Kenri Kodaka and Yuki Ishihara

Crossed hands strengthen and diversify proprioceptive drift in the
self-touch illusion

Frontiers in Human Neuroscience, 8, 422, 2014



In the self-touch illusion (STI), some can feel that both hands are
touching each other even when they are separated actually. This is achieved
by giving synchronized touches to both hands. Because the STI involves both
hands (an administrating hand and a receptive hand) of a single person, two
types of proprioceptive drifts (PDs) simultaneously occur in such a way
that both hands are attracted to each other. It is known that the PD
distance is generally larger for the administrating hand than for the
receptive hand when the two hands are uncrossed. However, it remains
unclear why such an asymmetrical relationship is observed universally. In
this study, we conducted two types of experiment to induce the STI. The
first experiment involved four conditions combining a factor of "whether
the hands are uncrossed or crossed" and a factor of "whether the
administrating hand is resting or active on the surface," with the
receptive (left) hand located at the body's midline. The result
demonstrated that crossing hands and resting on surface (ROS) induced the
STI. Specifically, crossing hands enhanced the amount of PD distance by
more than two or three times. Moreover, it is interesting that strong PD
with dominance of the receptive hand, which did not appear in the uncrossed
condition, was observed frequently. The second experiment collected seven
"illusion-sensitive" participants from the first experiment, all of whom
had a strong tendency to feel the self-touch, and examined the effect of
the location of the body midline on the PD when hands are crossed with the
administrating hand ROS. The result demonstrated that the dominant hand on
the PD completely differed among participants, but was relatively stable
over the midline position and time in the same person. We also found that a
small number of participants exhibited quite a different pattern of the PD
in the identical posture. On the basis of the results, we analyze in detail
how the dominant hand on the PD is determined in the STI.

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