Technologies in clinical neurophysiology for brain-body interfacing: IFCN handbook chapter.

Interfacing the brain with the body represents a central challenge in contemporary clinical neurophysiology. Neurological injuries such as stroke, spinal cord injury, and limb loss disrupt the bidirectional flow of information between central and peripheral circuits, impairing voluntary movement, sensation, and embodiment. Clinical neurophysiology provides the quantitative and methodological foundation necessary to understand and modulate this communication. By characterizing neural signals across cortical, spinal, and peripheral levels, it establishes the principles required to decode and stimulate the nervous system. Traditionally focused on diagnostic assessment through electrophysiological techniques, clinical neurophysiology provides the conceptual and technical basis for active brain-body interfacing. Within this framework, interfacing encompasses both "reading" and "writing" neural information. Decoding refers to the extraction of meaningful variables (e.g., movement intention) from neural activity. Actuation refers to the targeted delivery of electrical stimulation or mechanical assistance to modulate neural circuits or generate movement and sensation. This chapter reviews neurophysiology-based strategies across multiple neuroanatomical levels and how integrating decoding and actuation into closed-loop architectures enables the re-establishment of bidirectional information flow. By coupling neural control with functional feedback, these systems enhance control precision, support embodiment, and engage activity-dependent plasticity, providing a mechanistically grounded strategy for modulating disrupted neural communication.