Efficacy of perceptual learning in low vision: A systematic review and meta-analysis.

BACKGROUND: Visual perceptual learning (PL) shows promise for enhancing visual functions in individuals with visual impairment. OBJECTIVE: This systematic review aimed to evaluate the effectiveness of PL in improving visual function. STUDY ELIGIBILITY: Eligible studies were those examining the efficacy of PL in individuals with low vision. STUDY APPRAISAL AND SYNTHESIS METHODS: The review protocol was registered with the international Prospective Register of Systematic Reviews (ID CRD42022327545) and adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Screened studies were synthesized using random-effects meta-analysis and narrative synthesis following Synthesis Without Meta-analysis guidelines. The quality of the evidence was assessed using the Cochrane risk-of-bias tool and the JBI Critical Appraisal Tool for Quasi-Experimental studies. RESULTS: Fifty studies were included, covering various visual impairments and employing different PL interventions. Most studies had low risk of bias. Meta-analysis showed significant improvement in visual search for individuals with cortical blindness (Hedges' g = 0.71; 95% confidence interval, 0.48 to 0.93; p=0.002); all other analyses did not show significant improvements-reading in central vision loss and cortical blindness, and visual field in peripheral vision loss and cortical blindness. However, the narrative synthesis provided evidence showing effectiveness, particularly in individuals with central vision loss and cortical blindness, demonstrating positive effects on reading, contrast sensitivity, visual field, and motion perception. LIMITATIONS: Variations in study design, PL protocols, outcome measures, and measurement methods introduced heterogeneity, limiting the analysis. CONCLUSIONS: The efficacy of PL in vision rehabilitation remains uncertain. Although meta-analysis results were mostly inconclusive, the narrative synthesis indicated improved visual functions following PL, consistent with individual study findings. IMPLICATIONS OF KEY FINDINGS: Future research should optimize intervention parameters, explore long-term effects, and assess generalizability across diverse populations and visual impairment etiologies. Larger randomized controlled trials using standardized outcome measures are needed to advance the field.

The influence of retinal image motion on the perceptual grouping of temporally asynchronous stimuli.

Briefly presented stimuli can reveal the lower limit of retinal-based perceptual stabilization mechanisms. This is demonstrated in perceptual grouping of temporally asynchronous stimuli, in which alternate row or column elements of a regular grid are presented over two successive display frames with an imperceptible temporal offset. The grouping phenomenon results from a subtle shift between alternate grid elements due to incomplete compensation of small, fixational eye movements occurring between the two presentation frames. This suggests that larger retinal shifts should amplify the introduced shifts between alternate grid elements and improve grouping performance. However, large shifts are necessarily absent in small eye movements. Furthermore, shifts follow a random walk, making the relationship between shift magnitude and performance difficult to explore systematically. Here, we established a systematic relationship between retinal image motion and perceptual grouping by presenting alternate grid elements (untracked) during smooth pursuit of known velocities. Our results show grouping performance to improve in direct proportion to pursuit velocity. Any potential compensation by extraretinal signals (e.g., efference copy) does not seem to occur.

Eccentricity Dependency of Retinal Electrophysiological Deficits in People With Episodic Migraine.

Purpose: During the non-attack period, people with migraine may show retinal dysfunction. This study builds on previous work by exploring the possibility of foveal and non-foveal visual field and electroretinographic deficits and determining the overlap in eccentricity of such localized visual deficits in people with migraine. Methods: Visual fields and multifocal electroretinography (mf-ERG) were tested in 27 people with migraine (aged 19-45 years) and 18 non-headache controls (aged 20-46 years). Data were averaged according to 5 concentric rings at < 1.5 degrees (foveal) and 5 degrees, 10 degrees, 15.5 degrees, and 22 degrees eccentricities (non-foveal). Linear mixed effects modelling was used to predict mf-ERG amplitude, mf-ERG peak time, and visual field sensitivity with fixed effects of eye, group, and eccentricity. Results: Foveal mf-ERG responses, and visual field sensitivity across all eccentricities (foveal and non-foveal) were similar between the migraine and control groups (P > 0.05). In contrast, the non-foveal mf-ERG was reduced in amplitude in people with migraine relative to controls (P < 0.001), and this group difference depended on eccentricity (P < 0.001) - most prominently, in the parafoveal region (ring 2, P = 0.001). Conclusions: Retinal electrophysiological deficits were observed in people with migraine in the parafoveal region (between 1.5 degrees and 5 degrees eccentricity), without corresponding visual field deficits. This suggests a spatially localized area of retinal neuronal dysfunction in people with migraine that is insufficient to manifest as a visual field sensitivity loss using standard perimetric methods. Our study highlights the added confound of migraine when conducting standard clinical retinal electrophysiological tests for conditions such as glaucoma, particularly non-foveally.

Can visual cortex non-invasive brain stimulation improve normal visual function? A systematic review and meta-analysis.

