fNIRS Contains Usable Stimulus Information Much Earlier than the Canonical HRF Peak
Functional near-infrared spectroscopy (fNIRS) is often considered temporally limited due to the delayed peak of the hemodynamic response. This study investigates whether meaningful, stimulus-related information can be extracted from the early post-stimulus interval (0-2.5 s) during vibrotactile stimulation. Data from 13 participants across multiple stimulation locations and frequencies were analyzed using time-locked averaging, spectral validation, and linear mixed-effects modeling. Results show that early hemodynamic responses are reproducible, stimulus-locked, and spatially specific. While mean amplitude did not significantly differentiate conditions, maximum amplitude revealed significant stimulus- and location-dependent effects, and the slope of the response captured spatial organization consistent with somatosensory mapping. Control analyses confirmed that these effects are not driven by systemic noise or stimulation artifacts. These findings demonstrate that fNIRS contains usable and physiologically meaningful information earlier than traditionally assumed, supporting its potential for rapid detection of cortical activity in real-time applications.