Scientific Understanding of Consciousness
Association Cortices Connections
(Paraphrase of Purves; Neuroscience, 616ff)
Association Cortices Connections
The connectivity of the association cortices is appreciably different from primary and secondary sensory and motor cortices, particularly with respect to inputs and outputs. Two thalamic nuclei that are not involved in relaying primary motor or sensory information provide much of the subcortical input to the association cortices: the pulvinar projects to the parietal association cortex, while the medial dorsal nuclei project to the frontal association cortex. Several other thalamic nuclei, including the anterior and ventral anterior nuclei, innervate the association cortices as well.
Unlike the thalamic nuclei that receive peripheral sensory information and project to primary sensory cortices, signals via the thalamocortical projections to the association cortex originate in other regions of the cortex. Consequently, the signals coming into the association cortices via the thalamus reflect sensory and motor information that has already been processed in the primary sensory and motor areas of the cerebral cortex, and is being fed back to the association areas. The primary sensory cortices, in contrast, receive thalamic information that is more directly related to peripheral sense organs. Similarly, much of the thalamic input to primary motor cortex is derived from the thalamic nuclei related to the basal ganglia and cerebellum rather than to other cortical regions.
A second major difference in the sources of innervation to the association cortices is their enrichment in direct projections from other cortical areas, called corticocortical connections. Indeed, these corticocortical connections form the majority of the input to the association cortices. Ipsilateral corticocortical connections arise from primary and secondary sensory and motor cortices, and from other association cortices within the same hemisphere. Corticocortical connections also arise from both corresponding and noncorresponding cortical regions in the opposite hemisphere via the corpus callosum and anterior commissure, which together are referred to as interhemispheric connections. In the association cortices of humans and other primates, corticocortical connections often form segregated bands or columns in which interhemispheric projection bands are interdigitated with bands of ipsilateral corticocortical projections.
Another important source of innervation to the association areas is subcortical, arising from the dopaminergic nuclei in the midbrain, the noradrenergic and serotonergic nuclei in the brainstem reticular formation, and cholinergic nuclei in the brainstem and basal forebrain. These diffuse inputs project to different cortical layers and, among other functions, determine mental state along a continuum that ranges from deep sleep to high alert.
Although the association cortices have a high degree of interconnectivity, this should not be taken to imply that everything is connected to everything else. On the contrary, each association cortex is defined by a distinct, if overlapping, subset of thalamic, corticocortical, and subcortical connections. It is nonetheless difficult to conclude much about the functional role of these different cortical areas based solely on connectivity (this information is, in any event, quite limited for the human association cortices; most of the data comes from anatomical tracing in non-human primates). As a result, inferences about the function of human association areas continue to depend critically on observations of patients with cortical lesions. Damage to the association cortices in the parietal, temporal, and frontal lobes, respectively, results in specific cognitive deficits that indicate much about the operations and purposes of each of these regions. These deductions have largely been corroborated by patterns of neural activity observed in the homologous regions of the brains of experimental animals, as well as in humans using noninvasive imaging techniques.
(end of paraphrase)
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