Visual learning before sight: Mapping spontaneous synaptic input in the developing visual cortex at subcellular scale

Dendritic signal integration does not only happen linearly across the dendrite, but also supralinearly. Locally, active dendritic spikes can be generated as a result of clustered synaptic activity. Thus, a single neuron functions as multiple computational subunits, that allow for parallel processing and substantially boost the brain’s computational power. In addition, co-active synapses are clustered together on the dendrite, both during development and in mature neurons. This structure likely already arises in the mouse primary visual cortex during the spontaneous network events that occur during development, before its eyes open. In the two weeks between birth and eye-opening, many new synaptic connections are being formed, based on chemical cues and activity patterns. However, the timepoint and manner in which the clustered synaptic structure arises, remains to be elucidated. Calcium imaging was performed on L2/3 pyramidal neuron dendrites of the primary visual cortex in the second postnatal week (postnatal day 5 + 3-7 days-in-vitro). Here, for the first time, spontaneously induced synaptic inputs were analysed in organotypic slice cultures of the mouse primary visual cortex at the subcellular scale. Synapse density and the event frequency increase rapidly before eye opening. In addition, the organization of synapses along the dendrite is likely already spatially and functionally structured during the second postnatal week. This model allows for extensive further assays to unravel the underlying physiological mechanisms.