Acuity Crowding Feature Detection And Fixation In Normal And Amblyopic Vision
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Acuity Crowding Feature Detection And Fixation In Normal And Amblyopic Vision
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Author : Shuang Song
language : en
Publisher:
Release Date : 2009
Acuity Crowding Feature Detection And Fixation In Normal And Amblyopic Vision written by Shuang Song and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2009 with categories.
Both functional and physical properties related to vision are known to be abnormal in amblyopia. The former include spatial and temporal visual deficits such as reduced letter acuity and temporal sensitivity, crowding, and decreased position acuity; the latter refers to eccentric and unstable fixational eye movements in strabismic amblyopia. This dissertation represents parallel studies that aim to characterize amblyopic vision from these angles using different approaches. In the first series of experiments, we systematically explored the size and spacing requirements for identifying a letter among other letters by measuring flanked and unflanked letter acuities in normals and amblyopes, centrally and peripherally, while fixing the medium contrast and brief presentation duration. At fixation, for normals and non-strabismic amblyopes, no crowding has been found: legibility of a flanked letter is limited by overlap masking or acuity depending on the letter spacing. In normal peripheral vision and strabismic amblyopia, legibility is limited by crowding unless the letter spacing is very loose. We show that strabismic and non-strabismic amblyopia are much like normal vision at increased eccentricity or with added blur, respectively, as characterized by the crowding-acuity ratio. For clinical screening tests for strabismic and non-strabismic amblyopia, we recommend measuring both unflanked letter acuity and flanked letter acuity with tight spacing. In the second series of experiments, we explored the effects of various factors on both flanked and unflanked visual acuities in normal observers. Specifically, we varied polarity, contrast, and duration of the letter stimuli, as well as the pupil size of the observer by adding a pinhole. We find that the critical spacing for crowding in the fovea is 0.09 ̃0.1 deg. The main effects of and interactions between the above factors provide the context and supplement to the results from the above experiments, based on which we extend the letter legibility model developed from the above experiments by parameterization of each legibility limit to account for a wide range of conditions. In the third series of experiments, the first experiment estimated the spatiotemporal mechanism for detecting a luminance increment of a bright bar embedded in spatiotemporal noise for normal and amblyopic observers. The normal template is characterized by a temporal summation zone surrounded by symmetric spatial inhibition zones and followed by a temporal inhibition zone. The abnormal amblyopic template lacks inhibition but with normal temporal summation. Neither blurring the stimuli in space and time nor varying the signal-to-noise ratio caused any significant change in the normal or amblyopic template. However, decreasing the fundamental frequency of the stimuli restored the normal template in the amblyopic eye. Furthermore, the normal periphery shares spatial properties with amblyopia but the temporal properties may be different. The second experiment mapped the dynamic of spatiotemporal interactions of crowded stimulus orientations in normal and amblyopic eyes. For normal eyes, crowding occurs at locations adjacent to the target in space and around or before the target presentation in time. Different types of cues may affect the spatiotemporal interaction map differently, but the strongest crowding never coincides with the target presentation. The amblyopic spatiotemporal interaction map is not substantially different from that of the normal eyes, except that in general the spatiotemporal interaction is more widely distributed around and after the target presentation. "Anti-crowding"--Repulsion of perceived orientation that is induced by farther flankers is documented in both normal and amblyopic eyes. In the fourth series of experiments, an adaptive optics scanning laser ophthalmoscope (AOSLO) was used to record fixational eye movements, allowing direct viewing of retinal movements and fixation locations. Fixational eye movements were modeled as a stochastic process governed by a potential function assumed to have the form of a quadratic polynomial for both normal and strabismic amblyopic observers. Our results confirm that microsaccades, on average, correct for fixation inaccuracy. Microsaccades occur more frequently and tend to move faster towards the target at relatively large displacements. Drift is modeled as a Brownian motion with constant rate over time plus an error-correcting component initially following a microsaccade. The unstable eccentric fixational eye movements in strabismic amblyopia are characterized by frequent intrusive saccades with large amplitudes and high speeds. The strabismic amblyopic fixation pattern on multiple loci is task dependent, with less eccentric but unstable fixation induced by challenging tasks, and relatively stable fixation by small highly visible static targets.
Learning To See Better Improving Visual Deficits With Perceptual Learning
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Author : Gianluca Campana
language : en
Publisher: Frontiers Media SA
Release Date : 2015-07-09
Learning To See Better Improving Visual Deficits With Perceptual Learning written by Gianluca Campana and has been published by Frontiers Media SA this book supported file pdf, txt, epub, kindle and other format this book has been release on 2015-07-09 with Psychology categories.
