Cortical Networks Produce Three Distinct 7-12 Hz Rhythms during Single Sensory Responses in the Awake Rat

Cortical rhythms in the alpha/mu frequency range (7-12 Hz) have been variously related to "idling," anticipation, seizure, and short-term or working memory. This overabundance of interpretations suggests that sensory cortex may be able to produce more than one (and even more than two) distinct alpha/mu rhythms. Here we describe simultaneous local field potential and single-neuron recordings made from primary sensory (gustatory) cortex of awake rats and reveal three distinct 7-12 Hz de novo network rhythms within single sessions: an "early," taste-induced approximately 11 Hz rhythm, the first peak of which was a short-latency gustatory evoked potential; a "late," significantly lower-frequency (approximately 7 Hz) rhythm that replaced this first rhythm at approximately 750-850 ms after stimulus onset (consistently timed with a previously described shift in taste temporal codes); and a "spontaneous" spike-and-wave rhythm of intermediate peak frequency (approximately 9 Hz) that appeared late in the session, as part of a oft-described reduction in arousal/attention. These rhythms proved dissociable on many grounds: in addition to having different peak frequencies, amplitudes, and shapes and appearing at different time points (although often within single 3 s snippets of activity), the early and late rhythms proved to have completely uncorrelated session-to-session variability, and the spontaneous rhythm affected the early rhythm only (having no impact on the late rhythm). Analysis of spike-to-wave coupling suggested that the early and late rhythms are a unified part of discriminative taste process: the identity of phase-coupled single-neuron ensembles differed from taste to taste, and coupling typically lasted across the change in frequency. These data reveal that even rhythms confined to a narrow frequency band may still have distinct properties.