Ocular dominance plasticity: inhibitory interactions and contrast equivalence

Reduced interocular suppression after inverse patching in anisometropic amblyopia

Purpose

Recent investigations observed substantial enhancements in binocular balance, visual acuity, and stereovision among older children and adults with amblyopia by patching the amblyopic eye (i.e., inverse patching) for 2 h daily over 2 months. Despite these promising findings, the precise neural mechanisms underlying inverse patching remain elusive. This study endeavors to delve deeper into the neural alterations induced by inverse patching, focusing on steady-state visual evoked potentials (SSVEPs). We specifically investigate the changes in SSVEPs following monocular deprivation of either the fellow eye or the amblyopic eye in older amblyopic children and adults.

Method

Ten participants (17.60 ± 2.03 years old; mean ± SEM), clinically diagnosed with anisometropic amblyopia, were recruited for this study. Each participant underwent a 120 min patching session on their fellow eye on the first day, followed by a similar session on their amblyopic eye on the second day. Baseline steady-state visual evoked potentials (SSVEPs) measurements were collected each day prior to patching, with post-patching SSVEPs measurements obtained immediately after the patching session. The experimental design incorporated a binocular rivalry paradigm, utilizing SSVEPs measurements.

Results

The results revealed that inverse patching induced a heightened influence on neural plasticity, manifesting in a reduction of interocular suppression from the fellow eye to the amblyopic eye. In contrast, patching the fellow eye demonstrated negligible effects on the visual cortex. Furthermore, alterations in interocular suppression subsequent to inverse patching exhibited a correlation with the visual acuity of the amblyopic eye.

Conclusion

Inverse patching emerges as a promising therapeutic avenue for adolescents and adults grappling with severe anisometropic amblyopia that proves refractory to conventional interventions. This innovative approach exhibits the potential to induce more robust neural plasticity within the visual cortex, thereby modulating neural interactions more effectively than traditional amblyopia treatments.

The Suppressive Basis of Ocular Dominance Changes Induced by Short-Term Monocular Deprivation in Normal and Amblyopic Adults

Purpose

We aimed to study the effect of short-term monocular deprivation on the suppressive interocular interactions in normals and amblyopes by using a dichoptic masking paradigm.

Methods

Nine adults with anisometropic or mixed amblyopia and 10 control adults participated in our study. The contrast sensitivity in discriminating a target Gabor dichoptically masked was measured before and after 2 hours of monocular deprivation. The mask consisted of bandpass-filtered noise. Both the target and the mask were horizontally oriented at the spatial frequency of 1.31 cpd. Deprivation was achieved using an opaque patch on the amblyopic eye of amblyopes and the dominant eye of controls.

Results

Results were similar in both controls and amblyopes. After 2 hours of monocular deprivation, the previously patched eye showed a significant increase in contrast sensitivity under dichoptic masking, which also suggested reduced suppressive effect from the nonpatched eye. Meanwhile, the contrast sensitivity of the nonpatched eye remained almost unchanged under dichoptic masking.

Conclusions

We demonstrate that the ocular dominance changes induced by short-term monocular deprivation-namely, the strengthening of the deprived eye's contribution-are associated with the unilateral and asymmetric changes in suppressive interaction. The suppression from the nondeprived eye is reduced after short-term monocular deprivation. This provides a better understanding of how inverse patching (patching of the amblyopic eye) could, by reducing the suppressive drive from the normally sighted (nondeprived) eye, form the basis of a new treatment for the binocular deficit in amblyopia.

The duration effect of short-term monocular deprivation measured by binocular rivalry and binocular combination

The ocular dominance shift observed after short-term monocular deprivation is a widely used measure of visual homeostatic plasticity in adult humans. Binocular rivalry and binocular combination techniques are used interchangeably to characterize homeostatic plasticity, sometimes leading to contradictory results. Here we directly compare the effect of short-term monocular deprivation on ocular dominance measured by either binocular rivalry or binocular combination and its dependence on the duration of deprivation (15 or 120 min) in the same group of participants. Our results show that both binocular rivalry and binocular combination provide reliable estimates of ocular dominance, which are strongly correlated across techniques both before and after deprivation. Moreover, while 15 min of monocular deprivation induce a larger shift of ocular dominance when measured using binocular combination compared to binocular rivalry, for both techniques, the shift in ocular dominance exhibits a strong dependence on the duration of monocular deprivation, with longer deprivation inducing a larger and longer-lasting shift in ocular dominance. Taken together, our results indicate that both binocular rivalry and binocular combination offer very consistent and reliable measurements of both ocular dominance and the effect short-term monocular deprivation.

