Reduction of transcallosal connection upon awakening from REM sleep in humans as assessed by transcranial magnetic stimulation

The cerebral hemispheres are functionally interconnected by a tonic exchange of impulses via the corpus callosum. In the cat, callosal activity progressively decreases during the EEG synchronization process as compared to wakefulness; a further drop of activity is observed at the onset of REM sleep. Data on human sleep coming from studies on spectral inter-hemispheric EEG coherence seem to converge suggesting a decreased interhemispheric transmission during REM sleep. Unfortunately, no direct measurements of callosal function during sleep have been provided in humans, also due to intrinsic methodological difficulties in measuring callosal activity via non-invasive techniques. The aim of this study is to assess in humans transcallosal inhibition upon awakening from REM and non-REM sleep, by the paired-pulse Transcranial Magnetic Stimulation (TMS). Ten right-handed subjects participated in the experiment for four consecutive sleep recording nights. During daytime, a baseline session of motor evoked potentials (MEPs) was recorded. During the nighttime, the TMS sessions were administered just before sleep onset and upon awakenings from REM and stage 2 sleep, both in the early and final part of night. During the daytime a robust transcallosal inhibition was found; the MEP amplitude reduction ranged from 35 to 40%. During the nighttime, a decrease of transcallosal inhibition from right-to-left motor cortex as compared to that from left-to-right motor cortex was observed. The direct assessment of MEP changes, as a function of sleep stage and of the time of night, pointed to a drop of transcallosal inhibition after awakening from REM sleep. As a matter of fact, the inhibitory activity of transcallosal fibers observed after non-REM awakening almost disappeared after REM sleep awakenings. The drastic drop of transcallosal inhibition after awakenings from REM sleep represents the first evidence in humans of a change of inter-hemispheric connectivity mediated by the corpus callosum during this sleep stage, and may open new avenues for a better understanding of some aspects of dreaming function and dream mentation. Our results provide the first evidence in humans of a drop of transcallosal activity associated to REM sleep.