Interhemispheric EEG coherence during sleep and wakefulness in left- and right-handed subjects.

REM sleep is associated with the production of complex imagery sequences. Yet research is divided as to whether different brain regions are more or less coordinated in their functioning at this time. Some research suggests that there may occur a functional disconnection of the left and right cerebral hemispheres during REM sleep which is similar to the disconnection syndrome seen after corpus callosotomy. Other research suggests that an increase in interhemispheric coordination occurs. On the assumption that hemispheric coordination is reflected in the EEG coherence measure, we explored differences in interhemispheric coherence recorded in six left- and six right-handed normal subjects during periods of wakefulness, stage REM, stage 2, and stage 3/4 sleep. Strong evidence was found that mean EEG coherence values are larger during sleep than during waking and that they are approximately equal for the different stages of sleep. Frontal electrode placements demonstrated a slightly different pattern of coherence than central, parietal, or occipital placements. Furthermore, coherence values were larger for left-handed subjects over the occipital region during wakefulness, stage 2, and stage REM sleep, but not during stage 3/4 sleep. Coherence was not different for male and female subjects. These findings oppose the interpretation that a functional disconnection of hemispheres occurs during REM sleep and favor the interpretation that sleep in general is a state of heightened cortical coordination. Moreover, greater interhemispheric coherence over occipital brain regions in left-handed subjects suggests possible differences in the cognitive processes of these subjects during waking and dreaming states.