Lixia Feng

Binocular combination in anisometropic amblyopia.

Using a suprathreshold binocular summation paradigm developed by J. Ding and G. Sperling (2006, 2007) for normal observers, we investigated suprathreshold cyclopean perception in anisometropic amblyopia. In this paradigm, two suprathreshold sinewave gratings of the same spatial frequency but different spatial phases are presented to the left and right eyes of the observer. The perceived phase of the binocularly combined cyclopean image is measured as a function of the contrast ratio between the images in the two eyes. We found that both eyes contributed equally in normal subjects. However, stimulus of equal contrast was weighted much less in the amblyopic eye relative to the fellow eye in binocular combination. For the five amblyopes, the effective contrast of the amblyopic eye in binocular combination is equal to about 11%-28% of the same contrast presented to the fellow eye, much less than the ratio of contrast sensitivity between the two eyes (0.73-1.42). The results from the current study have many important implications in amblyopia research and treatment.

qCSF in Clinical Application: Efficient Characterization and Classification of Contrast Sensitivity Functions in Amblyopia

PURPOSE The qCSF method is a novel procedure for rapid measurement of spatial contrast sensitivity functions (CSFs). It combines Bayesian adaptive inference with a trial-to-trial information gain strategy, to directly estimate four parameters defining the observers CSF. In the present study, the suitability of the qCSF method for clinical application was examined. METHODS The qCSF method was applied to rapidly assess spatial CSFs in 10 normal and 8 amblyopic participants. The qCSF was evaluated for accuracy, precision, test-retest reliability, suitability of CSF model assumptions, and accuracy of amblyopia screening. RESULTS qCSF estimates obtained with as few as 50 trials matched those obtained with 300 Ψ trials. The precision of qCSF estimates obtained with 120 and 130 trials, in normal subjects and amblyopes, matched the precision of 300 Ψ trials. For both groups and both methods, test-retest sensitivity estimates were well matched (all R > 0.94). The qCSF model assumptions were valid for 8 of 10 normal participants and all amblyopic participants. Measures of the area under log CSF (AULCSF) and the cutoff spatial frequency (cutSF) were lower in the amblyopia group; these differences were captured within 50 qCSF trials. Amblyopia was detected at an approximately 80% correct rate in 50 trials, when a logistic regression model was used with AULCSF and cutSF as predictors. CONCLUSIONS The qCSF method is sufficiently rapid, accurate, and precise in measuring CSFs in normal and amblyopic persons. It has great potential for clinical practice.

Perceptual Learning Improves Stereoacuity in Amblyopia

PURPOSE Amblyopia is a developmental disorder that results in both monocular and binocular deficits. Although traditional treatment in clinical practice (i.e., refractive correction, or occlusion by patching and penalization of the fellow eye) is effective in restoring monocular visual acuity, there is little information on how binocular function, especially stereopsis, responds to traditional amblyopia treatment. We aim to evaluate the effects of perceptual learning on stereopsis in observers with amblyopia in the current study. METHODS Eleven observers (21.1 ± 5.1 years, six females) with anisometropic or ametropic amblyopia were trained to judge depth in 10 to 13 sessions. Red-green glasses were used to present three different texture anaglyphs with different disparities but a fixed exposure duration. Stereoacuity was assessed with the Fly Stereo Acuity Test and visual acuity was assessed with the Chinese Tumbling E Chart before and after training. RESULTS Averaged across observers, training significantly reduced disparity threshold from 776.7″ to 490.4″ (P < 0.01) and improved stereoacuity from 200.3″ to 81.6″ (P < 0.01). Interestingly, visual acuity also significantly improved from 0.44 to 0.35 logMAR (approximately 0.9 lines, P < 0.05) in the amblyopic eye after training. Moreover, the learning effects in two of the three retested observers were largely retained over a 5-month period. CONCLUSIONS Perceptual learning is effective in improving stereo vision in observers with amblyopia. These results, together with previous evidence, suggest that structured monocular and binocular training might be necessary to fully recover degraded visual functions in amblyopia. Chinese Abstract.

Training in contrast detection improves motion perception of sinewave gratings in amblyopia.

