Preserved cortical somatotopic and motor representations in tetraplegic humans

Abstract

A rich literature has documented changes in cortical representations of the body in somatosensory and motor cortex. Recent clinical studies of brain-machine interfaces designed to assist paralyzed patients have afforded the opportunity to record from and stimulate human somatosensory, motor, and action-related areas of the posterior parietal cortex. These studies show considerable preserved structure in the cortical somato-motor system. Motor cortex can immediately control assistive devices, stimulation of somatosensory cortex produces sensations in an orderly somatotopic map, and the posterior parietal cortex shows a high-dimensional representation of cognitive action variables. These results are strikingly similar to what would be expected in a healthy subject, demonstrating considerable stability of adult cortex even after severe injury and despite potential plasticity-induced new activations within the same region of cortex. Clinically, these results emphasize the importance of targeting cortical areas for BMI control signals that are consistent with their normal functional role.

Figure 1:

Neural prosthetic control of individual fingers enabled by preserved motor representations. a) The representational structure (RS) of population neural activity BMI control of a virtual hand. RS was computed as the pair-wise distances between fingers (see inset) revealing a clear reproducible pattern across repeitions (adapted from Guan et al. 2021). b) The RS of fMRI signals during overt finger movements in able-bodied individuals (adapted from Ejaz et al. 2015). c) Participant RS closely matches able-bodied RS. Similarity of patient RS (a) and able-bodied RS (b) across sessions (blue) compared to similarity of patient RS (a) and task-optimal RS (orange). Patient RS structure was consistent across time despite leading to BMI errors.

Figure 2:

Visual and planning activity is restricted to high-level cortices in tetraplegic individuals. a) Accuracy of target location decode through time (95% ci) during planning and execution of imagined reaches in primary somatosensory (S1, a) and posterior parietal cortices (PPC, b). Visual cue and planning activity is found in S1 but not PPC. Adapted from Jafari et al. 2020 and Aflalo et al. 2015.