NONLINEAR CHANGES IN BRAINS RESPONSE IN THE EVENT OF INJURY AS DETECTED BY ADAPTIVE COHERENCE ESTIMATION OF EVOKED-POTENTIALS

Injury-related changes in evoked potentials are studied with the aid o f the coherence function, which effectively measures the degree of lin ear association between a pair of signals recorded during normal and a bnormal states of the brain, The performance of an adaptive algorithm for estimating coherence function is studied, and the effects of addit ive noise on the estimated coherence function is discussed, Further, a linearity index is formulated and, through analysis and simulations, the index is shown to respond in a predictable manner to increasing no nlinearity while maintaining the robustness to the observation noise, Somatosensory evoked potentials are shown to be sensitive to injury re sulting from acute cerebral hypoxia, We analyze the somatosensory evok ed potentials recorded from anesthetized cats during inhalation of 8-9 % oxygen gas mixtures and during recovery with 100% oxygen, Analyses o f the experimental data show a very sharp drop in the magnitude cohere nce estimates during hypoxic injury and a corresponding rapid decline in the linearity index at the very early stages of the hypoxic injury, Thus, injury mag lead to nonlinearities in the electrical response of the brain, and such measurements analyzed by the adaptive coherence e stimation method may be used for diagnostic purposes.