Exploring high-frequency oscillation as a marker of brain ischemia using S-transform

Brain injury, such as hypoxic-ischemia produced in brain after cardiac arrest, is known to alter somatosensory evoked potential (SSEP) signals, thus serving a diagnostic role. This study explores the high-frequency oscillation (HFO) in SSEP recorded in a rat model of asphyxial cardiac arrest. To best characterize this complex oscillatory activity, several time-frequency representation strategies are implemented and compared. The S-transform (ST) is found to precisely localize the HFO in temporal-spectral space. More, the ´phase ST´-the inter-trial coherence (ITC) sensitively detects the phase-locked activities in HFO. Using ST and ITC, we explored the evolution of HFO during early recovery from brain injury. A discrepancy between the amplitude of HFO, which increases over time, and its phase, which stays time-invariant, is revealed here. The recovery dynamics of HFO mirrors that of N10 in terms of their amplitudes, which suggests HFO as a prelude of large-scale cortical responses. In addition, statistics shows the amplitudes of HFOs have different levels (p<;;0.05) and recovery dynamics (p=0.03) between the good- and bad-outcome groups. We consider the HFO to be reflective of the health of thalamocotical circuitry in brain ischemia.