DISTINCT LAYER-SPECIFIC PATTERNS OF SPONTANEOUS ACTIVITY IN DEVELOPING SENSORY CORTICES

Early brain sensory circuit formation is critically regulated by spontaneous neuronal activity during development, which is thought to prepare sensory systems for the processing of postnatal sensory inputs. Previous studies have described distinct sensory cortices exhibiting region-specific spontaneous activity patterns during early postnatal development; however, how these patterns evolve over early development and whether they arise from specific cell types remains unclear. Here, we monitor spontaneous neuronal activity in identified cell types using in vivo two-photon calcium imaging during the early postnatal period (P1–P4) in both primary visual (V1) and somatosensory (S1) cortices. To do so, we use transgenic mice expressing the calcium indicator GCaMP6f under layer-specific promoters, Rorβ for layer (L) 4 and Rbp4 for L5, or via in utero electroporation for L2/3. We observed a progressive increase in overall spontaneous activity from P1 to P4 in both V1 and S1, indicating a developmental strengthening of intrinsic network activity across sensory cortices. Neurons in L4 exhibited distinct spontaneous activity profiles across sensory modalities. Specifically, L4 neurons in V1 showed wave-like propagating events, while L4 neurons in S1 showed spot-like non-propagating activity. This modality-specific activity in L4, the main thalamo-recipient layer, suggests that spontaneous activity in the cortex may unfold upon arrival of thalamocortical axons. Together, these findings suggest that early, layer-specific spontaneous activity builds upon a common developmental trajectory to preconfigure cortical circuits in an area-dependent manner, potentially supporting flexible allocation of cortical territory and later developmental plasticity.