Diversity of correlated excitatory presynaptic inputs to pairs of cortical pyramidal neurons

Jami L. Milton-Dantzker and Edward M. Callaway

The Salk Institute for Biological Studies
The University of California, San Diego

Fundamental to an understanding of how neurons process information is basic knowledge about the underlying neuronal processing units. Functional units in primary sensory cortical areas that process stimuli from the periphery correlate to the columnar organization of cortical excitatory neuron dendrites and axons. This "functional column" hypothesis proposes that cortical neurons making up a processing unit would be joined by similar connections within a vertical column so that all cells within a unit are excited by incoming stimuli with small latency differences.

Although the existence of vertically connected neurons in the cortex is not in question, a complete understanding of minimal units and/or rules of connectivity that define these functional columns has eluded experimentalists. In this study, we examined the extent and specificity of vertical connectivity to pairs of layer 2/3 pyramidal neurons to determine the organization of fine-scale microcircuits that comprise functional units. Specifically, we simultaneously mapped functional input to pairs of layer 2/3 pyramidal neurons to determine spatial and temporal correlation of synaptic input by combining dual intracellular recordings and scanning laser photostimulation. We analyzed cross-correlations of stimulus-evoked EPSCs between layer 2/3 pyramidal cells after stimulating in cortical layers 2/3, 4 and 5. Correlated presynaptic input was examined for neurons within 50 microns of one another, increasing the probability of recording between pairs of neurons that make up the same functional units. We asked how selective source (presynaptic) layers are for individual neurons in target (postsynaptic) layers and whether selectivity varied depending on the connectivity or spatial relationships of the target neurons.