Short-term monocular patching boosts the patched eye's response in visual cortex

The interplay between Hebbian and homeostatic plasticity in the adult visual cortex

Homeostatic and Hebbian plasticity co-operate during the critical period, refining neuronal circuits; however, the interaction between these two forms of plasticity is still unclear, especially in adulthood. Here, we directly investigate this issue in adult humans using two consolidated paradigms to elicit each form of plasticity in the visual cortex: the long-term potentiation-like change of the visual evoked potential (VEP) induced by high-frequency stimulation (HFS) and the shift of ocular dominance induced by short-term monocular deprivation (MD). We tested homeostatic and Hebbian plasticity independently, then explored how they interacted by inducing them simultaneously in a group of adult healthy volunteers. We successfully induced both forms of plasticity: 60 min of MD induced a reliable change in ocular dominance and HFS reliably modulated the amplitude of the P1 component of the VEP. Importantly, we found that, across participants, homeostatic and Hebbian plasticity were negatively correlated, indicating related neural mechanisms, potentially linked to intracortical excitation/inhibition balance. On the other hand, we did not find an interaction when the two forms of plasticity were induced simultaneously. Our results indicate a largely preserved plastic potential in the visual cortex of the adult brain, for both short-term homeostatic and Hebbian plasticity. Crucially, we show for the first time a direct relationship between these two forms of plasticity in the adult human visual cortex, which could inform future research and treatment protocols for neurological diseases. KEY POINTS: Homeostatic and Hebbian plasticity co-operate during the critical period to refine neuronal circuits in the visual cortex. The interaction between these two forms of plasticity is still unknown, especially after the closure of the critical periods and in humans. We directly investigate the interplay between Hebbian and homeostatic visual plasticity in adult humans using non-invasive paradigms. We found a negative correlation between these forms of plasticity showing for the first time a direct relationship between Hebbian and homeostatic plasticity. Our results could inform future research and treatment protocols for neurological diseases.

Attention enhances short-term monocular deprivation effect

Patching one eye of an adult human for a few hours has been found to promote the dominance of the patched eye, which is called short-term monocular deprivation effect. Interestingly, recent work has reported that prolonged eye-specific attention can also cause a shift of ocular dominance toward the unattended eye though visual inputs during adaptation are balanced across the eyes. Considering that patching blocks all input information from one eye, attention is presumably deployed to the opposite eye. Therefore, the short-term monocular deprivation effect might be, in part, mediated by eye-specific attentional modulation. Yet this question remains largely unanswered. To address this issue, here we asked participants to perform an attentive tracking task with one eye patched. During the tracking, participants were presented with both target gratings (attended stimuli) and distractor gratings (unattended stimuli) that were distinct from each other in fundamental visual features. Before and after one hour of tracking, they completed a binocular rivalry task to measure perceptual ocular dominance. A larger shift of ocular dominance toward the deprived eye was observed when the binocular rivalry testing gratings shared features with the target gratings during the tracking compared to when they shared features with the distractor gratings. This result, for the first time, suggests that attention can boost the strength of the short-term monocular deprivation effect. Therefore, the present study sheds new light on the role of attention in ocular dominance plasticity.

Pupillometry indexes ocular dominance plasticity

Short-term monocular deprivation in normally sighted adult humans produces a transient shift of ocular dominance, boosting the deprived eye. This effect has been documented with both perceptual tests and through physiological recordings, but no previous study simultaneously measured physiological responses and the perceptual effects of deprivation. Here we propose an integrated experimental paradigm that combines binocular rivalry with pupillometry, to introduce an objective physiological index of ocular dominance plasticity, acquired concurrently with perceptual testing. Ten participants reported the perceptual dynamics of binocular rivalry, while we measured pupil diameter. Stimuli were a white and a black disk, each presented monocularly. Rivalry dynamics and pupil-size traces were compared before and after 2 h of monocular deprivation, achieved by applying a translucent patch over the dominant eye. Consistent with prior research, we observed that monocular deprivation boosts the deprived-eye signal and consequently increases ocular dominance. In line with previous studies, we also observed subtle but systematic modulations of pupil size that tracked alternations between exclusive dominance phases of the black or white disk. Following monocular deprivation, the amplitude of these pupil-size modulations increased, which is consistent with the post-deprivation boost of the deprived eye and the increase of ocular dominance. This provides evidence that deprivation impacts the effective strength of monocular visual stimuli, coherently affecting perceptual reports and the automatic and unconscious regulation of pupil diameter. Our results show that a combined paradigm of binocular rivalry and pupillometry gives new insights into the physiological mechanisms underlying deprivation effects.

