Evaluation of visual plasticity in patients with refractive amblyopia treated using a visual perceptual learning system

BACKGROUND

Amblyopia is a neurological deficit in binocular vision that affects 3% of the population and is the result of disruptions in early visual development.

OBJECTIVE

In this study, we used a visual perceptual learning system for the short-term treatment of children with ametropic amblyopia and evaluated the clinical efficacy of this system in terms of visual plasticity.

METHODS

We conducted a retrospective analysis of the clinical data of 114 children (228 eyes) with refractive amblyopia, who were aged 6.51 ± 1.51 years. Prior to the treatment, we evaluated all children with amblyopia using the visual information processing test. We determined the type of amblyopic defect according to the type of amblyopia, corrected visual acuity, and advanced visual function test results. Based on the type of defect, each child with amblyopia was given short-term visual perception training for 10 days. Finally, we compared the results of visual acuity and visual information processing tests before and after the treatment.

RESULTS

The best-corrected visual acuity of patients was better after 10 days of visual training than that before training (P< 0.05). The perceptual eye position after training improved with statistically significant differences in horizontal and vertical perceptual eye position (both P< 0.05) compared to that before training. The number of amblyopic children without suppression in both eyes was 81 cases (71.1%) after training which was higher than that (65 cases, or 57.0%) before training, with a statistically significant difference (P< 0.05). Binocular fine stereopsis and dynamic stereopsis improved after training with a statistically significant difference (both P< 0.05).

CONCLUSION

In this study, it was found that patients with amblyopia showed visual plasticity. Moreover, continuous visual perceptual learning improved the best-corrected visual acuity and recovered stereopsis in children with refractive amblyopia.

Metaplasticity: Dark exposure boosts local excitability and visual plasticity in adult human cortex

An interlude of dark exposure for about 1 week is known to shift excitatory/inhibitory (E/I) balance of the mammalian visual cortex, promoting plasticity and accelerating visual recovery in animals that have experienced cortical lesions during development. However, the translational impact of our understanding of dark exposure from animal studies to humans remains elusive. Here, we used magnetic resonance spectroscopy as a probe for E/I balance in the primary visual cortex (V1) to determine the effect of 60 min of dark exposure, and measured binocular combination as a behavioural assay to assess visual plasticity in 14 normally sighted human adults. To induce neuroplastic changes in the observers, we introduced 60 min of monocular deprivation, which is known to temporarily shift sensory eye balance in favour of the previously deprived eye. We report that prior dark exposure for 60 min strengthens local excitability in V1 and boosts visual plasticity in normal adults. However, we show that it does not promote plasticity in amblyopic adults. Nevertheless, our findings are surprising, given the fact that the interlude is very brief. Interestingly, we find that the increased concentration of the excitatory neurotransmitter is not strongly correlated with the enhanced functional plasticity. Instead, the absolute degree of change in its concentration is related to the boost, suggesting that the dichotomy of cortical excitation and inhibition might not explain the physiological basis of plasticity in humans. We present the first evidence that an environmental manipulation that shifts cortical E/I balance can also act as a metaplastic facilitator for visual plasticity in humans. KEY POINTS: A brief interlude (60 min) of dark exposure increased the local concentration of glutamine/glutamate but not that of GABA in the visual cortex of adult humans. After dark exposure, the degree of the shift in sensory eye dominance in favour of the previously deprived eye from short-term monocular deprivation was larger than that from only monocular deprivation. The neurochemical and behavioural measures were associated: the magnitude of the shift in the concentration of glutamine/glutamate was correlated with the boost in perceptual plasticity after dark exposure. Surprisingly, the increase in the concentration of glutamine/glutamate was not correlated with the perceptual boost after dark exposure, suggesting that the physiological mechanism of how E/I balance regulates plasticity is not deterministic. In other words, an increased excitation did not unilaterally promote plasticity.

