Hand Amputees Experience Distorted Near-Space Perception

Tamar R. Makin, PhD
Tamar Makin, PhD

In today’s world, with enough determination and hard work, anything the heart desires is just within reach. Those missing a hand, however, might have a problem perceiving the distance to those objects of desire. This was the subject of a recent study conducted by Tamar R. Makin, PhD, scientist from the Hebrew University of Jerusalem.

Makin’s interest in this topic began with her fascination with action space, which she defined as “the theater for our interaction with the immediate environment.” Action space does not describe the actual space surrounding the body, but rather our possibilities to act on objects within this space. This intricate tie to the the body allows the brain to use hand-centered coordinates to make further determinations about that space.

She wondered, though, how this affects people who have lost a hand — or who were born without a hand — and their ability to create or effectively process these hand-centered coordinates.

Near versus far

Together with her adviser Ehud Zohary, PhD, associate professor in the de partment of neurobiology at The Hebrew University of Jerusalem, and Meytal Wilf, an undergraduate student at the time, Makin collaborated with Isabella Schwartz, MD, in the department of physical medicine and rehabilitation at Hadassah University in Jerusalem.

The team enlisted the help of 12 unilateral hand amputees — seven left and five right, with 10 below elbow or at the hand, one at the elbow, and one above elbow. These study participants viewed hundreds of images projected onto a screen — two targets in varying positions, one always to the left and one always to the right of a stationary cross, which was positioned in the center of the screen — and, after viewing them for a brief time, determined which of the targets was farther away from the central cross.

Participants completed the exercise both in the near condition, where the amputees were close enough to reach the screen, and in the far condition, where they were farther from the screen.

Makin and her team found that hand amputation leads to consistent biases in visuospatial perception of the space close to the amputee, she said. When viewing the targets placed on their amputated side, amputees repeatedly underestimated the distance; compared with their intact side, which resulted in starkly contrasting reports from the left hand and right hand amputees.

When researchers asked the amputees to repeat the same task farther away from the screen, however — at a distance of 150 cm compared to the near distance of 50 cm — both left hand and right hand amputees performed the task more accurately.

According to Makin’s study, when viewing targets on their amputated side, participants underestimated the distance
According to Makin’s study, when viewing targets on their amputated side, participants underestimated the distance. However when seated further away, they performed this task more accurately.

Visuospatial perception

The fact that the biases were abolished in far space suggests that the possibilities for action may shape perception, but only within action space, Makin told O&P Business News.

“I think the most obvious conclusion that we can draw from this study is that hand amputation has far greater consequences than anyone has suggested before,” she said. “We discovered that hand amputation has far-reaching consequences for sensory perception, specifically visuospatial, but I believe this result is just the tip of an iceberg.”

These results have brought more questions for Makin. First, how does the use of prosthetic devices — functional or cosmetic — affect visuospatial biases? Further, how does this discrepancy between reality and what an amputee perceives affect the rehabilitation process? And is there a possibility that the difference between amputated side and the intact side stems from an overrepresentation of the intact side?

“Maybe the results of this study have absolutely no implications on their lives — I don’t know,” she said.

Ultimately, Makin said that the brain of an amputee undergoes just as much “stress” as the rest of the body after an amputation. The reorganization of the areas of the brain that previously controlled the missing limb may aid the body in rehabilitation, or get in the way of adaptation, causing menaces like phantom pain. Her goal is to determine which is true. She already is exploring this connection between neuroscience and rehabilitation in her current research, a collaboration between the University of Oxford and the Nuffield Orthopaedic Centre.

“We cannot assess the full challenges of rehabilitation following hand amputation before we can have a better understanding of what is happening inside the brain. — Stephanie Z. Pavlou, ELS


Tamar R. Makin, PhD, has touched on a very interesting area of research that demands further exploration, to be sure. What this means for prosthetists is hard to say at this point. Statisticians will immediately object to the small sample size and scoff at what can truly be gleaned from this research. But I like to think in “what if” scenarios and am of the belief the potential underlying reasons are as — if not more — important than the actual result. As such, I am very intrigued, as much as to why visuospatial perceptional bias exists, but also why the distance is underestimated on the prosthetic side. What were the experience levels of the prosthetic subjects? Did the amount of bias differ at the more distal lengths of amputation vs. the more proximal?

These and other questions need to be addressed; however, the upshot is that integration of prosthetic technology results in the brain attempting to adapt in ways we as patients and prosthetists heretofore perhaps didn’t consider to any great degree until now. What this means, with regard to prosthetic and OT training, as well as the prosthetic wearer’s individual adaptation to spatial awareness, remains to be seen. What Makin et al’s research has done is to encourage us to look more closely at the cognitive impact, rather than focusing largely on the physical, and how both the cognitive and physical impacts are inseparable.

— Randall Alley, BSc, CP, FAAOP, CFT
Chief Executive Officer, biodesigns inc. and Practitioner Advisory Council member, O&P Business News

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