Short-term homeostatic visual neuroplasticity in adolescents after two hours of monocular deprivation

In healthy adults with normal vision, temporary deprivation of one eye's visual experience produces transient yet robust homeostatic plasticity effects, where the deprived eye becomes more dominant. This shift in ocular dominance is short-lived and compensatory. Previous work shows that monocular deprivation decreases resting state gamma aminobutyric acid (GABA; inhibitory neurotransmitter) levels in visual cortex, and that those with the greatest reduction in GABA have stronger shifts due to monocular deprivation. Components of the GABAergic system in visual cortex vary with age (early childhood, early teen years, ageing); hence if GABA is critical to homeostatic plasticity within the visual system, adolescence may be a key developmental period where differences in plasticity manifest. Here we measured short-term visual deprivation effects on binocular rivalry in 24 adolescents (aged 10-15 years) and 23 young adults (aged 20-25 years). Despite differences in baseline features of binocular rivalry (adolescents showed more mixed percept p < 0.001 and a tendency for faster switching p = 0.06 compared to adults), deprived eye dominance increased (p = 0.01) similarly for adolescents and adults after two hours of patching. Other aspects of binocular rivalry - time to first switch (heralding the onset of rivalry) and mixed percept - were unaltered by patching. These findings suggest that binocular rivalry after patching can be used as a behavioral proxy for experience-dependent visual cortical plasticity in adolescents in the same way as adults, and that homeostatic plasticity to compensate for temporarily reduced visual input is established and effective by adolescence.

The shift in sensory eye dominance from short-term monocular deprivation exhibits no dependence on test spatial frequency

Background

Studies have shown that short-term monocular deprivation induces a shift in sensory eye dominance in favor of the deprived eye. Yet, how short-term monocular deprivation modulates sensory eye dominance across spatial frequency is not clear. To address this issue, we conducted a study to investigate the dependence of short-term monocular deprivation effect on test spatial frequency.

Methods

Ten healthy young adults (age: 24.7 ± 1.7 years, four males) with normal vision participated. We deprived their dominant eye with a translucent patch for 2.5 h. The interocular contrast ratio (dominant eye/non-dominant eye, i.e., the balance point [BP]), which indicates the contribution that the two eyes make to binocular combination, was measured using a binocular orientation combination task. We assessed if BPs at 0.5, 4 or 6 cycles/degree (c/d) change as a result of monocular deprivation. Different test spatial frequency conditions were conducted on three separate days in a random fashion.

Results

We compared the BPs at 0.5, 4 and 6 c/d before and after monocular deprivation. The BPs were found to be significantly affected by deprivation, where sensory eye dominance shift to the deprived eye (F_{1.86, 16.76} = 33.09, P < 0.001). The changes of BP were consistent at 0.5, 4, and 6 c/d spatial frequencies (F_2,18 = 0.15, P = 0.57).

Conclusion

The sensory eye dominance plasticity induced by short-term deprivation is not dependent on test spatial frequency, suggesting it could provide a practical solution for amblyopic therapy that was concerned with the binocular outcome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40662-022-00303-4.

The Mechanism of Short-Term Monocular Pattern Deprivation-Induced Perceptual Eye Dominance Plasticity

Previously published studies have reported that 150 min of short-term monocular deprivation temporarily changes perceptual eye dominance. However, the possible mechanisms underlying monocular deprivation-induced perceptual eye dominance plasticity remain unclear. Using a binocular phase and contrast co-measurement task and a multi-pathway contrast-gain control model (MCM), we studied the effect of 150 min of monocular pattern deprivation (MPD) in normal adult subjects. The perceived phase and contrast varied significantly with the interocular contrast ratio, and after MPD, the patched eye (PE) became dominant. Most importantly, we focused on the potential mechanisms of the deprivation effect. The data of an averaged subject was best fitted by a model, which assumed a monocular signal enhancement of the PE after the MPD. The present findings might have important implications for investigations of binocular vision in both normal and amblyopic populations.

