The Distinctive Sleep Pattern of the Human Calcarine Cortex: A Stereo-EEG Study

Study objectives: The aim of the study was to describe the spontaneous electroencephalographic (EEG) features of sleep in the human calcarine cortex, comparing them with the well-established pattern of the parietal cortex. Methods: We analysed pre-surgical intracerebral EEG activity in calcarine and parietal cortices during NREM and REM sleep in 7 patients with drug-resistant focal epilepsy. The time course of the EEG spectral power and NREM vs. REM differences were assessed. Sleep spindles were automatically detected. To assess homeostatic dynamics, we considered the 1 st vs. 2 nd half of the night ratio in the delta frequency range (0.5-4 Hz) and the rise rate of delta activity during the 1 st sleep cycle. Results: While the parietal area showed the classically described NREM and REM sleep hallmarks, the calcarine cortex exhibited a distinctive pattern characterized by: a) the absence of sleep spindles; b) a large similarity between EEG power spectra of NREM and REM; c) reduced signs of homeostatic dynamics, with a decreased delta ratio between the 1 st and the 2 nd half of the night, a reduced rise rate of delta activity during the 1 st NREM sleep cycle, and lack of correlation between these measures. Conclusions: Besides describing for the first time the peculiar sleep EEG pattern in the human calcarine cortex, our findings provide evidence that different cortical areas may exhibit specific sleep EEG pattern, supporting the view of sleep as a local process and promoting the idea that the functional role of sleep EEG features should be considered at a regional level.