Person: Bex, Peter
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Bex
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Peter
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Bex, Peter
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Publication Assessing Binocular Interaction in Amblyopia and Its Clinical Feasibility(Public Library of Science, 2014) Kwon, MiYoung; Lu, Zhong-Lin; Miller, Alexandra; Kazlas, Melanie; Hunter, David; Bex, PeterPurpose To measure binocular interaction in amblyopes using a rapid and patient-friendly computer-based method, and to test the feasibility of the assessment in the clinic. Methods: Binocular interaction was assessed in subjects with strabismic amblyopia (n = 7), anisometropic amblyopia (n = 6), strabismus without amblyopia (n = 15) and normal vision (n = 40). Binocular interaction was measured with a dichoptic phase matching task in which subjects matched the position of a binocular probe to the cyclopean perceived phase of a dichoptic pair of gratings whose contrast ratios were systematically varied. The resulting effective contrast ratio of the weak eye was taken as an indicator of interocular imbalance. Testing was performed in an ophthalmology clinic under 8 mins. We examined the relationships between our binocular interaction measure and standard clinical measures indicating abnormal binocularity such as interocular acuity difference and stereoacuity. The test-retest reliability of the testing method was also evaluated. Results: Compared to normally-sighted controls, amblyopes exhibited significantly reduced effective contrast (∼20%) of the weak eye, suggesting a higher contrast requirement for the amblyopic eye compared to the fellow eye. We found that the effective contrast ratio of the weak eye covaried with standard clincal measures of binocular vision. Our results showed that there was a high correlation between the 1st and 2nd measurements (r = 0.94, p<0.001) but without any significant bias between the two. Conclusions: Our findings demonstrate that abnormal binocular interaction can be reliably captured by measuring the effective contrast ratio of the weak eye and quantitative assessment of binocular interaction is a quick and simple test that can be performed in the clinic. We believe that reliable and timely assessment of deficits in a binocular interaction may improve detection and treatment of amblyopia.Publication Orientation-Specificity of Adaptation: Isotropic Adaptation Is Purely Monocular(Public Library of Science, 2012) Cass, John; Johnson, Ameika; Bex, Peter; Alais, DavidNumerous studies have found that prolonged exposure to grating stimuli reduces sensitivity to subsequently presented gratings, most evidently when the orientations of the adapting and test patterns are similar. The rate of sensitivity loss varies with angular difference indicating both the presence and bandwidths of psychophysical ‘orientation channels’. Here we study the orientation dependency of contrast adaptation measured both monoptically and dichoptically. Earlier psychophysical reports show that orientation bandwidths are broader at lower spatial frequencies, and we confirm this with a simple von Mises model using 0.25 vs. 2 c.p.d. gratings. When a single isotropic (orientation invariant) parameter is added to this model, however, we find no evidence for any difference in bandwidth with spatial frequency. Consistent with cross-orientation masking effects, we find isotropic adaptation to be strongly low spatial frequency-biased. Surprisingly, unlike masking, we find that the effects of interocular adaptation are purely orientation-tuned, with no evidence of isotropic threshold elevation. This dissociation points to isotropic (or ‘cross-orientation’) adaptation being an earlier and more magnocellular-like process than that which supports orientation-tuned adaptation and suggests that isotropic masking and adaptation are likely mediated by separate mechanisms.Publication Illusory Stimuli Can Be Used to Identify Retinal Blind Spots(Public Library of Science, 2007) Crossland, Michael D.; Dakin, Steven C.; Bex, PeterBackground: Identification of visual field loss in people with retinal disease is not straightforward as people with eye disease are frequently unaware of substantial deficits in their visual field, as a consequence of perceptual completion (“filling-in”) of affected areas. Methodology: We attempted to induce a compelling visual illusion known as the induced twinkle after-effect (TwAE) in eight patients with retinal scotomas. Half of these patients experience filling-in of their scotomas such that they are unaware of the presence of their scotoma, and conventional campimetric techniques can not be used to identify their vision loss. The region of the TwAE was compared to microperimetry maps of the retinal lesion. Principal Findings: Six of our eight participants experienced the TwAE. This effect occurred in three of the four people who filled-in their scotoma. The boundary of the TwAE showed good agreement with the boundary of lesion, as determined by microperimetry. Conclusion: For the first time, we have determined vision loss by asking patients to report the presence of an illusory percept in blind areas, rather than the absence of a real stimulus. This illusory technique is quick, accurate and not subject to the effects of filling-in.Publication Crowding Changes Appearance(Cell Press, 2010) Greenwood, John A.; Bex, Peter; Dakin, Steven C.Summary: Crowding is the breakdown in object recognition that occurs in cluttered visual environments and the fundamental limit on peripheral vision, affecting identification within many visual modalities and across large spatial regions. Though frequently characterized as a disruptive process through which object representations are suppressed or lost altogether, we demonstrate that crowding systematically changes the appearance of objects. In particular, target patches of visual noise that are surrounded (“crowded”) by oriented Gabor flankers become perceptually oriented, matching the flankers. This was established with a change-detection paradigm: under crowded conditions, target changes from noise to Gabor went unnoticed when the Gabor orientation matched the flankers (and the illusory target percept), despite being easily detected when they differed. Rotation of the flankers (leaving target noise unaltered) also induced illusory target rotations. Blank targets led to similar results, demonstrating that crowding can induce apparent structure where none exists. Finally, adaptation to these stimuli induced a tilt aftereffect at the target location, consistent with signals from the flankers “spreading” across space. These results confirm predictions from change-based models of crowding, such as averaging, and establish crowding as a regularization process that simplifies the peripheral field by promoting consistent appearance among adjacent objects.