Daily dose-response from short-term monocular deprivation in adult humans

Short-term monocular deprivation (MD) shifts sensory eye balance in favour of the previously deprived eye. The effect of MD on eye balance is significant but brief in adult humans. Recently, researchers and clinicians have attempted to implement MD in clinical settings for adults with impaired binocular vision. Although the effect of MD has been studied in detail in single-session protocols, what is not known is whether the effect of MD on eye balance deteriorates after repeated periods of MD (termed 'perceptual deterioration'). An answer to this question is relevant for two reasons. Firstly, the effect of MD (i.e., dose-response) should not decrease with repeated use if MD is to be used therapeutically (e.g., daily for weeks). Second, it bears upon the question of whether the neural basis of the effects of MD and contrast adaptation, a closely related phenomenon, is the same. The sensory change from contrast adaptation depends on recent experience. If the observer has recently experienced the same adaptation multiple times for consecutive days, then the adaptation effect will be smaller because contrast adaptation exhibits perceptual deterioration, so it is of interest to know if the effects of MD follow suit. This study measured the effect of 2-h MD for seven consecutive days on binocular balance of 15 normally sighted adults. We found that the shift in eye balance from MD stayed consistent, showing no signs of deterioration after subjects experienced multiple periods of MD. This finding shows no loss of effectiveness of repeated daily doses of MD if used therapeutically to rebalance binocular vision in otherwise normal individuals. Furthermore, ocular dominance plasticity, which is the basis of the effects of short-term MD, does not seem to share the property of 'perceptual deterioration' with contrast adaptation, suggesting different neural bases for these two related phenomena.

Adaptation of Ocular Opponency Neurons Mediates Attention-Induced Ocular Dominance Plasticity

Previous research has shown that ocular dominance can be biased by prolonged attention to one eye. The ocular-opponency-neuron model of binocular rivalry has been proposed as a candidate account for this phenomenon. Yet direct neural evidence is still lacking. By manipulating the contrast of dichoptic testing gratings, here we measured the steady-state visually evoked potentials (SSVEPs) at the intermodulation frequencies to selectively track the activities of ocular-opponency-neurons before and after the "dichoptic-backward-movie" adaptation. One hour of adaptation caused a shift of perceptual and neural ocular dominance towards the unattended eye. More importantly, we found a decrease in the intermodulation SSVEP response after adaptation, which was significantly greater when high-contrast gratings were presented to the attended eye than when they were presented to the unattended eye. These results strongly support the view that the adaptation of ocular-opponency-neurons contributes to the ocular dominance plasticity induced by prolonged eye-based attention.

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.

Temporal dynamics of normalization reweighting

For decades, neural suppression in early visual cortex has been thought to be fixed. But recent work has challenged this assumption by showing that suppression can be reweighted based on recent history; when pairs of stimuli are repeatedly presented together, suppression between them strengthens. Here we investigate the temporal dynamics of this process using a steady-state visual evoked potential (SSVEP) paradigm that provides a time-resolved, direct index of suppression between pairs of stimuli flickering at different frequencies (5 and 7 Hz). Our initial analysis of an existing electroencephalography (EEG) dataset (N = 100) indicated that suppression increases substantially during the first 2-5 seconds of stimulus presentation (with some variation across stimulation frequency). We then collected new EEG data (N = 100) replicating this finding for both monocular and dichoptic mask arrangements in a preregistered study designed to measure reweighting. A third experiment (N = 20) used source-localized magnetoencephalography and found that these effects are apparent in primary visual cortex (V1), consistent with results from neurophysiological work. Because long-standing theories propose inhibition/excitation differences in autism, we also compared reweighting between individuals with high versus low autistic traits, and with and without an autism diagnosis, across our three datasets (total N = 220). We find no compelling differences in reweighting that are associated with autism. Our results support the normalization reweighting model and indicate that for prolonged stimulation, increases in suppression occur on the order of 2-5 seconds after stimulus onset.

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.

Rapid adaptation of primate LGN neurons to drifting grating stimulation

The visual system needs to dynamically adapt to changing environments. Much is known about the adaptive effects of constant stimulation over prolonged periods. However, there are open questions regarding adaptation to stimuli that are changing over time, interrupted, or repeated. Feature-specific adaptation to repeating stimuli has been shown to occur as early as primary visual cortex (V1), but there is also evidence for more generalized, fatigue-like adaptation that might occur at an earlier stage of processing. Here, we show adaptation in the lateral geniculate nucleus (LGN) of awake, fixating monkeys following brief (1 s) exposure to repeated cycles of a 4-Hz drifting grating. We examined the relative change of each neuron's response across successive (repeated) grating cycles. We found that neurons from all cell classes (parvocellular, magnocellular, and koniocellular) showed significant adaptation. However, only magnocellular neurons showed adaptation when responses were averaged to a population response. In contrast to firing rates, response variability was largely unaffected. Finally, adaptation was comparable between monocular and binocular stimulation, suggesting that rapid LGN adaptation is monocular in nature.NEW & NOTEWORTHY Neural adaptation can be defined as reduction of spiking responses following repeated or prolonged stimulation. Adaptation helps adjust neural responsiveness to avoid saturation and has been suggested to improve perceptual selectivity, information transmission, and predictive coding. Here, we report rapid adaptation to repeated cycles of gratings drifting over the receptive field of neurons at the earliest site of postretinal processing, the lateral geniculate nucleus of the thalamus.

Is ocular dominance plasticity a special case of contrast adaptation?

The visual system can regulate its sensitivity depending on the prevailing contrast conditions. This is known as contrast adaptation and reflects contrast gain changes at different stages along the visual pathway. Recently, it has been shown that depriving an eye of visual stimulation for a short period of time can lead to neuroplastic changes in ocular dominance as the result of homeostatic changes in contrast gain. Here we examine, on the basis of previously published results, whether the neuroplastic ocular dominance changes are just manifestation of the mechanism responsible for contrast adaptation. The evidence suggests that these two visual processes are separate and do not have a common neural substrate.

