Asthenopia, Associated Phoria, and Self-Selected Prism.

Purpose: The vergence position of rest (phoria) has been found to vary over time. We asked whether the vergence position of rest is correlated with discomfort on reading.

Methods: We examined 20 subjects who reported strain on prolonged reading ("asthenopes") and 20 subjects who did not ("non-asthenopes").

Microstrabismus: Fixation point scotoma, a perimetric artefact.

The contribution of the strabismic eye to binocular vision has frequently been studied with stimuli presented solely to the strabismic eye, on a binocular background. These studies revealed a central suppression scotoma in the strabismic eye, the so-called fixation point scotoma. Considering that this scotoma might be an artefact due to the unnatural viewing condition, we employed stereoperimetry that allowed examining the contribution of the strabismic eye under natural viewing, and compared the stereoresolution with the Vernier resolution of the strabismic eye.

Do dissociated or associated phoria predict the comfortable prism?

Background: Dissociated and associated phoria are measures of latent strabismus under artificial viewing conditions. We examined to what extent dissociated and associated phoria predict the "comfortable prism", i.e.

Ocular prevalence and stereoacuity.

Background: Most people attribute a higher weight to the input from one eye than to that from the other eye when they have to align stereodisparate objects in the same visual direction. This preference for visual directions has been termed 'ocular prevalence', according to the Latin praevalentia = superior power.

Questions: (1) Is ocular prevalence of one eye (or its correlate, partial suppression of the other eye in the prevalence task) restricted to large stereodisparities, close to Panum's limit, or does it occur also at small stereodisparities, near the stereoscopic threshold? (2) Is ocular prevalence a handicap for stereoacuity?

Methods: Six non-strabismic observers with equal visual acuity of their two eyes were examined.

Vernier acuity for stereodisparate objects and ocular prevalence.

Question: How precisely can objects, located in different depth planes, be aligned to the same visual direction?

Methods: Twenty normal observers were presented with vertical Vernier lines at various stereodisparities. They had to judge whether the lower, anterior line was located on the right- or left-hand side of the upper, posterior line.

Results: Over a stereodisparity range from zero to 62'', the threshold for detecting a lateral offset between the Vernier lines remained at the "hyperacuity" level of about 7''.

Ocular prevalence versus ocular dominance.

Unlabelled: Ocular dominance manifests itself in tests that contain stereo-objects with a disparity beyond Panum's area, e.g. in pointing a finger.

[Do prisms according to Hans-Joachim Haase influence ocular prevalence?].

Background: Ocular prevalence is defined as an unequal weighting of the eyes in the directional perception of stereo objects. Opinions differ as to the cause and relevance of ocular prevalence. Hans-Joachim Haase suggested that ocular prevalence is due to fixation disparity, brought about by incomplete compensation of heterophoria.

Stereoacuity versus fixation disparity as indicators for vergence accuracy under prismatic stress.

Background: Fixation disparity has been widely used as an indicator for vergence accuracy under prismatic stress. However, the targets used for measuring fixation disparity contain artificial features in that the fusional contours are thinned out. We considered that stereoacuity might be a preferable indicator of vergence accuracy, as stereo targets represent natural viewing conditions.

Interindividual variability of learning in stereoacuity.

Background: In the evaluation of therapies aiming at binocular vision, for instance by the use of prisms or orthoptic training in the case of heterophoria, stereoacuity is often the primary outcome measure. To assess therapeutic effects it is necessary to separate them from perceptual learning with repeated testing. Learning stereoacuity has been investigated only in a few studies with up to six subjects.

[Do prisms according to Hans-Joachim Haase improve stereoacuity?].

Background: The "Measuring and Correcting Methodology" after H.-J. Haase (MKH) aims at converting "fixation disparity" into bicentral fixation, using prismatic spectacles.