Cell sorting using Lomb-Scargle periodogram. (a) Calcium profiles, (b) membrane action potential profiles, and (c) Lomb-Scargle periodogram, of a burster cell, a spiker cell and a silent cell. The burster has a clear peak frequency at f = 33 mHz (0.033 sec-1), whereas the spiker and silent cells have broad spectra. (d) Distribution of the principal peak p values. All cells with p < 10-12 were plotted at p = 10-12. The burster cells form a distinct group from others, with p < 0.005 (dashed black line).