Thalamic regulation of a visual critical period and motor behavior

During the visual critical period (CP), sensory experience refines the structure and function of visual circuits. The basis of this plasticity was long thought to be limited to cortical circuits, but recently described thalamic plasticity challenges this dogma and demonstrates greater complexity underlying visual plasticity. Yet how visual experience modulates thalamic neurons or how the thalamus modulates CP timing is incompletely understood. Using a larval zebrafish, thalamus-centric ocular dominance model, we show functional changes in the thalamus and a role of inhibitory signaling to establish CP timing using a combination of functional imaging, optogenetics, and pharmacology. Hemisphere-specific changes in genetically defined thalamic neurons correlate with changes in visuomotor behavior, establishing a role of thalamic plasticity in modulating motor performance. Our work demonstrates that visual plasticity is broadly conserved and that visual experience leads to neuron-level functional changes in the thalamus that require inhibitory signaling to establish critical period timing.

Low frequency repetitive transcranial magnetic stimulation promotes plasticity of the visual cortex in adult amblyopic rats

The decline of visual plasticity restricts the recovery of visual functions in adult amblyopia. Repetitive transcranial magnetic stimulation (rTMS) has been shown to be effective in treating adult amblyopia. However, the underlying mechanisms of rTMS on visual cortex plasticity remain unclear. In this study, we found that low-frequency rTMS reinstated the amplitude of visual evoked potentials, but did not influence the impaired depth perception of amblyopic rats. Furthermore, the expression of synaptic plasticity genes and the number of dendritic spines were significantly higher in amblyopic rats which received rTMS when compared with amblyopic rats which received sham stimulation, with reduced level of inhibition and perineuronal nets in visual cortex, as observed via molecular and histological investigations. The results provide further evidence that rTMS enhances functional recovery and visual plasticity in an adult amblyopic animal model.

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.

Impact of Temporal Visual Flicker on Spatial Contrast Sensitivity in Myopia

Purpose

To investigate whether short-term exposure to high temporal frequency full-field flicker has an impact on spatial visual acuity in individuals with varying degrees of myopia.

Methods

Thirty subjects (evenly divided between control and experimental groups) underwent a 5-min exposure to full-field flicker. The flicker rate was lower than critical flicker frequency (CFF) for the experimental group (12.5 Hz) and significantly higher than CFF for the controls (60 Hz). Spatial contrast sensitivity function (CSF) was measured before and immediately after flicker exposure. We examined whether the post flicker CSF parameters were different from the pre-exposure CSF values in either of the subject groups. Additionally, we examined the relationship between the amount of CSF change from pre to post timepoints and the degree of subjects’ myopia. The CSF parameters included peak frequency, peak sensitivity, bandwidth, truncation, and area under log CSF (AULCSF).

Results

There was no significant difference of all five pre-exposure CSF parameters between the two groups at baseline (P = 0.333 ∼ 0.424). Experimental group subjects exhibited significant (P < 0.005) increases in peak sensitivity and AULCSF, when comparing post-exposure results to pre-exposure ones. Controls showed no such enhancements. Furthermore, the extent of these changes in the experimental group was correlated significantly with the participants’ refractive error (P = 0.005 and 0.018, respectively).

Conclusion

Our data suggest that exposure to perceivable high-frequency flicker (but, not to supra-CFF frequencies) enhances important aspects of spatial contrast sensitivity, and these enhancements are correlated to the degree of myopia. This finding has implications for potential interventions for cases of modest myopia.

Multimodal Neuroimaging Study of Visual Plasticity in Schizophrenia

Schizophrenia is a severe mental illness with visual learning and memory deficits, and reduced long term potentiation (LTP) may underlie these impairments. Recent human fMRI and EEG studies have assessed visual plasticity that was induced with high frequency visual stimulation, which is thought to mimic an LTP-like phenomenon. This study investigated the differences in visual plasticity in participants with schizophrenia and healthy controls. An fMRI visual plasticity paradigm was implemented, and proton magnetic resonance spectroscopy data were acquired to determine whether baseline resting levels of glutamatergic and GABA metabolites were related to visual plasticity response. Adults with schizophrenia did not demonstrate visual plasticity after family-wise error correction; whereas, the healthy control group did. There was a significant regional difference in visual plasticity in the left visual cortical area V2 when assessing group differences, and baseline GABA levels were associated with this specific ROI in the SZ group only. Overall, this study suggests that visual plasticity is altered in schizophrenia and related to basal GABA levels.

