NONLINEAR BEHAVIOR OF HUMAN EEG - FRACTAL EXPONENT VERSUS CORRELATIONDIMENSION IN AWAKE AND SLEEP STAGES

The question of whether the finite values of the correlation dimension (D2), used as an index of EEG complexity are due to its chaotic natur e or they reflect its behaviour as linearly-correlated noise, remains open. This report aims at clarifying this by measuring D2 and analysin g the non-linear nature of EEG through the method of surrogate data as well as by calculating the fractal exponent (beta) via coarse grainin g spectral analysis (CGSA) in nine adult subjects during waking and sl eep states. The results show that even if it is possible to get an est imation of D2 in all slates, non-linear structure appears to be presen t only during slow wave sleep (SWS). EEG exhibits random fractal struc ture with 1/f(-beta) spectrum (1 < beta < 3) and a negative linear cor relation between D2 and beta in all states except during SWS. In conse quence, in those states, finite D2 values could be attributed to the f ractal nature of EEG and not to the presence of low-dimensional chaos, and therefore, it the use of beta would be more appropriate to descri be the complexity of EEG, due to its lower computational cost. (C) 199 8 Elsevier Science Ireland Ltd. All rights reserved.