PURPOSE. One critical concern about using perceptual learning to treat amblyopia is whether training with one particular stimulus and task generalizes to other stimuli and tasks. In the spatial domain, it has been found that the bandwidth of contrast sensitivity improvement is much broader in amblyopes than in normals. Because previous studies suggested the local motion deficits in amblyopia are explained by the spatial vision deficits, the hypothesis for this study was that training in the spatial domain could benefit motion perception of sinewave gratings. METHODS. Nine adult amblyopes (mean age, 22.1 ± 5.6 years) were trained in a contrast detection task in the amblyopic eye for 10 days. Visual acuity, spatial contrast sensitivity functions, and temporal modulation transfer functions (MTF) for sinewave motion detection and discrimination were measured for each eye before and after training. Eight adult amblyopes (mean age, 22.6 ± 6.7 years) served as control subjects. RESULTS. In the amblyopic eye, training improved (1) contrast sensitivity by 6.6 dB (or 113.8%) across spatial frequencies, with a bandwidth of 4.4 octaves; (2) sensitivity of motion detection and discrimination by 3.2 dB (or 44.5%) and 3.7 dB (or 53.1%) across temporal frequencies, with bandwidths of 3.9 and 3.1 octaves, respectively; (3) visual acuity by 3.2 dB (or 44.5%). The fellow eye also showed a small amount of improvement in contrast sensitivities and no significant change in motion perception. Control subjects who received no training demonstrated no obvious improvement in any measure. CONCLUSIONS. The results demonstrate substantial plasticity in the amblyopic visual system, and provide additional empirical support for perceptual learning as a potential treatment for amblyopia.

The amblyopic deficit for 2nd order processing: Generality and laterality.

A number of previous reports have suggested that the processing of second-order stimuli by the amblyopic eye (AE) is defective and that the fellow non-amblyopic eye (NAE) also exhibits an anomaly. Second-order stimuli involve extra-striate as well as striate processing and provide a means of exploring the extent of the cortical anomaly in amblyopia using psychophysics. We use a range of different second-order stimuli to investigate how general the deficit is for detecting second-order stimuli in adult amblyopes. We compare these results to our previously published adult normative database using the same stimuli and approach to determine the extent to which the detection of these stimuli is defective for both amblyopic and non-amblyopic eye stimulation. The results suggest that the second-order deficit affects a wide range of second-order stimuli, and by implication a large area of extra-striate cortex, both dorsally and ventrally. The NAE is affected only in motion-defined form judgments, suggesting a difference in the degree to which ocular dominance is disrupted in dorsal and ventral extra-striate regions.

Sensory eye balance in surgically corrected intermittent exotropes with normal stereopsis

Surgery to align a deviated or strabismic eye is often done for both functional as well as cosmetic reasons. Although amblyopia is often an impediment to regaining full binocularity in strabismics in general, intermittent exotropes, because their deviation is intermittent, have no amblyopia and some degree of stereopsis. Binocular function, including a balanced ocular dominance, could be expected to be normal after surgical correction if normal levels of stereopsis and visual acuity are postsurgically achieved. Here we used a binocular phase combination paradigm to quantitatively assess the ocular dominance in a group of surgically corrected intermittent exotropes who have normal stereo and visual acuity as defined clinically. Interestingly, we found significant interocular imbalance (balance point < 0.9) in most of the surgically treated patients (8 out 10) but in none of the controls. We conclude that the two eyes may still have a residual sensory imbalance in surgically corrected strabismus even if stereopsis is within normal limits. Our study opens the possibility that a further treatment aimed at re-balancing the ocular dominance might be necessary in surgically treated intermittent exotropia to provide more efficient binocular processing in the long term.

On the Maintenance of Normal Ocular Dominance and a Possible Mechanism Underlying Refractive Adaptation

Purpose Do humans with uncorrected anisometropia who have not developed anisometropic amblyopia exhibit a shift in ocular dominance nonetheless, reflecting a more subtle form of deprivation? Also, is such a change in dominance, if it occurs, permanent or could it be rectified by an extended period of optical correction? Methods A total of 25 normal controls (27.5 ± 2.1 years; mean ± SD); 28 anisometropes (20.7 ± 5.6 years) who were fully corrected for more than 16 weeks prior to this investigation; and 24 anisometropes who had never been corrected (21.2 ± 9.8 years) participated in this study. Sensory eye dominance of observers was measured using the binocular phase combination paradigm to find an interocular contrast ratio at which the contributions of each eye to the binocularly fused percept were equal (i.e., the balance point measure of ocular dominance). Results Controls exhibited a balance point close to unity (0.91 ± 0.05), while the two groups of anisometropes exhibited a clear binocular imbalance (uncorrected anisometropes, 0.51 ± 0.28; corrected anisometropes, 0.70 ± 0.19); both were significantly different from controls (P < 0.001). The imbalance was less severe in corrected anisometropes compared with uncorrected anisometropes (P = 0.004). Conclusions We find that anisometropia is associated with an ocular imbalance even in the absence of amblyopia. This abnormality is weaker in anisometropes who have worn an optical correction for some time, suggestive that a better optical status leads to a better binocular status.