Effects of Transcranial Direct Current Stimulation on Visual Neuroplasticity in Schizophrenia

People with schizophrenia (SZ) exhibit visual processing abnormalities that affect their daily functioning and remediating these deficits might help to improve functioning. Transcranial direct current stimulation (tDCS) is a potential tool for perceptual enhancement for this purpose, though there are no reports of tDCS applied to visual cortex in SZ. In a within-subject, crossover design, we evaluated the effects of tDCS on visual processing in 27 SZ. All patients received anodal, cathodal, or sham stimulation over the central occipital region in 3 visits separated by 1 week. In each visit, a backward masking task and an electroencephalography measure of visual neuroplasticity were administered after tDCS. Neuroplasticity was assessed with visual evoked potentials before and after tetanizing visual high-frequency stimulation. Masking performance was significantly poorer in the anodal and cathodal conditions compared with sham. Both anodal and cathodal stimulation increased the amplitude of P1 but did not change the plasticity index. We found significant plasticity effects of tDCS for only one waveform for one stimulation condition (P2 for anodal tDCS) which did not survive correction for multiple comparisons. The reason for the lack of tDCS stimulation effects on plasticity may be because tDCS was not delivered simultaneously with the tetanizing visual stimulus. The present findings emphasize the need for more research on the relevant parameters for stimulation of visual processing regions in clinical populations.

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.

Selective serotonin reuptake inhibitors may lead to improved cataract surgery outcomes in patients with amblyopia

Purpose

To evaluate the effects of selective serotonin reuptake inhibitor (SSRI)/serotonin norepinephrine reuptake inhibitor (SNRI) medications in combination with cataract surgery in treating amblyopia in adult patients.

Patients and methods

A retrospective chart review study was conducted on patients who had undergone cataract surgery at the Johns Hopkins Hospital Wilmer Eye Institute. Six inclusion criteria were used to assess patient eligibility: 1) >18 years of age, 2) diagnosis of amblyopia, 3) diagnosis of cataract and treatment with surgery, 4) electronic medical record contains pre-surgery and post-surgery visual acuity (VA) measurements, 5) electronic medical record contains information on whether the patient was ever prescribed a SSRI/SNRI and the treatment duration, and 6) interocular VA difference of two lines or more on Snellen chart prior to cataract surgery. From each record, preoperative VA, postoperative VA, date of surgery, date at which postoperative VA was measured, and age at surgery were collected.

Results

A total of 237 patients were included, with 38 of them being on SSRI/SNRI. The mean improvement in VA after surgery was not significantly greater in patients on SSRI/SNRI (SSRI/SNRI: −0.276 logMAR, control: −0.192 logMAR, p=0.15). Multivariable regression was subsequently performed and while holding all other variables constant, demonstrated a statistically significant improvement in VA in patients on SSRI/SNRI (95% CI: −0.194, −0.0116, p=0.03). The regression analysis further demonstrated that advanced age has an adverse effect on the change in post-op VA (CI: 3.34×10⁻³ logMAR, 9.77×10⁻³ logMAR, p<0.005). Worse baseline VA is associated with a greater improvement in post-op VA (95% CI: −0.659 logMAR, −0.463 logMAR, p<0.005) but adverse effect on the absolute post-op VA (95% CI: 0.341 logMAR, 0.544 logMAR, p<0.005).

Conclusion

This study suggests that patients with amblyopia undergoing cataract surgery may potentially have a greater visual improvement when treated with SSRI/SNRIs.

Plasticity in the Structure of Visual Space

Visual space embodies all visual experiences, yet what determines the topographical structure of visual space remains unclear. Here we test a novel theoretical framework that proposes intrinsic lateral connections in the visual cortex as the mechanism underlying the structure of visual space. The framework suggests that the strength of lateral connections between neurons in the visual cortex shapes the experience of spatial relatedness between locations in the visual field. As such, an increase in lateral connection strength shall lead to an increase in perceived relatedness and a contraction in perceived distance. To test this framework through human psychophysics experiments, we used a Hebbian training protocol in which two-point stimuli were flashed in synchrony at separate locations in the visual field, to strengthen the lateral connections between two separate groups of neurons in the visual cortex. After training, participants experienced a contraction in perceived distance. Intriguingly, the perceptual contraction occurred not only between the two training locations that were linked directly by the changed connections, but also between the outward untrained locations that were linked indirectly through the changed connections. Moreover, the effect of training greatly decreased if the two training locations were too close together or too far apart and went beyond the extent of lateral connections. These findings suggest that a local change in the strength of lateral connections is sufficient to alter the topographical structure of visual space.

