Transactions from the Xth International Orthoptics Congress 2004

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of 146″ and 114″ (sec arc) for PDs of 70 and 55mm respectively. These values are approximations and assume targets viewed in central vision with symmetrical gaze. The contrast between distance and near reflects the fact that disparities are scaled inversely with the square of the fixation distance. Given these values and that many other factors affect stereoacuity, this study used a large sample size of observers for the required statistical power. Methods Stereoacuities were measured using the Howard-Dolman task which requires setting two adjacent vertical rods to apparent equidistance. The two rods were black and seen against a brightly lit white background through a window in a white screen set about 20cm in front of the fixed rod. The screen obscured the supports for the rods. The participant adjusted the position in depth of one rod using either a long thin smooth hand-held pole (distance settings) or by rotating a knob that that operated a ratchet linkage to the moveable rod (near settings). In both cases, the participant could not make settings using unwanted cues from seeing the pole or the ratchet mechanism as these were obscured by black cloth. Head movement parallax cues were controlled with a chin rest. Pilot trials established that accuracy in rod settings deteriorated dramatically under monocular viewing, demonstrating that the task was performed using binocular cues. Various apparatus constraints meant that the visual angles subtended by rod width, rod separation and window size were different for the distant and near tasks. Thus, for the distance and near cases respectively, rod widths were approximately 10’ and 7’ (min arc), rod separations were 7.7º and 0.8º, and the width/height window sizes were 11.7º/4º and 2.5º/1º. Allowance was made for these differences in the analysis by normalising the data to group means. Each participant made 10 equidistance settings successively. They were encouraged to complete each one in fewer than 10 secs. There was a pause between settings during which the screen window was masked and the moveable rod set a few cm in front of or behind the fixed rod. Practice was limited to the minimum needed for the observer to understand the task. No feedback was given until all 10 settings were completed. The measure of stereoacuity was 0.6745 of the standard deviation (SD) of the 10 settings to equidistance. This gives an estimate of the semi-interquartile range of the psychometric function, which is a standard measure of a difference threshold. The participants were 186 psychology students of the University of Sheffield attending laboratory classes. They gave their informed consent. Inclusion criteria were: no strabismus or amblyopia history, 2.5SDs from the group means, suggesting that they were unable to do the task on the same basis as other participants). There were two groups of students run in successive years. One group (n=109, mean age 19.9 yrs, range 18-22 yrs) made distance settings with the fixed rod set at 5.5m or 4.9m according to available room size. The second group (n=77, mean age 18.7 yrs, range 17 -24 yrs) made near settings with the fixed rod set at 1m. PD was measured using a PD-82II Digital PD Meter (Towa Medical Instruments Co. Ltd) set for either distance or near viewing as appropriate. PDs were first measured a few weeks prior to the stereoacuity measurements in order to ensure that students from the ends of the PD range of the whole class (n > 300) were invited to be participants, as time did not permit including all of them in the study. This selection criterion did not lead to prominent bimodality in PDs as there were few students at the ends of the PD distribution. PDs were measured again on the days when stereoacuities were measured for confirmation, yielding the PD values used in the analyses. 2
Results The distance and near stereoacuities produced similar correlations with PD when analysed separately. Space limitations forbid a full report of each group’s data here (data from the distance group have been reported elsewhere; ref. 3). Instead, we concentrate on a combined analysis, in which each participant’s results were normalised to their group means, yielding relative measures as the data for Pearson product moment correlations. Using relative measures eliminated the variation in PD measurements arising from using distance and near settings on the PD meter. It also eliminated differences in group means for the distance and near stereoacuities. The group means prior to normalisation were: distance PD 60.3mm (SD 3.8mm); near PD 56.5mm (2.7mm), distance stereoacuity 5.3″ (2.4″), near stereoacuity 34″ (14″). We interpret the latter difference as caused by differences in the details of the two Howard-Dolman displays rather than a genuine difference in near and distance stereoacuities. For example, apparatus factors related to the ratchet mechanism meant that the rods had to be separated by about 7.7º in the near case, as against only 0.8º for the distance settings. This wider separation is likely to have made the near task harder. Fig. 1 Frequency distribution of relative PD (N=186). Fig. 2. Frequency distribution of relative stereoacuities ( N=186) Figs. 1 and 2 show frequency distributions of the relative PDs and relative stereoacuities (equivalently, stereo thresholds). The stereoacuities were not normally distributed, showing a longer tail for the larger values as is typical in this kind of data. This skew was removed for correlations using a log transformation. Fig. 3 shows a scattergram of the correlation (r = -0.24, p
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Page 19 and 20: Table 2 Patient Preoperative fusion
Page 21 and 22: Assessing The Dominant Eye: A New M
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Page 25 and 26: Does Dissociation Reduce Stereoacui
Page 27 and 28: Methods Observers were students in
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Page 41 and 42: The results of all four stereotests
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Page 49 and 50: Stereoacuity Following Unilateral A
Page 51 and 52: Table 1 Pre-operatively Post-operat
Page 53 and 54: Measurement And Improvement Of Visu
Page 55 and 56: Table 1 Criteria for visual skills
Page 57 and 58: Diagnosis And Treatment In Patients
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Transactions from the Xth International Orthoptics Congress 2004

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