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Motors & motion control 60%


Prosthesis users who reported being just as or more functional without their artificial limbs in 2007.


American Journal of Physical Medicine & Rehabilitation


anything perfect, but give it something functional with which to work.


“What I like to argue,” says Segil, “is that the other half of the equation in terms of why rejection rates are so high is the fact that the limb is external to the person, it’s not a part of them. If we can elicit the sense of embodiment – if we can allow the person to understand the limb as oneself, then they’ll be more forgiving and more accepting of the limb.”


Take control With the recent development of closed-loop prosthetic control and sensory feedback, talk of ‘embodiment’ has become inescapable. The same can’t be said for specific definitions of the term. One of Ortiz Catalan’s most recent projects has been the development of a practical understanding of “prosthetic embodiment” for use across the field. Although it has previously been used interchangeably with ‘ownership’, which is a person’s sense that the parts that make up their body belong to them, Ortiz Catalan and his colleagues stress that any model of embodiment also has to include ‘agency’ – a person’s sense that they initiate and control their body’s movements.


“If I want to reach down to grab something, my hand will do it right away, so I know I’m in control of it,” he explains. “That’s agency. And when I touch something,


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I have sensations that I don’t even need to think about. That’s called ownership. If your prosthesis is responsive and it provides a sensation, then you feel it’s a part of your body.”


The dual senses of ownership and agency need to be continually reinforced to maintain any sense of embodiment. Clearly, the absence of sensing technology in the current generation of widely available prosthetics limits the potential for them to instil a sense of ownership, as do their uncomfortable sockets, which draw attention to the difference between body and prosthetic. On top of that, their propensities for breaking down and moving of their own accord also regularly short the circuit of agency. It’s not hard to see why users who feel they have other options choose to forego the expense and discomfort. By contrast, Segil’s favourite thing about working on sense-restoring prosthetics is how closely their users seem to bond with them. “They refer to this device – which is plastic and metal – as themselves,” he says. “They talk about ‘my hand’ or they say ‘that felt like you touching me’. And those comments are, for me, somehow more impactful than the everyday functionality of the limb. It shows that there’s broader implications if we can restore sensation, and hopefully, eventually, embodiment of the device, for the person as a whole.” More generally, he’s learning how intimately psychology and engineering can be enmeshed. “What these feelings of ownership and agency are built upon is actually not far away from a lot of engineering principles,” he notes. “Ownership has to do with multisensory integration, which is like sensor fusion. You’re just combining multiple sensations together, and when they happen in synchrony, we make a decision based upon it. What we’re noticing is that we can apply some engineering principles to study these psychological ideas, because we have this portal into the body – because we have this neural interface and other experimental methods – to probe them.” As tantalising as that might be for psychologists and medical device designers, Ortiz Catalan warns that it’s a secondary concern. The problem with prosthetics has as little to do with the fact engineers haven’t read enough phenomenology as the fact not all artificial limbs have Iron Man paint jobs or 5G capabilities. The problem is they don’t work for patients. “If you focus on function, and that function is intuitive, then it’s rather logical that the patient will adopt the technology and feel it as part of their arm,” Ortiz Catalan says. “My concern is that there are a lot of engineering projects that are not considering the patient from the viewpoint of what their needs are. If you focus on the needs, and you satisfy those, everything else will fall into place.” There’s a mantra for every medical device manufacturer, no matter their product. ●


72 Medical Device Developments / www.nsmedicaldevices.com


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