NSWO 18, 2007 135 MODELING SLOW WAVE ACTIVITY PATTERNS ACROSS THE SCALP Andrei Zavada 1 , Arjen M. Strijkstra 1 , Ate S. Boerema 1 , Serge Daan 1 , Domien G.M. Beersma 1 1 Chronobiology, University of Groningen, Haren, The Netherlands INTRODUCTION In models of sleep regulation, timing of sleep is linked to the temporal dynamics of slow- wave activity (SWA, EEG spectral power in ~0.75–4.5 Hz range) in the cortical NREM sleep EEG. In the original two process model of sleep regulation, SWA was used as an indicator of sleep debt. The time constant of the SWA decrease over consecutive NREM–REM sleep cycles was used to estimate the time course of process S during sleep 1 . Later, SWA expression was assumed to reflect the decay rate of process S rather than sleep debt directly. A detailed model describing the overnight SWA pattern with its intrusions of REM sleep and waking was made by Achermann 2 , refining the description of the changes in SWA and process S over the night. In this model, process S and SWA are coupled by a ‘gain constant’, quantifying the efficiency of SWA in dissipating S. The time course of SWA during sleep after sleep deprivation varies between cortical locations 3 , suggesting that local differences in process S exist. In this study, we estimate local differences of S regulation by fitting ‘gain constants’ for 26 locations on the scalp. We observed higher frontal ‘gain constants’, suggesting that indeed regulation of S has local differences. METHODS Nine healthy young subjects (18–28 years) participated. Subjects did neither smoke nor use drugs, and abstained from consumption of alcohol and coffee throughout the experiment. They did not rate as extreme morning or evening types. The experiment was approved by the Medical Ethics Committee of the Academic Hospital of the University of Groningen. Subjects signed an informed consent form. Subjects had a habituation sleep night and a baseline sleep night in the. Only baseline sleep EEGs were used here. Before sleep nights, subjects were doing their normal daily routine, until they came to the sleep laboratory at 20:00. After application of scalp electrodes subjects were asked to perform computerized test series of ~35 min duration at 22:00 and 23:00. The test series contained questionnaires and visual event related potential trials. Subjects prepared for sleep at 23:40 and went to bed around 23:55, after which the electrodes were connected to the EEG amplifiers. At 00:00 hours, lights were turned off until 08:00 hours the next morning. EEGs were recorded using a cap system with Ag/AgCl electrodes (Electro-Cap International, Inc., Eaton, Ohio, USA), on 26 positions on the scalp (F7, F3, Fz, F4, F8, T3, C3, Cz, C4, T4, T5, P3, Pz, P4, T6, P9, P10, PO7, PO8, O1, Oz, O2, PO9, PO10, O9, O10). The left earlobe was used as reference, and the inion was used as ground. Data were amplified