The Neural Integration and Embodiment of Prosthetics in the Upper LimbS. L. Beattie, 2013

Department of Exercise Science, Health, and Physical Education, University of Victoria, Victoria, BC

Hundreds of years ago, a lost limb was simply replaced with a crude wooden or metal hook or stump and would result in a life forever without the function of that limb1.

Today, research is making rapid progress in developing bionic limbs that can generate movement from biological motor signals and can detect sensory information from the environment for human perception1.

Developing a limb that can produce the vast number of complex movements of a human is the ultimate goal, however, the methods of integration and embodiment are just as critical to the success of the limb5. Research that aims to develop these optimal methods has the potential to provide amputees with a chance to live a life free of the limitations that were once brought on by the loss of their limb1.

Introduction

Amputation is the loss of any appendage from the body, primarily due to birth defect, injury, or disease.

Limitations of an amputation depend on the limb involved and how much of the limb was removed, including which joints were lost1.

In the stump, nerve endings remain, which were previously associated with the lost limb5.

Without sensory input, they can generate phantom limb pain5. Despite these maladaptive symptoms, these remaining nerve endings have allowed researchers to explore neural integration of a prosthetic limb4.

The neuroplasticity of the brain also facilitates the methods of embodiment of the prosthesis into the individual’s body schema2.

Background

Methods of IntegrationMyoelectric Integration

• Surface electrodes are attached to the muscles in the remaining forearm ,and detect the muscle activity that is under conscious control1.• Computerized hardware in the prosthesis interprets the neural activity and transforms it into complex movements of the hand1 (see figure 1).

Targeted Motor Reinnervation (TMR)

• Preserved nerve endings from the lost limb are surgically rerouted to a large muscle group nearby (e.g. Pectoralis major) 1 5 (see figure 2).• Surface or implanted electrodes detect activity from the reinnervated muscle due to descending motor signals from the rerouted nerves, in order to control the prosthetic limb1.

Methods of EmbodimentCortical Activation

• A computer program connects to the remaining stump muscles via wired surface electrodes that are patient-selected, and enables the individual to practice controlling a virtual prosthesis2.• A study on a 13 year old girl with a congenital amputation showed that cortical activation practice can elicit activity in the side of the brain that controls the prosthesis that is similar to the other side that controls the intact arm (see figure 3)

2.

Transcutaneous Electrical Nerve Stimulation (TENS)

• TENS is the stimulation of mechanoreceptors on the stump that are associated with nerves that previously innervated the lost limb5. • This can be coupled with the Rubber Hand illusion method so that the individual views a rubber hand or prosthesis being stroked while they receive TENS5.• The success of this method suggests that embodiment is based on intermodal congruence between vision, touch and proprioception3 5 (see figure 4).

Progressive ResearchIntraneural Electrodes

• Current research is exploring the direct connection of prosthetics to motor and sensor y nerves to facilitate bidirectional flow to and from the prosthesis 4. • Results so far show that electroneurographic recordings of the nerves can be differentiated into “classes” of activity that likely correspond to distinct hand grips (e.g. Power grip) 4.• Facilitation of this method in commercialized prosthetics is still in progress, but shows promise4.

ConclusionFrom the research reviewed in this presentation, it is clear that there are many ways in which individuals associate with their limbs and, therefore, there are many methods that can be explored for enabling life-like integration and embodiment of prosthetic limbs.

Although modern prosthetics are fascinating in their ability to use muscle activity to produce movement, it will be very interesting to see the progression of integrating prosthetic to have both motor and sensory control in the next few years1. This presentation fully supports and recommends the continuation of research towards developing a prosthetic that can fully integrate into the human nervous system in both a motor and sensory capacity.

References

1. Clement, P.G.E., Bugler, K.E., & Oliver, C.W. (2011). Bionic prosthetic hands: a review of present techniques and future aspirations. The Surgeon, 9, 336- 340.

2. DaPaz, A.C. Jr. & Braga, L.W. (2007). Brain activation of myoelectric prosthetic hand: The role of the brain in the rehabilitation of amputees. Journal of Pediatric Orthopaedics, 27, 947-951.

3. Ehrsson, H.H., Spence, C., & Passingham, R.E. (2004). That’s my hand! Activity in premotor cortex reflects feeling of ownership of a limb. Science,305, 875–877.

4. Micera, S., Citi, L., Rigosa, J., Carpaneto, J., Raspopovic, S., Di Pino, G., Rossini, L., Yoshida, K., Denaro, L., Dario, P., & Rossini, P.M. (2010). Neural signals recorded using intraneural electrodes: Toward the development of a neurocontrolled hand prosthesis, Proceedings of the IEEE, 98, 407-417.

5. Mulvey, M.R., Fawkner, H.F., Radford, H., & Johnson, M.I. (2009). The use of transcutaneous electrical nerve stimulation (TENS) to aid perceptual embodiment of prosthetic limbs. Medical Hypotheses, 72, 140-142.

Figure 1. The ilimb hand prosthesis that is controlled by myoelectric connections.(retrieved from www.touchbionics.com)

Figure 2. Schematics diagram of how TMR is organized. ( Retrieved from Neogi et al., 2011).

Figure 3. fMRI showing near symmetrical activity in the two hemispheres.(Retrieved from DaPaz et al. 2007).

Figure 4. Significant relationship between premotor activity and illusion suggests that visual perception affects feelings of ownership. (Retrieved from Ehrsson et al., 2004)