fNIRS Characterization of Temporal Potentiation During Optimized Nerve Stimulation
Restoring stable somatosensory feedback through transcutaneous nerve stimulation (TNS) represents a significant challenge in neural engineering; however, the cortical dynamics underlying sensory habituation are not yet well characterized. This study utilizes a multimethod functional near-infrared spectroscopy framework to quantify the temporal evolution of cortical engagement during optimized TNS. It was hypothesized that stimulation parameters fundamentally constrain the spatiotemporal profile of the hemodynamic response function. A General Linear Model with temporal and dispersion derivatives was used to analyze hemodynamic responses in 10 participants across three phases: validation, duration optimization (2, 5, or 10 seconds), and sustained engagement (45 minutes). During the optimization phase, 5-second stimulation produced the highest canonical amplitude with optimal dispersion characteristics, while 10-second trains resulted in significant temporal delays. During the sustained phase, parametric modeling indicated no linear decay in cortical response. Instead, the cortex demonstrated temporal potentiation, characterized by a transition from sparse early-phase activation to widespread late-phase recruitment across the sensorimotor network. The left posterior parietal cortex (BA5) served as a continuous integration hub, whereas primary somatosensory regions exhibited dynamic late-onset engagement. These findings quantify a nonlinear adaptation mechanism whereby engineered temporal spacing prevents neural saturation. The observed intersubject heterogeneity highlights the necessity for adaptive, closed-loop TNS controllers guided by real-time hemodynamic biomarkers.