Effects of general anesthesia on high-frequency oscillations in somatosensory evoked potentials

To determine the characteristics of high-frequency oscillations (HFOs) of cortical somatosensory evoked potentials (SEPs), the effect of general anesthesia on HFOs and low-frequency primary cortical responses was studied. The authors recorded SEPs elicited by median nerve stimulation directly from human brains of seven patients who underwent implantation of subdural electrodes before surgical treatment of intractable epilepsy. Recordings were made before and during general anesthesia. Changes in the number of HFOs and amplitude ratios of HFOs/primary cortical responses were analyzed. Under general anesthesia, the number of HFO peaks and the amplitude ratios were significantly decreased. General anesthesia induced remarkably decreased HFO activities when compared to low-frequency SEPs, suggesting that each of those originated from different generators. Possible relations between gamma-amino-butyric acid (GABA)ergic inhibitory interneurons and HFOs are discussed.