2/ BRISC uses random molecular barcodes transsynaptically transmitted by rabies viruses to map inputs of many neurons simultaneously, while maintaining single cell resolution. Connections between individual neurons are then identified by matching their barcode sequences.

3/ Barcodes are read out from intact tissue sections using in situ sequencing, thus reconstructing presynaptic ensembles of individual starter neurons while preserving spatial information.

4/ We apply BRISC in the mouse primary visual cortex, mapping 7,814 putative synaptic connections of 385 starter neurons and identifying layer- and cell-type-specific connectivity rules.

\5 Long-range inputs onto single V1 neurons reveal the topography of feedback projections from higher visual areas, which mirrors the retinotopic organisation of the visual cortex.