Prosthetic users less affected by ‘size-weight illusion’, research shows

People with prosthetic arms or hands do not experience the "size-weight illusion" as strongly as other people, new research shows.

The size-weight illusion, which affects about 98 per cent of people, causes them to experience smaller objects as feeling heavier than larger objects of the same weight.

The study - led by the University of Exeter and the University of Strathclyde - compared the perception of people using their anatomical hands with that of amputees using prosthetic limbs.

The researchers were surprised to find that the size-weight illusion was twice as strong in non-amputees lifting with their hand as it was in the prosthetic users.

"This unexpected finding suggests that using a prosthesis might fundamentally affect the way people perceive the world," said Dr Gavin Buckingham of the Department of Sport and Health Sciences at the University of Exeter.

"People using a prosthetic hand perceive real weight differences just like everybody else, but the effect of the size-weight illusion is halved.

"The reasons for this are a little mysterious. It might be to do with the lack of sensory receptors in a prosthetic hand, or might depend on how the prosthetic hand is attached to the stump."

In a second experiment, the researchers tested how the illusion affected non-amputees who used a prosthetic hand simulator to lift objects.

The results were similar to those for amputees with prosthetic limbs - the effect of the size-weight illusion was halved.

Sarah Day, of the National Centre for Prosthetics and Orthotics in the Department of Biomedical Engineering at the University of Strathclyde, added: "Many amputees prefer not to use prosthetic arms or hands, but the reasons for this are not well understood. Research like this might help us better understand why."