The transcranial magnetic stimulation (TMS) as a new research tool in psychophysiology of sleep

Due to a low time resolution and to the specific procedures for data acquisition, the brain imaging measures cannot be extensively used in sleep research. Transcranial magnetic stimulation (TMS) is now an established technique in neurosciences allowing the non-invasive assessment of motor cortical excitability, but its systematic application to sleep has been limited by the fact that delivering a single suprathreshold magnetic stimulus awakens subjects, or lightens their sleep. Furthermore, the acquisition of multiple responses to magnetic pulses to record motor evoked potentials (MEPs), reflecting brain excitability, seems incompatible with the intrinsic peculiarities of sleep. Since several findings support the feasibility of the assessment upon awakening from different sleep stages to compare their pre-awakening cortical physiology, we have used TMS in three different protocols: 1. Motor thresholds (MTs) to assess corticospinal excitability upon REM and NREM awakenings; 2. Cortico-cortical paired pulses to assess intracortical facilitation and inhibition upon REM and SWS awakenings; 3. Inter-hemispheric paired pulses to assess callosal inhibition upon REM and NREM sleep awakenings. The main fingings are: (a) an increase of MTs upon REM sleep awakenings reflecting the brainstem inhibitory effect on spinal motoneurones [Bertini et al., J Sleep Res 13:31-6, 2004]; (b) a heightened cortical facilitation upon awakening from REM sleep than to SWS and wakefulness [De Gennaro et al. Eur J Neurosci 19:3099-104, 2004]; (c) a drop of callosal inhibition upon awakening from REM sleep [Bertini et al. Sleep 27:875-82, 2004]. The coherent pattern of empirical results encourages the psychophysiological study of the corticospinal organization in different sleep stages by means of the TMS protocols upon awakenings from different sleep stages, legitimating the introduction of this technique as a new tool for sleep research.