Internal neural states influence the short-term effect of monocular deprivation in human adults

The adult human visual system maintains the ability to be altered by sensory deprivation. What has not been considered is whether the internal neural states modulate visual sensitivity to short-term monocular deprivation. In this study we manipulated the internal neural state and reported changes in intrinsic neural oscillations with a patched eye open or closed. We investigated the influence of eye open/eye closure on the unpatched eye's contrast sensitivity and ocular dominance (OD) shifts induced by short-term monocular deprivation. The results demonstrate that internal neural states influence not only baseline contrast sensitivity but also the extent to which the adult visual system can undergo changes in ocular dominance.

Effect of fasting on short-term visual plasticity in adult humans

Brain plasticity and function is impaired in conditions of metabolic dysregulation, such as obesity. Less is known on whether brain function is also affected by transient and physiological metabolic changes, such as the alternation between fasting and fed state. Here we asked whether these changes affect the transient shift of ocular dominance that follows short-term monocular deprivation, a form of homeostatic plasticity. We further asked whether variations in three of the main metabolic and hormonal pathways affected in obesity (glucose metabolism, leptin signalling and fatty acid metabolism) correlate with plasticity changes. We measured the effects of 2 h monocular deprivation in three conditions: post-absorptive state (fasting), after ingestion of a standardised meal and during infusion of glucagon-like peptide-1 (GLP-1), an incretin physiologically released upon meal ingestion that plays a key role in glucose metabolism. We found that short-term plasticity was less manifest in fasting than in fed state, whereas GLP-1 infusion did not elicit reliable changes compared to fasting. Although we confirmed a positive association between plasticity and supraphysiological GLP-1 levels, achieved by GLP-1 infusion, we found that none of the parameters linked to glucose metabolism could predict the plasticity reduction in the fasting versus fed state. Instead, this was selectively associated with the increase in plasma beta-hydroxybutyrate (B-OH) levels during fasting, which suggests a link between neural function and energy substrates alternative to glucose. These results reveal a previously unexplored link between homeostatic brain plasticity and the physiological changes associated with the daily fast-fed cycle.

Exercise does not enhance short-term deprivation-induced ocular dominance plasticity: evidence from dichoptic surround suppression

The input from the two eyes is combined in the brain. In this combination, the relative strength of the input from each eye is determined by the ocular dominance. Recent work has shown that this dominance can be temporarily shifted. Covering one eye with an eye patch for a few hours makes its contribution stronger. It has been proposed that this shift can be enhanced by exercise. Here, we test this hypothesis using a dichoptic surround suppression task, and with exercise performed according to American College of Sport Medicine guidelines. We measured detection thresholds for patches of sinusoidal grating shown to one eye. When an annular mask grating was shown simultaneously to the other eye, thresholds were elevated. The difference in the elevation found in each eye is our measure of relative eye dominance. We made these measurements before and after 120 min of monocular deprivation (with an eye patch). In the control condition, subjects rested during this time. For the exercise condition, 30 min of exercise were performed at the beginning of the patching period. This was followed by 90 min of rest. We find that patching results in a shift in ocular dominance that can be measured using dichoptic surround suppression. However, we find no effect of exercise on the magnitude of this shift. We further performed a meta-analysis on the four studies that have examined the effects of exercise on the dominance shift. Looking across these studies, we find no evidence for such an effect.

Mutual interaction between visual homeostatic plasticity and sleep in adult humans

Sleep and plasticity are highly interrelated, as sleep slow oscillations and sleep spindles are associated with consolidation of Hebbian-based processes. However, in adult humans, visual cortical plasticity is mainly sustained by homeostatic mechanisms, for which the role of sleep is still largely unknown. Here, we demonstrate that non-REM sleep stabilizes homeostatic plasticity of ocular dominance induced in adult humans by short-term monocular deprivation: the counterintuitive and otherwise transient boost of the deprived eye was preserved at the morning awakening (>6 hr after deprivation). Subjects exhibiting a stronger boost of the deprived eye after sleep had increased sleep spindle density in frontopolar electrodes, suggesting the involvement of distributed processes. Crucially, the individual susceptibility to visual homeostatic plasticity soon after deprivation correlated with the changes in sleep slow oscillations and spindle power in occipital sites, consistent with a modulation in early occipital visual cortex.

Aerobic Exercise and Human Visual Cortex Neuroplasticity: A Narrative Review

There is compelling evidence from animal models that physical exercise can enhance visual cortex neuroplasticity. In this narrative review, we explored whether exercise has the same effect in humans. We found that while some studies report evidence consistent with exercise-induced enhancement of human visual cortex neuroplasticity, others report no effect or even reduced neuroplasticity following exercise. Differences in study methodology may partially explain these varying results. Because the prospect of exercise increasing human visual cortex neuroplasticity has important implications for vision rehabilitation, additional research is required to resolve this discrepancy in the literature.

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.