Perceptual learning can be defined as a long lasting improvement in a perceptual skill following a systematic training, due to changes in brain plasticity at the level of sensory or perceptual areas. Its efficacy has been reported for a number of visual tasks, such as detection or discrimination of visual gratings (De Valois, 1977; Fiorentini & Berardi, 1980, 1981; Mayer, 1983), motion direction discrimination (Ball & Sekuler, 1982, 1987; Ball, Sekuler, & Machamer, 1983), orientation judgments (Fahle, 1997; Shiu & Pashler, 1992; Vogels & Orban, 1985), hyperacuity (Beard, Levi, & Reich, 1995; Bennett & Westheimer, 1991; Fahle, 1997; Fahle & Edelman, 1993; Kumar & Glaser, 1993; McKee & Westheimer, 1978; Saarinen & Levi, 1995), visual search tasks (Ahissar & Hochstein, 1996; Casco, Campana, & Gidiuli, 2001; Campana & Casco, 2003; Ellison & Walsh, 1998; Sireteanu & Rettenbach, 1995) or texture discrimination (Casco et al., 2004; Karni & Sagi, 1991, 1993). Perceptual learning is long-lasting and specific for basic stimulus features (orientation, retinal position, eye of presentation) suggesting a long-term modification at early stages of visual analysis, such as in the striate (Karni & Sagi, 1991; 1993; Saarinen & Levi, 1995; Pourtois et al., 2008) and extrastriate (Ahissar & Hochstein, 1996) visual cortex. Not confined to a basic research paradigm, perceptual learning has recently found application outside the laboratory environment, being used for clinical treatment of a series of visually impairing conditions such as amblyopia (Levi & Polat, 1996; Levi, 2005; Levi & Li, 2009, Polat et al., 2004; Zhou et al., 2006), myopia (Tan & Fong, 2008) or presbyopia (Polat, 2009). Different authors adopted different paradigms and stimuli in order to improve malfunctioning visual abilities, such as Vernier Acuity (Levi, Polat & Hu, 1997), Gratings detection (Zhou et al., 2006), oculomotor training (Rosengarth et al., 2013) and lateral interactions (Polat et al., 2004). The common result of these studies is that a specific training produces not only improvements in trained functions, but also in other, untrained and higher-level visual functions, such as visual acuity, contrast sensitivity and reading speed (Levi et al, 1997a, 1997b; Polat et al., 2004; Polat, 2009; Tan & Fong, 2008). More recently (Maniglia et al. 2011), perceptual learning with the lateral interactions paradigm has been successfully used for improving peripheral vision in normal people (by improving contrast sensitivity and reducing crowding, the interference in target discrimination due to the presence of close elements), offering fascinating new perspectives in the rehabilitation of people who suffer of central vision loss, such as maculopathy patients, partially overcoming the structural differences between fovea and periphery that limit the vision outside the fovea. One of the strongest point, and a distinguishing feature of perceptual learning, is that it does not just improve the subject’s performance, but produces changes in brain’s connectivity and efficiency, resulting in long-lasting, enduring neural changes. By tailoring the paradigms on each subject’s needs, perceptual learning could become the treatment of choice for the rehabilitation of visual functions, emerging as a simple procedure that doesn’t need expensive equipment.
A Common Framework For Visual Crowding In Typical And Amblyopic Vision
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Author : Alexandra V. Kalpadakis-Smith
language : en
Publisher:
Release Date : 2019
A Common Framework For Visual Crowding In Typical And Amblyopic Vision written by Alexandra V. Kalpadakis-Smith and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2019 with categories.
Amblyopia is a developmental disorder of vision characterised by reduced acuity in one eye despite optical correction. When associated with strabismus, foveal vision is impaired by crowding: objects that are readily recognised in isolation become indistinguishable in clutter. In typical vision, crowding is minimal in the fovea and increases in the visual periphery. According to pooling accounts, the increase of crowding in the periphery arises from the integration of adjacent objects to promote perceptual homogeneity where sampling is insufficient and neurons have large receptive fields. It is unclear whether amblyopic crowding represents the same process. In this thesis I characterise amblyopic crowding, and investigate whether it can be understood within the same pooling framework as peripheral crowding. First, I show that amblyopic crowding systematically shifts the appearance of crowded objects to promote perceptual homogeneity, matching the perceptual effects in the periphery. A model simulating pooled responses of populations of visual neurons accurately characterises these effects in amblyopic and peripheral crowding, suggesting a common underlying mechanism. Second, I investigate the pattern of amblyopic crowding across the visual field and its neural correlates. I show that amblyopic crowding is elevated relative to typical vision in both fovea and periphery. At a group level, the increase of crowding in amblyopia and the typical periphery matches the increase of fMRI population receptive field (pRF) estimates in V1, V2, and V3, but at an individual level there is no correlation. Finally, I investigate the effects of higher-level grouping processes by examining whether uniformity in global clutter configuration modulates amblyopic crowding. I find that in most cases, clutter disrupts recognition in amblyopia regardless of global configuration, suggesting that contrary to the periphery, amblyopic crowding is largely unaffected by higher-level grouping processes. Therefore, on the whole pooling provides a successful framework for both amblyopic and peripheral crowding.
