Distributed representation of single touches in somatosensory and visual cortex

Multi-voxel pattern analysis (MVPA) was used to analyze blood-oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) data, which were acquired as human subjects received brief vibrotactile stimulation of their hands and feet. Support vector machines trained and tested on the whole brain fMRI data were able to accurately decode the body site of single touches, with mean performance of 92% in a two-way discrimination task (chance performance 50%) and 70% in a four-way discrimination task (chance performance 25%). Primary and secondary somatosensory areas (S1 and S2) alone decoded the touched body site with high accuracy. S1 was more accurate at decoding touches closely spaced on the body surface (different fingers of the same hand) whereas S2 and S1 were equally accurate at decoding widely spaced touches (hand vs. foot). The hand and foot regions of S1 (S1hand and S1foot) were separately examined in a two-way classification task. S1hand was better able to decode the hand of stimulation (left vs. right), and S1foot was better able to decode the foot of stimulation. In addition to S1 and S2, vibrotactile responses were observed in a region of visual cortex, areas MST and STP (MST/STP) in lateral occipito-temporal lobe. MST/STP was able to accurately decode the hand but not the foot of stimulation, supporting the idea of a role for MST/STP in eye-hand coordination.