Regions of visual cortex responding to tactile stimulation in an individual with longstanding low vision are not causally involved in tactile processing performance

Abstract

AbstractBraille reading and other tactile discrimination tasks recruit the visual cortex of both blind and normally sighted individuals undergoing short-term visual deprivation. Prior functional magnetic resonance imaging (fMRI) work in patient ‘S’, a visually impaired adult with the rare ability to read both highly magnified print visually and Braille by touch, found that foveal representations of S’s visual cortex were recruited during tactile perception, whereas peripheral regions were recruited during visual perception. Here, we test the causal nature of tactile responses in the visual cortex of S by combining tactile and visual psychophysics with repetitive transcranial magnetic stimulation (rTMS). First, we replicate this prior fMRI work in S. Second, we demonstrate that transient disruption of S’s foveal visual cortex has no measurable impact on S’s tactile processing performance compared to that of healthy controls – a pattern not predicted by the fMRI results. Third, stimulation of foveal visual cortex maximally disrupted visual processing performance in both S and controls, suggesting the possibility of preserved visual function within S’s foveal cortex. Finally, stimulation of somatosensory cortex induced the expected disruption to tactile processing performance in both S and controls. These data suggest that tactile responses in S’s foveal representation reflect unmasking of latent connections between visual and somatosensory cortices and not behaviourally relevant cross-modal plasticity. Unlike studies in congenitally blind individuals, it is possible that the absence of complete visual loss in S has limited the degree of causally impactful cross-modal reorganisation.Significance statementPrior fMRI work in patient ‘S’ identified that foveal portions of S’s visual cortex respond more to tactile processing, whereas peripheral portions respond more to visual processing. Here, we tested whether this foveal processing was causally related to either tactile or visual processing. First, using fMRI we replicate prior work. Second, we demonstrate that TMS of the foveal representation and of somatosensory cortex interfered with visual and tactile discriminations respectively in controls and crucially also in S. The foveal representation in S, which is responsive to tactile stimulation, does not however play a causal role in mediating S’s ability to discriminate Braille characters and likely reflects the unmasking of latent connections between visual and somatosensory cortices.