Rapid alternate monocular deprivation does not affect binocular balance and correlation in human adults

Recent studies show that the human adult visual system exhibits neural plasticity. For instance, short-term monocular deprivation shifts the eye dominance in favor of the deprived eye. This phenomenon is believed to occur in the primary visual cortex by reinstating neural plasticity. However, it is unknown whether the changes in eye dominance after monocularly depriving the visual input can also be induced by alternately depriving both eyes. In this study, we found no changes in binocular balance and interocular correlation sensitivity after a rapid (7 Hz), alternate and monocular deprivation for one hour in adults. Therefore, the effect of short-term monocular deprivation cannot seem to be emulated by alternately and rapidly depriving both eyes.Significance statementPrevious work has shown that short-term binocular function disruption, which its most extreme form is monocular deprivation, could induce neural plasticity in adult visual system. In this study, we found a balanced deprivation of binocular function could not induce a neuroplastic change in human adults. It appears that ocular dominance plasticity in human adults is unique in so far as it is only driven by an input imbalance not balanced deprivation of binocular function.

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.

Steady-state measures of visual suppression

In the early visual system, suppression occurs between neurons representing different stimulus properties. This includes features such as orientation (cross-orientation suppression), eye-of-origin (interocular suppression) and spatial location (surround suppression), which are thought to involve distinct anatomical pathways. We asked if these separate routes to suppression can be differentiated by their pattern of gain control on the contrast response function measured in human participants using steady-state electroencephalography. Changes in contrast gain shift the contrast response function laterally, whereas changes in response gain scale the function vertically. We used a Bayesian hierarchical model to summarise the evidence for each type of gain control. A computational meta-analysis of 16 previous studies found the most evidence for contrast gain effects with overlaid masks, but no clear evidence favouring either response gain or contrast gain for other mask types. We then conducted two new experiments, comparing suppression from four mask types (monocular and dichoptic overlay masks, and aligned and orthogonal surround masks) on responses to sine wave grating patches flickering at 5Hz. At the occipital pole, there was strong evidence for contrast gain effects in all four mask types at the first harmonic frequency (5Hz). Suppression generally became stronger at more lateral electrode sites, but there was little evidence of response gain effects. At the second harmonic frequency (10Hz) suppression was stronger overall, and involved both contrast and response gain effects. Although suppression from different mask types involves distinct anatomical pathways, gain control processes appear to serve a common purpose, which we suggest might be to suppress less reliable inputs.

Some psychophysical tasks measure ocular dominance plasticity more reliably than others

In the recent decade, studies have shown that short-term monocular deprivation strengthens the deprived eye's contribution to binocular vision. However, the magnitude of the change in eye dominance after monocular deprivation (i.e., the patching effect) has been found to be different between different methods and within the same method. There are three possible explanations for the discrepancy. First, the mechanisms underlying the patching effect that are probed by different measurement tasks might exist at different neural sites. Second, the test–retest variability of the same test can produce inconsistent results. Third, the magnitude of the patching effect itself within the same observer can vary across separate days or experimental sessions. To explore these possibilities, we assessed the test–retest reliability of the three most commonly used tasks (binocular rivalry, binocular combination, and dichoptic masking) and the repeatability of the shift in eye dominance after short-term monocular deprivation for each of the task. Two variations for binocular phase combination were used, at one and many contrasts of the stimuli. Also, two variations for dichoptic masking were employed; the orientation of the mask grating was either horizontal or vertical. Thus, five different tasks were evaluated. We hoped to resolve some of the inconsistencies reported in the literature concerning this form of visual plasticity. In this study, we also aimed to recommend a measurement method that would allow us to better understand its physiological basis and the underpinning of visual disorders.

Neuroplasticity in older adults revealed by temporary occlusion of one eye

Occluding one eye for several hours alters visual experience. Specifically, occluding one eye shifts the balance of ocular dominance to favour the recently deprived eye, which can be measured using binocular rivalry. This ocular dominance shift demonstrates homeostatic neuroplasticity within the visual system and has been explored in detail in younger adults. Here we measure whether the strength and general features of neuroplasticity revealed by monocular patching are maintained in older adults. Thirty younger (18-35 years) and 30 older adults (60-81 years) participated. Binocular rivalry features were measured before and after 2 h of occlusion. Post-patching, perceptual dominance of the non-patched eye decreased (p < .001) in both age groups. The effect of occlusion on all features of binocular rivalry did not significantly differ between groups. The older visual system maintains the ability to rapidly adjust to changes in perceptual experience induced by eye occlusion. This preservation of neuroplasticity suggests that visual training methods designed to improve visual performance based on eye occlusion should maintain effectiveness into older age.

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.

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.

Bariatric surgery restores visual cortical plasticity in nondiabetic subjects with obesity

BACKGROUND/OBJECTIVES

Obesity leads to changes in synaptic plasticity. We aimed at investigating the impact of bariatric surgery (RYGB) on visual neural plasticity (NP) and its relationship with the main gut peptides, leptin, and brain-derived neurotrophic factor (BDNF).