Perceptual training profoundly alters binocular rivalry through both sensory and attentional enhancements

The effects of attention, as well as its functional utility, are particularly prominent when selecting among multiple stimuli that compete for processing resources. However, existing studies have found that binocular rivalry-a phenomenon characterized by perceptual competition between incompatible stimuli presented to the two eyes-is only modestly influenced by selective attention. Here, we demonstrate that the relative resistance of binocular rivalry to selective modulations gradually erodes over the course of extended perceptual training that uses a demanding, feature-based attentional task. The final result was a dramatic alteration in binocular rivalry dynamics, leading to profound predominance of the trained stimulus. In some cases, trained observers saw the trained rival image nearly exclusively throughout 4-min viewing periods. This large change in binocular rivalry predominance was driven by two factors: task-independent, eye-specific changes in visual processing, as well as an enhanced ability of attention to promote predominance of the task-relevant stimulus. Notably, this strengthening of task-driven attention also exhibited eye specificity above and beyond that from observed sensory processing changes. These empirical results, along with simulations from a recently developed model of interocular suppression, reveal that stimulus predominance during binocular rivalry can be realized both through an eye-specific boost in processing of sensory information and through facilitated deployment of attention to task-relevant features in the trained eye. Our findings highlight the interplay of attention and binocular rivalry at multiple visual processing stages and reveal that sustained training can substantially alter early visual mechanisms.

Effects of PSA Removal from NCAM on the Critical Period Plasticity Triggered by the Antidepressant Fluoxetine in the Visual Cortex

Neuronal plasticity peaks during critical periods of postnatal development and is reduced towards adulthood. Recent data suggests that windows of juvenile-like plasticity can be triggered in the adult brain by antidepressant drugs such as Fluoxetine. Although the exact mechanisms of how Fluoxetine promotes such plasticity remains unknown, several studies indicate that inhibitory circuits play an important role. The polysialylated form of the neural cell adhesion molecules (PSA-NCAM) has been suggested to mediate the effects of Fluoxetine and it is expressed in the adult brain by mature interneurons. Moreover, the enzymatic removal of PSA by neuroaminidase-N not only affects the structure of interneurons but also has been shown to play a role in the onset of critical periods during development. We have here used ocular dominance plasticity in the mouse visual cortex as a model to investigate whether removal of PSA might influence the Fluoxetine-induced plasticity. We demonstrate that PSA removal in the adult visual cortex alters neither the baseline ocular dominance, nor the fluoxetine-induced shift in the ocular dominance. We also show that both chronic Fluoxetine treatment and PSA removal independently increase the basal FosB expression in parvalbumin (PV) interneurons in the primary visual cortex. Therefore, our data suggest that although PSA-NCAM regulates inhibitory circuitry, it is not required for the reactivation of juvenile-like plasticity triggered by Fluoxetine.

Sharpening coarse-to-fine stereo vision by perceptual learning: asymmetric transfer across the spatial frequency spectrum

Neurons in the early visual cortex are finely tuned to different low-level visual features, forming a multi-channel system analysing the visual image formed on the retina in a parallel manner. However, little is known about the potential 'cross-talk' among these channels. Here, we systematically investigated whether stereoacuity, over a large range of target spatial frequencies, can be enhanced by perceptual learning. Using narrow-band visual stimuli, we found that practice with coarse (low spatial frequency) targets substantially improves performance, and that the improvement spreads from coarse to fine (high spatial frequency) three-dimensional perception, generalizing broadly across untrained spatial frequencies and orientations. Notably, we observed an asymmetric transfer of learning across the spatial frequency spectrum. The bandwidth of transfer was broader when training was at a high spatial frequency than at a low spatial frequency. Stereoacuity training is most beneficial when trained with fine targets. This broad transfer of stereoacuity learning contrasts with the highly specific learning reported for other basic visual functions. We also revealed strategies to boost learning outcomes 'beyond-the-plateau'. Our investigations contribute to understanding the functional properties of the network subserving stereovision. The ability to generalize may provide a key principle for restoring impaired binocular vision in clinical situations.

