Wavelet analysis can sensitively describe dynamics of ethanol evoked local field potentials of the slug (Limax marginatus ) brain Atsuko Schu ¨tt a, *, Iori Ito b , Osvaldo A. Rosso a , Alejandra Figliola a a Instituto de Ca ´lculo, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello ´n II, Ciudad Universitaria, 1428 Buenos Aires, Argentina b Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received 5 March 2003; received in revised form 17 June 2003; accepted 18 June 2003 Abstract Odorants evoke characteristic, but complex, local field potentials (LFPs) in the molluscan brain. Wavelet tools in combination with Fourier analysis can detect and characterize hitherto unknown discrete, slow potentials underlying the conspicuous oscillations. Ethanol was one of the odorants that we have extensively studied (J. Neurosci. Methods, 119 (2002) 89). To detect new features and to elucidate their functions, we tested the wavelet tools on the ethanol-evoked LFP responses of the slug (Limax ) procerebrum. Recordings were made in vitro from the neuropile and the cell layer. The present study led to the following findings: (i) Mutual exclusion. Energy concentrated mainly in two ranges, (a) 0.1  /0.4 Hz and (b) 1.56 /12.5 Hz, and the sum of energy remained constant throughout experiments regardless of the condition. A redistribution of relative energy within this sum seemed to occur in the course of main, possible interactions between the two components excluding each other (‘mutual exclusion’). (ii) Transient signal ordering and disordering. Ethanol stimulation alternatingly evoked periods of strongly time evolving oscillation dominated by the energy of 1.56  /12.5 Hz (increase of entropy /disordered or complexly ordered state) and those of near-silence were predominated by the energy of 0.1  /0.4 Hz (decrease of entropy /ordered state). (iii) About 0.1 Hz slow wave oscillation. It was robust. The dominant energy oscillation and the resulting large entropy fluctuation were negatively correlated to each other, and revealed strong frequency-tuning or synchronization at this frequency. Our findings suggest that discrete slow waves play functionally important roles in the invertebrate brain, as widely known in vertebrate EEG. Wavelet tools allow an easy interpretation of several minutes of frequency variations in a single display and give precise information on stimulus-evoked complex change of the neural system describing the new state ‘more ordered’ or ‘non-ordered or more complexly ordered’. # 2003 Elsevier B.V. All rights reserved. Keywords: Wavelet transform; Limax procerebrum; Ethanol-evoked local field potentials; Energy; Entropy; Mutual exclusion 1. Introduction The difficulty in relating the properties of single neurons to those of neuronal assemblies has been one of the problems in considering the role of neuronal oscillations in brain function. In our previous investiga- tions we have tried to approach this problem, using the isolated snail brain as a model of cellular assembly and digital filters as well as the Fourier transform as tools. The rationale of starting experiments on the snail ganglia by using methods of systems theory (the Four- ier-based method, Bas ¸ar, 1980) was based on the descriptive efficiency of these methods. The descriptive efficiency of methods of systems theory makes it possible to detect and characterize the EEG-like oscilla- tory phenomena in such a relatively simple cellular mass as the Helix ganglia (Schu ¨tt et al., 1992; Schu ¨tt and Bas ¸ar, 1992). To test the hypothesis that different odorants are likely to cause distinctive changes in the ongoing electrical activity of populations of olfactory cells, we investigated local field potentials (LFP) in the Helix brain and their alterations by odorants, as recorded with semi-microelectrodes in an isolated preparation of the * Corresponding author. Address: Institute of Physiology, University of Lu ¨ beck, Ratzeburger Allee 160, 23538 Lu ¨ beck, Germany. Tel.:  /49-451-500-4180; fax:  /49-451-500-4171. E-mail address: schuett@physio.uni-luebeck.de (A. Schu ¨ tt). Journal of Neuroscience Methods 129 (2003) 135 /150 www.elsevier.com/locate/jneumeth 0165-0270/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0165-0270(03)00200-0