SUBJECTS/METHODS

NP was assessed testing binocular rivalry before and after 2 h of monocular deprivation (index of visual brain plasticity) in 15 subjects with obesity (age 42.3 ± 9.8 years; BMI 46.1 ± 4.9 kg/m²) before and after RYGB. Gut peptides, leptin, and BDNF were obtained at baseline and 6 months after surgery in 13 subjects.

RESULTS

A significant reduction in BMI (p < 0.001 vs. baseline) and a significant increase of disposition index (DI, p = 0.02 vs baseline) were observed after RYGB. Total and active GLP-1 release in response to glucose ingestion significantly increased after RYGB, while no changes occurred in VIP, GIP, and BDNF levels. Fasting leptin concentration was lower after RYGB (p = 0.001 vs. baseline). Following RYGB, NP was progressively restored (p < 0.002). NP was correlated with DI and fasting glucose at baseline (r = 0.75, p = 0.01; r = -0.7, p = 0.02; respectively), but not with BMI. A positive correlation between post-pre-RYGB changes in AUC_{active GLP-1} and NP was observed (r = 0.70, p < 0.01). Leptin was inversely correlated with NP 6 months after surgery (r = -0.63, p = 0.02). No correlation was observed between GIP, VIP, BDNF, and NP.

CONCLUSIONS

Visual plasticity is altered in subjects with obesity, and it can be restored after RYGB. The improvement may be mediated by amelioration of insulin sensitivity, increased GLP-1 levels, and reduced leptin levels.

Binocular Suppression in the Macaque Lateral Geniculate Nucleus Reveals Early Competitive Interactions between the Eyes

The lateral geniculate nucleus (LGN) of the dorsal thalamus is the primary recipient of the two eyes’ outputs. Most LGN neurons are monocular in that they are activated by visual stimulation through only one (dominant) eye. However, there are both intrinsic connections and inputs from binocular structures to the LGN that could provide these neurons with signals originating from the other (non-dominant) eye. Indeed, previous work introducing luminance differences across the eyes or using a single-contrast stimulus showed binocular modulation for single unit activity in anesthetized macaques and multiunit activity in awake macaques. Here, we sought to determine the influence of contrast viewed by both the non-dominant and dominant eyes on LGN single-unit responses in awake macaques. To do this, we adjusted each eye’s signal strength by independently varying the contrast of stimuli presented to the two eyes. Specifically, we recorded LGN single unit spiking activity in two awake macaques while they viewed drifting gratings of varying contrast. We found that LGN neurons of all types [parvocellular (P), magnocellular (M), and koniocellular (K)] were significantly suppressed when stimuli were presented at low contrast to the dominant eye and at high contrast to the non-dominant eye. Further, the inputs of the two eyes showed antagonistic interaction, whereby the magnitude of binocular suppression diminished with high contrast in the dominant eye, or low contrast in the non-dominant eye. These results suggest that the LGN represents a site of precortical binocular processing involved in resolving discrepant contrast differences between the eyes.

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.

Reduced surround suppression in monocular motion perception

Motion discrimination of large stimuli is impaired at high contrast and short durations. This psychophysical result has been linked with the center-surround suppression found in neurons of area MT. Recent physiology results have shown that most frontoparallel MT cells respond more strongly to binocular than to monocular stimulation. Here we measured the surround suppression strength under binocular and monocular viewing. Thirty-nine participants took part in two experiments: (a) where the nonstimulated eye viewed a blank field of the same luminance (n = 8) and (b) where it was occluded with a patch (n = 31). In both experiments, we measured duration thresholds for small (1 deg diameter) and large (7 deg) drifting gratings of 1 cpd with 85% contrast. For each subject, a Motion Suppression Index (MSI) was computed by subtracting the duration thresholds in logarithmic units of the large minus the small stimulus. Results were similar in both experiments. Combining the MSI of both experiments, we found that the strength of suppression for binocular condition (MSIbinocular = 0.249 ± 0.126 log10 (ms)) is 1.79 times higher than under monocular viewing (MSImonocular = 0.139 ± 0.137 log10 (ms)). This increase is too high to be explained by the higher perceived contrast of binocular stimuli and offers a new way of testing whether MT neurons account for surround suppression. Potentially, differences in surround suppression reported in clinical populations may reflect altered binocular processing.

Brief period of monocular deprivation drives changes in audiovisual temporal perception

The human brain retains a striking degree of plasticity into adulthood. Recent studies have demonstrated that a short period of altered visual experience (via monocular deprivation) can change the dynamics of binocular rivalry in favor of the deprived eye, a compensatory action thought to be mediated by an upregulation of cortical gain control mechanisms. Here, we sought to better understand the impact of monocular deprivation on multisensory abilities, specifically examining audiovisual temporal perception. Using an audiovisual simultaneity judgment task, we discovered that 90 minutes of monocular deprivation produced opposing effects on the temporal binding window depending on the eye used in the task. Thus, in those who performed the task with their deprived eye there was a narrowing of the temporal binding window, whereas in those performing the task with their nondeprived eye there was a widening of the temporal binding window. The effect was short lived, being observed only in the first 10 minutes of postdeprivation testing. These findings indicate that changes in visual experience in the adult can rapidly impact multisensory perceptual processes, a finding that has important clinical implications for those patients with adult-onset visual deprivation and for therapies founded on monocular deprivation.

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.

