Physiology of suppression in strabismic amblyopia (original) (raw)

A Limited Role for Suppression in the Central Field of Individuals with Strabismic Amblyopia

PLoS ONE, 2012

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.

Clinical suppression and amblyopia

Investigative ophthalmology & visual science, 1988

In individuals with abnormal binocular vision, such as strabismics and anisometropes, it is common for all or part of one eye's view to be suppressed so binocular confusion and diplopia are eliminated. We examined the relation between the depth of suppression (the amount by which the monocular contrast increment threshold for an eye was elevated by stimulation in the contralateral eye) and the degree of amblyopia (difference in monocular contrast thresholds for the two eyes). There was a significant negative correlation between suppression and amblyopia, so that clinical suppressors with no amblyopia exhibited deep suppression (ie, large threshold elevation) while observers with amblyopia exhibited weaker or no suppression. This negative correlation was found when the two eyes viewed orthogonally oriented contours as well as identically oriented contours. These results suggest that when an eye is amblyopic there is no longer a need for strong suppression of that eye by the contr...

The Perceptual Consequences of Interocular Suppression in Amblyopia

Investigative Opthalmology & Visual Science, 2011

PURPOSE. It is known that information from an amblyopic eye can be strongly suppressed when both eyes are open. The authors investigated the way in which suppression influences the relative perception of suprathreshold contrast and luminance between a person's eyes under dichoptic viewing conditions. METHODS. Stimuli consisted of four patches of luminance or four patches containing gratings. Two patches were presented to each eye. Ten amblyopes with mild suppression (six strabismic, three anisometropic and strabismic, and one deprivation; mean age, 34.5 years) and three control observers with normal vision (mean age, 33.0 years) matched the appearance of the stimuli presented to each eye. The match involved manipulation of either luminance or contrast. RESULTS. Amblyopes with mild suppression decreased stimulus luminance in the fellow fixing eye or increased luminance in the amblyopic eye to achieve a match (mean matching luminance, 21.1 and 39.6 cd/m 2 for the fellow fixing eye and the amblyopic eye, respectively; standard luminance, 30 cd/m 2). This interocular mismatch was also observed when luminance was variable and contrast was kept constant (mean matching luminance, 22.8 cd/m 2 for the fellow fixing eye). On the other hand, the amblyopic eye showed no loss of perceived contrast. There was little or no mismatch between the two eyes of control participants with normal binocular vision. CONCLUSIONS. Amblyopes have monocular deficits in contrast perception but dichoptic deficits in luminance perception, suggesting that suppression in its mild form involves luminance processing.

The Pathophysiology of Amblyopia: Electrophysiological Studies

Annals of the New York Academy of Sciences, 1980

Functional amblyopia may be defined as a loss of visual acuity caused by form deprivation and/or abnormal binocular interaction, for which no organic cause can be detected by the physical examination of the eye.' Amblyopia is usually associated with strabismus (turned eye), anisometropia (unequal refractive error), or form deprivation early in life, and thus represents a developmental disorder. There appears to be a critical period for the development of amblyopia in humans and it may be preventable if the causative factors are discovered and eliminated within this critical period.' Interestingly, amblyopia may also be reversible much later in life.* Over the past 15 to 20 years, there has been a renewed interest in the study of amblyopia, which is in large part attributable to the single-unit studies in cats and monkeys reared with experimentally induced amblyopia. These studies have shown that normal visual experience is essential for the development and maintenance of the physiological characteristics of cells in the visual cortex, and that disruption of the normal visual process during an early period of susceptibility by light or form deprivation, strabismus, or anisometropia, may result in a marked disturbance of the physiological organization of the visual cortex, cell shrinkage in the LGN, and severe a m b l y~p i a .~-~ However, it is not clear whether strabismus and form deprivation (due to lid suture or occlusion) affect similar cell populations in the visual pathway. There is evidence that the effects of monocular lid suture are selective for the Y or transient cells," while in kittens reared with unilateral strabismus, the X or sustained cells appear to be most affected."-" Although such methods of investigation are valuable in dealing with experimental animals, studies of human amblyopia have, for the most part been restricted to psychophysics and visual evoked potentials (VEP). In humans, with naturally occurring amblyopia, the VEP is generally the only direct method available for studying cortical responses to visual stimuli and may provide a valuable tool for the assessment of visual function in amblyopia for several reasons: (1) it provides an objective measure of function which may be useful in infants and nonverbal patients; (2) it may aid in localizing function and dysfunction; (3) it may be of value in assessing prognosis and monitoring therapy; and (4) it may be helpful in bridging the gap between psychophysics and physiology. Over the past few years we have been studying the VEP of observers with naturally occurring amblyopia using luminance and pattern stimuli. The present paper reviews some of the results. LUMINANCE EVOKED POTENTIALS There have been a large number of studies of the VEP in amblyopia using unpatterned stimuli; however, the results are quite equivocal. Some investigators

