The effect of illumination and foveal fusion lock on clinical fixation disparity measurements with the Sheedy disparometer

Wildsoet, C

doi:10.1016/0275-5408(85)90071-7

Cited by 9 publications

(9 citation statements)

References 14 publications

(30 reference statements)

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“…We realise that this is not clear from our Figure 4 and we are grateful to Professor Kommerell for bringing this to our attention.We believe that the presence of a central fusion lock is an important component of the Mallett unit fixation disparity test. This improves the stability of fixation disparity (Wildsoet and Cameron, 1985;Ukwade, 2000) and reduces the magnitude of fixation disparity (Pickwell and Stockley, 1960;Brownlee and Goss, 1988;Brautaset and Jennings, 2006). A very recent study measured objective eye position and demonstrated that the presence of a central fusion lock caused the subjective fixation disparity to more closely reflect the true objective eye position (Brautaset and Jennings, 2006).…”

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The Mallett Fixation Disparity Test: influence of test instructions and relationship with symptoms

Karania

Evans

2006

Ophthalmic Physiologic Optic

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Fixation disparity is a minute ocular misalignment under conditions of binocular single vision and is typically detected in primary eye care practices in the UK using the Mallett Unit Fixation Disparity Test. This instrument creates natural viewing conditions, when the patient's binocular system is fused using both central and peripheral fusion locks. This allows the examiner to determine the minimum prism power that eliminates the fixation disparity: the associated phoria or aligning prism. The spherical power that eliminates the fixation disparity, the aligning sphere, can also be determined. The near Mallett Unit Fixation Disparity Test has been shown to have good sensitivity and specificity for detecting symptomatic heterophoria. Cases of decompensated heterophoria tend to have a fixation disparity and the aligning prism or aligning sphere is a good indicator of the correction that will render the heterophoria compensated. The purpose of this study was, for the first time, to investigate the effect of test instructions on the results of the Mallett Unit Fixation Disparity Test. In study 1, we surveyed and observed practitioners to determine the instructions that are typically used. In study 2, we compared results obtained with this ÔstandardÕ method of questioning with a more ÔspecificÕ form of questioning that has been suggested in the literature. The participants for study 2 were 105 patients aged 7-70 years who were randomly selected from those attending a community optometric practice. Significantly different results were obtained with the two sets of instructions. The specific form of questioning revealed more cases of fixation disparity and the results with this method showed a better correlation with symptoms. This only held for near vision: for distance vision, symptoms were not significantly correlated with the presence of fixation disparity. This agrees with previous work with the Mallett unit, which showed a significant relationship with symptoms only at near. We also found that patients with more severe symptoms had greater degrees of aligning prism. Our study supports previous work indicating that the Mallett unit is a useful tool for detecting symptomatic heterophoria at near. However, we found that the testing method is important: patients need to be asked not just whether the nonius strips are aligned but also whether one or both of the strips ever moves. More research is needed to investigate the significance of precise test instructions in other optometric and orthoptic tests.

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The Mallett Fixation Disparity Test: influence of test instructions and relationship with symptoms

Karania

Evans

2006

Ophthalmic Physiologic Optic

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“…Firstly, it should be noted that Enright's sequential stereopsis technique may not relate directly to reading in terms of the fusional lock employed since participants were not actively seeking to look at a speci®c point on the target textures (e.g. Wildsoet and Cameron, 1985). It is also true that vergence angle changes during reading and does not actually need to be kept constant in the manner investigated here (see e.g.…”

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confidence: 99%

Vergence control across saccades in dyslexic adults

Moores

Frisby

Buckley

et al. 1998

Ophthalmic and Physiological Optics

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Many aspects of vision have been investigated in developmental dyslexia. Some research suggests deficits in vergence control (e.g. Buzzelli, 1991, Optom. Vision Sci. 68, 842-846), although ability to control vergence across saccades has not yet been investigated. We have explored this question indirectly using Enright's (1996 Vision Res. 36, 307-312.) sequential stereopsis task. The task requires observers to set two adjacent targets (whose textures cannot be resolved simultaneously if either is fixated) to appear equi-distant. Enright has argued that sequential stereopsis stereoacuity thresholds offer an indication of vergence control across saccades. We report two experiments using a total of 17 dyslexic and 18 control adults. Performance was measured on a sequential stereopsis task and an ordinary 'simultaneous' stereopsis task. No significant differences between groups were found. However, whereas practice of the sequential task lowered control group thresholds on the simultaneous task, for the dyslexic group it significantly raised thresholds, suggesting that visual fatigue is especially important in investigations of visual functions in dyslexia. Although the small samples used limit conclusions at this stage, the main sequential stereopsis results suggest that, if Enright is correct, dyslexic adults can show normal vergence control across saccades.

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“…Therefore, it was the aim of the present study to measure subjective fixation disparity as a function of the vertical gap between nonius lines (up to 7.9 deg). We used a central fusion stimulus because the resulting fixation disparity is more stable 27 and seems to be ecologically more valid, because most visual tasks (reading printed text or visual tasks at computer screens) involve central fusion stimuli. Furthermore, many studies reporting on the relation between fixation disparity and asthenopic complaints used a central fusion stimulus.…”

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confidence: 99%