Sparse recurrent excitatory connectivity in the microcircuit of the adult mouse and human cortex

Understanding cortical function will require a detailed knowledge of local circuit properties. Many prior studies have addressed this problem; however, generating a comprehensive description of cortical networks requires a large-scale, systematic approach. To that end, the Allen Institute for Brain Science is engaged in a pipeline project using multipatch electrophysiology, supplemented with 2-photon optogenetics, to characterize local connectivity and synaptic signaling between major classes of neurons in the adult mouse primary visual cortex and neurosurgical samples from human cortex. We focus on producing results that are detailed enough for the generation of computational models and enable rigorous comparison with future studies. Here we report our examination of the intralaminar connectivity within each of several classes of excitatory neurons. We find that connections are sparse but present among all excitatory cell types and layers we sampled, with the most sparse connections in layers 5 and 6. Almost all synapses in mouse exhibited short-term depression with similar dynamics. Synaptic signaling between a subset of layer 2/3 neurons, however, exhibited facilitation. These results contribute to a body of evidence describing recurrent excitatory connectivity as a conserved feature of cortical microcircuits.

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