Response to Miller: “Broadband” vs. “high gamma” electrocorticographic signals

In his Journal Club, Miller provides a cogent description of how researchers can use broadband signals in electrocorticographic (ECoG) recordings to estimate neuronal spiking without observing individual action potentials. The review brings up the important issue of whether broadband ECoG signals, which appear at all frequencies (1, 2), reflect the same physiological signal as “high gamma ” oscillations, which are typically reported as occurring in the ∼70–150-Hz range (3–6). Our research suggests that broadband ECoG activity is a non-oscillatory physiological signal. As we explain below, previous studies have measured broadband signals, but the results are often incorrectly described as “high gamma ” oscillations. Beginning with Crone et al. (3), a number of studies described ECoG power increases at ∼70–150 Hz when a brain region was active. This pattern was called “high gamma ” because it appeared to be a faster version of the well-studied gamma oscillation, which generally appears as a rhythmic signal at ∼40 Hz (7). The use of the term “high gamma ” implied that this faster signal reflected a true rhythmic oscillation. At the time this nomenclature appeared to be appropriate, because the evidence indicated that this activity was limited to a frequency band below ∼150 Hz. Subsequent studies examined the ECoG correlates of cortical activity using recording systems with faster sampling rates. These studies found that this signal was a broadband ECoG power increase that appeare