Objective: Multiple studies have explored the use of visual cortex non-invasive brain stimulation (NIBS) to enhance visual function. These studies vary in sample size, outcome measures, and methodology. We conducted a systematic review and meta-analyses to assess the effects of NIBS on visual functions in human participants with normal vision. Methods: We followed the PRISMA guidelines, and a review protocol was registered with PROSPERO before study commencement (CRD42021255882). We searched Embase, Medline, PsychInfo, PubMed, OpenGrey and Web of Science using relevant keywords. The search covered the period from 1st January 2000 until 1st September 2021. Comprehensive meta-analysis (CMA) software was used for quantitative analysis. Results: Fifty studies were included in the systematic review. Only five studies utilized transcranial magnetic stimulation (TMS) and no TMS studies met our pre-specified criteria for meta-analysis. Nineteen transcranial electrical stimulation studies (tES, 38%) met the criteria for meta-analysis and were the focus of our review. Meta-analysis indicated acute effects (Hedges's g = 0.232, 95% CI: 0.023-0.442, p = 0.029) and aftereffects (0.590, 95% CI: 0.182-0.998, p = 0.005) of tES on contrast sensitivity. Visual evoked potential (VEP) amplitudes were significantly enhanced immediately after tES (0.383, 95% CI: 0.110-0.665, p = 0.006). Both tES (0.563, 95% CI: 0.230-0.896, p = 0.001) and anodal-transcranial direct current stimulation (a-tDCS) alone (0.655, 95% CI: 0.273-1.038, p = 0.001) reduced crowding in peripheral vision. The effects of tES on visual acuity, motion perception and reaction time were not statistically significant. Conclusion: There are significant effects of visual cortex tES on contrast sensitivity, VEP amplitude, an index of cortical excitability, and crowding among normally sighted individuals. Additional studies are required to enable a comparable meta-analysis of TMS effects. Future studies with robust experimental designs are needed to extend these findings to populations with vision loss. Clinical trial registration: ClinicalTrials.gov/, identifier CRD42021255882.

Selective postsaccadic enhancement of motion perception.

Saccadic eye movements can drastically affect motion perception: during saccades, the stationary surround is swept rapidly across the retina and contrast sensitivity is suppressed. However, after saccades, contrast sensitivity is enhanced for color and high-spatial frequency stimuli and reflexive tracking movements known as ocular following responses (OFR) are enhanced in response to large field motion. Additionally, OFR and postsaccadic enhancement of neural activity in primate motion processing areas are well correlated. It is not yet known how this postsaccadic enhancement arises. Therefore, we tested if the enhancement can be explained by changes in the balance of centre-surround antagonism in motion processing, where spatial summation is favoured at low contrasts and surround suppression is favoured at high contrasts. We found motion perception was selectively enhanced immediately after saccades for high spatial frequency stimuli, consistent with previously reported selective postsaccadic enhancement of contrast sensitivity for flashed high spatial frequency stimuli. The observed enhancement was also associated with changes in spatial summation and suppression, as well as contrast facilitation and inhibition, suggesting that motion processing is augmented to maximise visual perception immediately after saccades. The results highlight that spatial and contrast properties of underlying neural mechanisms for motion processing can be affected by an antecedent saccade for highly detailed stimuli and are in line with studies that show behavioural and neuronal enhancement of motion processing in non-human primates.

The influence of perceptual stabilisation on perceptual grouping of temporally asynchronous stimuli.

Even during fixation, our eyes constantly make small, involuntary eye movements that cause the retinal image to be swept across our retinae. Despite this, our world appears completely stable, due to powerful perceptual stabilisation mechanisms. Whether these mechanisms are of functional consequence for visual performance remains largely unexplored, however. We directly tested this by using a perceptual grouping task, where physically aligned alternate grid elements were presented with an imperceptible temporal offset. Observers' abilities to reliably group the grid into rows (or columns) is posited to arise from the failure in compensation of retinal slip arising from the small eye movements that occur during the temporal offset, effectively introducing a spatial shift in the arrangement of grid elements. We incorporated this perceptual grouping task within the on-line jitter illusion, which temporarily disables perceptual stabilisation mechanisms through a 10 Hz flickering annulus of random noise (Vision Res 43 (2003) 957-969). Observers' abilities to correctly group the grid stimulus were measured with and without perceptual stabilisation mechanisms engaged (i.e. non-flickering vs. flickering annulus). Grouping performance was better when eye movements were perceived, suggesting that the influence of retinal slip is increased when perceptual stabilisation mechanisms are disabled. We therefore find that perceptual stabilisation can measurably influence visual function, in addition to its perceptual effects.

Masking of random-walk motion by flicker, and its role in the allocation of motion in the on-line jitter illusion.

Typically, perceptual stabilization mechanisms make us unaware of the retinal image motion produced by the small, involuntary eye movements our eyes constantly make during fixation. The breakdown of perceptual stability is demonstrated by the on-line jitter illusion, in which a circular static pattern appears to jitter coherently when surrounded by a flickering annular pattern. Although both regions of the stimulus are subject to retinal motion from eye movements, the visual system attributes this motion to the central static region in the form of visual jitter, while the surrounding flickering region remains perceptually stable. We investigated factors influencing this allocation of motion and reference frame in the on-line jitter illusion. The flickering of the surround was found to impair the detection of simultaneous random-walk motion in this area, giving a detection reliability of around 80% for motion approximating that from fixational eye movements. Changes to spatial texture and location of flicker (centre vs. surrounding annulus) had little effect on the final percept. However, use of a nonconcentric stimulus resulted in a marked reduction in apparent jitter in all subjects. Our results suggest for the on-line jitter illusion, allocation of motion and reference frame is influenced by the general principle that, if one region surrounds another, the surrounding region tends to be allocated as the frame of reference. When this factor is controlled for, spatial textures, location of flicker, and the masking of motion by flicker have a smaller but measurable influence on the final percept.