Perceiving prosthesis as lighter thanks to neurofeedback

Signals from tactile sensors under the sole of the prosthetic foot and from angle sensors in the electronic prosthetic knee joint are passed on to the nervous system. (Visualisations: ETH Zurich)

Trans­mit­ting sens­ory sig­nals from pros­theses to the nervous sys­tem helps leg am­putees to per­ceive pros­thesis as part of their body. While am­putees gen­er­ally per­ceive their pros­theses as heavy, this feed­back helps them to per­ceive the pros­theses as sig­ni­fic­antly lighter, ETH re­search­ers have shown.

Leg am­putees are of­ten not sat­is­fied with their pros­thesis, even though the soph­ist­ic­ated pros­theses are be­com­ing avail­able. One im­port­ant reason for this is that they per­ceive the weight of the pros­thesis as too high, des­pite the fact that pros­thetic legs are usu­ally less than half the weight of a nat­ural limb. Re­search­ers led by Stan­isa Raspop­ovic, a pro­fessor at the De­part­ment of Health Sci­ences and Tech­no­logy, have now been able to show that con­nect­ing the pros­theses to the nervous sys­tem helps am­putees to per­ceive the pros­thesis weight as lower, which is be­ne­fi­cial for their ac­cept­ance.

To­gether with an in­ter­na­tional con­sor­tium, Raspop­ovic has de­veloped in re­cent years pros­theses that provide feed­back to the wearer’s nervous sys­tem. This is done via elec­trodes im­planted in the thigh, which are con­nec­ted to the leg nerves present there. In­form­a­tion from tact­ile sensors un­der the sole of the pros­thetic foot and from angle sensors in the elec­tronic pros­thetic knee joint are con­ver­ted into pulses of cur­rent and passed in to the nerves.

“To trick an above-​knee am­putee’s brain into the be­lief that the pros­thetic leg was sim­ilar to his own leg, we ar­ti­fi­cially re­stored the lost sens­ory feed­back,” says ETH pro­fessor Raspop­ovic. In a study pub­lished last year, he and his team showed that wear­ers of such neur­o­feed­back pros­theses can move more safely and with less ef­fort.

Be­ne­fi­cial in­volve­ment

In a fur­ther study, the sci­ent­ists were now able to show that neur­o­feed­back also re­duces the per­ceived weight of the pros­thesis. They pub­lished the res­ults in the journal Cur­rent Bio­logy.

In or­der to de­term­ine how heavy a trans­femoral am­putee per­ceives their pros­thetic leg to be, they had a vol­un­tary study par­ti­cipant com­plete gait ex­er­cises with either neur­o­feed­back switched on or off. They weighed down the healthy foot with ad­di­tional weights and asked the study par­ti­cipant to rate how heavy he felt the two legs were in re­la­tion to each other. Neur­o­feed­back was found to re­duce the per­ceived weight of the pros­thesis by 23 per­cent, or al­most 500 grams.

The sci­ent­ists also con­firmed a be­ne­fi­cial in­volve­ment of the brain by a motor-​cognitive task, dur­ing which the vo­lun­teer had to spell back­wards five-​letter words while walk­ing. The sens­ory feed­back not only al­lowed him to have a faster gait but also to have a higher spelling ac­cur­acy.

“Neur­o­feed­back not only en­ables faster and safer walk­ing and pos­it­ively in­flu­ences weight per­cep­tion,” says Raspop­ovic. “Our res­ults also sug­gest that, quite fun­da­ment­ally, it can take the ex­per­i­ence of pa­tients with an ar­ti­fi­cial device closer to that with a nat­ural limb.”

Ref­er­ence

Preatoni G, Valle G, Pet­rini FM, Raspop­ovic S: Light­en­ing the per­ceived pros­thesis weight with neural em­bod­i­ment pro­moted by sens­ory feed­back. Cur­rent Bio­logy, 7 Janu­ary 2021, doi: 10.1016/j.cub.2020.11.069

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Fa­bio Ber­ga­min
ETH Zurich

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The development of medical equipment, products and technical procedures is characterized by high research and development costs in a variety of fields related to the study of human medicine.

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