Short-term plasticity in the human visual thalamus

While there is evidence that the visual cortex retains a potential for plasticity in adulthood, less is known about the subcortical stages of visual processing. Here we asked whether short-term ocular dominance plasticity affects the human visual thalamus. We addressed this question in normally sighted adult humans, using ultra-high field (7T) magnetic resonance imaging combined with the paradigm of short-term monocular deprivation. With this approach, we previously demonstrated transient shifts of perceptual eye dominance and ocular dominance in visual cortex (Binda et al., 2018). Here we report evidence for short-term plasticity in the ventral division of the pulvinar (vPulv), where the deprived eye representation was enhanced over the non-deprived eye. This ventral-pulvinar plasticity was similar as previously seen in visual cortex and it was correlated with the ocular dominance shift measured behaviorally. In contrast, there was no effect of monocular deprivation in two adjacent thalamic regions: dorsal pulvinar (dPulv), and Lateral Geniculate Nucleus (LGN). We conclude that the visual thalamus retains potential for short-term plasticity in adulthood; the plasticity effect differs across thalamic subregions, possibly reflecting differences in their cortico-fugal connectivity.

Short-term monocular deprivation induces an interocular delay

Short term monocular deprivation modulates ocular dominance, such that the previously deprived eye's contribution to the binocular percept increases, supposedly as a result of changes in contrast-gain. Therefore, the processing time of the previously patched eye would be expected to speed up as a result of an increase in contrast gain. In order to test this hypothesis, this study examines the effects of short-term monocular deprivation on interocular synchronicity. The present study uses a paradigm based on the Pulfrich phenomenon. The stimulus used for testing consists of elements defining a cylinder rotating in depth, that allows measurement of any interocular delay. The interocular delay was measured at baseline before patching and at outcome, after one hour of monocular deprivation with an opaque or translucent patch. Contrary to expectations, short-term monocular deprivation induces an interocular delay, albeit not always significant, in the previously patched eye. The amplitude of this effect is larger with opaque patching compared to translucent patching. These results are the first report of a non-beneficial effect - i.e. a slowing down in the processing time of the previously patched-eye. They indicate that the plasticity effects of monocular deprivation are not exclusively mediated by contrast gain mechanisms and that light adaptation mechanisms might also be involved in the plasticity resulting from short-term monocular deprivation.

The ups and downs of sensory eye balance: Monocular deprivation has a biphasic effect on interocular dominance

Classic studies of ocular dominance plasticity in early development showed that monocular deprivation suppresses the neural representation and visual function of the deprived eye. However, recent studies have shown that a short period of monocular deprivation (<3 h) in normal adult humans, shifts sensory eye dominance in favor of the deprived eye. How can these opposing effects be reconciled? Here we argue that there are two systems acting in opposition at different time scales. A fast acting, stabilizing, homeostatic system that rapidly decreases gain in the non-deprived eye or increases gain in the deprived eye, and a relatively sluggish system that shifts balance toward the non-deprived eye, in an effort to reduce input of little utility to active vision. If true, then continuous deprivation should produce a biphasic effect on interocular balance, first shifting balance away from the non-deprived eye, then towards it. Here we investigated the time course of the deprivation effect by monocularly depriving typical adults for 10 h and conducting tests of sensory eye balance at six intervening time points. Consistent with previous short-term deprivation work, we found shifts in sensory eye dominance away from the non-deprived eye up until approximately 5 h. We then observed a turning point, with balance shifting back towards the non-deprived eye, -, a biphasic effect. We argue that this turning point marks where the rapid homeostatic response saturates and is overtaken by the slower system responsible for suppressing monocular input of limited utility.

The role of astrocyte-mediated plasticity in neural circuit development and function

Neuronal networks are capable of undergoing rapid structural and functional changes called plasticity, which are essential for shaping circuit function during nervous system development. These changes range from short-term modifications on the order of milliseconds, to long-term rearrangement of neural architecture that could last for the lifetime of the organism. Neural plasticity is most prominent during development, yet also plays a critical role during memory formation, behavior, and disease. Therefore, it is essential to define and characterize the mechanisms underlying the onset, duration, and form of plasticity. Astrocytes, the most numerous glial cell type in the human nervous system, are integral elements of synapses and are components of a glial network that can coordinate neural activity at a circuit-wide level. Moreover, their arrival to the CNS during late embryogenesis correlates to the onset of sensory-evoked activity, making them an interesting target for circuit plasticity studies. Technological advancements in the last decade have uncovered astrocytes as prominent regulators of circuit assembly and function. Here, we provide a brief historical perspective on our understanding of astrocytes in the nervous system, and review the latest advances on the role of astroglia in regulating circuit plasticity and function during nervous system development and homeostasis.

