Sensory and Motor Building Blocks of an Extra Robotic Digit Representation

Longitudinal Third Thumb training linked to stronger fMRI activity in the M1 hand area during both Third Thumb control and sensory consequences alone, compared with short-term exposure.

• Why it matters: The work addresses how M1 builds representations for a completely novel effector within an existing sensorimotor system, and how experience reshapes that mapping.
• What they did: Participants were scanned with fMRI after short-term Third Thumb exposure or after longitudinal Third Thumb training; the device was toe-controlled, while its sensory effects were felt on the hand. Multivariate and pattern component modelling quantified how the Third Thumb representation related to toe motor versus hand sensory patterns.
• The result: With prolonged experience, the Third Thumb representation became more distinct from its biological toe controller and shifted from toe-dominated to sensory-dominated contributions, implying flexible recombination in M1.

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