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PubMed2006

Are orthoptic exercises an effective treatment for convergence and fusion deficiencies?

PURPOSE: To investigate whether orthoptic exercises are an effective way to influence the near point of convergence, fusion range and asthenopic symptoms. METHODS: Seventy-eight patients met the inclusion criteria of visual acuity 6/9 or better, no history of orthoptic treatment, squint surgery or Meares Irlen syndrome/dyslexia. Information was collected from case records related to diagnosis, near point of convergence, fusion range, prism and cover test measurements and symptoms. Type, duration and frequency of exercises were also recorded. Non-parametric statistics were applied. RESULTS: Patients ranged in age from 5 to 73 years (mean 11.9). Females outnumbered males (46:32). The diagnoses were: decompensating heterophoria (n = 50) or convergence insufficiency (n = 28: primary 27; secondary 1). Exophoria was more common (n = 65), than esophoria (n = 11) or orthophoria (n = 1). Treatments were aimed at improving near point of convergence and/or reduced fusional reserves. The mean treatment period was 8.2 months. Reduced near point of convergence normalized following treatment in 47/55 cases, and mean near point of convergence improved from 16.6 to 8.4 cm (p = 0.0001). Fusional reserves normalized in 29/50. Fusional convergence improved significantly for those with exodeviation (p > 0.0006). Asthenopic symptoms improved in 65 patients. A reduction in deviation of 5 pd or more occurred in 20 patients. CONCLUSIONS: Orthoptic exercises are an effective means of reducing symptoms in patients with convergence insufficiency and decompensating exophoria, and appear to target the proximal and fusional components of convergence. Their role in esophoria is unclear and needs further study.

Adolescent↗
PubMed1998

A neural model of binocular integration and rivalry based on the coordination of action-potential timing in primary visual cortex.

In normal vision, the inputs from the two eyes are integrated into a single percept. When dissimilar images are presented to the two eyes, however, they compete for perceptual dominance, so that one eye's view suppresses that of the other. Recent evidence suggests that this phenomenon, known as binocular rivalry, arises through competition between alternative stimulus interpretations in extrastriate cortex. Because eye-specific information appears to be lost at this stage, it remains unclear how the stimulus conditions that yield binocular rivalry are distinguished from those that produce stable single vision. Using a neural network that models the mammalian early visual system, I investigate here the hypothesis that congruent and conflicting stimuli are distinguished by their different effects on the relative timing of action potentials in primary visual cortex (V1), where monocular inputs are first combined. In the model, congruent stimulation of both eyes results in synchronization of discharges among binocular neurons in V1. By contrast, conflicting stimulation of the two eyes results in neuronal asynchrony in this area. This asynchrony then produces rivalrous response suppression at later stages in the visual pathway. Synchronization of firing in V1, however, prevents such competition, thereby ensuring non-rivalrous responses. These novel effects of spike timing on competition emerge naturally from the network dynamics. The results suggest that input-related differences in relative spike timing at an early stage of visual processing may play an important part in the phenomena both of binocular integration and rivalry; furthermore, they indicate that the temporal patterning of cortical activity may be a fundamental mechanism of selection among competing stimulus representations.

Action Potentials↗
PubMed1995

Interocular suppression in cat striate cortex is not orientation selective.

For the majority of neurones in cat striate cortex, the response to an optimal stimulus presented to one eye is suppressed when a stimulus of substantially different orientation is presented to the other eye. In order to determine the true orientational tuning of the underlying inhibitory interactions in the absence of binocular facilitation for matched stimuli, we tested how the response of such cells to an optimal grating in one eye is affected by gratings in the other eye of spatial frequencies too high or low to elicit an excitatory response through either eye: the vast majority of cells displayed suppression that was essentially independent of orientation. Our results indicate that interocular inhibition derives from cells representing all orientations, but is swamped by interocular facilitation for stimuli matched in orientation and spatial frequency.

Animals↗
PubMed2000

Early postnatal development of functional ocular dominance columns in cat primary visual cortex.

During postnatal development of the visual cortex the thalamocortical afferents serving the two eyes segregate into alternating patches called ocular dominance (OD) columns. Interested in the dynamics of this segregation process we studied the appearance of functional OD columns in the primary visual cortex of normally raised and strabismic kittens aged 2-6 weeks using 2-deoxyglucose labelling in awake animals. In both experimental groups, OD columns covering the entire area 17 and spanning all cortical laminae are first visible at 3 weeks and appear already adult-like at 4 weeks, much earlier than thought on the basis of previous anatomical studies. We hypothesize that a small and anatomically undetectable imbalance between the afferents from the two eyes is amplified by intracortical interactions so that their activity patterns become different and may guide the segregation process of the afferents in cortical layer IV.

Afferent Pathways↗
PubMed2005

Increased gamma-band synchrony precedes switching of conscious perceptual objects in binocular rivalry.

