Influence of the Somatic Rubber Hand Illusion on Maximum Grip Aperture

Figures & data

Table 1. The table summarises the types of movement and corresponding movement parameters assessed by the studies.

Authors	Type of movement	Movement parameter
Kammers et al. (2009)	Reaching own’s hand	Movement time
		Peak velocity
		Relative time to peak velocity
		Mean velocity
		Endpoint error
	Bimanual grasping of a stick	Kinematic metric distance
		Peak velocity
		Maximum GA
Kammers et al. (2010)	Grasping a cylinder	Maximum GA*
		Peak velocity*
		Time to maximum GA
		Time to peak velocity
Zopf et al. (2011)	Reaching a line on a display	Reaching endpoint*
		Initial movement direction*
		Curvature
		Mean velocity
		Movement onset
		Movement duration
		Peak velocity
		Time to peak
Heed et al. (2011)	Grasping a cylinder	Reaching endpoint*
		Maximum GA

*Presence of an influence of the visual RHI on such a movement parameter.

Figure 1. Panel A shows the setup for the somatic RHI. Panel B zooms into the hands (highlighting in red the fingers used for stroking and the stimulated fingers to induce the illusion) and panel C shows how the synchronous and asynchronous strokes differ.

Table 2. Table shows the original statements by Lopez et al. (2012) for the somatic RHI used in the current study.

Question	Statements
Q1	I felt as if I was touching my right hand with my left index finger
Q2	It felt as if I had more than one right hand
Q3	My right hand felt larger than normal
Q4	I experienced that my right hand was moving
Q5	I could not feel my own right hand
Q6	It felt as if my right hand were drifting towards the left
Q7	I found that experience enjoyable
Q8	It seemed like the rubber hand belonged to me
Q9	It seemed like my right hand was in the location where the rubber hand was
Q10	It seemed like the rubber hand was in the location my right hand was
Q11	I felt as if the rubber hand were drifting towards my real right hand
Q12	I had the sensation that my right hand was numb
Q13	It seemed like the experience of my right hand was less vivid than normal

Table 3. Table reports the areas of interest of the study and the corresponding channels over the brain regions. * = indicates right side of the brain, which is ipsilateral to the limb under illusion.

Anatomical region		Channels
Frontal Areas	Premotor cortex/Supplementary motor area	1,2,*3,4,5,6,*7,10
	Motor cortex	5,8,9,13,*14
Parietal Areas	Intraparietal sulcus and Superior parietal lobe	17,18,19,20,21,22,23,24
	Primary somatic sensory cortex	11,12,15,16

Figure 2. Panel a) shows the NIRS cap utilised to carry out the NIRS data recording. Coloured circles indicate cap holes (default). Panel b) shows the disposition of the transmitters (red circles) and receivers (blue circles) of the template adopted (image produced by Homer2 NIRS Processing package) and the channels provided.

Figure 3. Figure shows the flowchart of the data processing of NIRS data from the recording of raw data till the calculation of the averaged HbO and HbR concentrations. HbO = oxygenated haemoglobin; HbR deoxygenated haemoglobin.

Figure 4. Figure shows the flowchart of the experiment. R&G = reach to grasp movement; ½ = half of participants; As = asynchronous stimulation; Sy = Synchronous stimulation.

Figure 5. Figure shows the average of proprioceptive drift observed after synchronous and asynchronous stimulation. Bars represent the average of the group, while dots individual data points. Participants experienced greater drift (*) when the synchronous condition was experienced.

Figure 6. Panel a) shows the average rating for each of the 13 items (statements) in the ownership questionnaire after the synchronous and asynchronous stimulation. While panel b) shows the average for the five questionnaire clusters, and the differences (*) observed between synchronous and asynchronous stimulation. The bars indicate the Standard Error (SE). Panel c) shows individual data plotting for the ownership cluster of the questionnaire. This information is relevant given the known variability in RHI studies, and helps the Reader appreciating the spread of Group data, that would not otherwise be visible. Panel d) Compliance individual data plotting shows that the group average, which is significantly different between conditions, is not reflected so clearly as for other clusters when the individual data points are considered.

Figure 7. Figure shows the average of the maximum GA as observe before and after the exposure to the somatic RHI (treatment). Bars represent the average of the group, while dots and individual data points. The maximum GA remained stable between measurements (p>.05).

Kammers, M. P., de Vignemont, F., Verhagen, L., & Dijkerman, H. C. (2009). The rubber hand illusion in action. Neuropsychologia, 47(1), 204–211. https://doi.org/10.1016/j.neuropsychologia.2008.07.028

 PubMed Web of Science ®Google Scholar

Kammers, Marjolein, P. M., Kootker, J. A., Hogendoorn, H., & Dijkerman, H. C. (2010). How many motoric body representations can we grasp? Experimental Brain Research, 202(1), 203–212. https://doi.org/10.1007/s00221-009-2124-7

 PubMed Web of Science ®Google Scholar

Zopf, R., Truong, S., Finkbeiner, M., Friedman, J., & Williams, M. A. (2011). Viewing and feeling touch modulates hand position for reaching. Neuropsychologia, 49(5), 1287–1293. https://doi.org/10.1016/j.neuropsychologia.2011.02.012

 PubMed Web of Science ®Google Scholar

Heed, T., Gründler, M., Rinkleib, J., Rudzik, F. H., Collins, T., Cooke, E., & O'Regan, J. K. (2011). Visual information and rubber hand embodiment differentially affect reach-to-grasp actions. Acta Psychologica, 138(1), 263–271. https://doi.org/10.1016/j.actpsy.2011.07.003

 PubMed Web of Science ®Google Scholar

Lopez, C., Bieri, C. P., Preuss, N., & Mast, F. W. (2012). Tactile and vestibular mechanisms underlying ownership for body parts: A non-visual variant of the rubber hand illusion. Neuroscience Letters, 511(2), 120–124. https://doi.org/10.1016/j.neulet.2012.01.055

 PubMed Web of Science ®Google Scholar

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are openly available in OSF at https://osf.io/963cq/ [DOI 10.17605/OSF.IO/963CQ].