Brief localised monocular deprivation in adults alters binocular rivalry predominance retinotopically and reduces spatial inhibition

Short-term deprivation (2.5 h) of an eye has been shown to boost its relative ocular dominance in young adults. Here, we show that a much shorter deprivation period (3–6 min) produces a similar paradoxical boost that is retinotopic and reduces spatial inhibition on neighbouring, non-deprived areas. Partial deprivation was conducted in the left hemifield, central vision or in an annular region, later assessed with a binocular rivalry tracking procedure. Post-deprivation, dominance of the deprived eye increased when rivalling images were within the deprived retinotopic region, but not within neighbouring, non-deprived areas where dominance was dependent on the correspondence between the orientation content of the stimuli presented in the deprived and that of the stimuli presented in non-deprived areas. Together, these results accord with other deprivation studies showing V1 activity changes and reduced GABAergic inhibition.

Short-term monocular deprivation reduces inter-ocular suppression of the deprived eye

The adult visual system was traditionally thought to be relatively hard-wired, but recent studies have challenged this view by demonstrating plasticity following short-term monocular deprivation. Depriving one eye of spatial information for 2-3 h increased subsequent sensory dominance of that eye. However, the mechanism underlying this phenomenon is unclear. The present study sought to address this issue and determine the consequences of short-term monocular deprivation on inter-ocular suppression of each eye. Sensory eye dominance was examined before and after depriving an eye of all input using an opaque patch for 2.5 h, in six adult participants with normal binocular vision. We used a percept tracking task during binocular rivalry (BR) to assess the relative eye dominance, and an objective probe detection task under continuous flash suppression (CFS) to quantify each eye's susceptibility to inter-ocular suppression. The monocular contrast increment threshold of each eye was also measured using the probe task to ascertain if the altered eye dominance is accompanied by changes in monocular perception. Our BR results replicated previous findings of a shift of relative dominance towards the eye that has been deprived of form information. More crucially, using CFS we demonstrated reduced inter-ocular suppression of the deprived eye with no complementary changes in the other eye, and no monocular changes in increment threshold. These findings imply that short-term monocular deprivation alters binocular interactions. The differential effect on inter-ocular suppression between eyes may have important implications for the use of patching as a therapy to recover visual function in amblyopia.

An Unexpected Spontaneous Motion-In-Depth Pulfrich Phenomenon in Amblyopia

The binocular viewing of a fronto-parallel pendulum with a reduced luminance in one eye results in the illusory tridimensional percept of the pendulum following an elliptical orbit in depth, the so-called Pulfrich phenomenon. A small percentage of mild anisometropic amblyopes who have rudimentary stereo are known to experience a spontaneous Pulfrich phenomenon, which posits a delay in the cortical processing of information involving their amblyopic eye. The purpose of this study is to characterize this spontaneous Pulfrich phenomenon in the mild amblyopic population. In order to assess this posited delay, we used a paradigm where a cylinder rotating in depth, defined by moving Gabor patches at different disparities (i.e., at different interocular phases), generates a strong to ambiguous depth percept. This paradigm allows one to accurately measure a spontaneous Pulfrich phenomenon and to determine how it depends on the spatio-temporal properties of stimulus. We observed a spontaneous Pulfrich phenomenon in anisometropic, strabismic, and mixed amblyopia, which is posited to be due to an interocular delay associated with amblyopic processing. Surprisingly, the posited delay was not always observed in the amblyopic eye, was not a consequence of the reduced contrast sensitivity of the amblyopic eye, and displayed a large variability across amblyopic observers. Increasing the density, decreasing the spatial frequency, or increasing the speed of the stimulus tended to reduce the observed delay. The spontaneous Pulfrich phenomenon seen by some amblyopes was variable and depended on the spatio-temporal properties of the stimulus. We suggest it could involve two conflicting components: an amblyopic delay and a blur-based acceleration.

