Scaling of optogenetically evoked signaling in a higher-order corticocortical pathway in the anesthetized mouse

Quantitative analysis of corticocortical signaling is needed to understand and model information processing in cerebral networks. However, higher-order pathways, hodologically remote from sensory input, are not amenable to spatiotemporally precise activation by sensory stimuli. Here, we combined parametric channelrhodopsin-2 (ChR2) photostimulation with multi-unit electrophysiology to study corticocortical driving in a parietofrontal pathway from retrosplenial cortex (RSC) to posterior secondary motor cortex (M2) in mice in vivo. Ketamine anesthesia was used both to eliminate complex activity associated with the awake state and to enable stable recordings of responses over a wide range of stimulus parameters. Photostimulation of ChR2-expressing neurons in RSC, the upstream area, produced local activity that decayed quickly. This activity in turn drove downstream activity in M2 that arrived rapidly (5–10 ms latencies), and scaled in amplitude across a wide range of stimulus parameters as an approximately constant fraction (∼0.1) of the upstream activity. A model-based analysis could explain the corticocortically driven activity with exponentially decaying kernels (∼20 ms time constant) and small delay. Reverse (antidromic) driving was similarly robust. The results showthat corticocortical signaling in this pathway drives downstreamactivity rapidly and scalably, in a mostly linear manner. These properties, identified in anesthetized mice and represented in a simple model, suggest a robust basis for supporting complex non-linear dynamic activity in corticocortical circuits in the awake state.