Several of the properties that characterize the interaction between excitation and inhibition in response to sensory stimuli are also found during beta and gamma oscillations (Fig. 7). During hippocampal gamma oscillations for example, despite the fact that the magnitude of excitation and inhibition can vary on a cycle-by-cycle basis, their overall ratio remains approximately constant (Fig. 7A) (Atallah and Scanziani, 2009). Furthermore, there is a phase difference between the excitatory and inhibitory components of the oscillation. During hippocampal gamma oscillations the inhibitory phase is delayed by 1–2 milliseconds relative to the phase of excitation (Fig. 7B) (Atallah and Scanziani, 2009). Similarly, inhibition has a lag of 5–10 ms relative to excitation during beta frequency oscillations (20 – 40 Hz) in olfactory cortex (Fig. 7C,D) (Poo and Isaacson, 2009). As a consequence, the ratio between excitation and inhibition, favors excitation early during these oscillation cycles while shifting towards inhibition later in the cycle. This sequence of excitation and inhibition leads to relatively narrow time windows for spiking, as is apparent in the tightly phase-locked firing behavior of pyramidal cells relative to the oscillations in the hippocampus and olfactory cortex (Fig. 7B, D) (Atallah and Scanziani, 2009; Poo and Isaacson, 2009).
