Sensory stimulation triggers spindles during sleep stage 2.

Study Objectives: Toward understanding the function of sleep spindle, we examined whether sensory stimulation triggers sleep spindles.

Participants: Eleven normal subjects participated in the experiments.

Intervention: The subjects had a nap in the afternoon, and sensory stimulation was applied during sleep stage 2.

Measurements: 21-channel EEG was recorded during the 2-3 hour experiment carried out between 13:00 and 16:00. Somatosensory, auditory, or visual stimulation was performed over a 5-minute period during stage 2. The frequency and duration of spindles were compared in 2 different segments of 5 minutes, with and without sensory stimulation. The latency from the onset of a sensory stimulus to the succeeding spindle was also analyzed. To estimate the active brain regions during a spindle, the EEG recordings were modeled with a single equivalent moving dipole (SEMD) model.

Results: In the period with stimulation, spindle frequency and duration increased compared with the period without stimulation. Statistical tests revealed that with stimulation, the interval between 2 consecutive spindles was significantly shorter (p < 0.05, regardless of the modality) and that the duration of the spindles was significantly longer with stimulation (p < 0.05, regardless of the modality). The latency was approximately 2 s. During a spindle after somatosensory stimulation brain activities were observed near the somatosensory area, while with auditory stimulation active regions were observed near the auditory cortex.

Conclusions: A sensory stimulus appeared to trigger a sleep spindle during sleep stage 2. SEMD trajectories suggest that active brain regions during spindle are different according to the modality of the preceding stimulus.