Can Short-Term Ocular Dominance Plasticity Provide a General Index to Visual Plasticity to Personalize Treatment in Amblyopia?

Purpose

Recently, Lunghi et al. (2016) showed that amblyopic eye’s visual acuity per se after 2 months of occlusion therapy could be predicted by a homeostatic plasticity, that is, the temporary shift of perceptual eye dominance observed after a 2-h monocular deprivation, in children with anisometropic amblyopia. In this study, we assess whether the visual acuity improvement of the amblyopic eye measured after 2 months of occlusion therapy could be predicted by this plasticity.

Methods

Seven children (6.86 ± 1.46 years old; SD) with anisometropic amblyopia participated in this study. All patients were newly diagnosed and had no treatment history before participating in our study. They finished 2 months of refractive adaptation and then received a 4-h daily fellow eye patching therapy with an opaque patch for a 2-month period. Best-corrected visual acuity of the amblyopic eye was measured before and after the patching therapy. The homeostatic plasticity was assessed by measuring the temporary shift of perceptual eye dominance from 2-h occlusion of the amblyopic eye before treatment. A binocular phase combination paradigm was used for this study.

Results

We found that there was no significant correlation between the temporary shift of perceptual eye dominance observed after 2-h occlusion of the amblyopic eye and the improvement in visual acuity in the amblyopic eye from 2 months of classical patching therapy. This result, although in disagreements with the conclusions of Lunghi et al. involving the short-term patching of the amblyopic eye, is in fact consistent with a reanalysis of Lunghi and colleagues’ data.

Conclusion

The short-term changes in perceptual eye dominance as a result of short-term monocular deprivation do not provide an index of cortical plasticity in the general sense such that they are able to predict acuity outcomes from longer-term classical patching.

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.

Action Video Gaming Does Not Influence Short-Term Ocular Dominance Plasticity in Visually Normal Adults

Action video gaming can promote neural plasticity. Short-term monocular patching drives neural plasticity in the visual system of human adults. For instance, short-term monocular patching of 0.5–5 h briefly enhances the patched eye’s contribution in binocular vision (i.e., short-term ocular dominance plasticity). In this study, we investigate whether action video gaming can influence this plasticity in adults with normal vision. We measured participants’ eye dominance using a binocular phase combination task before and after 2.5 h of monocular patching. Participants were asked to play action video games, watch action video game movies, or play non-action video games during the period of monocular patching. We found that participants’ change of ocular dominance after monocular patching was not significantly different either for playing action video games versus watching action video game movies (Comparison 1) or for playing action video games versus playing non-action video games (Comparison 2). These results suggest that action video gaming does not either boost or eliminate short-term ocular dominance plasticity, and that the neural site for this type of plasticity might be in the early visual pathway.

Short-Term Deprivation Does Not Influence Monocular or Dichoptic Temporal Synchrony at Low Temporal Frequency

Studies on binocular combination and rivalry show that short-term deprivation strengthens the contribution of the deprived eye in binocular vision. However, whether short-term monocular deprivation affects temporal processing per se is not clear. To address this issue, we conducted a study to investigate the effect of monocular deprivation on dichoptic temporal synchrony. We tested ten adults with normal vision and patched their dominant eye with an opaque patch for 2.5 h. A temporal synchrony paradigm was used to measure if temporal synchrony thresholds change as a result of monocular pattern deprivation. In this paradigm, we displayed two pairs of Gaussian blobs flickering at 1 Hz with either the same or different phased- temporal modulation. In Experiment 1, we obtained the thresholds for detecting temporal asynchrony under dichoptic viewing configurations. We compared the thresholds for temporal synchrony between before and after monocular deprivation and found no significant changes of the interocular synchrony. In Experiment 2, we measured the monocular thresholds for detecting temporal asynchrony. We also found no significant changes of the monocular synchrony of either the patched eye or the unpatched eye. Our findings suggest that short-term monocular deprivation induced-plasticity does not influence monocular or dichoptic temporal synchrony at low temporal frequency.

Dynamics of Visual Perceptual Echoes Following Short-Term Visual Deprivation

The visual impulse-response function to random input as measured by EEG is dominated by the perceptual echo, a reverberation of stimulus information in the alpha range believed to represent active rhythmic sampling. How this response is generated on a cortical level is unknown. To characterize the underlying mechanisms, we investigated the echoes’ dynamics following short-term visual deprivation, which is known to modify the excitation/inhibition balance in visual cortex. We subjected observers to 150 min of light deprivation (LD) and monocular contrast deprivation (MD). Perceptual echoes were measured by binocular and dichoptic stimulation, respectively, and compared with a baseline condition. Our results show that the echo response is enhanced after LD, but not affected in temporal frequency or spatial propagation. Consistent with previous studies, MD shifted early response (0–150 ms) amplitudes in favor of the deprived eye, but had no systematic effect on the echoes. Our findings demonstrate that the echoes’ synchrony scales with cortical excitability, adding to previous evidence that they represent active visual processing. Their insensitivity to modulation at the monocular level suggests they are generated by a larger region of visual cortex. Our study provides further insight into how mechanisms of rhythmic sampling are implemented in the visual system.

