Populations of cortical neurons generate rhythmic fluctuations in their ongoing spontaneous activity. These fluctuations can be seen in the local field potential (LFP), which reflects summed return currents from synaptic activity in the local population near a recording electrode. The LFP is spectrally broad and many researchers view this breadth as containing many narrowband oscillatory components which may have distinct functional roles. This view is supported by the observation that the phase of narrowband oscillations are often correlated with cortical excitability and can relate to the timing of spiking activity and the fidelity of sensory evoked responses. Accordingly, researchers commonly “tune in'' to these channels by narrowband filtering the LFP. Alternatively, neural activity may be fundamentally broadband and composed of transient, non-stationary rhythms that are difficult to approximate as oscillations. In this view, the instantaneous state of the broad ensemble relates directly to the excitab...