EM data was visualized in Neuroglancer and Matlab R2019b.

To identify CCs, we first screened all the small neurons within ± 20μm of the center of the PCL, discarded all the granule cells and MLIs, based on their characteristic morphology and focused on PLIs that matched closely the description of CCs, as well as our 2-photon fills. We restricted our analysis to the most completely reconstructed cells, prioritizing the presence of a beaded axon in the molecular layer. Synapses and cell labels were manually generated. Extended Data Fig. 10 illustrates the importance of using serial EM to identify connections.