Traveling waves along the cortical depth reflect structured synaptic inputs

Traveling waves (TWs) are widely observed in the brain and are thought to play multiple functional roles, yet what LFP traveling waves represent remains unclear. Using translaminar recordings in macaque V1 and V4, we observed local field potential (LFP) TWs that were modulated by sensory experience, but no corresponding TWs in the multi-unit activity envelope (MUAe). Nevertheless, LFP TW periods were associated with increased depth synchrony of MUAe. Crucially, TW speeds predicted population spiking dynamics: faster waves were associated with larger MUAe amplitude, whereas slower waves are associated with smaller MUAe amplitude. We propose that LFP TWs reflect spatiotemporally structured excitatory inputs, while MUAe reflects their leaky integration. This framework provides a parsimonious explanation for both the observed traveling waves and their relationship to population spiking, offering a physiological interpretation of laminar LFP traveling waves, in which TW speed reflects the temporal organization of structured synaptic inputs.