Check out the full pre-print now: doi.org/10.1101/2025.06.16.658246. This work was truly a collaborative and interdisciplinary effort, and we’d like to thank all of our collaborators, and the over 100 participants that made this work possible! @plasticity-lab.bsky.social

Together, these results demonstrate that our somatosensory system can facilitate an immediate and accessible sensory representation of the Third Thumb. This is then refined through experience, allowing integration of the Third Thumb into the hand representation across the sensorimotor hierarchy

Finally, we not only found increased subjective somatosensory embodiment following Third Thumb training, but this increase also correlated with increased similarity between the Third Thumb and biological fingers in S1

Within the S1 hand representation, we saw experience-dependent refinement of the Third Thumb’s sensory representation following training, as it became more similar to the biological fingers in a way not seen for our control group

Following Third thumb training, markerless hand tracking revealed reduced co-usage amongst the biological fingers as the Third Thumb became integrated into the hand’s coordination patterns

To explore how sensorimotor experience refines this representation, participants then underwent 7 days of motor training involving Third Thumb-biological hand collaboration, or our control group instead trained to play the piano keyboard

Using RSA, we found the immediate emergence of a topographically organised Third Thumb sensory representation within S1. This representation is also distinct from the biological palm representation

But how does the brain then represent this natural sensory information? We next stimulated the Third Thumb and the biological hand, and used fMRI to map their sensory representations in primary somatosensory cortex

Despite the tasks being designed for the artificial systems, participants performed equally, or even outperformed, with the natural feedback, demonstrating that meaningful, interpretable information can be extracted from the natural feedback across versatile task demands

To put this natural feedback to the test, we created 2 new Third Thumb systems - one integrated artificial skin stretch feedback, the other integrated artificial vibrotactile feedback. We then compared the artificial to the 'natural' feedback across perceptual discrimination tasks

This is possible through the previously neglected role of the ‘natural’ feedback received as a natural by-product of how artificial limbs interface with the biological body. For example, the Third Thumb is worn and moves on the hand, this naturally generates sensory information felt across the palm

However, despite having no artificial feedback, we have previously demonstrated that people can perform a coordination task with the Third Thumb when blindfolded, relying on a sense of proprioceptive awareness of the robotic digit

Somatosensory feedback is crucial for motor learning, yet artificial limbs are thought to lack such feedback. Research has therefore focused on creating artificial sensory feedback, but these signals do not replicate the rich, multimodal information available from natural touch-see shorturl.at/2PMyU