By Emily Sohn / American Psychological Association
The oldest known prosthetic device was a 3,000-year-old toe made out of leather and wood for the daughter of an Egyptian priest. In the centuries since, advances in engineering and neuroscience have led to the development of prosthetics that were once confined to the realm of science fiction.
Devices have become lighter, more flexible, and more lifelike. Some connect to nerves, bones, muscles, and neurons in the brain to restore hearing, speech, and movement of artificial limbs. Experimental models are even beginning to restore the sense of touch.
But there is a lot more to prosthetics than gizmos, gadgets, and conduits to the nervous system. Psychology matters, too, not only in determining whether people will want and accept a device as part of their own body but also in increasing the chances of acceptance by designing these prosthetic devices to feel more real and integrated with the human body. As prosthetics research continues alongside a growing need for prosthetic devices, experts say, psychology is increasingly playing a role in both the design and rehabilitation processes.
Boosting integration with tech
Based in part on a classic research paradigm called the rubber hand illusion, which demonstrates the brain’s ability to experience sensations from a body part that is not its own, researchers have been working for decades to improve this sense of embodiment in the field of prosthetics (Castro, F., et al., Neuroscience Biobehavioral Reviews, Vol. 153, No. 105351, 2023). The effort has become a multidisciplinary collaboration encompassing neurosurgery, engineering, rehabilitation therapy, psychology, philosophy, and other fields.
On the technology side, scientists are developing strategies for restoring motor and sensory functions in increasingly realistic ways, based on the idea that it will be easier to embody an artificial limb that works as much as possible like a human one. Myoelectric prostheses, for example, connect a device to electrical signals in the muscles. Neuroprosthetics, which include cochlear implants, can replace or improve the function of the nervous system. Sensors on some prosthetic devices now send messages about pressure and temperature to the brain by electrically stimulating residual nerves in the limbs. Bidirectional prostheses aim to give users both control over and sensation from a prosthetic device.
Many of these technologies are tapping into a growing understanding of neurology by zeroing in on specific neurons that control movement and the sense of touch, said Nicholas Hatsopoulos, PhD, a behavioral neuroscientist at the University of Chicago. Hatsopoulos has a master’s degree in experimental psychology and is part of a team developing brain-computer interfaces that could give people with upper limb paralysis or amputation the ability to move limbs with their thoughts.
Based on animal studies that map neural activity to behaviors, Hatsopoulos is developing biomimetic prosthetics that use electrical brain stimulation to replicate the same patterns that normally happen when someone touches an object. Often, for example, neurons start firing strongly at first touch and then quiet down before another increase upon letting go. His team is also developing computer algorithms that translate these signals from the brain into manipulation of a prosthetic to reflect a person’s intentions. “What does he want to do?” Hatsopoulos said. “How does he want to move? And can we give him that functionality?”
The research is still in its early stages and is not yet integrated into devices that can help people with a full loss of sensation. But when he and colleagues test their techniques, Hatsopoulos said, study participants express amazement that they can actually feel sensations from a robotic hand.
Studies suggest that improving sensory feedback should improve embodiment (Zbinden, J., et al., Journal of NeuroEngineering and Rehabilitation, Vol. 19, No. 37, 2022). The ultimate goal, Valle said, is a realistic-looking prosthetic that could completely restore a sense of touch alongside fine motor control. “This can create, in theory, the highest embodiment possible,” he said, “because you can achieve the perfect incorporation.”