Perception
Does being able to move a hand help you recognise hands?
Open access · cc by · source: Europe PMC
People born with one hand judged pictured hands less accurately and more slowly on awkward postures than amputees, and amputees who could move their phantom hand better made faster judgements, suggesting that motor control, not just visual experience, feeds into how we see bodies.
Study at a glance
- Design
- Case-control — Three-group comparison (controls, congenital one-handers, acquired amputees) on a vocal hand-laterality judgement task with easy vs hard postures, plus correlations with phantom-hand finger-tapping speed and an exploratory drift-diffusion simulation
- N
- N=54 · 54 analysed participants: 21 two-handed controls (one of 22 excluded), 17 congenital one-handers and 16 amputees; a separate group of 13 controls rated image difficulty. One amputee could not move the phantom fingers and was left out of the phantom motor analysis.
- Population
- Adults in the UK with congenital unilateral hand absence or acquired unilateral upper-limb amputation, plus age-matched two-handed controls
- Outcome
- Accuracy and reaction time for left/right hand-laterality judgements; response bias (signal detection criterion); correlation of amputees' judgement speed with phantom-hand motor control and age at amputation
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Key findings
Congenital one-handers were less accurate when judging images of their intact hand than their missing hand, and signal detection analysis showed this was a bias towards answering 'missing hand' rather than poorer discrimination. On hard postures, congenital one-handers were slower than amputees. Among amputees, faster phantom-hand tapping went with faster laterality judgements, while age at amputation (a proxy for past visuomotor experience) did not predict performance. A model with two parallel hand 'simulators' for controls and one for congenital one-handers reproduced the group patterns.
Methodology
Participants saw photographs of left and right hands in postures ranging from easy to biomechanically awkward and said 'left' or 'right' as fast as possible. The three groups had similar experience of watching other people's hands but differed in their own motor experience: two-handed controls, people born without one hand, and people who had lost a hand through amputation. Amputees also performed a finger-to-thumb tapping task with their phantom hand to measure how much motor control they still felt over it.
Limitations
Groups were small and naturally formed, so they may differ in ways other than hand experience, and the key phantom-motor correlation rests on the small amputee group alone. Most amputees had lost their non-dominant hand, so the study cannot say how hand dominance or which side is missing affects the results. The drift-diffusion simulation was built after the data were seen and was hand-tuned to group averages, so it is an illustration rather than a test. The correlation between phantom motor control and judgement speed does not by itself show that one causes the other.
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