Perceiving prosthesis as lighter thanks to neurofeedback

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.”

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