Spontaneous activity as a key modulator of sensory circuit development

Spontaneous activity as a key modulator of sensory circuit development Sensory perception relies on the existence of primary sensory pathways, each of which is specialized to process a specific sensory modality. However, it was unclear whether these pathways arise directly as unimodal or whether they are initially multimodal and become specified over time. We recently demonstrated that somatosensory and visual circuits are not by default segregated but require the earliest retinal activity to do so. In the embryo, somatosensory and visual circuits are intermingled in the superior colliculus and thus, a whisker pad stimulus leads to a cortical multimodal response, that is, a response that engages both primary somatosensory (S1) and primary visual (V1) cortices. At birth, these circuits segregate and cortical responses switch to unimodal and this segregation depends on the arrival of stage I retinal waves to the superior colliculus. Simultaneously to these fundamental processes of development, patterns of neuronal activity emerge spontaneously in the cortex. The role of these patterns of activity in endowing cortical territories with specific sensory modalities remains unclear. Using mesoscale functional imaging in embryonic and early postnatal mice, we show that distinct sensory cortical territories exhibit specific patterns of spontaneous activity. Interestingly, the properties of these patterns have a divergent trajectory during development, being more similar when the circuits are intermingled, and progressively becoming distinct from one another as the circuits segregate. Furthermore, the patterns of spontaneous activity rapidly change in response to sensory deprivation, reflecting the underlying circuit reorganizations. Altogether, our work suggests that patterned spontaneous activity observed in the developing cortex contains crucial information about the construction of emerging circuits in normal and pathological conditions and may be used to predict or modulate circuit assembly during development and disease. None