Measurement of suprathreshold binocular interactions in amblyopia

Vision research, 2008

It has been established that in amblyopia, information from the amblyopic eye (AME) is not combined with that from the fellow fixing eye (FFE) under conditions of binocular viewing. However, recent evidence suggests that mechanisms that combine information between the eyes are intact in amblyopia. The lack of binocular function is most likely due to the imbalanced inputs from the two eyes under binocular conditions [Baker, D. H., Meese, T. S., Mansouri, B., & Hess, R. F. (2007b). Binocular summation of contrast remains intact in strabismic amblyopia. Investigative Ophthalmology & Visual Science, 48(11), 5332-5338]. We have measured the extent to which the information presented to each eye needs to differ for binocular combination to occur and in doing so we quantify the influence of interocular suppression. We quantify these suppressive effects for suprathreshold processing of global stimuli for both motion and spatial tasks. The results confirm the general importance of these suppr...

Strabismic amblyopia. Part 2. Neural processing

Clinical and Experimental Optometry

This is the second of a two-part survey of current literature concerning strabismic amblyopia. The aim of this review is to bring the optometric community up to date on the status of scientific research into strabismic amblyopia. Part 1 in this series discussed research into strabismic amblyopia from the viewpoint of psychophysical experiments, which investigate both spatial and temporal behavioural deficits accompanying strabismic amblyopia. These include deficits in contrast sensitivity, spatial localisation, fixation, ocular motility, accommodation, crowding, attention, motion perception and temporal processing. Part 2 concerns neural processing in regards to strabismic amblyopia. It discusses current understanding of more fundamental aspects of central processing of visual information and in particular current theories regarding neural sites and mechanisms involved in amblyopia.

Strabismic amblyopia: Part 1: Psychophysics

Clinical and Experimental Optometry, 2000

This is a two-part survey of current literature concerning strabismic amblyopia. The aim of this review is to bring the optometric practitioner up to date on the status of scientific research into strabismic amblyopia.Part 1 in this series discusses research into strabismic amblyopia from the viewpoint of psychophysical experiments that investigate both spatial and temporal behavioural deficits accompanying strabismic amblyopia. These include deficits in contrast sensitivity, spatial localisation, fixation, ocular motility, accommodation, crowding, attention, motion perception and temporal processing.Part 2 will evaluate neural processing in regard to strabismic amblyopia. It will discuss current understanding of aspects of central processing of visual information and theories regarding neural sites and mechanisms involved in amblyopia.

Experimental analysis of amblyopia and strabismus

British Journal of Ophthalmology, 1974

In the past few years physiological experiments have brought us a little closer to an understanding of some forms of developmental amblyopia-deficits of visual acuity of central, not retinal, origin, which are not correctable by optical means. While amblyopia has become a catch-all term that covers the symptoms of a multitude of disorders, there are certain stereotyped experimental procedures that invariably lead to gross disturbances in the visual system. In this paper we shall describe a few of these experiments and attempt to relate them to some of the clinical origins of amblyopia in man. The cat has been the subject of a great deal of research on the development of the visual system, though it is clear that most of the observations, with some minor differences, also apply to monkeys (see von Noorden, 1974). By extrapolation, the situation is probably similar in man. Cats, monkeys, and man are all highly binocular animals. They have roughly the same amount of binocular overlap of the two visual fields, and their eyes are extremely mobile, showing disjunctive as well as conjugate movements. There is now behavioural evidence that both cats (Fox and Blake, 197i) and monkeys (Bough, I970) normally have stereoscopic vision, being able to discriminate the relative distances of objects solely on the basis of the retinal disparity of their images. Indeed, from neurophysiological experiments there is now a fair understanding of the actual neural mechanism of stereopsis.

A Binocular Approach to Treating Amblyopia: Antisuppression Therapy

Optometry and Vision Science, 2010

Purpose. We developed a binocular treatment for amblyopia based on antisuppression therapy. Methods. A novel procedure is outlined for measuring the extent to which the fixing eye suppresses the fellow amblyopic eye. We hypothesize that suppression renders a structurally binocular system, functionally monocular. Results. We demonstrate using three strabismic amblyopes that information can be combined normally between their eyes under viewing conditions where suppression is reduced. Also, we show that prolonged periods of viewing (under the artificial conditions of stimuli of different contrast in each eye) during which information from the two eyes is combined leads to a strengthening of binocular vision in such cases and eventual combination of binocular information under natural viewing conditions (stimuli of the same contrast in each eye). Concomitant improvement in monocular acuity of the amblyopic eye occurs with this reduction in suppression and strengthening of binocular fusion. Furthermore, in each of the three cases, stereoscopic function is established. Conclusions. This provides the basis for a new treatment of amblyopia, one that is purely binocular and aimed at reducing suppression as a first step.