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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 (about 250 ms)
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).
The experiment was analogous to that of Bertelson and Aschersleben [1], which investigated
visual influences on auditory localization. Based on this, we expected the synchronous light to
facilitate multimodal integration and therefore affect tactile 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
0.9 cm 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.
