Responsiveness of cerebral cortex to hippocampal inputs depends on brain region and brain state

Abstract

Gamma oscillations (30–90 Hz) are a prominent signature of cortical network state, but whether they facilitate or hinder inter-areal communication remains unresolved. The communication-through-coherence hypothesis posits that gamma enhances transmission between areas, whereas recent computational work suggests that high-amplitude gamma oscillations may instead filter incoming inputs and reduce their impact. To distinguish between these accounts, we used a well-defined physiological input—sharp-wave ripple (SWR) complexes—to probe cortical responsiveness via two parallel monosynaptic pathways: from ventral CA1 to prefrontal cortex (PFC), and from dorsal CA1 to retrosplenial cortex (RSC). By classifying the cortical state immediately preceding each ripple as low- or high-amplitude gamma, we found that PFC responses to ripples were significantly larger during low-amplitude gamma states, an effect carried by the ventral CA1–PFC pathway and driven primarily by ripples during quiet wakefulness. RSC showed no such state-dependent modulation. A mean-field model of PFC reproduced the enhanced responsiveness during low-amplitude gamma and revealed that this modulation depends on the excitatory–inhibitory balance of the afferent input and on the level of recurrent excitation—providing a mechanistic explanation for the distinct behaviors of PFC and RSC, which differ in their local recurrent connectivity. Extending the model to a chain of cortical areas predicted that low-amplitude gamma supports robust propagation of activity across regions, whereas high-amplitude gamma confines it locally. Together, these results argue that low-amplitude gamma, rather than strong gamma synchronization, constitutes a favorable substrate for communication between brain areas.