Role of corpus callosum in interhemispheric coherent activity during sleep M. Corsi-Cabrera a, * , R. Ondarza b , V. Martı ´nez-Gutie ´rrez a , Y. del Rı ´o-Portilla a , M.A. Guevara c , J. Ramos-Loyo c a Facultad de Psicologı ´a, Posgrado, Universidad Nacional Auto ´noma de Me ´xico, Me ´xico City, Mexico b Instituto Nacional de Neurologı ´a y Neurocirugı ´a MVS, Mexico, DF, Mexico c Instituto de Neurociencias, Universidad de Guadalajara, Jalisco, Mexico Accepted 21 May 2006 Abstract Objective: To investigate to what extent the increase in interhemispheric coherent activity observed from wakefulness to sleep depends on the integrity of the corpus callosum (CC). Methods: Interhemispheric coherent activity was analyzed in two epileptic patients selected for callosotomy because of multifocal refractory epilepsy, before and 4 months after callosotomy. One patient underwent complete callosotomy and another was subjected to callosotomy of the anterior 2/3, which offered the possibility of comparing the role of the CC in the coherent activity increase from wakefulness to sleep, between anterior regions with interrupted CC communication (in the two patients) and posterior regions with intact communication (in one of them). Results were compared with a group of normal subjects. Results: Both patients showed increased coherent activity from wakefulness to sleep after surgery. Conclusions: Results demonstrate that interhemispheric coherent activity, despite an attenuation after surgery, is higher during SWS than during wakefulness after sectioning the CC; however, they have to be taken with caution because they come from two patients only. Significance: Present results show that the increase in coherent activity during sleep does not depend exclusively on callosal integrity but also on state-dependent influences from sleep-promoting mechanisms, probably spread throughout the thalamo–cortical network. q 2006 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Epilepsy; Callosotomy; Corpus Callosum; EEG coherence; EEG correlation; Sleep 1. Introduction In the last decades, the importance of coherent activity between neuronal populations of different brain regions for cognitive processes has been stressed, and this cooperative activity has been proposed as a binding mechanisms serving to integrate disperse information in cortical regions into a unified experience (Crick, 1994; Singer, 1999; Gray, 1999; Edelman and Tononi, 2000). In addition to the stimulus time-locked increase of coherent activity between the cortical regions directly involved in processing a specific task, the basal level of coherent activity between homo- logous regions of the left and right hemisphere undergoes dramatic changes with sleep. In independent groups of subjects, we found a significant increase in interhemispheric coherent activity of slow Hertz and spindle frequencies from wakefulness (W) to stage 2, and stage 4 of slow wave sleep (Corsi-Cabrera et al., 1987, 1996; Guevara et al., 1995; Pe ´rez-Garci et al., 2001). The increase of coherent activity during sleep has been confirmed by other studies (Nielsen et al., 1990; Achermann and Borbe ´ly, 1998). The meaning of the increase in interhemispheric coherent activity during sleep is not fully understood yet but it suggests a reorganization of functional relationships among cortical and thalamic areas that may be relevant for the reprocessing of information; the changes in intrinsic excitability of cortical neurons and coherent oscillations in the cortex and thalamus that take place during sleep may provide an Clinical Neurophysiology 117 (2006) 1826–1835 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2006 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2006.05.008 * Corresponding author. Address: Laboratorio de Suen ˜o, Facultad de Psicologı ´a, Universidad Nacional Auto ´noma de Me ´xico, Av. Universidad 3004, Copilco-Universidad, Me ´xico City DF 04510, Mexico. Tel.: C52 55 56 22 22 51/55 68 95 14. E-mail address: corsi@servidor.unam.mx (M. Corsi-Cabrera).