Gamma-band neural synchrony has been proposed as a correlate of consciousness and as a solution for the binding problem. Research on visual working memory and the perception of coherent images in ambiguous figures supports this view. To test the relationship between perceptual consciousness and gamma synchrony, we recorded electroencephalogram activity while participants were presented with rivalling visual images to each eye. Participants pressed buttons to indicate which image they were perceiving, and response-locked phase-locking values (6-60 Hz) were calculated from the electroencephalogram time series. Results revealed transient bursts of increased global phase synchrony in the gamma-band peaking approximately 425 and approximately 260 ms preresponse. This suggests that global gamma synchrony is associated with the emergence of a coherent conscious percept.

Adolescent↗
PubMed2001

Binocular vision recovery in bilateral keratoplasty.

PURPOSE: To determine the benefit of bilateral penetrating keratoplasty with regard to binocular vision. METHODS: We compared patients who underwent corneal transplantation bilaterally with patients who had successful corneal transplantation in one eye and corneal disease in the other eye. Specifically, changes in fusion, stereopsis, and binocular vision function were analyzed. A questionnaire regarding performance of daily tasks was given. RESULTS: Patients with bilateral keratoplasty performed better in all the analyzed functions. Fusion was achieved by 81.25% in the unilateral group versus 100% in the bilateral group (p = 0.15). Stereopsis was present more in the bilateral group (100% vs. 62.5%, p = 0.008) and the quantity of stereopsis was significantly better in the bilateral group (121 seconds of arc vs. 1,284 seconds of arc, p = 0.014). 88.8% of the patients subjectively improved in daily activities after second eye surgery. CONCLUSION: There are objective and subjective improvements after bilateral penetrating keratoplasty.

Activities of Daily Living↗
PubMed1988

Accommodative adaptation to monocular and binocular stimuli.

Pre- and post-task measures of dark-focus (DF) were used to assess accommodative adaptation induced by a 45-s near-vision task at 33 cm. Adaptation was measured under monocular and binocular conditions for a group of 10 young emmetropic subjects (mean age 21.6 years). The accommodative response was measured objectively using an infrared optometer (Canon Autoref R-1). Post-task DF was sampled immediately after the task at 1-s intervals over a 90-s period. No significant difference in accommodative adaptation was observed between the monocular and binocular near-vision tasks. The implications of this finding are discussed with regard to the oculomotor constituents of the closed-loop accommodative response.

Accommodation, Ocular↗
PubMed1989

Central and peripheral sensitivity to temporally modulated stimuli presented to the sighting and nonsighting eyes.

Thirty-two visually normal observers showed reduced sensitivity to uniform field flicker in the fovea of the nonsighting eye relative to the sighting eye. The two eyes showed equal sensitivity to flicker in the peripheral retina. This result with visually normal observers is analogous to that found in observers with functional amblyopia. It is also consistent with the hypothesis that amblyopic and normal suppressive processes share a common mechanism.

Contrast Sensitivity↗
PubMed1990

Effect of induced static aniseikonia on fixation performance during oblique gaze.

In a previous study, it was found that oblique gaze and the prismatic effect inherent in dynamic aniseikonia combine to affect the ability to fixate centrally. In the current study, retinal image differences were introduced, whereas prismatic effects were eliminated. This simulates static aniseikonia as opposed to dynamic aniseikonia. At the same time, oblique gaze was simulated by presenting the stimuli in directions other than straight ahead. The directions of presentation contained both horizontal and vertical components. Fixation eccentricity was monitored by using the border enhancement method, as described in previous experiments. It was found that fixation eccentricity increased as the eyes were directed toward stimuli oriented in directions other than straight ahead. When vertical and horizontal components of oblique gaze were combined, the fixation eccentricity was found to be greater than with either component alone. When differential magnification between left and right retinal images was introduced, the fixation eccentricity increased further, the increment being approximately constant for all directions of gaze.

Aniseikonia↗
PubMed1991

Stability of retinal correspondence in normal binocular vision.

Although many studies have suggested absolute stability of retinal correspondence, perhaps more have concluded that the correspondence of adult subjects with normal binocular vision is capable of small variation, particularly with strong sustained demands upon vergence, in order for binocular vision to be maintained. In this comprehensive review of the literature, the reasons for these differences are critically discussed and areas still to be resolved are pointed out.

Depth Perception↗
PubMed1991

Diagnostic occlusion and clinical management of latent hyperphoria.

Occasionally in patients who have symptoms suggestive of a vertical heterophoria no deviation is found, even on careful examination. Six days of occlusion have been recommended for uncovering such "latent" vertical deviations. We investigated prolonged monocular occlusion (Part I) and found that vertical deviations of varying amounts manifested on symptomatic and asymptomatic subjects. Thus, results of prolonged occlusion can be difficult to interpret. Nonadaptive vertical vergence systems have been implicated in development of symptoms. Therefore, it may be that diagnostic monocular occlusion is not appropriate unless patients have symptoms of vertical imbalance. We used (24 h) occlusion and associated phoria measurements (Part II) to determine vertical prism prescriptions which eliminated symptoms of seven symptomatic patients who did not show significant vertical heterophoria on routine clinical testing. We present data and case reports which elucidate the efficacy of this procedure.

Adolescent↗
PubMed1991

Effect of induced blur on visual acuity and stereoacuity.

