Judging roughness by sight--a 7-Tesla fMRI study on responsivity of the primary somatosensory cortex during observed touch of self and others

Abstract

Observing another person being touched activates our own somatosensory system. Whether the primary somatosensory cortex (S1) is also activated during the observation of passive touch, and which subregions of S1 are responsible for self- and other-related observed touch is currently unclear. In our study, we first aimed to clarify whether observing passive touch without any action component can robustly increase activity in S1. Secondly, we investigated whether S1 activity only increases when touch of others is observed, or also when touch of one's own body is observed. We were particularly interested in which subregions of S1 are responsible for either process. We used functional magnetic resonance imaging at 7 Tesla to measure S1 activity changes when participants observed videos of their own or another's hand in either egocentric or allocentric perspective being touched by different pieces of sandpaper. Participants were required to judge the roughness of the different sandpaper surfaces. Our results clearly show that S1 activity does increase in response to observing passive touch, and that activity changes are localized in posterior but not in anterior parts of S1. Importantly, activity increases in S1 were particularly related to observing another person being touched. Self-related observed touch, in contrast, caused no significant activity changes within S1. We therefore assume that posterior but not anterior S1 is part of a system for sharing tactile experiences with others.

Keywords: 7 Tesla; S1; fMRI; observed touch; primary somatosensory cortex; self and other.

Figure 1

(A) Video stimuli of an example pair. Different types of video sequences were varied in a 2 × 2 × 2 factorial design with the factors Touch (Observed Touch, No‐Touch), Hand Identity (Self, Other), and Viewing Perspective (Ego = Egocentric, Allo = Allocentric); (B) Activity changes in S1 for the contrast Observed Touch > No‐Touch across experimental conditions superimposed on the MNI reference brain (visualized at P < 0.005 (uncorrected) and masked with left S1).

Figure 2

Trial structure of scanning session.

Figure 3

Activity changes in S1 for the contrasts Self Allo (=Allocentric) Observed Touch > No‐Touch and Other Allo Observed Touch > No‐Touch superimposed on coronal and axial slices of an individual's normalized T1‐image (visualized at P < 0.005 (uncorrected) and masked with left S1).

Figure 4

Contrast estimates extracted from left posterior (Area 1+2) and left anterior (Area 3a+3b) S1 for the contrast Observed Touch > No‐Touch in different experimental conditions, i.e., for the main effect (ME), and for the self egocentric (SE), self allocentric (SA), other egocentric (OE), and other allocentric (OA) conditions.

Figure 5

Individual activity profiles of all participants' (n = 14) unsmoothed functional data for the main effect of Observed Touch > No‐Touch superimposed on coronal and axial slices of an individual's normalized T1 image (visualized at P ≤ 0.001 (uncorrected) and masked with left S1); (A) Spatial consistency map of individual participants, bright yellow colors represent areas of high overlap, dark orange colors areas of low overlap between participants; (B) Individual participants' activity profiles are shown separately, each color represents active voxels of an individual subject.