Sleep Spindles Are Characteristic Brain Waves during Nrem Sleep. as Its Name Suggests, a Sleep Spindle Has a Waxing-and-waning Waveform

SLEEP SPINDLES ARE CHARACTERISTIC BRAIN WAVES DURING NREM SLEEP. AS ITS NAME SUGGESTS, A SLEEP SPINDLE HAS A WAXING-AND-WANING WAVEFORM lasting between 1 to 3 s at a frequency of 11-15 Hz. Spindle waves recur every 3 to 10 s. Several studies indicate that spindles originate in the thalamus. Through extensive animal investigations, Steriade et al. showed that networks of neurons in the thalamus and the cerebral cortex play a major role in the genesis of brain oscillations including the sleep spindle.1 Hofle et al. investigated changes in regional cerebral blood flow (rCBF) in humans during the progression from wake through stages of slow wave sleep.2 They evaluated rCBF using positron emission tomography (PET) and employed electroencephalographic (EEG) signals to determine the sleep stage. A significant negative covariation between spindle activity and the rCBF was obtained in the midline medial thalamus, suggesting marked inhibition of the thalamus in association with sleep spindles. Ueda et al. applied a single equivalent moving dipole (SEMD) model3 to a sleep spindle to estimate the active brain region during the spindle and showed that the equivalent dipole sources were estimated in/near the thalamus.4 The function of the sleep spindle is not fully understood. Steriade et al. showed that during spindle occurrence, the response of forebrain to external stimuli was markedly reduced.1 Taking into account the fact that inhibitory neurons in the reticular thalamic nucleus are involved in the spindle oscillations, they hypothesized that one of the roles of the spindle is to gate the transmission of sensory information from the external environment.5,6 To examine this hypothesis, Elton et al. compared event-related potentials (ERPs) to binaural stimuli in waking and NREM states.7 The latter was further divided into 2 cases, according to whether a spindle was present or absent during a stimulus. The positivity in the ERPs without spindles was increased compared to that in the waking state. The amplitude of the positive peak in the ERPs with spindles was further increased. Hansen and Hillyard have reported that when a subject is actively attending to the stimulus, the positivity in ERPs decreases and the negativity increases.8 Comparing their results to Hansen’s, Elton et al. concluded that their results support the hypothesis of the gating role.7 With a modified method using monaural stimuli, Cote et al. showed that as the stimulus became louder, the positivity increased, and that the positivity was greatest when spindles occurred following the stimulus.9 If the spindle plays a role in the gating process, a spindle could be induced by a sensory stimulation. The ERP studies did not address this issue, although Cote et al. reported the enhanced effect with spindles occurred just after stimulation.9 To address this issue, the present study investigated the relationship between spindles and sensory stimulation. Specifically, the numbers of spindles which occurred in a 5-minute period with and without stimulation were compared. This study also addressed the question of whether sensory stimuli in different modalities have the same effect on the succeeding spindles. Sensory Stimulation Triggers Spindles During Sleep Stage 2