  In measurement, a reference frame is needed to compare the measured object to
something already known. This raises the neuroscientific question of which
reference frame is used by humans when exploring the environment. Previous
studies suggested that, in touch, the body employed as measuring tool also
serves as reference frame. Indeed, an artificial modification of the perceived
dimensions of the body changes the tactile perception of external object
dimensions. However, it is unknown if such a change in tactile perception would
occur when the body schema is modified through the illusion of owning an limb
altered in size. Therefore, employing a virtual hand illusion paradigm with an
elongated forearm of different lengths, we systematically tested the subjective
perception of distance between two points (tactile distance perception task,
TDP task) on the corresponding real forearm following the illusion. Thus, TDP
task is used as a proxy to gauge changes in the body schema. Embodiment of the
virtual arm was found significantly greater after the synchronous visuo-tactile
stimulation condition compared to the asynchronous one, and the forearm
elongation significantly increased the TDP. However, we did not find any link
between the visuo-tactile induced ownership over the elongated arm and TDP
variation, suggesting that vision plays the main role in the modification of
the body schema. Additionally, significant effect of elongation found on TDP
but not on proprioception suggests that these are affected differently by body
schema modifications. These findings confirm the body schema malleability and
its role as reference frame in touch.
