“…Very little connectivity data is currently available at this sub-dendrite scale, though several observations, taken together, suggest that withindendrite biases of this kind are biologically feasible: (1) the axonal projections of inhibitory neurons show famously strong spatial biases at the sub-dendrite scale (Bloss et al, 2016;DeFelipe, Ballesteros-Yáñez, Inda, & Muñoz, 2006;Karube, Kubota, & Kawaguchi, 2004;Tremblay et al, 2016); (2) excitatory pathways have well known spatial biases of other kinds, for example, they can selectively target dendrites in specific layers or parts of layers (Harris & Shepherd, 2015;J. S. Lund, 1988;Petreanu, Mao, Sternson, & Svoboda, 2009); (3) excitatory axons are subject to activity-dependent clustering, producing a tendency for co-activated axons to form contacts on nearby spines (DeBello et al, 2014;Iacaruso, Gasler, & Hofer, 2017;Lee, Soares, Thivierge, & Béïque, 2016;van Bommel & Mikhaylova, 2016Weber et al, 2016); (4) individual excitatory axons can show strongly biased projections at the sub-dendrite scale (Bloss et al, 2018;Morgan, Berger, Wetzel, & Lichtman, 2016) (5) proximal vs. distal synapses can be subject to different plasticity rules, which could lead to a spatial sorting-out of functionally distinct input pathways (Froemke, Poo, & Dan, 2005;Gordon, Gribble, Syrett, & Granato, 2012;Sandler, Shulman, & Schiller, 2016); and (6) differences in EPSP rise times suggest horizontal vs. vertical axons (Yoshimura et al, 2000) and near vs. far horizontal connections onto pyramidal neurons (Schnepel, Kumar, Zohar, Aertsen, & Boucsein, 2014) do, on average, terminate at different distances from the soma.…”