Sensorimotor cortical responses to visuotactile synchrony during virtual foot mirror feedback: an fNIRS study

To determine whether temporally synchronous vibrotactile stimulation during virtual foot mirror feedback increases medial sensorimotor cortical oxygenation and alters subjective foot ownership. Twenty-five healthy adults completed Visual-only, Synchronous visuotactile, and Random-delay visuotactile conditions in a within-subject design. Participants performed right ankle dorsiflexion while viewing mirror-like virtual left-foot movement. The primary confirmatory comparison tested oxygenated haemoglobin (HbO) responses between the Synchronous and Random-delay conditions in a predefined medial sensorimotor region of interest around Cz using functional near-infrared spectroscopy. Subjective ownership and synchrony-related ratings were also assessed. The primary hypothesis was not supported. HbO did not increase in the Synchronous condition relative to the Random-delay condition (mean difference = −0.0022 μM, 95% confidence interval [−0.0140, 0.0097], t(24) = −0.3743, p = 0.711, Cohen’s dz = −0.075). Corrected secondary and supplementary region-of-interest analyses were also not significant. Synchrony-related ownership change was not positively associated with HbO change (Spearman ρ = −0.244, p = 0.239, bootstrap 95% confidence interval [−0.578, 0.139]). In contrast, subjective ratings showed a structured ownership-related change, and exploratory item-level analyses suggested reduced disownership in the Synchronous condition. The Synchronous condition was associated with changes in subjective attribution of the virtual foot without a detectable difference in medial sensorimotor HbO. These observations should be regarded as hypothesis-generating rather than as established evidence that subjective embodiment and local sensorimotor hemodynamic responses represent distinct response levels.