Authors:P. Halásza*, R. Bódizsc, M. Terzanob, L. Parrinob aNational Institute of Psychiatry and Neurology, Huvosvolgyi ut 16, Budapest, Hungary *Corresponding author. Address: Neurology Department, National, Institute of Psychiatry and Neurology, Huvosvolgyi ut 16, Budapest, Hungary. To the editor:We wish to state that the dynamic events of sleep can be meaningfully studied by combining neuroimaging (fMRI, and PET) and electrophysiological methods. From this standpoint we cite the work of Czisch et al. [1] and would like to make some comments on this topic. The study illustrates interesting correlations between BOLD signal decrease (BSD) and the frequency of K-complexes and delta waves evoked by acoustic stimulation during stage 2 of NREM sleep. In previous papers, we described a kind of ‘anti-arousal’ response in the form of K-complexes and delta waves to acoustic stimulation during NREM sleep [2]. The results of Czisch et al. nicely support our assumptions. The physiological mechanism of this anti-arousal response is still unknown. In natural sleep the ‘syncronization type’ response is linked to the first part of the night and to the descending slope of the sleep cycles, and a sleep-promoting function is atttributed to this type of response. On the contrary, ‘desynchronization type’ response occurs mainly in the ascending slope of the sleep cycle and is attached more to arosual function. The elicitability of K-complexes is higher during the ascending, compared with the descending, slope of the sleep cycle.According to these results it would be interesting to know whether there are differences in the BSD between stage 2 periods situated on the descending slope compared to the stage 2 periods of the ascending slope of sleep cycles [3]. In their paper, Czisch et al. state that “the standard Rechtschaffen and Kales sleep stage classification was found insufficient to predict specific fMRI responses” [1]. The existence of the ‘cyclic alternating pattern (CAP)’ [4] suggests that also the ongoing microstates should be taken into account when brain reactivity to sensorial stimulation is evaluated during NREM sleep. The rhythmic or random character of stimulation is also an important issue since regular stimulation seems to exert a sleep-promoting function. The described BSD was found most consistently in the visual cortex, precuneus and auditory association cortex, although the K-complex and delta is obviously a frontal pattern. This discrepancy needs some explanation. Does it mean that emanation of the frontal K-complex and delta response is preceded by a more local suppression of acoustic and visual reactivity? This question could be answered only if time relations of the EEG events with the BSD phenomenon could be detected in association with the single stimulus-elicited responses.
References[1] Czisch M, Wehrle R, Kaufmann C, Wetter TC, Holsboer F, Pollmacher T, Auer DP. Functional MRI during sleep: BOLD signal decreases and their electrophysiological correlates. Eur J Neurosci 2004;20(2):566–74. [2] Halász P, Terzano M, Parrino L, Bódizs R. The nature of arousal in sleep. J Sleep Res 2004;13:1–23. [3] Terzano MG, Parrino L, Borselli M, Smerieri A, Spaggiari MC. CAP components and EEG synchronization in the first three sleep cycles. Clin Neurophysiol 2000;111:283–90. [4] Terzano MG, Parrino L. Functional relationship between micro- and macrostructure of sleep. In: Terzano MG, Halász P, Declerck A, editors. Phasic events and dynamic organization of sleep. New York: Raven Press; 1991 p. 101–19.
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