Proceedings of The Physiological Society

Physiology 2012 (Edinburgh) (2012) Proc Physiol Soc 27, C79

Oral Communications

In the absence of vision muscle receptors contribute to body representation

S. C. Gandevia1,2, M. E. Héroux1, A. A. Butler1,2, L. D. Walsh1,2

1. Neuroscience Research Australia, Sydney, New South Wales, Australia. 2. University of New South Wales, Sydney, New South Wales, Australia.

The sense of body ownership, a sense of which things belong to our body, is presumably generated from sensory information. This sense can be manipulated using tactile stimuli to induce an illusion of ownership over a rubber hand (Botvinick and Cohen, 1998). We recently showed that muscle receptor signals can also be used to manipulate the sense of body ownership, using a passive movements stimulus (Walsh et al., 2011). When muscle receptor signals are manipulated to induce an illusion of ownership over a plastic magician's finger, subjects report that they feel that the plastic magician's finger is their finger, and that the perceived position of their finger moves closer to the plastic finger. However, it is unclear if muscle receptors contribute to the sense of ownership on their own or whether vision is required. We further developed this plastic finger illusion to exclude any contribution from seeing the finger move. Both of the subject's hands were covered and the experimenter used the subject's left index and thumb to passively hold and move a plastic finger that was coupled to the subject's right finger. The coupling placed the plastic finger 12 cm above the subject's finger and ensured that any movement of the proximal interphalageal joint of the plastic finger or the subject's finger was reproduced in the other, i.e. movement was congruent for the two fingers. This illusion was induced in naïve human subjects (n=20) when intact and after a digital nerve block of the right index finger (lignocaine 1%, 3-5 ml). This removed input from skin and joint receptors but left intact the muscle receptors in the long flexor and extensor muscles of the fingers and the intrinsic hand muscles. We asked subjects to report the perceived vertical spacing between their left and right index fingers, and also the perceived height of each finger above the table. After 3 mins of congruent movement, subjects reported a significantly smaller spacing of 1.5 [0.5,4.0] cm (median [interquartile range]) between their left and right index finger when compared to a perceived spacing of 3.5 [1.0, 6.0] cm after 3 minutes of incongruent movement (Wilcoxon rank test, p < 0.05). Interestingly, the perceived height of the index fingers did not change. There was no effect of blocking the digital nerves. Furthermore, in a separate study we found that simply passively grasping the plastic finger (n=10) with the left index and thumb was enough to reduce the perceived spacing between the fingers from 8.0 [6.0,10.0] cm to 4.0 [2.0,8.0] cm (Wilcoxon rank test, p < 0.01). There are three novel findings. First, in the absence of vision muscle receptors can generate a sense of body ownership. Second, holding a plastic finger with vision excluded is enough to alter perceived hand position. Finally, perceived finger position depends upon whether the fingers are considered separately or together.

Where applicable, experiments conform with Society ethical requirements