Temporary monocular occlusion facilitates binocular fusion during rivalry

A few hours of monocular patching temporarily enhances the deprived eye's contribution to binocular vision, constituting a form of adult brain plasticity. Although the mechanism for this plasticity is currently unknown, several imaging studies present evidence that monocular deprivation achieves its effects by changing excitatory-inhibitory balance in the visual cortex. Much of the past work on adult monocular patching utilized binocular rivalry to quantify the patching-induced shift in perceptual eye dominance, extracting periods of exclusive visibility (in which one eye's signal is suppressed from perception) to assess each eye's contribution to binocular vision while overlooking the occurrence of mixed visibility (in which information from both eyes is combined). In this paper, we discuss two experiments to investigate the effects of short-term monocular occlusion on the relative predominance of mixed and exclusive percepts during binocular rivalry. In addition to the known perceptual eye-dominance shift, we hypothesized patching would also increase the perception of mixtures during rivalry due to deprivation-induced changes in excitatory-inhibitory balance. Our data point to two previously unknown effects of monocular deprivation: (a) a significant increase in the overall fraction and median duration of mixed visibility during rivalry that is detectable up to at least an hour after removing the patch and (b) the overall fraction of superimposition; rather than piecemeal, mixed percepts are specifically enhanced after monocular deprivation. In addition to strengthening the contribution of the deprived eye, our results show that temporary monocular patching enhances the visibility of fused binocular percepts, likely the result of attenuated interocular inhibition.

Visual Cortical Plasticity in Retinitis Pigmentosa

Purpose

Retinitis pigmentosa is a family of genetic diseases inducing progressive photoreceptor degeneration. There is no cure for retinitis pigmentosa, but prospective therapeutic strategies are aimed at restoring or substituting retinal input. Yet, it is unclear whether the visual cortex of retinitis pigmentosa patients retains plasticity to react to the restored visual input.

Methods

To investigate short-term visual cortical plasticity in retinitis pigmentosa, we tested the effect of short-term (2 hours) monocular deprivation on sensory ocular dominance (measured with binocular rivalry) in a group of 14 patients diagnosed with retinitis pigmentosa with a central visual field sparing greater than 20° in diameter.

Results

After deprivation most patients showed a perceptual shift in ocular dominance in favor of the deprived eye (P < 0.001), as did control subjects, indicating a level of visual cortical plasticity in the normal range. The deprivation effect correlated negatively with visual acuity (r = −0.63, P = 0.015), and with the amplitude of the central 18° focal electroretinogram (r = −0.68, P = 0.015) of the deprived eye, revealing that in retinitis pigmentosa stronger visual impairment is associated with higher plasticity.

Conclusions

Our results provide a new tool to assess the ability of retinitis pigmentosa patients to adapt to altered visual inputs, and suggest that in retinitis pigmentosa the adult brain has sufficient short-term plasticity to benefit from prospective therapies.

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.

Individual differences in continuous flash suppression: Potency and linkages to binocular rivalry dynamics

Binocular rivalry (BR) and continuous flash suppression (CFS) are compelling psychophysical phenomena involving interocular suppression. Using an individual differences approach we assessed whether interocular suppression induced by CFS is predictable in potency from characteristics of BR that are plausibly governed by interocular inhibition. We found large individual differences in BR dynamics and, in addition, in the strength of CFS as gauged by the incidence and durations of breakthroughs in CFS during an extended viewing periods. CFS's potency waned with repeated trials, but stable individual differences persisted despite these mean shifts. We also discovered large individual differences in the strength of the post-CFS shift in BR dominance produced by interocular suppression. While CFS breakthroughs were significantly negatively correlated with shifts in BR dominance after CFS, there were no significant associations between individual differences in alternation rate during pre-CFS binocular rivalry and either breakthroughs during CFS or post-CFS dominance shifts. Bayesian hypothesis tests and highest posterior density intervals confirmed the weak association between these two forms of interocular suppression. Thus, our findings suggest that the substantial individual differences in BR dynamics and CFS effectiveness are modestly related but not entirely mediated by one common neural substrate.

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.

Comparison of head-mounted perimeter (imo®) and Humphrey Field Analyzer

Purpose

The head-mounted automated perimeter imo^® is a new portable perimeter that does not require a dark room and can be used to examine patients in any setting. In this study, imo 24plus (1-2) AIZE examinations were compared with previous Humphrey Field Analyzer (HFA) 30-2 (SITA standard) examinations within the same patient.

Patients and methods

imo examinations (either head-mounted [i-H] or fixed [i-F] type) were performed in patients with glaucoma or suspected glaucoma who had already experienced HFA five or more times. Measurement time and correlations of mean deviation (MD) and visual field index (VFI) values were compared between groups for HFA, i-H, i-F, and imo total (i-T). Fixation loss (FL), false-positive (FP), and false-negative (FN) detection rates were compared. The percentage of binocular random single-eye tests under possible non-occlusion conditions using imo was determined. Mann–Whitney U test was performed, and Spearman’s rank-order correlation coefficient was calculated.

