In bodily ownership illusion studies, most illusory finger deformation works from proximal to distal, and there are few reports concerning deformation that leads to finger widening. We hypothesized that the difference in the number of reports between the two types of finger deformations suggests anisotropy in illusory finger deformation. To examine this, we conducted an experiment to compare the cognitive cost of finger deformation between the distal and lateral directions of the finger using nonvisual rubber hand illusion (self-touch illusion). Specifically, we measured subjective ratings of the ownership and deformation as well as the proprioceptive drift of the involved finger during the illusory experience in the two directions. The experimental results showed that (1) the sense of illusory ownership in the distal layout was significantly greater than that in the lateral layout, and (2) the sense of illusory deformation correlated significantly positively with the proprioceptive drift of the receptive finger. The results support our hypothesized anisotropy.
