CIRCUIT SUPPORTING CORTICAL RIPPLES IN THE MACAQUE VISUAL CORTEX

Evidence across studies points to a link between short high‑frequency oscillatory (HFO) bursts and elevated spiking activity. Yet, the precise mechanisms underlying the generation of HFO remain poorly understood. In our work, we demonstrate the occurrence of such bursts, termed cortical ripples, in the primary visual cortex of macaque monkeys during rest. The ripples appear both with eyes closed and open, exhibiting distinct spatial and temporal organization. In either condition, increased ripple‑band power coincides with elevated spiking. Through spike sorting and waveform analysis, we classify six single‑unit types: two matching signatures predominantly characteristic of Layer 4, one ubiquitous unit type, and three units showing properties consistent with deeper cortical layers. These unit classes display differential modulation by ripple rhythms, varying in firing rate changes, phase‑locking strength, and preferred ripple phase. Based on these findings, we outline plausible mechanisms for the generation of ripple rhythms. We provide evidence for an oscillation-generating excitatory-inhibitory circuit in Layer 4, and propose a complementary mechanism accounting for oscillations observed in deeper layers. Finally, we suggest that the spontaneously active oscillation‑generating circuit corresponds to the circuit engaged also during natural images viewing.