Results

The inclusion period was July to December 2016. Among 273 subjects (543 eyes), 147 (292 eyes) were tested with i-H type and 126 (251 eyes) with i-F type. Mean MD values for HFA and i-T were -6.1±7.8 and -6.2±7.1 dB, respectively. Mean measurement times for HFA, i-H, i-F, and i-T were 15.23±2.07, 10.47±2.11, 11.04±2.31, and 10.54±2.19 minutes, respectively (P<0.01 for HFA vs i-H/i-F). Total mean measurement time was shorter by 30.8% for i-T vs HFA. Correlation coefficients of MD and VFI were R²>0.81 for HFA vs i-H and i-F. FP and FN detection rates were significantly higher with i-T than HFA; there was no significant difference in FL. Binocular random single-eye tests were possible in 85% of cases.

Conclusion

imo reduced measurement time by 30.8%. imo VFI and MD values were highly correlated with HFA. As i-F and i-H types produced similar results, imo can be used in accordance with the patient’s situation.

A new counterintuitive training for adult amblyopia

Objectives

The aim of this study was to investigate whether short-term inverse occlusion, combined with moderate physical exercise, could promote the recovery of visual acuity and stereopsis in a group of adult anisometropic amblyopes.

Methods

Ten adult anisometropic patients underwent six brief (2 h) training sessions over a period of 4 weeks. Each training session consisted in the occlusion of the amblyopic eye combined with physical exercise (intermittent cycling on a stationary bike). Visual acuity (measured with ETDRS charts), stereoacuity (measured with the TNO test), and sensory eye dominance (measured with binocular rivalry) were tested before and after each training session, as well as in follow-up visits performed 1 month, 3 months, and 1 year after the end of the training.

Results

After six brief (2 h) training sessions, visual acuity improved in all 10 patients (0.15 ± 0.02 LogMar), and six of them also recovered stereopsis. The improvement was preserved for up to 1 year after training. A pilot experiment suggested that physical activity might play an important role for the recovery of visual acuity and stereopsis.

Conclusions

Our results suggest a noninvasive training strategy for adult human amblyopia based on an inverse-occlusion procedure combined with physical exercise.

Cholinergic Potentiation Alters Perceptual Eye Dominance Plasticity Induced by a Few Hours of Monocular Patching in Adults

A few hours of monocular deprivation with a diffuser eye patch temporarily strengthens the contribution of the deprived eye to binocular vision. This shift in favor of the deprived eye is characterized as a form of adult visual plasticity. Studies in animal and human models suggest that neuromodulators can enhance adult brain plasticity in general. Specifically, acetylcholine has been shown to improve certain aspects of visual function and plasticity in adulthood. We investigated whether a single administration of donepezil (a cholinesterase inhibitor) could further augment the temporary shift in perceptual eye dominance that occurs after 2 h of monocular patching. Twelve healthy adults completed two experimental sessions while taking either donepezil (5 mg, oral) or a placebo (lactose) pill. We measured perceptual eye dominance using a binocular phase combination task before and after 2 h of monocular deprivation with a diffuser eye patch. Participants in both groups demonstrated a significant shift in favor of the patched eye after monocular deprivation, however our results indicate that donepezil significantly reduces the magnitude and duration of the shift. We also investigated the possibility that donepezil reduces the amount of time needed to observe a shift in perceptual eye dominance relative to placebo control. For this experiment, seven subjects completed two sessions where we reduced the duration of deprivation to 1 h. Donepezil reduces the magnitude and duration of the patching-induced shift in perceptual eye dominance in this experiment as well. To verify whether the effects we observed using the binocular phase combination task were also observable in a different measure of sensory eye dominance, six subjects completed an identical experiment using a binocular rivalry task. These results also indicate that cholinergic enhancement impedes the shift that results from short-term deprivation. In summary, our study demonstrates that enhanced cholinergic potentiation interferes with the consolidation of the perceptual eye dominance plasticity induced by several hours of monocular deprivation.

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.

Chromatic and achromatic monocular deprivation produce separable changes of eye dominance in adults

Temporarily depriving one eye of its input, in whole or in part, results in a transient shift in eye dominance in human adults, with the patched eye becoming stronger and the unpatched eye weaker. However, little is known about the role of colour contrast in these behavioural changes. Here, we first show that the changes in eye dominance and contrast sensitivity induced by monocular eye patching affect colour and achromatic contrast sensitivity equally. We next use dichoptic movies, customized and filtered to stimulate the two eyes differentially. We show that a strong imbalance in achromatic contrast between the eyes, with no colour content, also produces similar, unselective shifts in eye dominance for both colour and achromatic contrast sensitivity. Interestingly, if this achromatic imbalance is paired with similar colour contrast in both eyes, the shift in eye dominance is selective, affecting achromatic but not chromatic contrast sensitivity and revealing a dissociation in eye dominance for colour and achromatic image content. On the other hand, a strong imbalance in chromatic contrast between the eyes, with no achromatic content, produces small, unselective changes in eye dominance, but if paired with similar achromatic contrast in both eyes, no changes occur. We conclude that perceptual changes in eye dominance are strongly driven by interocular imbalances in achromatic contrast, with colour contrast having a significant counter balancing effect. In the short term, eyes can have different dominances for achromatic and chromatic contrast, suggesting separate pathways at the site of these neuroplastic changes.

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.