Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Cortical Columns Generate Streams of Information


Nature 457, 1133-1136 (26 February 2009)

Intracortical circuits of pyramidal neurons reflect their long-range axonal targets

Solange P. Brown & Shaul Hestrin

Department of Comparative Medicine, Stanford University School of Medicine, 300 Pasteur Drive, Edwards Building, R314, Stanford, California 94305-5342, USA


Cortical columns generate separate streams of information that are distributed to numerous cortical and subcortical brain regions. We asked whether local intracortical circuits reflect these different processing streams by testing whether the intracortical connectivity among pyramidal neurons reflects their long-range axonal targets. We recorded simultaneously from up to four retrogradely labelled pyramidal neurons that projected to the superior colliculus, the contralateral striatum or the contralateral cortex to assess their synaptic connectivity. Here we show that the probability of synaptic connection depends on the functional identities of both the presynaptic and postsynaptic neurons. We first found that the frequency of monosynaptic connections among corticostriatal pyramidal neurons is significantly higher than among corticocortical or corticotectal pyramidal neurons. We then show that the probability of feed-forward connections from corticocortical neurons to corticotectal neurons is approximately three- to fourfold higher than the probability of monosynaptic connections among corticocortical or corticotectal cells. Moreover, we found that the average axodendritic overlap of the presynaptic and postsynaptic pyramidal neurons could not fully explain the differences in connection probability that we observed. The selective synaptic interactions we describe demonstrate that the organization of local networks of pyramidal cells reflects the long-range targets of both the presynaptic and postsynaptic neurons.

The long-range axonal projections of cortical pyramidal neurons target unique sets of cortical and subcortical brain regions and define different functional classes of pyramidal neuron. In addition, each pyramidal neuron elaborates extensive intracortical axon collaterals that generate the majority of excitatory input in neighbouring cortical neurons.

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