Almost-lifelike hands perceived as creepy

If you had to have a prosthetic hand, would you want it to look like a real hand? Or would you prefer a gleaming metallic number, something that doesn’t even try to look human?

A new study looks at one of the issues that prosthetic designers and wearers face in making this decision: the creepy factor. People tend to get creeped out by robots or prosthetic devicesthat look almost, but not quite, human. So Ellen Poliakoff and colleagues at the University of Manchester in England had people rate the eeriness of various prosthetic hands.

Forty-three volunteers looked at photographs of prosthetic and real hands. They rated both how humanlike (realistic) the hands were and how eerie they were, defined as “mysterious, strange, or unexpected as to send a chill up the spine.” Real human hands were rated both the most humanlike and the least eerie (a good thing for humans). Metal hands that were clearly mechanical were rated the least humanlike, but less eerie overall than prosthetic hands made to look like real hands, the team reports in the latest issue of Perception.

The realistic prosthetics, like the rubber hand shown above, fell into what’s known as the uncanny valley. That term, invented by roboticist Matsuhiro Mori in 1970, describes how robots become unnerving as they come to look more humanlike. The superrealistic Geminoid DK robot and the animated characters in the movie The Polar Express suffer from this problem . They look almost human, but not quite, and this mismatch between expectation and reality is one of the proposed explanations for the uncanny valley. In particular, if something looks like a human but doesn’t quite move like one, it’s often considered eerie.

A bit of brain research supports the mismatch idea, with functional MRI brain scans showing that parts of the brainthat process visual and motor cues are more active when watching videos of an android robot than of a real person or of the android with its “skin” removed. This may reflect the brain working overtime when an android “appears human, but does not move biologically,” the authors write.

According to the new study, artificial human hands also show the uncanny valley effect. Yet something was different. Within the photos of realistic-looking prostheses, people rated some of the most realistic ones less eerie (see numbers 8 and 9 in the chart above). That’s not generally true for robotic faces; people tend to find even the most realistic of them eerie.

The idea thatthe most realistic prosthetic hands might avoid the uncanny valley is intriguing, and promising for prosthetic wearers, who naturally don’t want people to recoil from their prostheses. But the new data are very preliminary. People in the study didn’t see the hands moving, or even see them in three dimensions. A mismatch in movement or touch — say, a very realistic-looking hand that felt rubbery or moved mechanically — would almost certainly plunge a realistic prosthetic back into the uncanny valley.

Efforts to develop affordable functional prosthetics, for users in developing countries for example, sometimes dress up the robotics with a rubberized hand for a more “natural” look, but they may not be doing the users a favor unless the result looks and moves very realistically.

What’s considered eerie may be different for prosthetic hands that move compared with ones that don’t. Prostheses that move may be better off not trying to look realistic at all, to avoid that mismatch effect. But many prosthetic hands are still simple nonfunctional devices that are mainly cosmetic. These nonfunctional hands might be able to avoid eeriness if they are realistic enough. Keep in mind that exceedingly few people missing limbs have access to or could afford the latest mind-blowing functional prosthetics like the one demonstrated by this man in England.

I would love to know what people with hand prostheses have experienced and what reactions they get to mechanical versus realistic versions. If I needed a prosthetic, I might want one of the amazing bespoke alternative limbs that some artists are creating. There are so many ways to find beauty in the human form, after all, without trying to copy nature directly.