The median nerve stimulation induces long latency gamma modulations in the primary somatosensory cortex, which differentiates the conscious state.

Abstract OBJECTIVE While the intraoperative assessment of the short-latency somatosensory evoked potentials (SSEPs) phase reversal in electrocorticogram (ECoG) is routinely used to locate the central sulcus (CS), its spatial-spectral patterns in the primary sensorimotor cortex and the relation to consciousness is not well characterized. Our goal was to evaluate the functional utility of using median nerve stimulation-induced ECoG sub-band modulations in pre-and post-CS to differentiate between anesthetized and awake states. Method: SSEPs were recorded from the sensorimotor cortex with ECoG during routine intraoperative cortical mapping of 8 patients in the anesthetized and awake states. We conducted a time-frequency analysis on the SSEP trace in each state to extract the spectral modulations up to 900Hz and contrasted their intensity in M1 and S1 to discern awake from anesthetized states. RESULTS We observed late gamma activity ranging from 60-250Hz, starting approximately 50ms after stimulation onset and extending up to 250ms. The late gamma activity was suppressed in the anesthetized state in both M1 (p< 0.01) and S1 (p=0.0369). Whiles in the awake state, the late gamma power increased significantly with respect to baseline both in M1 (p=0.0180) and S1 (p< 0.01). The difference in late gamma power between the anesthetized and the awake state was highly significant for M1 (p< 0.01) and for S1 (p< 0.01). CONCLUSION The results show that besides CS delineation, SSEPs long latency gamma modulations can serve as an additional biomarker to monitor the level of consciousness in neurosurgical practice.