Multiple Roles for Nogo Receptor 1 in Visual System Plasticity

During the developmental critical period for visual plasticity, discordant vision alters the responsiveness of neurons in visual cortex. The subsequent closure of the critical period not only consolidates neural function but also limits recovery of acuity from preceding abnormal visual experience. Despite species-specific differences in circuitry of the visual system, these characteristics are conserved. The nogo-66 receptor 1 (ngr1) is one of only a small number of genes identified thus far that is essential to closing the critical period. Mice lacking a functional ngr1 gene retain developmental visual plasticity as adults and their visual acuity spontaneously improves after prolonged visual deprivation. Experiments employing conditional mouse genetics have revealed that ngr1 restricts plasticity within distinct circuits for ocular dominance and visual acuity. However, the mechanisms by which NgR1 limits plasticity have not been elucidated, in part because the subcellular localization and signal transduction of the protein are only partially understood. Here we explore potential mechanisms for NgR1 function in relation to manipulations that reactivate visual plasticity in adults and propose lines of investigation to address relevant gaps in knowledge.

Pharmacology, distribution and development of muscarinic acetylcholine receptor subtypes in the optic tectum of Rana pipiens

Visually evoked behaviors mediated by the frog optic tectum require cholinergic activity, but the receptor subtypes through which acetylcholine acts are not yet identified. Using quantitative autoradiography and scintillation spectrometry, we examined the binding of [3H]pirenzepine and [3H]AF-DX 384 in the laminated optic tectum of the frog. In mammalian systems, these substances bind excitatory (m1 and m3 subtypes) and inhibitory (m2 and m4 subtypes) muscarinic acetylcholine receptors, respectively. Pharmacological analyses, including the use of specific muscarinic toxins, confirmed the subtype selectivity of the radioligands in the frog brain. Binding sites for [3H]pirenzepine were distinct from those for [3H]AF-DX 384. In the adult tectum, [3H]pirenzepine demonstrated specific binding in tectal layers 5-9. [3H]Pirenzepine binding was also present in tadpoles as young as stage V, but all sampled stages of tadpole tectum had significantly less binding when compared to adults. Lesioning of the optic nerve had no effect on [3H]pirenzepine binding. Specific [3H]AF-DX 384 binding was found in all layers of the adult tectum. All sampled tadpole stages exhibited binding sites for [3H]AF-DX 384, but the densities of these sites were also significantly higher in adults than they were in developing stages. Short-term lesions of the optic nerve reduced [3H]AF-DX 384 binding in all tectal layers of the deafferented lobe when compared to the afferented one. Long-term lesions decreased [3H]AF-DX 384 sites in both lobes.These results indicate that multiple muscarinic acetylcholine receptor binding sites reside in the frog optic tectum at all stages of development, and their pharmacology resembles that of mammalian m1/m3, m2 and m4 subtypes. Our data indicate that few, if any, of these receptors are likely to be located on retinal ganglion cell terminals. Furthermore, the expression of inhibitory muscarinic subtypes seems to be regulated by different mechanisms than that for excitatory subtypes.

Activity-dependent regulation of substance P expression and topographic map maintenance by a cholinergic pathway

We have assessed the role of activity in the adult frog visual system in modulating two aspects of neuronal plasticity: neurotransmitter expression and topographic map maintenance. Chronic treatment of one tectal lobe with the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione decreased the percentage of substance P-like immunoreactive (SP-IR) tectal cells in the untreated lobe while disrupting topographic map formation in the treated one. Treatment with the NMDA receptor antagonist d-(-)-2-amino-5-phosphonovaleric acid (d-AP-5) disrupted the topographic map but had no affect on SP-IR cells. These results indicate that maintenance of the topographic map is dependent on direct input from the glutamatergic retinal ganglion cells, whereas substance P (SP) expression is being regulated by a pathway that relays activity from one tectal lobe to the other. Such a pathway is provided by the cholinergic nucleus isthmi, which is reciprocally connected to the ipsilateral tectum and sends a projection to the contralateral one. Mecamylamine and atropine, antagonists of nicotinic and muscarinic receptors, respectively, were used together to block all cholinergic activity or alone to block receptor subclass activity. All three treatments decreased SP expression and disrupted the topographic map in the treated tectal lobe. We conclude that both SP expression and topographic map maintenance in the adult optic tectum are activity-dependent processes. Although our results are consistent with the maintenance of the topographic map through an NMDA receptor-based mechanism, they suggest that SP expression is regulated by a cholinergic interaction that depends on retinal ganglion cell input only for its activation.