Rebecca Camilleri

Improvement of uncorrected visual acuity and contrast sensitivity with perceptual learning and transcranial random noise stimulation in individuals with mild myopia

Perceptual learning has been shown to produce an improvement of visual acuity (VA) and contrast sensitivity (CS) both in subjects with amblyopia and refractive defects such as myopia or presbyopia. Transcranial random noise stimulation (tRNS) has proven to be efficacious in accelerating neural plasticity and boosting perceptual learning in healthy participants. In this study, we investigated whether a short behavioral training regime using a contrast detection task combined with online tRNS was as effective in improving visual functions in participants with mild myopia compared to a 2-month behavioral training regime without tRNS (Camilleri et al., 2014). After 2 weeks of perceptual training in combination with tRNS, participants showed an improvement of 0.15 LogMAR in uncorrected VA (UCVA) that was comparable with that obtained after 8 weeks of training with no tRNS, and an improvement in uncorrected CS (UCCS) at various spatial frequencies (whereas no UCCS improvement was seen after 8 weeks of training with no tRNS). On the other hand, a control group that trained for 2 weeks without stimulation did not show any significant UCVA or UCCS improvement. These results suggest that the combination of behavioral and neuromodulatory techniques can be fast and efficacious in improving sight in individuals with mild myopia.

Improving myopia via perceptual learning: is training with lateral masking the only (or the most) efficacious technique?

Perceptual learning produces an improvement in visual functions such as an increase in visual acuity (VA) and contrast sensitivity in participants with both amblyopia and refractive defects. This improvement has been observed in the presence of lateral masking, which is known to bring about lateral interactions between detectors in early cortical pathways. Improvement has also been revealed in the absence of flankers in healthy individuals and those with amblyopia. This study seeks to understand whether a perceptual training regime really needs to be based on lateral interactions in cases where poor vision is not due to cortical dysfunction, such as myopia. Ten participants with mild myopia (max –2D) were recruited. A battery of tests measuring visual function was administered prior to (pre-test) and following (post-test) the training. The participants carried out an 8-week behavioural training using a single Gabor perceptual learning paradigm, completing a total of 24 sessions. Results indicate that training using a single Gabor protocol results in a VA improvement of 0.16 logMAR. The present study supports the idea that, in the absence of cortical deficits, as is the case in myopia, some sort of compensatory mechanism can take place at the cortical level by means of perceptual learning, resulting in more effective processing of the received blurred input. However, regarding training based on lateral masking, here we found that improvement of visual functions was smaller and limited to VA. This might suggest that training based on lateral masking, which is able to modify the strength of facilitatory and inhibitory lateral interactions, could be more effective for optimal recovery of blurred vision.

Probing the involvement of the earliest levels of cortical processing in motion extrapolation with rapid forms of visual motion priming and adaptation

In this study, we investigated the effect of brief motion priming and adaptation, occurring at the earliest levels of the cortical visual stream, on time-to-contact (TTC) estimation of a target passing behind an occluder. By using different exposure times of directional motion presented in the occluder area prior to the target’s disappearance behind it, our aim was to modulate (prime or adapt) the extrapolated motion of the invisible target, thus producing different TTC estimates. Our results showed that longer (yet subsecond) exposures to motion in the same direction as the target produced late TTC estimates, whereas shorter exposures produced shorter TTC estimates, indicating that rapid forms of motion adaptation and motion priming affect extrapolated motion. Our findings suggest that motion extrapolation might occur at the earliest levels of cortical processing of motion, at which these rapid mechanisms of priming and adaptation take place.

The application of online transcranial random noise stimulation and perceptual learning in the improvement of visual functions in mild myopia

It has recently been demonstrated how perceptual learning, that is an improvement in a sensory/perceptual task upon practice, can be boosted by concurrent high-frequency transcranial random noise stimulation (tRNS). It has also been shown that perceptual learning can generalize and produce an improvement of visual functions in participants with mild refractive defects. By using three different groups of participants (single-blind study), we tested the efficacy of a short training (8 sessions) using a single Gabor contrast-detection task with concurrent hf-tRNS in comparison with the same training with sham stimulation or hf-tRNS with no concurrent training, in improving visual acuity (VA) and contrast sensitivity (CS) of individuals with uncorrected mild myopia. A short training with a contrast detection task is able to improve VA and CS only if coupled with hf-tRNS, whereas no effect on VA and marginal effects on CS are seen with the sole administration of hf-tRNS. Our results support the idea that, by boosting the rate of perceptual learning via the modulation of neuronal plasticity, hf-tRNS can be successfully used to reduce the duration of the perceptual training and/or to increase its efficacy in producing perceptual learning and generalization to improved VA and CS in individuals with uncorrected mild myopia.

