Choosing spindles from time-frequency atoms

Sleep spindles play a major role in the analysis of cerebral activity in sleep. Their morphology, described and defined mainly for the purpose of visual analysis [Rechtschaffen and Kales, 1968], corresponds well to the basic shape of waveforms from the Gabor dictionary (section ), so each spindle should be represented by one time-frequency atom. It remains to choose, from the waveforms fitted to the analyzed segment, those corresponding to sleep spindles. According to generally accepted criteria and our previous experience, time-frequency conditions for a structure to be considered a sleep spindle were defined as follows: frequency: 11-15 Hz, time width: 0.5-2 s (octaves 6-8, eq. 10). Amplitude presents a separate problem--in this study the threshold was set at 25 $\mu$V (min), based upon comparison of MP results with visual detection of sleep spindles. Time of occurrence and phase had no influence on discrimination, since information from different channels was treated separately.