Seeing touch in the somatosensory cortex: a TMS study of the visual perception of touch

Abstract

Recent studies suggest the existence of a visuo-tactile mirror system, comprising the primary (SI) and secondary (SII) somatosensory cortices, which matches observed touch with felt touch. Here, repetitive transcranial magnetic stimulation (rTMS) was used to determine whether SI or SII play a functional role in the visual processing of tactile events. Healthy participants performed a visual discrimination task with tactile stimuli (a finger touching a hand) and a control task (a finger moving without touching). During both tasks, rTMS was applied over either SI or SII, and to the occipital cortex. rTMS over SI selectively reduced subject performance for interpreting whether a contralateral visual tactile stimulus contains a tactile event, whereas SII stimulation impaired visual processing regardless of the tactile component. These findings provide evidence for a multimodal sensory-motor system with mirror properties, where somatic and visual properties of action converge. SI, a cortical area traditionally viewed as modality-specific, is selectively implicated in the visual processing of touch. These results are in line with the existence of a sensory mirror system mediating the embodied simulation concept.

Figure 1

Schematic representation of the experimental tasks. Upper panels depict the types of stimuli used in the Touch Task (A) and in the No‐Touch Task (B). The target trial is shown on the left and the catch trial is shown on the right. The white lines represent the starting point of the moving hand and the white arrows indicate the direction of the movement and the different amplitudes in the target and catch trials. In both tasks, the visual stimuli (video clips of the moving index finger) exhibited identical motion duration (60 ms) and amplitude across the observed actions. C: Sequence of events in the rTMS session. Each trial started with the presentation of a central fixation cross. After a delay (>5 sec) the video clip started (60 ms of duration). At the onset of the clip, rTMS (13 Hz, three TMS pulses at 0, 75, 150 ms after stimulus onset) was delivered over one of the targeted areas (i.e., right hemisphere SI, SII, or VI). In the Touch Task, subjects were asked to indicate by pressing the response key whether the moving finger touched the hand below (target trial), while refraining from pressing if the finger approached the hand, but did not touch it (catch trial). In the No‐Touch Task, subjects were asked to discriminate the range of the finger's movement by pressing the response key to report the greater drop of the finger (target trial), and to refrain from responding to the smaller drop (catch trial).

Figure 2

rTMS Effects on behaviour. A and B: Panels describe the main effect of Session, showing the mean values in each experimental session for the percentages of errors (F = 3.86, P < 0.02) (A), the perceptual sensitivity (F = 3.48, P < 0.03) (B). C: The graph describes the Task by Session by Side Interaction (F = 4.15, P < 0.02), showing the selective decrement of sensitivity induced by rTMS over SI for contralateral (left‐sided) visual stimuli depicting touch events. White bars represent the Baseline, blacks bars represent the effects of rTMS over SI (SI‐rTMS session), grey bars represent the effects of rTMS over SII (SII‐rTMS session), grey and black bars represent the effects of rTMS over VI (VI‐rTMS session). Asterisks indicate a significant rTMS effect. Error bars depict standard errors.