Within-subject reliability and inter-session stability of EEG power and coherent activity in women evaluated monthly over nine months M. Corsi-Cabrera a, * , L. Galindo-Vilchis a , Y. del-Rı ´o-Portilla a , C. Arce b , J. Ramos-Loyo c a Facultad de Psicologı ´a, Posgrado, Universidad Nacional Auto ´ noma de Me ´xico, Mexico b Escuela Nacional Preparatoria, Universidad Nacional Auto ´noma de Me ´xico, Mexico c Instituto de Neurociencias, Universidad de Guadalajara, Jalisco, Mexico Accepted 27 August 2006 Available online 19 October 2006 Abstract Objective: Quantitative EEG parameters during resting conditions are used as baseline in research on cognition and in serial-EEG recordings. Despite its increasing use in cognitive research and the numerous evidences of the existence of sex differences in EEG, EEG stability has been mainly investigated in men. Particularly, studies on stability of coherent activity are scarce. The aim of this study was to investigate within-subject reliability and inter-session stability of resting EEG over a nine-month period in women. Methods: Within-subject reliability and inter-session stability were analyzed for absolute power and inter- and intrahemispheric coherent activity at central and posterior regions, once a month, in resting conditions, with eyes open and closed. Results: Within-subject reliability was very high (r > 0.89) for all subjects and EEG parameters. Inter-session stability was higher with eyes closed and for interhemispheric coherent activity, and poorer with eyes open especially in the alpha band. Conclusions: Present results indicate high reliability of the pattern of power and coherent activity of each individual woman during rest, and group stability of EEG activity with eyes closed at least over a nine-month period. Significance: These results provide information on EEG stability in women over a long period. Ó 2006 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Spectral power; EEG coherence; EEG correlation; Women; EEG intra-subject reliability; EEG stability 1. Introduction Quantitative analysis of EEG activity during resting conditions is frequently used as baseline in the research of cognitive processes and in serial-EEG recordings for clinical evaluation over time. The study of background EEG is also important to understand the relationship between back- ground EEG and event-related potentials (Polich, 1997). The need of reliable information on the stability of quantita- tive EEG background variables has been recognized for many years. There are several reports on stability of spectral power during resting conditions and during performance of cognitive tasks; however, EEG stability has been investi- gated mainly in men, or men and women have not been separated as independent groups, with a few exceptions where power was studied (Hawkes and Prescott, 1973; Van Dis et al., 1979). A considerable amount of evidence demonstrates sex differences in brain structural and func- tional organization (Kimura, 1987; McEwen et al., 1997) and particularly in EEG activity (Matsuura et al., 1985; Corsi-Cabrera et al., 1989; Harmony et al., 1990; Ramos et al., 1993), which points to the necessity of investigating stability of EEG parameters in women independently. Some studies have reported the stability of spectral power during resting conditions within the same recording session (John et al., 1983; Gasser et al., 1985; Oken and 1388-2457/$32.00 Ó 2006 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2006.08.013 * Corresponding author. Tel.: +52 55 56 22 22 51; fax: +52 55 56 22 23 10. E-mail address: corsi@servidor.unam.mx (M. Corsi-Cabrera). www.elsevier.com/locate/clinph Clinical Neurophysiology 118 (2007) 9–21