Nonlinear changes in evoked potentials during recovery from hypoxic-ischemic injury

Evoked potential (EP) recordings remain a sensitive indicator of the cerebral response to episodes of brain injury. Previous investigations focused primarily on power and spectral (Fourier) indicators of injury. These parameters are derived from second order statistics. Nonlinear changes also occur after the induction of hypoxic-ischemic (HI) injury. These alterations may be captured using higher order statistics such as the bicorrelation or third order cumulants. Higher order statistics are able to accurately capture signal shape changes due to phase alterations. The H-statistic examines the size of the lagged terms of the third order cumulant. Large H-statistics reveal embedded quadratic nonlinearities. Additionally, this statistic operates on single evoked potentials. In an animal model of HI injury we show that the baseline evoked potential morphology maintains a high degree of nonlinearity. More than one third of these baseline cases have high H-statistics (p>.95). After recovery from HI injury, the H-statistic shows a uniform distribution where an expected 5% of the cases adopt this probability value. H-statistic changes reveal a distinctive loss of nonlinear character after HI injury.