Psychology’s role in developing pioneering prosthetics

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.

“Psychology is ideal for tying things together—the engineering aspects, the technical possibilities, and the desired technical advancements with the psychosocial characteristics of the individual,” said Marcia Scherer, PhD, MPH, a research psychologist and president of the Institute for Matching Person and Technology in Webster, New York.

For decades, she has seen examples of products designed by engineers to dazzle other engineers that don’t appeal to potential users because they haven’t considered what the experience of using the device will be like. Examples include a vision-enhancement device that resembles a motorcycle helmet and a giant wheelchair for children. “Just because it’s possible,” she said, “doesn’t mean the consumer’s going to want it.”

Prostheses, which include artificial limbs as well as devices that restore hearing and speech, are in high—and growing—demand. Some 1.9 million people in the United States are living with limb loss, according to a 2008 report, a number that is expected to double by 2050 (Ziegler-Graham, K., et al., Archives of Physical Medicine and Rehabilitation, Vol. 89, No. 3, 2008opens in new window).

Diabetes and aging are major contributors. The tally also includes some 1,700 military service members with deployment-related amputations in the past 2 decades, according to the Military Health Systemopens in new window, and an estimated 1.8 million veterans currently at risk for amputation because of diseases like diabetes. Hearing problems are another major area of concern, among both veterans and the general population. As many as 29% of post-9/11 veterans have experienced hearing loss, according to the National Center for Rehabilitative Auditory Researchopens in new window (NCRAR). At some point, according to the World Health Organizationopens in new window (WHO), most people will need help from assistive technologies, which include prostheses.

In addition to their quality-of-life benefits, prostheses can help mitigate the mental health consequences of impairments in hearing and speech, as well as of limb loss. These are life-altering experiences that, many studies show, can lead to social isolation, unemployment, anxiety, and depression (Alessa, M., et al., Cureus, Vol. 14, No. 11, 2022opens in new window). Untreated hearing loss has also been linked to a higher risk of dementia, cognitive decline, and mental health issues. In studies of veterans, NCRAR has linked hearing problems, including tinnitus, with posttraumatic stress disorder and substance abuse.

One goal of prosthetic devices is to improve mental health by restoring function, providing independence, and improving self-esteem. Yet many people never get devices that could help. That’s especially true in certain countries and communities, Scherer said. Hearing loss affects 1.5 billion people globally, for example, but less than 10% of people who could benefit from hearing aids get access to them, according to the WHOopens in new window. Even when devices are available to people, many don’t bother. Fewer than half of Medicare recipients who have lost a limb go to an initial appointment to start the process, Scherer said.

People who do try prostheses frequently give up on them, especially for upper limb devices. Among 68 people with upper arm amputations in Austria, 44% abandoned prosthetics, despite recent advances in technology (Salminger, S., et al., Disability and Rehabilitation, Vol. 44, No. 14, 2022opens in new window). For those with amputation at the shoulder, rates of abandonment reach 60% (Brack, R., & Amalu, E. H., Journal of Orthopaedics, Vol. 23, No. 1, 2021opens in new window). Previous studies of upper limb amputees have found rejection rates as high as 81%. Even for the most expensive electric prosthetics, reported a 2021 study, rejection rates still hovered around 20% (Moore, C. H., et al., Frontiers in Neurorobotics, Vol. 15, No. 662397, 2021opens in new window).

One of the main reasons for giving up on prostheses, studies show, is that people often struggle to feel like the device belongs to them, especially when it doesn’t work, look, or feel like a natural body part. “There is a huge rate of abandonment of prostheses because the patient maybe has low functionality,” said Giacomo Valle, a neural engineer at the University of Chicago. “But also the fact that these prostheses are not perceived as part of your body.”

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, 2023opens in new window). 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, 2022opens in new window). 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.”

A device has to be more than just functional for a person to fully embody it, said Gianluca Saetta, PhD, a neuroscientist with a background in clinical psychology at ETH Zurich in Switzerland. He is part of a multidisciplinary effort to unravel and reconnect the relationship between body and self.