Visual plasticity and exercise revisited: No evidence for a "cycling lane"

Experiments using enriched environments have shown that physical exercise modulates visual plasticity in rodents. A recent study (Lunghi & Sale, 2015) investigated whether exercise also affects visual plasticity in adult humans. The plastic effect they measured was the shift in ocular dominance caused by 2 hr of monocular deprivation (e.g., by an eye patch). They used a binocular rivalry task to measure this shift. They found that the magnitude of the shift was increased by exercise during the deprivation period. This effect of exercise was later disputed by a study that used a different behavioral task (Zhou, Reynaud, & Hess, 2017). Our goal was to determine whether the difference in task was responsible for that study's failure to find an exercise effect. We set out to replicate Lunghi and Sale (2015). We measured ocular dominance with a rivalry task before and after 2 hr of deprivation. We measured data from two conditions in 30 subjects. On two separate days, they either performed exercise or rested during the deprivation period. Contrary to the previous study, we find no significant effect of exercise. We hypothesize that exercise may affect rivalry dynamics in a way that interacts with the measurement of the deprivation effect.

Inverse Occlusion: A Binocularly Motivated Treatment for Amblyopia

Recent laboratory findings suggest that short-term patching of the amblyopic eye (i.e., inverse occlusion) results in a larger and more sustained improvement in the binocular balance compared with normal controls. In this study, we investigate the cumulative effects of the short-term inverse occlusion in adults and old children with amblyopia. This is a prospective cohort study of 18 amblyopes (10-35 years old; 2 with strabismus) who have been subjected to 2 hours/day of inverse occlusion for 2 months. Patients who required refractive correction or whose refractive correction needed updating were given a 2-month period of refractive adaptation. The primary outcome measure was the binocular balance which was measured using a phase combination task; the secondary outcome measures were the best-corrected visual acuity which was measured with a Tumbling E acuity chart and converted to logMAR units and the stereoacuity which was measured with the Random-dot preschool stereogram test. The average binocular gain was 0.11 in terms of the effective contrast ratio (z = -2.344, p = 0.019, 2-tailed related samples Wilcoxon Signed Rank Test). The average acuity gain was 0.13 logMAR equivalent (t(17) = 4.76, p < 0.001, 2-tailed paired samples t-test). The average stereoacuity gain was 339 arc seconds (z = -2.533, p = 0.011). Based on more recent research concerning adult ocular dominance plasticity, we conclude that inverse occlusion in adults and old children with amblyopia does produce long-term gains to binocular balance and that acuity and stereopsis can improve in some subjects.

The shift in ocular dominance from short-term monocular deprivation exhibits no dependence on duration of deprivation

Deprivation of visual information from one eye for a 120-minute period in normal adults results in a temporary strengthening of the patched eye's contribution to binocular vision. This plasticity for ocular dominance in adults has been demonstrated by binocular rivalry as well as binocular fusion tasks. Here, we investigate how its dynamics depend on the duration of the monocular deprivation. Using a binocular combination task, we measure the magnitude and recovery of ocular dominance change after durations of monocular deprivation ranging from 15 to 300 minutes. Surprisingly, our results show that the dynamics are of an all-or-none form. There was virtually no significant dependence on the duration of the initial deprivation.

How Ocular Dominance and Binocularity Are Reflected by the Population Receptive Field Properties

Purpose

The neural substrate of binocularity and sighting ocular dominance in humans is not clear. By utilizing the population receptive field (pRF) modeling technique, we explored whether these phenomena are associated with amplitude and pRF size differences.

Methods

The visual field maps of 13 subjects were scanned (3-T Skyra) while viewing drifting bar stimuli. Both eyes (binocular condition), the dominant eye and the nondominant eye (two monocular conditions) were stimulated in separate sessions. For each condition, pRF size and amplitude were assessed. Binocular summation ratios were calculated by dividing binocular by mean monocular values (amplitude and pRF size).

Results

No differences in pRF size were seen between the viewing conditions within each region, that is, either between binocular and monocular or between dominant and nondominant viewing conditions. Binocular amplitudes were higher than the monocular amplitudes, but similar among the dominant and nondominant eyes. Binocular summation ratios derived from amplitudes were significantly higher than one (∼1.2), while those ratios derived from pRF size were not. These effects were found in all studied areas along the visual hierarchy, starting in V1.

