Modulation of cortical excitability can speed up blindsight but not improve it

How to test blindsight without light-scatter artefacts?

Light-scatter artefacts are a methodological problem in testing residual visual capacities (RVCs), for instance blindsight, in patients with homonymous visual field defects (HVFDs). The term light-scatter artefact describes the phenomenon that light from targets directed towards the HVFD can stray into the sighted visual field. This might enable an observer to respond correctly to information directed at her blind field despite the fact that she is unable to process that information in the blind field itself. In this manuscript, we present a review of the relevance of light-scatter in visual neuroscience, discuss factors that influence the impact of light-scatter and evaluate means to test for light-scatter artefacts. Furthermore, we present findings from an empirical study that was aimed at developing tests for RVCs that are free of light-scatter artefacts. Previous studies on light scatter only used small sample sizes and equipment that is no longer in use. Hence, their results cannot be generalized to future experiments making it necessary to run laborious light-scatter tests for every new study on RVCs. To avoid this, we hereby start a pool of stimuli and paradigms which demonstrably do not elicit light-scatter artefacts. To this end, we investigated 21 healthy young participants in three frequently used RVC-paradigms: (1) temporal 2AFC task, (2) movement direction discrimination, and (3) redundant target paradigm. For each paradigm, we applied the blind-spot method. But first, we had to establish that our testing paradigm was sufficiently sensitive to detect light-scatter artefacts. For this, we used conditions that are known to produce strong light scatter and a paradigm that is very sensitive to such effects. Specifically, we presented white targets on a black background in a dark room. The stimuli were presented to observers' blind spot. To check for light-scatter artefacts, we used a target-detection task in a temporal 2AFC format. We obtained clear light-scatter artefacts. Participants produced reliably above-chance detection performance under these conditions. The other two luminance conditions, measured in an illuminated room, did not produce light-scatter artefacts. Accuracy in the temporal 2AFC task was at chance level for white targets on a grey background at the blind-spot position. Additionally, black targets on a grey background avoided light-scatter artefacts in all three RVC-paradigms. In future, researchers can apply these stimulus and illumination conditions when using one of the three above paradigms in their studies. Using these conditions, they will be able to avoid light-scatter artefacts without having to perform their own blind-spot tests.

Non-invasive current stimulation in vision recovery: a review of the literature

Background:

Around 253 million people worldwide suffer from irreversible visual damage. Numerous studies have been carried out in order to unveil the effects of electrical stimulation (ES) as a useful tool for rehabilitation for different visual conditions and pathologies.

Objective:

This systematic review aimed to 1) examine the current evidence of ES efficacy for the treatment of visual pathologies and 2) define the corresponding degree of the recommendation of different ES techniques.

Methods:

A systematic review was conducted in MEDLINE and Cochrane Library database to collect documents published between 2000 and 2018. For each study, Level of Evidence of Effectiveness of ES as well as the Class of Quality for the treatment of different visual pathologies were determined.

Results:

Thirty-eight articles were included. Studies were grouped according to the pathology treated and the type of stimulation administered. The first group included studies treating pre-chiasmatic pathologies (age-related macular degeneration, macular dystrophy, retinal artery occlusion, retinitis pigmentosa, glaucoma, optic nerve damage, and optic neuropathy) using pre-chiasmatic stimulation; the second group included studies treating both pre-chiasmatic pathologies (amblyopia, myopia) and post-chiasmatic pathologies or brain conditions (hemianopsia, brain trauma) by means of post-chiasmatic stimulation. In the first group, repetitive transorbital alternating current stimulation (rtACS) reached level A recommendation, and transcorneal electrical stimulation (tcES) reached level B. In the second group, both high-frequency random noise stimulation (hf-RNS) and transcranial direct current stimulation (tDCS) reached level C recommendation.

Conclusions:

Study’s findings suggest conclusive evidence for rtACS treatment. For other protocols results are promising but not conclusive since the examined studies assessed different stimulation parameters and endpoints. A comparison of the effects of different combinations of these variables still lacks in the literature. Further studies are needed to optimize existing protocols and determine if different protocols are needed for different diseases.

Transcranial direct current stimulation (tDCS) combined with blindsight rehabilitation for the treatment of homonymous hemianopia: a report of two-cases

[Purpose] Homonymous hemianopia is one of the most common symptoms following neurologic damage leading to impairments of functional abilities and activities of daily living. There are two main types of restorative rehabilitation in hemianopia: "border training" which involves exercising vision at the edge of the damaged visual field, and "blindsight training," which is based on exercising the unconscious perceptual functions deep inside the blind hemifield. Only border effects have been shown to be facilitated by transcranial direct current stimulation (tDCS). This pilot study represents the first attempt to associate the modulatory effects of tDCS over the parieto-occipital cortex to blindsight treatment in the rehabilitation of the homonymous hemianopia. [Subjects and Methods] Patients TA and MR both had chronic hemianopia. TA underwent blindsight treatment which was combined with tDCS followed by blindsight training alone. MR underwent the two training rounds in reverse order. [Results] The patients showed better scores in clinical-instrumental, functional, and ecological assessments after tDCS combined with blindsight rehabilitation rather than rehabilitation alone. [Conclusion] In this two-case report parietal-occipital tDCS modulate the effects induced by blindsight treatment on hemianopia.

Improved reading measures in adults with dyslexia following transcranial direct current stimulation treatment

To better understand the contribution of the dorsal system to word reading, we explored transcranial direct current stimulation (tDCS) effects when adults with developmental dyslexia received active stimulation over the visual extrastriate area MT/V5, which is dominated by magnocellular input. Stimulation was administered in 5 sessions spread over two weeks, and reading speed and accuracy as well as reading fluency were assessed before, immediately after, and a week after the end of the treatment. A control group of adults with developmental dyslexia matched for age, gender, reading level, vocabulary and block-design WAIS-III sub-tests and reading level was exposed to the same protocol but with sham stimulation. The results revealed that active, but not sham stimulation, significantly improved reading speed and fluency. This finding suggests that the dorsal stream may play a role in efficient retrieval from the orthographic input lexicon in the lexical route. It also underscores the potential of tDCS as an intervention tool for improving reading speed, at least in adults with developmental dyslexia.