The acuities of three subjects with good visual acuity and stereoacuity were reduced by the addition of diffusing filters placed before one or both eyes; these subjects then underwent automated stereoacuity and visual acuity tests. Results were idiosyncratic and differed from subject to subject to such an extent that no general rule sufficed to describe the effect of different degrees of blur on visual acuity and stereoacuity. It had been thought that blur degraded stereoacuity more than visual acuity but most of the results from one subject indicated the opposite effect. The effects of monocular vs. binocular blur are discussed.

Form Perception↗
PubMed1992

New developments in the application of the multimeridional apparent frontoparallel plane.

This study is a sequel to a previous publication that introduced the multimeridional apparent frontoparallel plane (MAFPP) as a method of measuring aniseikonia. As the term implies, apparent frontoparallel plane (AFPP) responses are obtained not only in the horizontal meridian, but also in oblique meridians. Since the introductory publication, the method has been applied extensively for measuring aniseikonia and has been found to be reliable, accurate, and easily adapted to clinical purposes. This study presents a more detailed and comprehensive account of the underlying theory than did the introductory report. It clarifies some problems inherent in the method originally presented and describes a simplified and improved procedure for measuring aniseikonia. It describes the application of the theory to a number of specific clinical situations. Finally, it is suggested that the method has a much broader application than the clinical measurement of aniseikonia.

Aniseikonia↗
PubMed1992

Effects of interocular suppression on spectral sensitivity.

A stable form of interocular suppression, referred to as "permanent suppression," can be produced by presenting a contoured field to one eye and a spatially homogenous field to the fellow eye, a viewing condition analogous to the classic "hole-in-the-hand" illusion. We examined the relation between permanent suppression and binocular rivalry suppression by comparing the changes in the increment-threshold spectral sensitivity function produced by these two forms of suppression. Permanent suppression produced a reduction in spectral sensitivity; however, in contrast to binocular rivalry suppression, the sensitivity alterations associated with permanent suppression were independent of the test-probe wavelength. The different patterns of sensitivity loss observed during binocular rivalry and permanent suppression indicate that different neural mechanisms mediate these two forms of interocular suppression.

Humans↗
PubMed1992

Monocular acuity in the presence and absence of fusion.

Because little is known about monocular acuity during binocular fusion, acuity of the right eye was measured in binocularly normal subjects, with the fellow eye open or fully occluded. The target was flashed for 65 ms or presented sinusoidally for 1 s. Subjects had both eyes optimally focused, or the right eye defocused by +3.00 D. Monocular acuities were equivalent during fusion or occlusion when both eyes were optimally focused. Acuity of the defocused eye was reduced when the fellow eye was open compared to when it was occluded. Suppression of the blur, therefore, produced a measurable reduction in recognition thresholds. As monocular acuity was not measurably affected by the fellow eye when each eye was in focus, the finding that monocular vision is improved with the fellow eye occluded should be ascribed to something other than suppression or more sensitive spatial measures of the suppression should be obtained.

Humans↗
PubMed1993

Acceptance of different multifocal contact lenses depending on the binocular findings.

In a clinical study the subjective acceptance as well as the quality of different visual functions was examined during the wearing of three multifocal lenses (Ciba Spectrum Bifocal, Allergan Echelon, and Essilor Variations). Ten subjects wore each of the 3 contact lens types for a period of 12 to 21 days after the binocular finding (bicentral or disparate vision within the Panum's area); further functions such as contrast or scotopic vision were qualified and/or quantified in a basic examination. All subjects (N = 5) with the binocular finding orthophoria/motorcompensated heterophoria preferred the Ciba Spectrum Bifocal. Furthermore, all visual functions of the 10 test persons were clearly reduced with all 3 multifocal contact lenses. A correlation between the "preferred contact lens and the detailed subjective acceptance" and/or the "subjective acceptance and the functional measurements" could not be proven as statistically significant.

Adult↗
PubMed1994

Interocular suppression produced by rivalry stimuli: a comparison of normal and abnormal binocular vision.

This study compares interocular suppression in subjects with early strabismus and/or anisometropia with binocular rivalry suppression in subjects with normal binocular vision. A psychophysical test-probe paradigm was used to measure the changes in luminance-increment detection thresholds associated with periods of phenomenal suppression. In subjects with normal binocular vision, rivalry suppression induced by viewing orthogonally oriented grating pairs produced a distinctive wavelength-dependent change in visual sensitivity; specifically, there was a greater reduction in sensitivity for short (e.g., 450 nm) vs. middle or long wavelength stimuli (e.g., 560 nm). In contrast, subjects with abnormal binocular vision, regardless of the type of early abnormal visual experience, showed more reduction in sensitivity for a 560-nm stimulus than for the short-wavelength, 450-nm stimulus. Moreover, the pattern of sensitivity change in subjects with abnormal binocular vision was the same for suppression induced by either rivalry stimuli or stimuli that would normally promote fusion. The results clearly indicate that interocular suppression in subjects with abnormal binocular vision is qualitatively different from normal binocular rivalry. Evidently, the processes that mediate binocular rivalry are very susceptible to environmental influences during early vision development and can be disrupted easily.

Adolescent↗