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

Abstract

ObjectiveMultiple 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.MethodsWe 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.ResultsFifty 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.ConclusionThere 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 registrationClinicalTrials.gov/, identifier CRD42021255882.

Improving peripheral reading with non-invasive transcranial electrical stimulation of early visual areas

“…While both tDCS and tRNS are implicated in the reduction of visual crowding (Bello et al, 2023), it is also possible that the improvement due to brain stimulation found in the current study was not strictly driven by reduced visual crowding. Although peripheral RSVP reading tasks are beholden to visual crowding, they do not inherently measure visual crowding.…”

“…Applying non-invasive transcranial electrical stimulation (tES) to early visual areas can reduce the strength of visual crowding without changing the visual characteristics of the stimulus, enabling better processing of cluttered visual scenes (Bello et al, 2023). The effect of tES on visual crowding has been examined in participants with normal vision (Chen et al, 2021; Contemori et al, 2019; Raveendran et al, 2020) as well as in patients with visual dysfunction, including macular degeneration (Raveendran et al, 2021), and amblyopia (Donkor et al, 2021).…”

Visual cortex anodal transcranial direct current stimulation does not alter reading performance for Chinese presented character-by-character to normal peripheral vision in older adults

“…Anodal transcranial direct current stimulation (a-tDCS), a common NIBS technique ( Nitsche and Paulus, 2000 ), has been demonstrated to improve a range of visual functions. Results from a recent meta-analysis illustrated that visual cortex a-tDCS improved contrast sensitivity, visual evoked potential amplitude (an index of cortical excitability), and crowding in peripheral vision among normally sighted individuals ( Bello et al, 2023 ). TDCS involves a weak 1 – 2 mA electrical current delivered through two head-mounted electrodes (the anode and cathode) and induces regional changes in cortical excitability and neurotransmitter concentrations that outlast the duration of stimulation [see Roche et al (2015) for a review].…”

“…In summary, a-tDCS of the visual cortex can reduce crowding in peripheral vision ( Spiegel et al, 2012 ; Raveendran et al, 2020 ; Bello et al, 2023 ), and, on this basis, we explored whether a-tDCS could improve peripheral reading in patients with AMD ( Silva et al, 2022 ). A differential effect for English and Chinese writing systems was found whereby there was no improvement for Chinese readers.…”

The effect of montages of transcranial alternating current stimulation on occipital responses—a sham-controlled pilot study

“…Increasing evidence has demonstrated that the tES targeting on the occipital region (Oz) can enhance, preserve, and even restore visual functions in healthy and visually impaired individuals ( 4 ). However, the effects of tES on vision remain inconclusive in the literature [refer to ( 5 ) for a recent systematic review]. This variability may stem from diverse stimulation parameters employed across studies, influencing the neuroplastic aftereffects and the subsequent efficacy of tES.…”