Thalamic and Cortical Activation in Sleep

Based on electroencephalographic recordings (EEG), the human sleep/wake cycle has been subdivided in 4 major stages. The passage from wakefulness to light (lSWS) and deep (dSWS) slow wave sleep (SWS) is characterized by a progressive slowing of the EEG, more or less associated with transient elements (spindle; K complex), while the 4th vigilance stage (rapid eye movements or paradoxical sleep PS) exhibits activity largely comparable to that of wakefulness.1 The high frequency content and the low voltage of the EEG activity during wake and PS are generally viewed as reflecting a high level of cortical activation; conversely low frequency and high voltage EEG activity in lSWS and dSWS stages are thought to be characteristic of lower cortical activation levels. This interpretation is supported by recent metabolic neuroimaging studies showing a decrease in cortical blood flow during SWS.2 The reciprocal activation/deactivation of the cortex and the thalamus fluctuates during the sleep wake cycle. During SWS, 2 EEG features are observed: sleep spindles and slow waves. A large body of evidence suggests that “spindle” oscillations are locally generated in the reticular nucleus of the thalamus.3 Among slow waves, 2 types can be distinguished on the basis of their respective frequencies and putative generating mechanisms. The slow oscillations (<1Hz) can be generated and sustained by the cerebral cortex alone,4,5 while clocklike delta oscillations (1-4 Hz) are generated in thalamo-cortical cells. Thus one can assume that during these 2 types of SWS EEG activity, the deactivation levels could be different in the thalamus and the cortex. During PS, both the cortex and the thalamus are supposed to be reactivated, as they both show a predominance of low voltage activity at relatively high frequencies.1 However, in at least 2 thalamic nuclei, the ventral posterior lateral nucleus and the medial pulvinar (PuM), PS activity has been shown to be frequently interrupted by long periods of delta waves.6,7 This suggests that some degree of thalamic deactivation and transmission block of sensorial inputs to the cortex, similar to that observed in SWS, is likely to occur also in PS.6,7 The aim of this study was to assess the activation-deactivation pattern at cortical and thalamic levels during the complete sleep wake cycle. For this purpose, we analysed intracranial recordings performed simultaneously at thalamic and cortical levels during the different sleep stages using both a classical spectral parameter (SEF95) and a recently developed type of EEG analysis allowing the quantification of neuronal activity. This new technical approach gives an indicator called “dimension of activation” (DA) which has been already validated for assessing cortical dynamics in different conditions of brain activation.8