Functional hierarchy for tactile processing in the visual cortex of sighted adults

Łukasz Bola,Jacek Matuszewski,Michał Szczepanik,Dawid Droździel,Magdalena W Sliwinska,Małgorzata Paplińska,Katarzyna Jednoróg,Marcin Szwed,Artur Marchewka

doi:10.1016/j.neuroimage.2019.116084

Abstract

Perception via different sensory modalities was traditionally believed to be supported by largely separate brain systems. However, a growing number of studies demonstrate that the visual cortices of typical, sighted adults are involved in tactile and auditory perceptual processing. Here, we investigated the spatiotemporal dynamics of the visual cortex’s involvement in a complex tactile task: Braille letter recognition. Sighted subjects underwent Braille training and then participated in a transcranial magnetic stimulation (TMS) study in which they tactually identified single Braille letters. During this task, TMS was applied to their left early visual cortex, visual word form area (VWFA), and left early somatosensory cortex at five time windows from 20 to 520 ms following the Braille letter presentation’s onset. The subjects’ response accuracy decreased when TMS was applied to the early visual cortex at the 120–220 ms time window and when TMS was applied to the VWFA at the 320–420 ms time window. Stimulation of the early somatosensory cortex did not have a time-specific effect on the accuracy of the subjects’ Braille letter recognition, but rather caused a general slowdown during this task. Our results indicate that the involvement of sighted people’s visual cortices in tactile perception respects the canonical visual hierarchy—the early tactile processing stages involve the early visual cortex, whereas more advanced tactile computations involve high-level visual areas. Our findings are compatible with the metamodal account of brain organization and suggest that the whole visual cortex may potentially support spatial perception in a task-specific, sensory-independent manner.

Highlights

Until recently, perception via different sensory modalities was thought to be supported by largely separate brain systems—it was generally assumed that the visual cortex processes solely visual input, the somatosensory cortex processes solely tactile input, and so on (e.g., Fig. 18-2 in Kandel et al, 2012)
Our subjects progressed significantly in tactile reading during their Braille training, reaching an average performance of 8.5 Braille words read per minute (WPM; SD 1⁄4 4.2 WPM; range 1⁄4 3–16 WPM) and 17.3 Braille letters read per minute (LPM; SD 1⁄4 4.2 LPM; range 1⁄4 11–29 LPM)
We observed specific spatiotemporal dynamics of this cross-modal involvement; the early visual cortex was critically involved in the Braille letter recognition 120–220 ms following the letter presentation, whereas the visual word form area (VWFA) was critical for this task 320–420 ms following the letter presentation

Summary

Introduction

Perception via different sensory modalities was thought to be supported by largely separate brain systems—it was generally assumed that the visual cortex processes solely visual input, the somatosensory cortex processes solely tactile input, and so on (e.g., Fig. 18-2 in Kandel et al, 2012).

Methods

Results

Conclusion