Revisiting face-to-hand area remapping in the human primary somatosensory cortex after a cervical spinal cord injury

Spinal cord injury (SCI) leads to profound disruptions in sensorimotor processing. Seminal research in non-human primates suggests that this deprivation of sensory input causes functional remapping within the primary somatosensory cortex (S1). Crucially, cortical somatotopic representations of deprived body parts, such as the hand in cervical SCI, become responsive to touch on intact body parts, such as the face. However, in humans, the evidence for cortical remapping following SCI remains inconclusive.

Here, sixteen chronic cervical SCI patients (mean age ± s.e.m. = 52.4 ± 3.5 years; 1 female) and 21 able-bodied controls (age: 49.9 ± 3.4; 2 females) participated in two experiments in which we used a systematic and sensitive approach to determine the full extent of face-to-hand area remapping. In Experiment 1, we employed a lip movement fMRI paradigm, as previously used in human remapping studies. In Experiment 2, we investigated the full architecture of S1 face reorganisation through vibrotactile stimulation of the forehead, lips and chin. Firstly, we assessed whether the level of face activity in the anatomical S1 hand area was increased in SCI patients compared to controls. Secondly, we evaluated potential cortical shifts in peak face activity using geodesic distances. Thirdly, we used representational similarity analysis, where, in the instance of S1 face-to-hand remapping, we anticipated larger representational distances (i.e., more somatotopic information) between the different face sites in the S1 hand area. Finally, we explored what clinical characteristics may be driving face-to-hand area remapping.

Our results revealed no significant differences between control participants and tetraplegic patients in any of our three markers for reorganisation during lip movement (Experiment 1) or vibrotactile stimulation (Experiment 2). Furthermore, we found no significant correlations between remapping and the clinical traits of the SCI patients.

These findings suggest that cortical reorganisation is not apparent in human cervical SCI patients. As such, the extent of face-to-hand area reorganisation after a human SCI appears to differ fundamentally from what has been observed in experimental models of SCI. Beyond providing essential insights into the limitations of cortical reorganisation in humans, these findings call for a reassessment of rehabilitation strategies based on the assumption of face-to-hand reorganisation after an SCI.