Modifying an underlying component of perceived arm length: adaptation of tactile location induced by spatial discordance.

In four experiments we examined the adaptation and aftereffect that resulted from a treatment yielding tactile/kinesthetic length discordance between the arms. Perceived discordance diminished with trials and tended to zero. Subsequent visual/tactile cross-modal judgments of distance showed the aftereffect to be a change in the perceived location of an unseen probed spot on each hand with respect to the location of a truly coincident visual marker. This occurred toward the body for the probed spot on one arm and away from the body on the other. There were three other main findings: (a) Arm movement was not a necessary condition for adaptation or aftereffect; (b) with intrinsic length information about the right arm present, but touch information from the right index finger absent during treatment, adaptation and aftereffect were abolished; (c) aftereffects of tactile location that were manifest at the hand and wrist tended to zero when a point close to the elbow was tested with a cross-modal procedure. The experiments provide evidence that the mapping of the tactile sheet onto an internal length domain had been modified by the treatment. The sensory consequences of the treatment led many subjects to report spontaneously that their arms felt to be of different lengths.