Analysis of sleep spindles in different NREM-REM cycles by means of bispectra

Spindles are periodic oscillations that characterize the predominant stage 2 during sleep. The aim of this paper is to study the spindle activity in different cycles of NREM-REM sleep. Higher order spectral analysis, particularly the bispectrum is used to analyze the quadratic nonlinear interaction. Ten one-second spindles from the first and the last NREM-REM cycles, with two normal subjects, are analyzed. Spindle waves are characterized using the power spectral density (PSD) function and the bispectrum to obtain: (1) differences between sleep spindles of the first and the last NREM-REM cycles, and (2) the interaction between the different components in the EEG signal. Similar results were obtained in both subjects and channels presented: no differences are observed neither in time nor in frequency domain with PSD function between spindles from the first and the last NREM-REM cycles. However, more frequency couplings were observed in the last cycle than in the first one by means of the bispectrum.