Touch to see: neuropsychological evidence of a sensory mirror system for touch.

The observation of touch can be grounded in the activation of brain areas underpinning direct tactile experience, namely the somatosensory cortices. What is the behavioral impact of such a mirror sensory activity on visual perception? To address this issue, we investigated the causal interplay between observed and felt touch in right brain-damaged patients, as a function of their underlying damaged visual and/or tactile modalities. Patients and healthy controls underwent a detection task, comprising visual stimuli depicting touches or without a tactile component. Touch and No-touch stimuli were presented in egocentric or allocentric perspectives. Seeing touches, regardless of the viewing perspective, differently affects visual perception depending on which sensory modality is damaged: In patients with a selective visual deficit, but without any tactile defect, the sight of touch improves the visual impairment; this effect is associated with a lesion to the supramarginal gyrus. In patients with a tactile deficit, but intact visual perception, the sight of touch disrupts visual processing, inducing a visual extinction-like phenomenon. This disruptive effect is associated with the damage of the postcentral gyrus. Hence, a damage to the somatosensory system can lead to a dysfunctional visual processing, and an intact somatosensory processing can aid visual perception.