The rubber foot illusion

Nineteen able-bodied volunteers (10 females) participated in this study after signing a written informed consent. All experimental procedures were performed according to the standards set by the declaration of Helsinki for medical research involving human subjects. Each participant sat comfortably on a bed with both legs lying in a supine position and with his/her right leg out of sight, behind a screen. The participant was instructed to fix his/her sight on a rubber foot (single-axis foot, Ottobock Gmbh) during the experiment. Modality-matched and modality-mismatched stimulation conditions were tested. During the modality-matched conditions, congruent stimulation was delivered on the real and the rubber feet, using two paintbrushes, as in the original RHI experiment. In the modality-mismatched conditions the stimulation was incongruent; the rubber foot, in full view, was manually stimulated with the paintbrush while the hidden real foot was stimulated by small vibrators taped on the first and second toes, triggered off by a keypad, as in our previous studies [9, 10] (as depicted in (Fig. 1)). Each vibrator (8 mm diameter, 3.4 mm height, 0.7 g weight) could be selectively activated to vibrate at a pre-defined vibration frequency (165 Hz) and force amplitude (0.36 N; i.e. largely supra-threshold) or deactivated. As soon as a specific key on the keypad was pressed, the corresponding unit would start vibrating, until the key was released. The time delay between the pressure of a key and starting of perceivable vibration can be considered negligible (i.e. <10 ms). Paintbrush stimulations on the rubber foot were applied on the areas corresponding to where the vibrators were placed on the real foot, i.e. on the first and second toes. Details on this set-up can be found in [14]. Both the congruent and incongruent conditions were tested with either synchronous or asynchronous stimulation (a temporal delay of about 0.5 s was included between stimulations) for a total of four combinations. The asynchronous conditions were included as control conditions for the synchronous ones. The four combinations of modality and timing (congruent synchronous - CS, congruent asynchronous - CA, incongruent synchronous - IS and incongruent asynchronous - IA) were tested twice for each subject; 50 % of the trials (balanced across conditions) ended with a threat stimulus, i.e. a needle was used to stab the rubber foot, in order to measure the SCR (as in the conventional RHI experiment [7]). Each trial lasted 150 s; in previous studies the stimulation duration was ranged from a minimum of 45 s [9] to a maximum of 240 s [15]. The order of execution of the trials was randomized across the participants.

We quantified the embodiment of the rubber foot by means of the three state-of-art control measures, i.e. self-assessment questionnaires, proprioceptive drift and SCR [15]. The questionnaire (Table 1) comprised of nine statements adapted from [1] for application on the foot instead of on the hand and translated into Italian. Three of the statements, namely illusion statements, referred to the extent of sensory transfer into the rubber foot and self-attribution of it during the trial (S1–S3). The remaining six statements, namely control statements, served as controls, for assessing suggestibility and “placebo effect”. Moreover, in addition to the nine statements, subjects were asked to rate vividness and prevalence of self-attribution of the rubber foot. The vividness was defined as how life-like and realistic the illusion was when it was experienced; it was rated from 0 to 10. The prevalence rating (from 0–100 %) reflected the percentage of time that the illusion was experienced (equivalent to the continuance of the illusion). To assess the proprioceptive drift, the participant had to indicate the position of his/her big toe on a ruler placed over his/her foot, before and immediately after the stimulation. The subtraction of the two measurements resulted in the proprioceptive drift. In order to avoid any memory effect the ruler was positioned with different offsets all the times [15]. When the trial ended with a threat stimulus (i.e. a needle attached to a syringe, which was used to stab the rubber foot), we measured the SCR first, then the proprioceptive drift; in the other trials first we measured the proprioceptive drift then we asked to fill-in the questionnaire.

The Kolgomorov-Smirnov test was used to verify whether the measurements were normally distributed. Then, two-way repeated-measures ANOVA (factors: timing and modality) was performed singularly for each measurement (i.e. questionnaires, drifts or SCR) in order to assess significant differences in the stimulation conditions. Similarly the results were pairwise compared using two-tailed paired t-test [15, 16], with Tukey-Kramer correction. Statistical significance was set to 0.05.

The body schema is usually defined as a continuously updated sensorimotor map of the body that informs the brain about what parts belongs to the body and where those parts are located [18]. The trick of the RHI is a direct consequence of multisensory integration, which basically misleads the brain as to the status of ownership of a fake hand, provoking its inclusion in the body schema. The largest part of neurophysiological studies has explored the body schema using stimulation to and near the hands. Hands may be considered special in that they are used for almost any kind of action we perform, but it seems that processing principles revealed for the hands may not generalize to other body parts [19]. Indeed, combining multisensory information has different implications in case the stimuli come from different parts of the body’s surface. For example tactile information takes different amounts of time to be processed by the brain depending on the distance between the stimulated surface and the brain and some studies showed how the integration of visual and tactile stimulation of the foot can lead to bad perception of simultaneity of the stimuli [20]. Our results provide evidence that it is possible to elicit the perception of possessing a rubber foot when modality-matched stimulations are provided synchronously on the biological foot and to the equivalent rubber foot areas. Indeed, all the three independent measurements of embodiment, classically employed in similar experiments on the RHI [9, 15, 21], gave closely matched results supporting this main outcome, thus confirming that even though visual and tactile stimulations applied on the foot undergo different processing mechanisms, the multisensory integration can still effectively compound the illusion and the brain integrates the fake foot in the body schema. Second to that, we investigated the RFI approach for application in the development of sensory feedback systems for lower-limb prostheses in order to provide the individual with information from the foot sole [22, 23]. Results proved that it is possible to enhance the illusion even with modality-mismatched stimulations, even though illusion was lower than in case of modality-matched stimulation. Notably asynchronous stimulation, which is used as a standard control test, in both modality congruent and incongruent cases did not induce the illusion, as reported also in previous studies [1, 3, 7‐9, 13, 15]. These results open important perspectives in the field of lower-limb prosthetics; indeed, vibrotactile elements can be easily integrated into the socket of lower-limb prostheses in order to stimulate the referred phantom foot areas and induce the RFI on a daily basis. This feature could improve the controllability of the prosthesis [24] and enhance the satisfaction of the amputee in using the prosthesis even more [12].

Future studies will be devoted at investigating whether the proposed mismatched-stimulation paradigm can promote the embodiment of the prosthesis in the body schema of lower-limb amputees.

¹ Eighteen of nineteen participants replied to vividness and prevalence questions.