Thesis (doctoral)--Universität Hamburg, 1979.

Note:

published_or_final_version / Public Health / Master / Master of Public Health

No description available.

No description available.

Yes / Background: Although their eyes are pointing in different directions, people with long-standing strabismic amblyopia typically do not experience double-vision or indeed any visual symptoms arising from their condition. It is generally believed that the phenomenon of suppression plays a major role in dealing with the consequences of amblyopia and strabismus, by preventing images from the weaker/deviating eye from reaching conscious awareness. Suppression is thus a highly sophisticated coping mechanism. Although suppression has been studied for over 100 years the literature is equivocal in relation to the extent of the retina that is suppressed, though the method used to investigate suppression is crucial to the outcome. There is growing evidence that some measurement methods lead to artefactual claims that suppression exists when it does not. Methodology/Results: Here we present the results of an experiment conducted with a new method to examine the prevalence, depth and extent of suppression in ten individuals with strabismic amblyopia. Seven subjects (70%) showed no evidence whatsoever for suppression and in the three individuals who did (30%), the depth and extent of suppression was small. Conclusions: Suppression may play a much smaller role in dealing with the negative consequences of strabismic amblyopia than previously thought. Whereas recent claims of this nature have been made only in those with micro-strabismus our results show extremely limited evidence for suppression across the central visual field in strabismic amblyopes more generally. Instead of suppressing the image from the weaker/deviating eye, we suggest the visual system of individuals with strabismic amblyopia may act to maximise the possibilities for binocular co-operation. This is consistent with recent evidence from strabismic and amblyopic individuals that their binocular mechanisms are intact, and that, just as in visual normals, performance with two eyes is better than with the better eye alone in these individuals.

The aim of this study was to add to our understanding of interocular interactions in normally sighted children (Group I, N=20), normal adults (Group II, N=20) and adults with anisometropic amblyopia (N=12) by investigating responses to a range of visual functions under three kinds of viewing condition. Visual functions tested were visual acuity, contrast sensitivity and alignment sensitivity. Stimuli were generated on a Cambridge VSG card driving a high resolution monitor and FE liquid crystal goggles, enabling three kinds of viewing conditions: 1. Monocular (non-tested eye occluded), used as a baseline for most functions. 2. Dichoptic (uniform field presented to the non-tested eye but with a binocular fusion lock). 3. Binocular. In general, binocular performance was better than monocular (binocular summation) but so too was dichoptic performance (dichoptic advantage). However there was much variation within individuals (the three functions showing different summation/advantage pattern) and between individuals. Significant conclusions include: (a) Maturational windows for interocular interactions differ for different spatial visual functions. (b) Interpretations of results from one visual function cannot be applied automatically to other functions. (c) Care must be taken in interpreting results based on 5 or fewer subjects.

The human visual system comprises two neural pathways, the magnocellular/M and parvocellular/P pathways that process aspects of motion and form perception, respectively. Amblyopia is a developmental condition which may affect an otherwise healthy eye if it experiences abnormal visual stimulation due to ocular misalignment (strabismus), unequal refractive errors (anisometropia), or both. Amblyopia has been associated with deficits in both form and motion perception. Random-dot kinematograms (RDKs) which are created by shifting a computer-generated dot display in one direction by a given displacement can be used to assess motion processing. Maximum motion displacement (Dmax) is the largest dot displacement at which the direction of motion for a RDK can be correctly discriminated. Strabismic and anisometropic amblyopia represent two distinct subtypes of amblyopia and have been proposed to have different neural substrates. They have also been reported to have different Dmax deficits (Ho et al., 2005). The intentions of this thesis were: 1) to characterize deficits in Dmax for direction discrimination in the fellow and amblyopic eyes of participants with anisometropic and strabismic amblyopia using psychophysical methods; and 2) to investigate the relationship between psychophysical Dmax deficits and dysfunction in motion-sensitive extrastriate cortex of the M pathway using functional MRI techniques. The psychophysical results showed that Dmax thresholds are smaller in both amblyopic and fellow eyes for both subtypes of amblyopia relative to controls, although the deficits were greatest for strabismic amblyopia. Functional MRI results revealed decreased extrastriate cortical activation in both the strabismic and anisometropic groups relative to the control group when either eye viewed the RDK stimulus, although the lack of cortical activation was greatest for strabismic amblyopia. Taken together, this evidence suggests that dysfunctional binocular motion processing mechanisms in extrastriate cortex are part of the neural deficit underlying anisometropic and strabismic amblyopia and implies that strabismic amblyopia may be affected to a greater degree. For both amblyopic groups, there was a robust correlation between depth perception (stereoacuity) and Dmax thresholds. Specifically, direction discrimination was better when stereoacuity was worse. Abnormal binocular integration may have a significant role in predicting motion deficits in both anisometropic and strabismic amblyopia.

The human visual system comprises two neural pathways, the magnocellular/M and parvocellular/P pathways that process aspects of motion and form perception, respectively. Amblyopia is a developmental condition which may affect an otherwise healthy eye if it experiences abnormal visual stimulation due to ocular misalignment (strabismus), unequal refractive errors (anisometropia), or both. Amblyopia has been associated with deficits in both form and motion perception. Random-dot kinematograms (RDKs) which are created by shifting a computer-generated dot display in one direction by a given displacement can be used to assess motion processing. Maximum motion displacement (Dmax) is the largest dot displacement at which the direction of motion for a RDK can be correctly discriminated. Strabismic and anisometropic amblyopia represent two distinct subtypes of amblyopia and have been proposed to have different neural substrates. They have also been reported to have different Dmax deficits (Ho et al., 2005). The intentions of this thesis were: 1) to characterize deficits in Dmax for direction discrimination in the fellow and amblyopic eyes of participants with anisometropic and strabismic amblyopia using psychophysical methods; and 2) to investigate the relationship between psychophysical Dmax deficits and dysfunction in motion-sensitive extrastriate cortex of the M pathway using functional MRI techniques. The psychophysical results showed that Dmax thresholds are smaller in both amblyopic and fellow eyes for both subtypes of amblyopia relative to controls, although the deficits were greatest for strabismic amblyopia. Functional MRI results revealed decreased extrastriate cortical activation in both the strabismic and anisometropic groups relative to the control group when either eye viewed the RDK stimulus, although the lack of cortical activation was greatest for strabismic amblyopia. Taken together, this evidence suggests that dysfunctional binocular motion processing mechanisms in extrastriate cortex are part of the neural deficit underlying anisometropic and strabismic amblyopia and implies that strabismic amblyopia may be affected to a greater degree. For both amblyopic groups, there was a robust correlation between depth perception (stereoacuity) and Dmax thresholds. Specifically, direction discrimination was better when stereoacuity was worse. Abnormal binocular integration may have a significant role in predicting motion deficits in both anisometropic and strabismic amblyopia.

Thesis--University of Florida. / Description based on print version record. Typescript. Vita. Includes bibliographical references (leaves 245-258).