Stimulating the deprived motor ‘hand’ area causes facial muscle responses in one-handers

How the brain adapts to the absence of sensory inputs and motor outputs from early development is a key question in neuroscience. While sensory deprivation has long been known to trigger changes to cortical maps in sensory and motor cortex, e.g. due to blindness [1], deafness [2] or arm malformation [3], there is an ongoing debate on their functional relevance. For example, using fMRI, we have previously reported that the deprived sensorimotor hand territory of individuals born without a hand shows increased activity evoked by movements of the lips, feet and residual arm, when compared to two-handed controls [4,5]. This is consistent with the idea that the missing-hand area may be recruited to support increased representation of alternative intact body parts. However, changes to cortical map boundaries (i.e., remapping), as measured through fMRI, do not necessarily entail changed representational content and readout [3]. As such, the observed remapping may not subserve a functional role. By allowing to directly stimulate the motor cortex and measure the evoked muscular responses in various body parts, TMS provides a unique opportunity to causally assess the functional consequences of motor remapping. Few previous studies have used this technique to investigate the functional content of the missing-hand area of individuals with congenital limb deficiencies. In people born without both hands and with exceptional foot dexterity, TMS over the missing-hand area elicited MEPs in the feet and interfered with foot movements [6,7]. In congenital one-handers, studies have focused on the residual arm [8e10], with mixed evidence: two studies reported an expansion of the residual arm representation [8,9], but this has not been replicated [10]. Here, we used single-pulse TMS to investigate the functional relevance of the previously observed remapping of the lips in the missing-hand motor cortex of one-handers [4,5]. We hypothesised that functional reorganisation should result in facial MEPs when the missing-hand area is activated.