'Brain activation and hypothalamic functional connectivity during human non-rapid eye movement sleep: an EEG/fMRI study'--its limitations and an alternative approach

Abstract

Sir, We have read with great interest the recent paper by Kaufmann et al . (2006) which describes the first study of human spontaneous non-rapid eye movement sleep using electroencephalography (EEG)-combined with functional magnetic resonance imaging (fMRI). The mainstay of sleep imaging has been EEG-combined Positron Emission Tomography (PET) (Maquet, 2000). This methodology, however, cannot be used to study brief sleep phenomena such as spindles or K-complexes because of the limited temporal resolution of PET. This is different for EEG–fMRI which has been able to demonstrate brain activations with brief paroxysmal EEG events, such as thalamic activation in response to individual spike and wave discharges (Laufs et al ., 2006 b ). Therefore, the paper of Kaufmann et al . applying EEG–fMRI to the study of sleep is of particular interest, and they present a number of interesting findings such as the involvement of the hypothalamus and mamillary bodies in sleep. The authors emphasize that—as they have used fMRI operating at a temporal resolution superior to that of PET—their findings cannot directly be compared with previous PET findings (Kaufmann et al ., 2006). There are however limitations in the use of EEG–fMRI for studying sleep, which may also account for differences between their and previous findings e.g. compare results reported by Kaufmann in the occipital lobe during stage I sleep with those of others (Hofle et al ., 1997; Kjaer et al ., 2002; Nofzinger et al ., 2002) or in the thalamus and cerebellum during slow wave sleep compared to wakefulness with those of others (Maquet et al ., 1997; Born et al ., 2002) and (Braun et al ., 1997; Hofle et al ., 1997; Born et al ., 2002), respectively. Here we discuss limitations of EEG–fMRI in the study of sleep and …