A functional near-infrared spectroscopy study to detect activation of somatosensory cortex by peripheral nerve stimulation.

INTRODUCTION: Multi-channel near-infrared spectroscopy (NIRS) is a method for non-invasively monitoring of relative concentrations of oxygenated, deoxygenated, and total hemoglobin. This technique has found expanding application in brain mapping and functional imaging. The purpose of this study was to investigate whether activation of somatosensory cortex can be detected without the necessity of the patient's cooperation in performing a task.
METHODS: Real-time bilateral parietotemporal cerebral oxygenation was monitored in 12 healthy volunteers. The median nerve at the wrist was electrically stimulated repeatedly at an amplitude below the threshold of discomfort. Interstimulus intervals were randomized between 13 and 31 s to minimize synchronization with respiration or other natural oscillations in cerebral oxygenation.
RESULTS: In 8 of the 12 subjects, activation over the contralateral primary somatosensory cortex was detected, correlating significantly with the predicted hemodynamic response function.
CONCLUSIONS: To our knowledge, this is the first time functional NIRS has been used to detect activation of somatosensory cortex with peripheral nerve stimulation. While the sensitivity for detection of the functional hemodynamic response was inadequate for clinical diagnostics, these findings are uniquely important in critical care imaging in that the regional blood flow and oxygenation changes can be detected without the requirement of a volitional task. This advancement potentially expands the capability of this modality to be used in brain mapping and in the evaluation of patients with impaired cognitive or motor function at the bedside.