Cerebral cortex is hierarchical in two different ways . Microscopically, layer 4 receives primary sensory inputs from the thalamus and is thus on the bottom. Geography aside, layers 1-3 and 6 are more or less in the middle. In layer 5 giant pyramidal cells (which convey the verdicts of cortical processing to subcortical regions) are at the top of the hierarchy. This arrangement is nested in a larger-scale anatomical hierarchy with primary sensory areas (such as V1 for vision) at the bottom, and prefrontal executive cortex at the top. Consistent with Jackendoff's intermediate theory, shifting assemblies of many types of neurons sandwiched throughout numerous cortical regions appear to act as the NCC.
Particular Hebbian assemblies may be formed and strengthened primarily by alterations in dendritic morphology leading to enhanced synaptic activity and lowered threshold for specific circuits. Assemblies sculpted by postsynap-tic changes - synaptic plasticity - are the cornerstone of theoretical mechanisms for learning, memory and the NCC. The mechanisms of plasticity include altered number, sensitivity and clustering of postsynaptic receptors, optimal geometry of dendritic spines and branchings, dendro-dendritic connections, and changes in decremental conductance of postsynaptic potentials (e. g. Hausser et al. ). All these changes are mediated by structures within neuronal dendritic interiors, namely the cytoskeleton (e.g. Dayhoff ).
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