Tool Embodiment: The Tool's Output Must Match the User's Input

Abstract

The embodiment of tools and rubber hands is believed to involve the modification of two separate body representations: the body schema and the body image, respectively. It is thought that tools extend the capabilities of the body's action schema, whereas prosthetics like rubber hands are incorporated into the body image itself. Contrary to this dichotomy, recent research demonstrated that chopsticks can be embodied perceptually during a modified version of the rubber hand illusion (RHI) in which tools are held by the rubber hand and by the participant. In the present research, two experiments examined tool morpho-functional (tool output affordance, e.g., precision grasping) and sensorimotor (tool input, e.g., precision grip) match as a mechanism for this tool-use dependent change to the body image. Proprioceptive drift in the RHI occurred when the tool's output and the user's input matched, but not when this match was absent. This suggests that this factor may be necessary for tools to interact with the body image in the RHI.

Keywords: body representation; embodiment; expertise; rubber hand illusion; tools.

FIGURE 1

(A) The life cast of a hand holding needle-nose pliers. The pliers measured 13 cm in length, with a 10 cm handle and jaws 3 cm in length. (B) The wooden block used as the control viewed object in both experiments presented here and in Weser et al. (2017).

FIGURE 2

For each condition, the proprioceptive judgment phase (A) was conducted before and after the visuo-tactile stimulation phase (B). The viewed object was visible during (B) and hidden during (A) by changing the direction of the illumination from above the surface of the mirror to below.

FIGURE 3

The non-significant interaction of timing of visuo-tactile stimulation and viewed object. The significant main effect of viewed object is apparent, as drift was larger when participants viewed a rubber hand holding pliers than when a wooden block was viewed. Error bars represent ±1 SEM.

FIGURE 4

The significant interaction between the timing of visuo-tactile stimulation, and the number of beads transferred during the tool-skill task. Shaded areas indicate ±1 SEM.

FIGURE 5

The non-significant interaction between RHI condition and questionnaire component for the pliers-version of the RHI. Error bars represent ±1 SEM.

FIGURE 6

The life cast of a hand holding tweezers. The tweezers measured 9 cm in length.

FIGURE 7

The significant interaction of viewed object, timing of visuo-tactile stimulation and the median split of self-report of tweezers-use. Error bars represent ±1 SEM.

FIGURE 8

The non-significant interaction between RHI condition and questionnaire component for the tweezers-version of the RHI. Error bars represent ±1 SEM.

FIGURE 9

An analysis of the combined Experiments 1 and 2 data. The trending interaction suggests that across experiments, participants who self-report using the tool frequently (Pliers: n = 8, Tweezers n = 35), experience more proprioceptive drift in the synchronous visuo-tactile stimulation condition than do the participants who do not report frequent tool-use (Pliers: n = 58, Tweezers: n = 37). The effect reverses for the asynchronous condition. Error bars represent ±1 SEM.