To enhance the experience of embodiment, Saetta has been investigating phantom limb sensations, which are common after amputation and, some experts suspect, can interfere with the adoption of prosthetics. Phantom limb sensationsopens in new window emerge, according to one leading theory, from maladaptive rewiring of the brain after amputation. Pain is a common result, but sensation can also get transferred from the missing limb to another body part. Saetta has seen some patients who felt their amputated hand when touched in a specific spot on their face.

Taking advantage of the brain’s plasticity, Saetta proposes, might make it possible to train the mind to accept a prosthesis and improve embodiment. As part of a project called IntelliManopens in new window that is working toward artificial intelligence-powered prosthetics among other goals, he and colleagues trained participants without limb loss to use either a virtual hand or a virtual prosthesis to move an object within a virtual reality (VR) environment. Experimenting with the device’s haptics, particularly by compensating for a feeling that an object is slipping from the hand when grasped, enhanced embodiment, they found in a study that has been submitted for publication. There were no significant differences when using a virtual hand or a virtual prosthesis, suggesting that people were equally able to accept and embody either. Individuals who reported heightened embodiment also tended to allocate more attention to the object rather than the hand, indicating that a seamlessly integrated prosthesis demands less visual focus from the user.

In another study that has been posted as a preprint but not yet peer-reviewedopens in new window, VR allowed participants to play a game as an amputated avatar. The researchers found that people with body integrity dysphoria, a condition that causes some to express the desire for amputation, more fully embodied the avatar, experiencing a stronger sense of ownership of the virtual body. Besides demonstrating the potential usefulness of VR in both prosthetics research and therapies for a variety of conditions, these studies illustrate the importance of understanding the nuances of embodiment, Saetta said.

Psychological and personality traits also matter in how successfully someone will embody a prosthetic, Saetta said. In some of his newest research, he is attempting to find correlations between embodiment and a range of factors, including age, socioeconomic status, gender, attitudes about robots, and the “big five” personality traits: openness, conscientiousness, extraversion, agreeableness, and neuroticism.

His findings are still under review for publication, but he said, understanding all the pieces involved in acceptance of prosthetics could help researchers improve embodiment on an individual basis. “A person with neuroticism, for example, may be less able to accept the prostheses as part of them,” he said. “Here comes the psychology into play.”

Bioengineers, neuroscientists, and technology alone cannot boost acceptance rates, Scherer added. “Those professions can benefit from a more interdisciplinary team that involves a rehabilitation psychologist,” she said. Rehabilitation psychologists help people with disabilities and chronic illnesses achieve independence and pursue opportunities. Taking into account psychological factors, studies show, is essential for helping a person incorporate a prosthetic into the perceptions of their own bodiesopens in new window.

Because it requires conscious concentration, using a prosthesis imposes a greater cognitive loadopens in new window than using a natural limb. When asked to spell a 5-letter word backward, Valle said, people with leg prosthetics do the task more slowly while walking, presumably, electroencephalogram data suggest, because walking with a prosthesis saps a finite amount of mental resourcesopens in new window.

Improving the embodiment of prostheses might be able to reduce the cognitive load, suggests Valle’s recent research. In one study, he and colleagues tested the abilities of three people with lower limb amputations who were equipped with robotic feet and an implanted device that connected to the nerves in their legs. These prostheses incorporated sensors that delivered information about pressure to the nervous system in an attempt to mimic the electrical stimulation of walking on a healthy leg. Using the system this way, the participants were able to walk faster up and down stairs, and they said that walking felt more natural than when the device sent basic electrical pulses that didn’t mimic natural signals. With the biomimetic setting, Valle and colleagues found in a 2024 study, users were also able to spell words backward 20% faster (Nature Communications, Vol. 15, No. 1151, 2024opens in new window).

Improving sensory feedback to enhance embodiment might also make a prosthetic feel less burdensome. A typical prosthetic leg weighs half as much as a natural leg, but nearly 70% of amputees find their prosthetic to be uncomfortably heavy. When Valle and colleagues delivered sensory feedback via electrical signals directly to the nerves in the leg of a study subject with an above-the-knee prosthesis, measurements suggested a 23% reduction in perception of how heavy the leg was, a 36% increase in confidence while walking, and a 60% increase in the sense of embodiment (Current Biology, Vol. 31, No. 5, 2021opens in new window).

