Abstract
The primary source of information to determine where on the body we are being touched is derived from the somatosensory system. However, can visual information influence the perceived location of touch on the body? Ten participants localized a brief air puff (ca. 250ms) applied to the smooth ventral surface of the right forearm somewhere between wrist and elbow. Localization was measured with a 2AFC paradigm in which participants judged the location of the tactile stimulus relative to a visual reference using two opposing 1-up/2-down staircases. Participants' task was to indicate whether the air puff was closer or further from the wrist relative to the reference. In one condition the visual reference was a line drawn on the forearm midway between elbow and wrist (AIR ONLY). In another the air puff was accompanied by a temporally synchronous line of laser light projected onto the reference location (AIR+LASER). We expected the synchronous light to facilitate multimodal integration and therefore affect discrimination performance without introducing a bias. In the AIR ONLY condition the PSE was on average on the reference location. Surprisingly, in the AIR+LASER condition we did not find a change in discriminability relative to the AIR ONLY condition. We found, however, a significant shift of the PSE by 0.9cm towards the elbow. This bias was evident in eight out of the ten participants. This demonstration of a visual effect on tactile localization may indicate that judging the location of a visual reference on the body is not free from biases.