixation disparity curve components. Monocular defocus was obtained by placing convex lenses before one eye. In the second instance monocular image degradation was achieved by placing light-scattering filters before one eye. A small percentage of subjects displayed unequal distribution of the monocular components of fixation disparity with no image degradation. For both conditions of monocular image degradation, the distribution of the monocular components was found to be biased in favor of the clear image. However, defocus, which affects the high spatial frequencies relatively more than scatter, was found to have a greater effect on the distribution of the monocular components of fixation disparity than scatter.

Comparison of Fixation Disparity Curve Variables Measured with the Sheedy Disparometer and the Wesson Fixation Disparity Card

Previous studies suggest differences in the fixation disparity curves obtained with the Sheedy Disparometer and the Wesson Fixation Disparity Card, the two most commonly used methods for measuring fixation disparity. In one study the investigators proposed that the differences do not exist for subgroups divided by phoria. The purpose of this paper is to try to clarify this issue by use of two large sets of data. Dissociated phorias were measured by the von Graefe method. Fixation disparity curves were plotted using the Disparometer and the Wesson card. Type I fixation disparity curves were most common with the Wesson card. Type II curves were found more often with the Disparometer than with the Wesson card. The x-intercepts were shifted in the base-in (BI) direction with the Wesson card compared to the Disparometer. The y-intercepts were shifted in the exo direction with the Wesson card compared to the Disparometer. The differences were statistically significant regardless of whether the dissociated phoria was exo or eso. The slope of the fixation disparity curve was steeper with the Wesson card than with the Disparometer. The difference was statistically significant for exophores but not for esophores. The differences between results obtained with the two instruments are not consistent from one subject to another as shown by high standard deviations of the differences. The fixation disparity curves measured with these two instruments are different. Fixation disparity parameters obtained from one of these instruments cannot be used with normative findings from the other.