Bare hands and attention: evidence for a tactile representation of the human body.

If brain lesions impair the allocation of attention to a representation of the body surface and the hand may serve as an attentional focus or "wand", one might expect that somatosensory deficits caused by cerebral lesions would be ameliorated by contact with the ipsilesional hand. To test this prediction, tactile detection tasks were administered to two subjects with right hemisphere lesions. Subject CB's left tactile extinction was investigated in conditions in which the degree of contact between the right and left hands and the spatial relationship between his hands was systematically varied. His left tactile extinction was significantly reduced by touch of the right hand. Similarly, extinction at the left knee was ameliorated by touch of the knee by the right hand; touch of the right foot had no effect. Subject NC's ability to detect a tactile stimulus delivered to the left side was systematically assessed in conditions in which the hands touched and the spatial relationship between the hands was varied. His ability to detect a touch on the left hand improved in conditions in which the left hand was touched by the right hand. This effect was not observed if direct contact between the two hands was prevented by inserting a thin cloth between the hands. For both subjects, placing the right hand in close proximity to the left hand or altering the spatial location of the hands relative to the body did not influence performance. These data demonstrate that the hand may serve as a conduit for attention and provide strong evidence for a distinct representation of the body surface that is at least in part independent of spatial representations.