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.

Analysis of shape uses local apparent position rather than physical position.

Objects are often identified by the shapes of their boundaries. Here, by measuring threshold amplitudes for detection of sinusoidal modulation of local position, orientation and centrifugal speed in a closed path of Gabor patches, we show that the positions of such boundaries are misperceived to accommodate local illusions of orientation context and motion induced positional bias. These two types of illusion are shown to occur independently, but the misperception of position is additive. We conclude that, in the analysis of shape, the visual system uses the apparent rather than the veridical boundary conformation.

Impact of microsaccades on visual shape processing.

Sensitivity to subtle changes in the shape of visual objects has been attributed to the existence of global pooling mechanisms that integrate local form information across space. Although global pooling is typically demonstrated under steady fixation, other work suggests prolonged fixation can lead to a collapse of global structure. Here, we ask whether small ballistic eye movements that naturally occur during periods of fixation affect the global processing of radial frequency (RF) patterns-closed contours created by sinusoidally modulating the radius of a circle. Observers were asked to discriminate the shapes of circular patterns and RF-modulated patterns while fixational eye movements were recorded binocularly at 500 Hz. Microsaccades were detected using a velocity-based algorithm, allowing trials to be sorted according to the relative timing of stimulus and microsaccade onset. Results revealed clear perisaccadic changes in shape discrimination thresholds. Performance was impaired when microsaccades occurred close to stimulus onset, but facilitated when they occurred shortly afterward. In contrast, global integration of shape was unaffected by the timing of microsaccades. These findings suggest that microsaccades alter the discrimination sensitivity to briefly presented shapes but do not disrupt the spatial pooling of local form signals.NEW & NOTEWORTHY Microsaccades cause rapid displacement of visual images during fixation and dramatically alter the perception of basic image features. However, their effect on more complex aspects of visual processing is not well understood. Here, we demonstrate a dissociation in the impact of microsaccades on shape perception. Although overall shape discrimination performance is modulated around the time of microsaccades, the pooling efficiency of global mechanisms that combine local form information across space remains unaffected.

In unison: First- and second-order information combine for integration of shape information.

The modulation of orientation around radial frequency (RF) patterns and RF textures is globally processed in both cases. This psychophysical study investigates whether the combination-a textured RF path obtained by applying an RF texture to an RF contour-is processed like a texture or a contour when making judgements about shape. Unlike RF textures, the impression of a closed flow was not required for global integration of textured RF paths, suggesting that these paths were processed as second-order, or contrast-defined contours. Luminance-defined (LD) RF paths were shown to globally integrate but with thresholds approximately half of those for the proposed second-order textured paths. The next experiment investigated whether this benefit was due to LD stimuli possessing double the amount of information (first- and second-order information). A mixed three-part contour composed of two different second-order texture components and an LD component was then employed to determine how the different cues combined. The mixed path thresholds matched predictions derived from a linear combination of first- and second-order cues. The conclusion is that the shape of isolated contours is processed using both first- and second-order information equally and that the contribution of texture is to carry additional second-order signal.

Discrete annular regions of texture contribute independently to the analysis of shape from texture.

Radial frequency (RF) textures (created by applying a sinusoidal modulation of orientation to an otherwise circular texture) have been shown to be globally processed. RF textures differ from RF patterns (paths deformed from circular by a sinusoidal modulation in radius) in that the elements need not be constrained to a specific path. In the natural environment, objects differ from their background in texture, and a bounding contour can mark this textural change. This study examines the extent to which modulation of texture sums across space and whether the inclusion of a boundary between two areas provides a segmentation cue that limits the area over which summation occurs. RF textures were split into two annular regions and signal introduced to inner, outer, or both annuli Thresholds for the detection of RF modulation of orientation were not affected by the presence of a boundary. Further, it was found that the thresholds matched predictions for the independent contribution of the inner and outer areas to performance and that changing the relative phase of the modulation in the inner and outer annuli had no impact on performance, implying independent integration within the two annuli. Finally, integration of modulation information within the annuli was confirmed to ensure these results do apply to textures that are globally processed.

Modulated textures with shape structures implied by a closed flow are processed globally.

Radial frequency (RF) patterns, shapes deformed from circular by a sinusoidal modulation of radius, have been used to demonstrate global integration of shape information around a closed path by showing that the modulation depth required to detect shape deformation decreases rapidly as larger segments of the contour are modulated. In this psychophysical study we use a field of Gabor patches to examine integration of shape information in sampled RF patterns either alone or placed within an orientation-noise background and show that orientation-noise can be disregarded during the integration of modulation information. We also examine integration in modulated textures with local orientations that flow parallel or perpendicular to an underlying RF shape-structure. In using modulated textures comprising of elements with a random radial position but with orientation modulated such that it conforms to the local orientation of an RF pattern (RF texture) we demonstrate integration around texture patterns that imply shape. Texture patterns with element orientations locally orthogonal (RFO textures) to those of RF textures, however, exhibit a rate of decrease in modulation threshold, which is substantially reduced. When the textures are scrambled by permuting the polar positions of the patches the rate of decrease in threshold with increasing number of patches modulated in orientation is reduced for RF textures but not RFO textures. Detection of modulation in both scrambled textures is shown to be consistent with the detection of local cues. We conclude that implied closure in a modulated flow appears to be critical for global integration of textures.

Detecting shape change: characterizing the interaction between texture-defined and contour-defined borders.

The human visual system's extreme sensitivity to subtle changes in shape can often be attributed to global pooling of local information. This has been shown for shapes described by paths of contiguous elements, but it was unknown whether this global pooling translated to shapes defined by texture-segmentation borders. Also, previous research suggests that texture and luminance cues-to-shape are integrated by the visual system for shape detection but it has not been established whether they combined for shape discrimination. Controlled shapes defined either by an explicit path of Gabors, texture-segmentation borders, or both of these cues were used. Results show that all stimuli used were globally processed. Thresholds for shapes defined by both cues matched predictions based on an independent-cue vector sum of individual thresholds. Thus, while local elements are integrated around the contour and are processed by global shape-detection mechanisms, integration did not occur across different shape-cues.