Seeing the hand boosts feeling on the cheek

Tactile acuity improves when subjects look at the stimulated body part, even when vision does not provide any information relevant to touch. This “visual enhancement of touch” (VET), might involve modulation of primary somatosensory cortex (SI) processing by multimodal information related to the body. SI shows a characteristic somatotopic organization, with the face and hand represented laterally and adjacent to each other, and the foot represented more medially.The aim of this study was to investigate whether VET is limited to the viewed body part, spreads to all body parts, or generalizes only to body parts represented in SI closely to the viewed body part. Tactile acuity was assessed in healthy subjects on the hand, face and foot, while subjects viewed either their stimulated hand, their foot or were blindfolded: viewing the hand, compared to blindfold condition, enhanced tactile sensitivity on the hand and also on the face, but not on the foot. Conversely, viewing the foot, compared to blindfold condition, improved touch only on the foot. Two control experiments assured this effect was due to viewing the body and not to directing visuo-spatial attention toward the location of tactile stimulation. The present results show that VET acts accordingly to a somatotopic gradient based on SI organization, suggesting that this multisensory effect may occur within SI. This finding might have a possible application to facilitate the recovery of tactile deficits in patients with a lesion of somatosensory cortices.