Abstract
The current literature on Intra-Spinal Micro-Stimulation (ISMS) for motor prostheses is reviewed in light of neurobiological data on spinal organization, and a neurobiological perspective on output motor modularity, ISMS maps, stimulation combination effects, and stability. By comparing published data in these areas, the review identifies several gaps in current knowledge that are crucial to the development of effective intraspinal neuroprostheses. Gaps can be categorized into a lack of systematic and reproducible details of: (a) Topography and threshold for ISMS across the segmental motor system, the topography of autonomic recruitment by ISMS, and the coupling relations between these two types of outputs in practice. (b) Compositional rules for ISMS motor responses tested across the full range of the target spinal topographies. (c) Rules for ISMS effects' dependence on spinal cord state and neural dynamics during naturally elicited or ISMS triggered behaviors. (d) Plasticity of the compositional rules for ISMS motor responses, and understanding plasticity of ISMS topography in different spinal cord lesion states, disease states, and following rehabilitation. All these knowledge gaps to a greater or lesser extent require novel electrode technology in order to allow high density chronic recording and stimulation. The current lack of this technology may explain why these prominent gaps in the ISMS literature currently exist. It is also argued that given the “known unknowns” in the current ISMS literature, it may be prudent to adopt and develop control schemes that can manage the current results with simple superposition and winner-take-all interactions, but can also incorporate the possible plastic and stochastic dynamic interactions that may emerge in fuller analyses over longer terms, and which have already been noted in some simpler model systems.
Highlights
The goal of this review is to bring together our current neurobiological understanding of spinal organization, and of motor modularity, together with the needs and issues arising in designing and implementing an intraspinal microstimulation (ISMS) neuroprosthetic device
The co-stimulation experiments indicated that the Intra-Spinal Micro-Stimulation (ISMS) superposition and modularity results together could be used as a compositional system to artificially construct force patterns in the limb, using modules drawn from the basis set embedded in spinal cord
This framework naturally handles the simple linear superposition results of resting spinal cord as special cases, but can extend to the more complex control conditions elaborated as potential issues in Section A Program of ISMS Research Based on the Current “Known Unknowns.”
Summary
The goal of this review is to bring together our current neurobiological understanding of spinal organization, and of motor modularity, together with the needs and issues arising in designing and implementing an intraspinal microstimulation (ISMS) neuroprosthetic device. The co-stimulation experiments indicated that the ISMS superposition and modularity results together could be used as a compositional system to artificially construct force patterns in the limb, using modules drawn from the basis set (otherwise termed a collection of primitives) embedded in spinal cord This was definitively demonstrated as an ISMS control method in frog by Lemay et al (2001). Neurobiological analyses showed a collection of spinal primitives to construct reflex behaviors could be identified in the frog These were based on systematic recruitment of a few (spatial) muscle synergies in welldefined pulsed patterns. Most researchers have routinely examined ISMS functional effects while working from a quiescent cord baseline state, and stayed restricted to a hemicord rather than considering bilateral coordination and control issues This is in part a limitation imposed by the constraints of preparation stability and longevity that are currently possible even with state of the art acute and chronic electrodes. The new neurotechnologies anticipated will hopefully allow areas to be explored, identified and estimated with much greater precision than the community’s current sketches of spinal stimulation effects to date have allowed
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