Developing sensory cortices exhibit spatially-organized spontaneous activity, which is critical for the cortical circuit maturation. We previously reported “patchwork-type” spatial organization of spontaneous activity in the mouse barrel cortex at postnatal day 5 (P5) (Press release at 2018)

In the present study, we analyzed in detail how the spatial organization of barrel cortex spontaneous activity changes during the first two postnatal weeks. We found that spontaneous activity between P1 to P5 exhibited a patchwork-type pattern (Phase I). While around P9, a new type of pattern, showing wide area synchronization (Phase II), was observed, and at P11, neurons fired sparsely as observed in the adult brain (Phase III).

When thalamus was genetically silenced, Phase I cortical activity was abolished but Phase II and III activity remained intact, suggesting that the Phase I to II transition is associated with the shift of the activity source. On the other hand, the Phase II to III transition was impaired by cortical Rac1 inhibition. Phase II to III transition may be facilitated by Rac1-mediated synapse maturation.

Figure: Three phases of spontaneous network activity were found in the barrel cortex layer 4 during the first two postnatal weeks. The Phase I to II transition is associated with the loss of thalamocortical input dependency. The Phase II to III transition may rely on cortical Rac1-dependent synapse maturation.