Shifting eye balance using monocularly directed attention in normal vision

Monocular eye-cueing shifts eye balance in amblyopia

Here, we investigate the shift in eye balance in response to monocular cueing in adults with amblyopia. In normally sighted adults, biasing attention toward one eye, by presenting a monocular visual stimulus to it, can shift eye balance toward the stimulated eye, as measured by binocular rivalry. We investigated whether we can modulate eye balance by directing monocular stimulation/attention in adults with clinical binocular deficits associated with amblyopia and larger eye imbalances. In a dual-task paradigm, eight participants continuously reported ongoing rivalry percepts and simultaneously performed a task related to the cueing stimulus. Time series of eye balance dynamics, aligned to cue onset, are averaged across trials and participants. In different time series, we tested the effect of monocular cueing on the amblyopic and fellow eyes (compared to a binocular control condition) and the effect of an active versus passive task. Overall, we found a significant shift in eye balance toward the monocularly cued eye, when both the fellow eye or the amblyopic eye were cued, F(2, 14) = 27.649, p < 0.01, ω2 = 0.590. This was independent of whether, during the binocular rivalry, the cue stimulus was presented to the perceiving eye or the non-perceiving eye. Performing an active task tended to produce a larger eye balance change, but this effect did not reach significance. Our results suggest that the eye imbalance in adults with binocular deficits, such as amblyopia, can be transiently reduced by monocularly directed stimulation, at least through activation of bottom-up attentional processes.

Towards a principled and efficacious approach to the treatment of amblyopia. A review

There is currently a diverse array of treatment for amblyopia. In addition to the traditional penalization therapy, which has been used for over 200 years, there are not only more active treatments to recover the monocular visual loss of the amblyopic eye involving both behavioral (visual training) as well as non-invasive brain stimulation but also a variety of methods designed specifically to restore binocular function. Our understanding of visual function in general and of the etiology of the amblyopic loss in particular has progressed a great deal over the last 50 years and it is now time to take a more principled approach to how we treat, when we treat and why we treat.

Oculomotor Contributions to Foveal Crowding

Crowding, the phenomenon of impaired visual discrimination due to nearby objects, has been extensively studied and linked to cortical mechanisms. Traditionally, crowding has been studied extrafoveally; its underlying mechanisms in the central fovea, where acuity is highest, remain debated. While low-level oculomotor factors are not thought to play a role in crowding, this study shows that they are key factors in defining foveal crowding. Here, we investigate the influence of fixational behavior on foveal crowding and provide a comprehensive assessment of the magnitude and extent of this phenomenon (N = 13 human participants, four males). Leveraging on a unique blend of tools for high-precision eyetracking and retinal stabilization, we show that removing the retinal motion introduced by oculomotor behavior with retinal stabilization, diminishes the negative effects of crowding. Ultimately, these results indicate that ocular drift contributes to foveal crowding resulting in the same pooling region being stimulated both by the target and nearby objects over the course of time, not just in space. The temporal aspect of this phenomenon is peculiar to crowding at this scale and indicates that the mechanisms contributing to foveal and extrafoveal crowding differ.

Flash Suppression Reveals an Additional Nonvisual Extrastriate Contribution for Amblyopic Suppression

Purpose

A growing body of evidence suggests that anomalous binocular interactions underlie the deficits in amblyopia, but their nature and neural basis are still not fully understood.

Methods

We examined the behavioral and neural correlates of interocular suppression in 13 adult amblyopes and 13 matched controls using a flash suppression paradigm while recording steady-state visual evoked potentials. The strength of suppression was manipulated by changing the contrast (10%, 20%, 30%, or 100%) of the flash stimulus, or the suppressor, presented either in the dominant (fellow) or nondominant (amblyopic) eye.

Results

At the behavioral level, interocular suppression in normal observers was found, regardless of the eye origin of the flash onset. However, the pattern of suppression in the amblyopes was not symmetric, meaning that the suppression from the dominant eye was stronger, supporting a putative chronic suppression of the amblyopic eye. Interestingly, the amblyopic eye was able to suppress the dominant eye but only at the highest contrast level. At the electrophysiology level, suppression of the steady-state visual evoked potential responses in both groups in all conditions was similar over the occipital region, but differed over the frontal region.

Conclusions

Our findings suggest that, although suppression in amblyopia involves an imbalanced interaction between the inputs to the two eyes in the visual cortex, there is also involvement of nonvisual extrastriate areas.

Attentional eye selection modulates sensory eye dominance

Brief periods of monocular deprivation significantly modify binocular visual processing. For example, patching one eye for a few hours alters the inter-ocular balance, with the previously patched eye becoming dominant once the patch is removed. However, the contribution of higher-level visual processing to this phenomenon is still unclear. Here, we compared changes in sensory eye dominance produced by three types of monocular manipulations in adult participants with normal binocular vision. One eye was covered for 150 min using either an opaque patch, a diffusing lens, or a prism that inverted the image. All three manipulations altered dominance duration and predominance during binocular rivalry (BR) in favour of the treated eye and the time courses of the changes were similar. These results indicate that modifications of luminance or contrast are not strictly necessary to drive shifts in eye dominance, as both were unaltered in the prism condition. Next, we found that shifts in eye dominance were dependent on attentional demands during the monocular treatment period, providing support for the role of attentional eye selection in modulating eye dominance. Finally, we found relatively rapid build-up of the ocular dominance shift after the onset of monocular treatment. Taken together, our results suggest that modifications to monocular input alter inter-ocular balance via selective attentional mechanisms that bias output towards the deprived eye. Eye-based attention may play an important role in conditions where normal input to one eye is disrupted, such as childhood amblyopia.