Behavioral/Systems/Cognitive Relationships between Hippocampal SharpWaves, Ripples, and Fast Gamma Oscillation: Influence of Dentate and Entorhinal Cortical Activity

Hippocampal sharp waves (SPWs) and associated fast (“ripple”) oscillations (SPW-Rs) in the CA1 region are among the most synchro-nous physiological patterns in themammalian brain. Using two-dimensional arrays of electrodes for recording local field potentials and unit discharges in freely moving rats, we studied the emergence of ripple oscillations (140–220 Hz) and compared their origin and cellular–synaptic mechanisms with fast gamma oscillations (90–140 Hz). We show that (1) hippocampal SPW-Rs and fast gamma oscillations are quantitatively distinct patterns but involve the samenetworks and share similarmechanisms; (2) both the frequency and magnitude of fast oscillations are positively correlated with themagnitude of SPWs; (3) during both ripples and fast gamma oscillations the frequency of network oscillation is higher in CA1 than in CA3; and (4) the emergence of CA3 population bursts, a prerequisite for SPW-Rs, is biased by activity patterns in the dentate gyrus and entorhinal cortex, with the highest probability of ripples associated with an “optimum ” level of dentate gammapower.Wehypothesize that eachhippocampal subnetworkpossesses distinct resonant properties, tuned by the magnitude of the excitatory drive