A reduction in phantom pain is another potential benefit of improved embodiment. Arm prostheses that deliver sensations through neural stimulation have alleviated phantom sensationsopens in new window in multiple studies. Getting rid of pain and phantom sensations, experts say, makes it more likely that someone will accept a prosthesis as part of them.

There is plenty of technological and neurological work yet to be done to make prostheses that feel like natural body parts and that come to be fully integrated into a person’s sense of self. Some of today’s most advanced prosthetics that stimulate neurons in the brain to mimic real sensations, for example, involve only a small number of the brain’s many billions of cells, Hatsopoulos said. We have, in other words, likely only scratched the surface of the kind of sensation that people have in their natural limbs. The goal of full restoration is still a significant way off.

Longevity is another challenge, Hatsopoulos said. Eventually, electrodes and other components wear out and need to be replaced. That raises ethical and philosophical questions. What happens, Valle asks, if you come to view a prosthesis as part of your personal identity and then it, too, breaks?

That’s not the only unanswered question raised by rapid advances in prostheses. In a comprehensive review of studies on neuroprostheses, researchers from the Netherlands identified 169 ethical implications (van Velthoven, E. A. M., et al., Journal of Neural Engineering, Vol. 19, No. 026055, 2022opens in new window). For instance, implantation of electrodes into the brain carries health risks, and there are long-term uncertainties about safety. Other questions concern autonomy, the relationship between people and machines, and threats to communities, like Deaf culture. In addition, although many studies mention a goal of improving well-being and life satisfaction with neuroprosthetic devices that restore hearing or speech, devices don’t always work perfectly, and they can end up causing suffering when results don’t match expectations.

Hearing aids, in particular, face barriers to acceptance that include lack of awareness. Even though 1 in 8 U.S. adults has problems with hearing, which affects more than two thirds of people older than 70, more than 90% of people couldn’t identify what a normal range of hearing is in a survey of 1,250 adults ages 50 to 80 (Carlson, M. L., et al., Otology & Neurotology, Vol. 43, No. 3, 2022opens in new window). Most adults don’t know that there are long-term health consequences (such as cognitive decline) to untreated hearing loss or that there are treatments that can help.

As technological advances continue, rehabilitation psychologists remain an essential part of an interdisciplinary team of prosthetists, physical therapists, engineers, and others who can work together to help people learn to use and embody prostheses, Scherer said. Rehab psychologists make good managers of rehabilitation teams, she added, but they are in short supply. Even though APA Division 22 (Rehabilitation Psychology) was formed in 1958, fewer than 1% of psychologists today self-report a specialty in rehabilitation psychology, according to recent APA data, and just 4% hold an American Board of Professional Psychology certification in the specialty.

Many rehabilitation patients get help from psychologists not trained in rehab psychology, found an analysis of postdoctoral rehabilitation psychology training in the United States and Canada (Stiers, W., & Stucky, K., Rehabilitation Psychology, Vol. 67, No. 3, 2022opens in new window). Solutions will need to include more outreach and advocacy to expand the pipeline and visibility of the profession, concluded researchers in a 2020 report on the state of the field which was the first of its kind (Baker, L. N., et al., Rehabilitation Psychology, Vol. 67, No. 2, 2020opens in new window).

“A rehabilitation psychologist is focused on coping, adaptation, maximizing functioning, enhancing quality of life, helping people find employment, helping them achieve what they want to do,” Scherer said. “There are not enough of them.”

If prostheses eventually reach the point where they can truly mimic or improve sensation and performance, there will be yet more issues to grapple with, Hatsopoulos said. Should prosthetics be reserved for those with paralysis, for example, or should they become available as performance enhancers for people with ordinary abilities or to make high-performing athletes even more competitive? “When the time comes when you can actually enhance people’s capabilities, even people without spinal cord injury,” he said, “there are definitely ethical considerations.”