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Abstract:
This article deals with the anatomy of the cortex and shows how its particular structure is suited for higher cognitive functions. The cortex is one of several pieces of gray matter with a mainly two-dimensional layout. It is composed of layers which are to do with the arrangement of input and output systems but can also be visualized in cell body and myelin stains. The variety of neuronal types can be divided into two main classes: neurons with many dendritic spines, excitatory synapses, and both short-range and long-range connections, and neurons with few or no spines, inhibitory synapses, and only short-range connections. Most of the neurons in the cortex are of the spiny type (pyramidal cells) and most of the synapses are synapses between neurons of this type. The main distinction between the cortex and the other major parts of the brain are (a) the overwhelming connectivity of the cortex with itself, and (b) the fact that this network is excitatory. This enables the cortex to deal with correlations in the outside world, a prerequisite for higher cognitive functions, as it is assumed in the Hebbian theory of cell assemblies and the Abelesian theory of synfire chains. The human cortex can be divided into more than 50 areas on the basis of variations in the appearance of layers, input and output systems, and electrophysiological properties. Areas often have a columnar substructure, which is in many cases related to the local separation of different kinds of input. Cortical areas are richly connected among themselves, the pattern of connections reflecting the preprocessing within modalities, preceding global operations.
