Vision of the body affects touch accordingly to SI organization

Tactile acuity improves when subjects look at their body (Kennett et al., 2001). However it is not yet clear whether this effect strictly depends upon the vision of the tactilely stimulated body part or arises because any body part provides an arousing visual stimulus relevant to bodily sensation. To study this issue, 32 normal subjects’ tactile acuity was assessed, by means of a two point discrimination task (2PDT), in three different body parts: the hand, the cheek and the foot. The test was performed twice, i.e. with or without visual stimulation. This consisted for half subjects in viewing their hand and for half in viewing a neutral object, presented in the same spatial position as the hand. All subjects were seated so that the hand was midway in external space between cheek and foot.. Viewing the hand, but not the neutral object, improved subjects’ 2pdt thresholds on the hand and on the cheek, but not on the foot. We suggest that viewing the hand modulated tactile representation in early somatosensory cortex (SI). The hand and face representations are adjacent and lateral SI, while the foot representation is distant and more medial. The benefit of viewing the hand for touch on the hand and the cheek but not the foot shows that viewing the body modulates SI activity according to a somatotopic principle. Vision may provide a top-down modulatory input that spreads across areas co-represented in the SI homunculus.