Conclusions

Neither the amplitude nor the pRF size show intereye difference and therefore cannot explain the different roles of the dominant and the nondominant eyes. Binocular, as compared to monocular vision, resulted in higher amplitudes, while receptive fields' sizes were similar, suggesting increased binocular response intensity as the basis for the binocular summation phenomenon. Our results could be applicable in imaging studies of monocular disease and studies that deal with nondisparity binocularity effects.

Interocular correlation sensitivity and its relationship with stereopsis

Stereoscopic vision uses the disparity between the images received by the two eyes to derive three-dimensional estimates. Here, we were interested in providing a measure of the strength of binocular vision alternate to disparity processing. In particular, we wanted to assess the spatial dependence of sensitivity to detect interocular correlation (IOC). Thus we designed dichoptic stimuli composed of bandpass textures whose IOC is sinusoidally modulated at different correlation frequencies and compared sensitivity to these stimuli to that of analogous stimuli modulated in disparity. We observed that the IOC sensitivity is low pass/band pass and increases with stimulus duration and contrast in a similar way to that of disparity sensitivity. IOC sensitivity is only weakly, though significantly, correlated with disparity sensitivity in the population. It could provide an alternate measure of binocular sensitivity.

The mechanism of short-term monocular deprivation is not simple: separate effects on parallel and cross-oriented dichoptic masking

Short-term deprivation of the input to one eye increases the strength of its influence on visual perception. This effect was first demonstrated using a binocular rivalry task. Incompatible stimuli are shown to the two eyes, and their competition for perceptual dominance is then measured. Further studies used a combination task, which measures the contribution of each eye to a fused percept. Both tasks show an effect of deprivation, but there have been inconsistencies between them. This suggests that the deprivation causes multiple effects. We used dichoptic masking to explore this possibility. We measured the contrast threshold for detecting a grating stimulus presented to the target eye. Thresholds were elevated when a parallel or cross-oriented grating mask was presented to the other eye. This masking effect was reduced by depriving the target eye for 150 minutes. We tested fourteen subjects with normal vision, and found individual differences in the magnitude of this reduction. Comparing the reduction found in each subject between the two masks (parallel vs. cross-oriented), we found no correlation. This indicates that there is not a single underlying effect of short-term monocular deprivation. Instead there are separate effects which can have different dependencies, and be probed by different tasks.

Interocular contrast difference drives illusory 3D percept

Any processing delay between the two eyes can result in illusory 3D percepts for moving objects because of either changes in the pure disparities over time for disparity sensors or by changes to sensors that encode motion/disparity conjointly. This is demonstrated by viewing a fronto-parallel pendulum through a neutral density (ND) filter placed over one eye, resulting in the illusory 3D percept of the pendulum following an elliptical orbit in depth, the so-called Pulfrich phenomenon. Here we use a paradigm where a cylinder rotating in depth, defined by moving Gabor patches is presented at different interocular phases, generating strong to ambiguous depth percepts. This paradigm allows one to manipulate independently the contrast and the luminance of the patches to determine their influence on perceived motion-in-depth. Thus we show psychophysically that an interocular contrast difference can itself result in a similar illusory 3D percept of motion-in-depth. We argue that contrast, like luminance (ND filter) can modify the dynamics of visual neurons resulting in an interocular processing delay or an interocular velocity difference.

Short-term monocular occlusion produces changes in ocular dominance by a reciprocal modulation of interocular inhibition

Ocular dominance can be modulated by short-term monocular deprivation. This changes the contribution that each eye makes to binocular vision, an example of adult cortical neuroplasticity. Optical imaging in primates and psychophysics in humans suggest these neuroplastic changes occur in V1. Here we use brain imaging (MEG) in normal adults to better understand the nature of these neuroplastic changes. The results suggest that short-term monocular deprivation, whether it be by an opaque or translucent patch, modulates dichoptic inhibitory interactions in a reciprocal fashion; the unpatched eye is inhibited, the patched eye is released from inhibition. These observations locate the neuroplastic changes to a level of visual processing where there are interocular inhibitory interactions prior to binocular combination and help to explain why both binocular rivalry and fusional tasks reveal them.