The Double-Edged Nature of Delay: Toward a Functional Latency Window in Neurorehabilitation and BrainMachine Interaction

Delay pervades humanmachine systemsfrom synaptic transmission and axonal conduction to sensing, computation, and actuation. Traditionally viewed as a nuisance to minimize, latency can instead serve as a functional design parameter when it is internal, structured, and predictively compensated. Theoretically, the FLW connects timing-dependent plasticity and predictive coding in neural systems to an analysis of stability in delay-differential equations which shows that moderate delays with structure can stabilize dynamics rather than break them. Practically, this framework informs delay-aware design in brain-computer interfaces and rehabilitation robotics wherein user-experienced delay has to be minimized so as to retain implicit learning and agency. In contrast, internal controller delay can be structured to enhance stability and adaptation. By integrating neuroscience, psychophysics, and control theory, this review proposes delay as both a constraint and a resource, offering a biologically grounded principle for building robust, human-compatible feedback systems.