Prosthetic arm with a bionic hand which detects tactile stimulation and sends that sensory signal to ... [+]
Neuroscientists have found that people with bionic hands can't seem to retrain the brain so that tactile sensations match locations on their robotic limbs.
When a person touches something with the artificial thumb, for instance, they might think that sensory input came from another part of their prosthetic hand, which suggests that neural circuits are less flexible than scientists thought.
The finding was made possible thanks to recent advances in prosthetic limbs. In early 2020, a team led by Max Ortiz-Catalan at Chalmers University of Technology in Gothenburg, Sweden, implanted bionic limbs in four people whose arms had been amputated above the elbow. Each prosthesis is anchored to the humerus bone and has an artificial skeleton with muscles controlled by a user's neural activity.
That 'neuromusculoskeletal' prosthesis also includes sensors in its bionic hand that send tactile feedback to nerves in a person's upper arm — signals that are relayed to the brain to provide a sense of touch.
Surgeons can connect the hand's sensors to a person's nerves via electrodes — but not necessarily at the same point in a circuit that was used by the original arm (because nerves are organised in a relatively arbitrary way), which means an artificial thumb might become wired to circuitry that expects to receive signals from an index finger, for example.
Neural circuits aren't always hard-wired, however: there's some flexibility or 'plasticity' in the connections at synapses between nerve cells (neurons), which is what caused many scientists to assume that the brain would be able to rewire synaptic connections. For a prosthesis, that would mean sensory signals from a bionic thumb could be relayed via nerves originally used by an index finger to be correctly interpreted as sensations from a thumb, rather than the finger.
People cannot remap perceived location of a sensation to the stimulated area.
But according to the latest research by Ortiz-Catalan and colleagues, the brain can't rewire circuits to match the correct tactile sensation from a bionic hand.
The new study involved three people with amputations who had used a robotic hand to manipulate objects for more than 12 hours at home every day, for over a year. And yet despite their extensive practice, the study's participants never managed to remap their subjective sensations from a perceived location to the stimulated area, from where they thought they felt something to where it actually touched.
So it appears that people can't simply fix their sense of touch, which implies that developing prosthetic limbs that feel like real ones will be more challenging than researchers once believed.
