Aperiodic (1/f) component

The neural power spectrum is not just a set of oscillatory peaks sitting on zero; it has a pervasive aperiodic (background) component whose power falls off with increasing frequency roughly as 1/f raised to some exponent, appearing as a straight line of negative slope on log-log axes. This 1/f-like background is not mere noise: its slope and vertical offset vary with age, arousal, anaesthesia and cortical state, and are thought to reflect, among other things, the balance of excitation and inhibition and aggregate synaptic activity.

The aperiodic component is a confound for any band-power measure. Because a true oscillation is a narrowband bump sitting on top of this sloping background, comparing band power between conditions can be driven entirely by a change in the aperiodic slope or offset, with no change in oscillatory power at all. Separating the two is therefore a prerequisite for any claim about a specific rhythm.