Deficient Binocular Combination of Second-Order Stimuli in Amblyopia

PURPOSE Sensory imbalances in humans with amblyopia have been well documented using luminance-modulated (first-order) stimuli. However, little is known regarding whether there is a deficient binocular combination in amblyopes for stimuli defined by modulations in contrast (second-order stimuli). To address this, we asked two questions: Does a sensory imbalance also exist in the binocular combination of second-order stimuli, and if so, is it more severe than that expected on the basis of the imbalance for first-order stimuli? METHODS The sensory imbalances of 14 adult amblyopes (mean age: 30.5 ± 11.5 years; 5 with strabismus and 9 without) were measured using a dichoptic phase combination task. Three types of second-order dichoptic stimulus pairs were used in the study, where the carriers in the two eyes were either correlated, anticorrelated, or uncorrelated. Results were compared with those obtained using first-order stimuli in all observers. RESULTS We found that second-order binocular combination in amblyopes was not affected by the interocular carrier correlations. The amblyopic eyes contribution to the binocularly fused percept was much less than that of the nonamblyopic eye. The resulting sensory imbalance in binocular combination for second-order images was comparable to that for first-order images in 8 of the observers but was more severe in the other 6 amblyopes. CONCLUSIONS These results indicate that amblyopia does not disrupt the normal architecture of binocular combination for second-order signals; however, there is an additional deficit in binocular combination of second-order image in some amblyopes that cannot be fully accounted for by the known first-order sensory imbalance.

The effect of Lasik surgery on myopic anisometropes’ sensory eye dominance

Lasik is a common surgery for treating anisometropia. In this study, we asked a specific question: what’s the effect of Lasik surgery on anisometropes’ sensory eye dominance? Fifteen myopic anisometropes (mean age: 23 ± 6.9 years old; 6 females) participated in our experiment. We quantitatively measured participants’ sensory eye dominance before and after the Lasik surgery using a binocular phase combination paradigm. We found no significant change of sensory eye dominance within 16 weeks (measured between 8 to 96 days, for one or two repetitions) after the surgery (t(14) = −1.44, p = 0.17). A further following on eight patients showed that patients’ two eyes were much more balanced at 16 weeks or more (measured one or two times between 112 to 408 days) after the surgery (t(7) = −3.79, p = 0.007). Our results suggest that the benefit of Lasik surgery on anisometropes’ sensory eye dominance is not immediate, a long-term ‘adaptation’ period (16 weeks or more) is necessary to enable the surgery to be truly effective.

Reduced response cluster size in early visual areas explains the acuity deficit in amblyopia.

Focal visual stimulation typically results in the activation of a large portion of the early visual cortex. This spread of activity is attributed to long-range lateral interactions. Such long-range interactions may serve to stabilize a visual representation or to simply modulate incoming signals, and any associated dysfunction in long-range activation may reduce sensitivity to visual information in conditions such as amblyopia. We sought to measure the dispersion of cortical activity following local visual stimulation in a group of patients with amblyopia and matched normal. Twenty adult anisometropic amblyopes and 10 normal controls participated in this study. Using a multifocal stimulation, we simultaneously measured cluster sizes to multiple stimulation points in the visual field. We found that the functional MRI (fMRI) response cluster size that corresponded to the fellow eye was significantly larger as opposed to that corresponding to the amblyopic eye and that the fMRI response cluster size at the two more central retinotopic locations correlated with amblyopia acuity deficit. Our results suggest that the amblyopic visual cortex has a diminished long-range communication as evidenced by significantly smaller cluster of activity as measured with fMRI. These results have important implications for models of amblyopia and approaches to treatment.