Discussion of jsiegle's comment: The authors combine focal optogenetic stimulation (they estimate that they activate 2-20 neurons at a time) with extracellular recording to investigate the mechanisms of theta oscillations in behaving animals. Previous studies have shown that injecting sinusoidal currents into CA1 pyramidal cells in vitro causes preferential resonance in the 4-11 Hz range, demonstrating that intrinsic cellular properties can contribute to theta generation. This paper provides an updated perspective by using sinusoidally modulated blue light to alter membrane currents in cells expressing ChR2. The authors found that nearby pyramidal cells preferentially phase-lock to theta-frequency stimuli (they tested a range of frequencies from 1-40 Hz), but only when the stimulated cells are parvalbumin-positive interneurons (PV). Directly activating pyramidal cells leads to phase locking at a wide range of frequencies. As hypothesized, infusion of an HCN1 channel blocker abolished PV-dependent theta resonance, indicating that this "hyperpolarization-activated" channel (what the "H" stands for) mediates the effects they observed.