Fig. 3.

Construction of the spectrogram. A. A ten-second electroencephalogram trace recorded under propofol-induced unconsciousness. B. The electroencephalogram trace in A filtered into its two principal oscillations: the blue curve, an alpha (8 to 12 Hz) oscillation and the green curve, a slow-delta (0.1 to 4 Hz) oscillation. C. The spectrum provides a decomposition of the electroencephalogram in A in to power by frequency for all of the frequencies in a specified range. The range here is 0.1 to 30 Hz. Power at a given frequency is defined in decibels as the 10 times the log base 10 of the squared amplitude (10 log₁₀ (amplitude)²). The green horizontal line underscores the slow-delta frequency band and the blue horizontal line underscores the alpha frequency band used to compute the filtered signals in B. The median frequency, 3.4 Hz, (dashed vertical line) is the frequency that divides the power in the spectrum in half. The spectral edge frequency, 15.9 Hz (solid vertical line) is the frequency such that 95% of the power in the spectrum lies below this value. D. The three-dimensional spectrogram (compressed spectral array) displays the successive spectra computed on a 32-minute electroencephalogram recording from a patient anesthetized with propofol. Each spectrum is computed on a 3-second interval and adjacent spectra have 0.5 seconds of overlap. The black curve at minute 24 is the spectrum in C. D. The spectrogram in C plotted in two-dimensions (density spectral array). The black vertical curve is the spectrum in C. The lower white curve is the time course of the median frequency and the upper white curve is the time course of the spectral edge frequency. Panels A–E were adapted from Purdon and Brown, Clinical Electroencephalography for the Anesthesiologist (2014), with permission from the Partners Healthcare Office of Continuing Professional Development.¹³¹