Collinear flanking stimuli can reduce the detectability of a Gabor target presented in peripheral vision. This phenomenon is called collinear lateral inhibition and it may contribute to crowding in peripheral vision. Perceptual learning can reduce collinear lateral inhibition in peripheral vision, however intensive training is required. Our aim was to assess whether modulation of collinear lateral inhibition can be achieved within a short time-frame using a single 20-minute session of primary visual cortex anodal transcranial direct current stimulation (a-tDCS). Thirteen observers with normal vision performed a 2AFC contrast detection task with collinear flankers positioned at a distance of 2λ from the target (lateral inhibition) or 6λ (control condition). The stimuli were presented 6˚to the left of a central cross and fixation was monitored with an infra-red eye tracker. Participants each completed two randomly sequenced, single-masked stimulation sessions; real anodal tDCS and sham tDCS. For the 2λ separation condition, a-tDCS induced a significant reduction in detection threshold (reduced lateral inhibition). Sham stimulation had no effect. No effects of a-tDCS were observed for the 6λ separation condition. This result lays the foundation for future work investigating whether a-tDCS may be useful as a visual rehabilitation tool for individuals with central vision loss who are reliant on peripheral vision.
Purpose The purpose of this study was to assess whether motion information from suppressed amblyopic eyes can influence visual perception. Methods Participants with normal vision ( n = 20) and with amblyopia ( n = 20; 11 anisometropic and 9 strabismic/mixed) viewed dichoptic, orthogonal drifting gratings through a mirror stereoscope. Participants continuously reported form and motion percepts as gratings rivaled for 60 seconds. Responses were binned into categories ranging from binocular integration to complete suppression. Periods when the grating presented to the nondominant/amblyopic eye was suppressed were analyzed further to determine the extent of binocular integration of motion. Results Individuals with amblyopia experienced longer periods of non-preferred eye suppression than controls. When the non-preferred eye grating was suppressed, binocular integration of motion occurred 48.1 ± 6.2% and 31.2 ± 5.8% of the time in control and amblyopic participants, respectively. Periods of motion integration from the suppressed eye were significantly non-zero for both groups. Conclusions Visual information seen only by a suppressed amblyopic eye can be binocularly integrated and influence the overall visual percept. These findings reveal that visual information subjected to interocular suppression can still contribute to binocular vision and suggest the use of appropriate optical correction for the amblyopic eye to improve image quality for binocular combination.
People with central vision loss (CVL) due to macular degeneration are forced to rely on their residual peripheral vision and often develop a preferred retinal locus (PRL), a region of intact peripheral retina that is used for fixation. At the PRL, visual processing is impaired due to crowding (cluttering of visual objects). The problem of crowding still persists when images are magnified to account for the lower resolution of peripheral vision. We assessed whether anodal transcranial direct stimulation (a-tDCS), a neuro-modulation technique that alters cortical inhibition, would reduce collinear inhibition (an early component of crowding) when applied to the visual cortex in patients with CVL. Our results showed that applying a-tDCS to the visual cortex for 20mins reduced crowding in three patients with CVL and that the effect was sustained for up to 30mins. Sham stimulation delivered in a separate session had no effect. These initial observations mandate further research into the use of a-tDCS to enhance cortical processing of residual retinal input in patients with CVL.
Abnormal visual experience during an early critical period of visual cortex development can lead to a neurodevelopmental disorder of vision called amblyopia. A key feature of amblyopia is interocular suppression, whereby information from the amblyopic eye is blocked from conscious awareness when both eyes are open. Suppression of the amblyopic eye is thought to occur at an early stage of visual processing and to be absolute. Using a binocular rivalry paradigm, we demonstrate that suppressed visual information from the amblyopic eye remains available for binocular integration and can influence overall perception of stimuli. This finding reveals that suppressed visual information continues to be represented within the brain even when it is blocked from conscious awareness by chronic pathological suppression. These results have direct implications for the clinical management of amblyopia.
Objective Transcranial magnetic stimulation (TMS)–evoked phosphenes are oculocentric; their perceived location depends upon eye position. We investigated the accuracy and precision of TMS-evoked phosphene oculocentric mapping. Methods We evoked central phosphenes by stimulating early visual cortical areas with TMS, systematically examining the effect of eye position by asking participants to report the location of the evoked phosphene. We tested whether any systematic differences in the precision or accuracy of responses occurred as a function of eye position. Results Perceived phosphene locations map veridically to eye position, although there are considerable individual differences in the reliability of this mapping. Conclusions Our results emphasize the need to carefully control eye movements when carrying out phosphene localization studies and suggest that individual differences in the reliability of the reported position of individual phosphenes must be considered.
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