Contingent sensory / proprioceptive feedback

The afferent side of the rehabilitation loop: delivering sensory feedback — proprioceptive from an orthosis or FES-driven movement, cutaneous, or via direct somatosensory stimulation — that is contingent on and time-locked to the patient's decoded motor command. The rationale is that recovery depends on re-pairing efferent intent with matching reafferent input within the plasticity window, so the brain receives the sensory consequence of its own command. Feedback that is delayed, non-contingent, or mismatched fails to close the Hebbian loop and can even be counterproductive.

Honesty: the primacy of contingency over mere sensory bombardment is supported by sham-controlled BCI-FES results, but the optimal modality, the precise timing tolerance, and how far this generalizes across patients and tasks are unsettled. Providing rich, correctly-timed proprioceptive feedback non-invasively is itself hard, which is part of why FES and robotic orthoses are used as the feedback actuators.