Nonlinear, fractal, and spectral analysis of the EEG of lizard, Gallotia galloti. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R86-R93, 1999.—Electroencephalogram (EEG) from dorsal cortex of lizard Gallotia galloti was analyzed at different temperatures to test the presence of fractal or nonlinear structure during open (OE) and closed eyes (CE), with the aim of comparing these results with those reported for human slow-wave sleep (SWS). Two nonlinear parameters characterizing EEG complexity (correlation di- mension (D2)) and predictability (largest Lyapunov exponent (l1)) were calculated, and EEG spectrum and fractal expo- nent b were determined via coarse graining spectral analysis. At 25°C, evidence of nonlinear structure was obtained by the surrogate data test, with EEG phase space structure suggest- ing the presence of deterministic chaos (D2 ,6, l1 ,1.5). Both nonlinear parameters were greater in OE than in CE and for the right hemisphere in both situations. At 35°C the evidence of nonlinearity was not conclusive and differences between states disappeared, whereas interhemispheric differences remained for l1. Harmonic power always increased with temperature within the band 8-30 Hz, but only with OE within the band 0.3-7.5 Hz. Qualitative similarities found between lizard and human SWS EEG support the hypothesis that reptilian waking could evolve into mammalian SWS. EVOLUTIONARY STUDIES searching for the origin of mam- malian and avian sleep using reptiles as experimental animals proliferated in the sixties and in the seventies. Unfortunately, the results were discouraging, because neither slow-wave sleep (SWS) nor rapid eye movement sleep (REMS) was unquestionably found in these ani- mals. However, recent works (27) have demonstrated unequivocal REMS in the platypus, a primitive mam- mal, although it showed rather deviant characteristics from the well-known REMS of mammals and birds. After this result, it seems compulsory to analyze reptil- ian neurophysiology, trying to find the minimum set of characteristics needed to define both SWS and REMS. Using signal analysis techniques (10), our group showed that the reptilian electroencephalogram (EEG) exhib- ited slow-wave spindles and high-voltage spikes that could appear both spontaneously and after sensory stimulation, a set of traits very similar to those found in mammals during SWS. This evidence was used to support the idea that the reptilian waking, being mainly due to the activity of brain stem structures, could evolve into the mammalian SWS. As a result, the true evolutionary acquisition of mammals would not be sleep, but cortical wakefulness achieved after the devel- opment of multisensory motor processing areas in the neocortex. The development of new methods for time series analysis could help to overcome the problems found in earlier studies. In particular, mathematical tools for the study of nonlinear dynamical systems have made possible the analysis of complex signals, formerly re
