Upper limb amputees can be induced to experience a rubber hand as their own

Abstract

We describe how upper limb amputees can be made to experience a rubber hand as part of their own body. This was accomplished by applying synchronous touches to the stump, which was out of view, and to the index finger of a rubber hand, placed in full view (26 cm medial to the stump). This elicited an illusion of sensing touch on the artificial hand, rather than on the stump and a feeling of ownership of the rubber hand developed. This effect was supported by quantitative subjective reports in the form of questionnaires, behavioural data in the form of misreaching in a pointing task when asked to localize the position of the touch, and physiological evidence obtained by skin conductance responses when threatening the hand prosthesis. Our findings outline a simple method for transferring tactile sensations from the stump to a prosthetic limb by tricking the brain, thereby making an important contribution to the field of neuroprosthetics where a major goal is to develop artificial limbs that feel like a real parts of the body.

Fig. 1

The experimental set-up. The stump was hidden under a table and synchronous brushstrokes were applied to the stump and the index finger of a rubber hand placed in full view in front of the participant (A). After experiencing the illusion, the participant was asked to demonstrate where he or she had felt the touches by making a horizontal pointing movement along a ruler with eyes closed (B). We also stabbed the rubber hand with a needle whilst simultaneously measuring the associated changes in the participant's skin conductance as an objective measure of any fear and anticipated pain. (See the Methods section for details.)

Fig. 2

The results of the questionnaire. The responses to Questions 1–3 reflect the experiences of the illusion: Q1—‘I felt the touch of the brush on the prosthetic hand’; ‘Q2—It seemed as if the brush caused the sensation touch that I experienced’; Q3—It felt as if the prosthetic hand was my hand’. The responses to Questions 4–9 served as controls for suggestibility and task compliance (see Methods section). The scores for the illusion questions (Q1–Q3) were significantly greater (P < 0.01) than those for the control conditions after the period of synchronous stimulation on the stump and the prosthetic hand (blue). Further, on average, the scores on the three illusion-related questions were greater in the stump condition than in the control condition when contralateral intact arm was stimulated (P < 0.05). Finally, it can be noted that the illusion ratings when stroking the participants’ stumps were lower than when testing the classical rubber hand illusion by stroking their intact contralateral hand (yellow). For details, see the Results section.

Fig. 3

Behavioural evidence that people perceived a change in the location of the sensation of touch from the stump (and phantom in the cases of referred sensations) towards the rubber hand. When asked to indicate where they had sensed the touches of the paintbrush, by pointing with the intact hand with their eyes closed, the participant indicated greater drift in the perceived location of the touch towards the rubber hand after the illusion condition with synchronous stimulation (Sync) than after the asynchronous control condition (Async; P < 0.05).

Fig. 4

Objective physiological evidence that the participants experienced an increase in the ownership of the prosthetic hand when we brushed the stump and the prosthetic hand synchronously. Greater psychologically induced sweating, as measured with the skin conductance response (in micro Sievert), was observed when the prosthetic hand was stabbed with a needle in the illusion condition (sync-stump) than in the asynchronous control condition (async-stump; P < 0.05).

Fig. 5

Significant relationship between the time since amputation and the strength of the illusion as indicated by the illusion-items in the questionnaire (Pearson's correlation = −0.474, N = 18, P = 0.047, two-tailed P < 0.05). See the Results section for details.