Cortical mapping of real-time ultrasound-imaging visual feedback for ankle dorsiflexion in patients after stroke: a fNIRS study

BACKGROUND: Foot drop in patients with stroke compromises gait and mobility. Active ankle movement with real-time ultrasound-imaging visual feedback (UVF) is found to be a clinically effective rehabilitation protocol; however, its underlying neural mechanisms in poststroke brains remains unclear. This functional near-infrared spectroscopy (fNIRS) study aimed to investigate UVF's neuromodulatory effects during ankle dorsiflexion (ADF) execution. METHODS: Fifteen post-stroke patients (SP) and fifteen healthy controls (HC) performed active ADF tasks with and without UVF. The fNIRS measured the change of oxyhemoglobin (ΔHbO) and deoxygenated hemoglobin (ΔHbR) over the dorsolateral prefrontal cortex (DLPFC), the primary motor cortex (M1), and the premotor cortex/supplementary motor areas (PMC/SMA). Functional connectivity (FC) between these regions was calculated. The muscle morphology parameters [muscle thickness (MT), pennation angle (PA), fascicle length (FL)] were quantified via ultrasound imaging during movement execution. Two-way mixed ANOVA was used to assess the main and interaction effects of "task" and "group" on the local cortical activation levels and the FC among pairwise regions of interest (ROI). Correlations between muscle parameters and M1-related fNIRS data were analyzed. RESULTS: UVF did not increase ΔHbO at any ROIs; instead, it reduced ΔHbR in the I PMC/SMA in both groups. UVF also decreased the interhemispheric DLPFC connectivity in both groups, based on residual FC data. Regardless of whether UVF was provided or not, SP consistently showed stronger I PMC/SMA - I DLPFC connectivity than HC. SP also exhibited a UVF‑specific increase in the I PMC/SMA - I M1 connectivity relative to HC. During UVF and with comparable number of ADF, SP showed reduced PA and FL modulation than that of HC. Among MT, FL, and PA, only FL showed a trend-level positive association with interhemispheric M1 connectivity in SP, on top of the local M1 activation. CONCLUSIONS: Generally, UVF during ADF was associated with the reduced bilateral prefrontal coupling and the enhanced ipsilesional premotor-motor connectivity in post-stroke patients. Ankle muscle FL displayed certain level of task sensitivity, and showed a preliminary association with interhemispheric motor coordination, implying for possible network‑level influences during UVF‑guided motor execution.