The Influence Of Accommodation And Fixation On The Visual Evoked Response And Visual Acuity Of Normal And Amblyopic Subjects
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Author : Kevin Katz
language : en
Publisher:
Release Date : 1978
The Influence Of Accommodation And Fixation On The Visual Evoked Response And Visual Acuity Of Normal And Amblyopic Subjects written by Kevin Katz and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1978 with Amblyopia categories.
Subjective And Visual Evoked Potential Measures Of Acuity In Normal And Amblyopic Adults And Children
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Author : Deborah Ann Orel-Bixler
language : en
Publisher:
Release Date : 1989
Subjective And Visual Evoked Potential Measures Of Acuity In Normal And Amblyopic Adults And Children written by Deborah Ann Orel-Bixler and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1989 with Amblyopia categories.
Clinical Ophthalmology Contemporary Perspectives E Book
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Author : A.K. Gupta
language : en
Publisher: Elsevier Health Sciences
Release Date : 2012-05-14
Clinical Ophthalmology Contemporary Perspectives E Book written by A.K. Gupta and has been published by Elsevier Health Sciences this book supported file pdf, txt, epub, kindle and other format this book has been release on 2012-05-14 with Medical categories.
Many new developments have taken place in ophthalmology since the publication of the last edition. It therefore became mandatory to revise and update the present edition. To accomplish this many new chapters have been added and the existing ones revised. Thoroughly revised and updated chapters Recent Advances in Lamellar Keratoplasty Principles of Molecular Genetics Intraocular Tumours Recent Advances in Oculoplastic Surgery Ophthalmic Electrodiagnostic Techniques Ultrasonography in Ophthalmology Ocular Coherence Tomography Anterior Segment Trauma Corneal Topography and Wavefront Sensing
Clinical Ophthalmology
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Author : Gupta
language : en
Publisher: Elsevier India
Release Date : 2009-11-20
Clinical Ophthalmology written by Gupta and has been published by Elsevier India this book supported file pdf, txt, epub, kindle and other format this book has been release on 2009-11-20 with Medical categories.
Many new developments have taken place in ophthalmology since the publication of the last edition. It therefore became mandatory to revise and update the present edition. To accomplish this many new chapters have been added and the existing ones revised. We have also included several new illustrations, flowcharts, tables and boxes to make the coverage more comprehensive and contemporary. About the Author : - A.K. Gupta, Director, Shroff Eye Centre, Kailash Colony, New Delhi.V. Krishna, Senior Consultant, Department of Ophthalmology, St. Stephen s Hospital, New Delhi.
Ophthalmology E Book
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Author : Myron Yanoff
language : en
Publisher: Elsevier Health Sciences
Release Date : 2018-08-09
Ophthalmology E Book written by Myron Yanoff and has been published by Elsevier Health Sciences this book supported file pdf, txt, epub, kindle and other format this book has been release on 2018-08-09 with Medical categories.
Long considered one of ophthalmology’s premier texts, this award-winning title by Drs. Myron Yanoff and Jay S. Duker remains your go-to reference for virtually any topic in this fast-changing field. It offers detailed, superbly illustrated guidance on nearly every ophthalmic condition and procedure you may encounter, making it a must-have resource no matter what your level of experience. Extensive updates throughout keep you current with all that’s new in every subspecialty area of the field.
Impaired Feature Integration And Sensitivity To Synchronicity In The Amblyopic Visual System For Biological Motion
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Author : Jennifer Yen Luu
language : en
Publisher:
Release Date : 2008
Impaired Feature Integration And Sensitivity To Synchronicity In The Amblyopic Visual System For Biological Motion written by Jennifer Yen Luu and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2008 with categories.
Contemporary Perspectives On Ophthalmology 10e
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Author : A.K. Gupta
language : en
Publisher: Elsevier Health Sciences
Release Date : 2019-09-26
Contemporary Perspectives On Ophthalmology 10e written by A.K. Gupta and has been published by Elsevier Health Sciences this book supported file pdf, txt, epub, kindle and other format this book has been release on 2019-09-26 with Medical categories.
Contemporary Perspectives on Ophthalmology is a specialty text targeting all learners of modern-day ophthalmology, particularly who are making the first steps in the discovery of this vibrant branch of medicine after completing their general medicine course as undergraduates. Contemporary Ophthalmology as a branch of medicine has grown to enormous magnitudes in the last decade. Hence the students need a comprehensive compilation detailing the updates in the subject for easy and quick perusal before their examination. The current edition is completely redesigned, and many chapters are rewritten in order to make it relevant. Some of the key features include: Wide and comprehensive coverage of updated topics in all specialties of ophthalmology. Succinct and focused format for quick reading and understanding. Key updates on evolving topics such as refractive surgery, ocular therapeutics, stem cells and corneal surgery, advances in cataract surgery, glaucoma investigations and treatment, uveal disorders, retinal surgery, oculoplastic procedures and ocular trauma. Enhanced emphasis on diagnosis of ocular disorders and investigations. A chapter on community ophthalmology. A new chapter on statistics for core understanding required for research techniques. Many illustrations and artworks for better elucidation of key concepts. Videos on selected topics.