Opposite effects of high- and low-frequency transcranial random noise stimulation probed with visual motion adaptation

Transcranial random noise stimulation (tRNS) is a recent neuro-modulation technique whose effects at both behavioural and neural level are still debated. Here we employed the well-known phenomenon of motion after-effect (MAE) in order to investigate the effects of high- vs. low-frequency tRNS on motion adaptation and recovery. Participants were asked to estimate the MAE duration following prolonged adaptation (20 s) to a complex moving pattern, while being stimulated with either sham or tRNS across different blocks. Different groups were administered with either high- or low-frequency tRNS. Stimulation sites were either bilateral human MT complex (hMT+) or frontal areas. The results showed that, whereas no effects on MAE duration were induced by stimulating frontal areas, when applied to the bilateral hMT+, high-frequency tRNS caused a significant decrease in MAE duration whereas low-frequency tRNS caused a significant corresponding increase in MAE duration. These findings indicate that high- and low-frequency tRNS have opposed effects on the adaptation-dependent unbalance between neurons tuned to opposite motion directions, and thus on neuronal excitability.

Differential effects of high-frequency transcranial random noise stimulation (hf-tRNS) on contrast sensitivity and visual acuity when combined with a short perceptual training in adults with amblyopia

ABSTRACT Amblyopia is a neuro‐developmental disorder characterised by several functional impairments in spatial vision even with the best optical correction. There is evidence that extensive perceptual training can improve visual acuity (VA) and contrast sensitivity (CS) in adults with amblyopia. In the present study, we assessed the efficacy of a recently developed neuro‐modulatory technique (i.e., high‐frequency transcranial random noise stimulation; hf‐tRNS) combined with a short perceptual training in adults with amblyopia. One group of ten participants underwent a short (8 sessions) monocular training in a contrast detection task with concurrent hf‐tRNS, whereas another group of ten participants underwent the same training protocol but with Sham stimulation (control group). The training consisted of a two‐interval forced choice (2IFC) contrast detection task in which participants had to detect the presence of a central Gabor patch flanked by two high‐contrast collinear Gabors (lateral masking). The results showed a significant and similar improvement of CS for both groups, suggesting that hf‐tRNS is not crucial for the improvement of CS. However, for VA, a significant improvement was only observed in the hf‐tRNS group with a mean VA improvement of 0.19 LogMAR in the amblyopic eye. Most notably, this improvement was achieved after eight training sessions. The results are discussed in terms of the influence of hf‐tRNS on short‐term neural plasticity. HIGHLIGHTSPerceptual training improves visual functions in adults with amblyopia.With a short training we improved contrast sensitivity in adults with amblyopia.When the training was coupled with hf‐tRNS improvement transferred to visual acuity.No transfer to visual acuity was observed with Sham stimulation.hf‐tRNS might boost transfer of perceptual learning to untrained visual functions.

Boosting vision through combined perceptual learning and non-invasive transcranial random noise stimulation

Introduction Perceptual learning has been shown to produce an improvement of visual functions such as an increase of visual acuity and contrast sensitivity both in participants with cortical visual defects such as amblyopia as well as refractive defects such as myopia or presbyopia. Additionally, transcranial random noise stimulation (tRNS), a type of alternating current stimulation, has proven to be efficacious in accelerating neural plasticity and boosting visual perceptual learning in healthy participants. This work investigates whether a short visual behavioural training regime using a contrast detection task, combined with online hf-tRNS, is more effective than the training plus sham stimulation, or stimulation alone, in improving visual functions (VA and CS) in participants with visual defects, specifically: mild myopia and amblyopia. Method A battery of tests measuring visual functions was administered prior to (pre-test) and following the training (post-test). Eight consecutive training sessions on a contrast detection task (combined or not with tRNS) were undertaken by the participants. A third group of participants in the myopia group underwent solely brain stimulation, in the absence of any visual training. Results The experimental groups (tRNS + PL) reached a larger improvement, with respect to the control groups, in both VA (mean improvement of 0.18 LogMAR) and CS at intermediate spatial frequencies. Conclusions These results suggest that neuromodulatory techniques can boost perceptual learning and its generalization to non-trained visual functions, strongly reducing the duration of behavioural trainings needed to improve sight in people both with non-corrected refractive defects as well as with cortical visual defects such as amblyopia.