During early development, even before the senses are active, bursts of activity travel across the nervous system. This spontaneously generated activity drives the refinement of synaptic connections, preparing young networks for patterned sensory input. Synaptic fine-tuning relies not only on the presence of spontaneous activity, but also on the specific characteristics of these activity patterns, such as their frequency, amplitude and synchronicity. Here, we provide evidence that these crucial characteristics are shaped by the relative balance of excitation and inhibition, where patterns with distinct characteristics have different excitatory/inhibitory ratios. Inhibition can control whether cells participate during a spontaneous event, as pharmacogenetic suppression of the somatostatin (SST) expressing subtype of inhibitory interneurons increased cell recruitment and lateral spread of events. KeywordsMouse, in vivo, pharmacogenetics, somatostatin expressing interneuron, VIP expressing interneuron, calcium imaging, DREADD

Highlights d Spontaneous retinal ''L-events'' activate small areas of neonatal visual cortex d Somatostatin interneurons restrict activation area and cell recruitment d Retina-independent ''H-events'' activate larger areas of cortex than L-events d H-and L-events are characterized by distinct excitatory/ inhibitory input ratios