The Cerebral Cortex and Complex Cerebral Functions

Abstract

The cerebral cortex can be divided into a large isocortex or neocortex, a smaller allocortex (the hippocampal formation and the olfactory cortex) and a transition zone (the mesocortex) in between. The heterogeneous allocortex and the mesocortex have been discussed in Chap. 14. The various parts of the neocortex show large variations in the development of their constituent layers. The cortical areas that receive the primary sensory pathways via the thalamus form the granular cortex, in which layers II and IV are especially well developed. In the motor cortex, these layers are poorly developed (the agranular cortex), whereas the pyramidal layers III and V are well developed. Based on such differences in cytoarchitecture, Brodmann, the Vogts, von Economo and Koskinas and Sarkissov et al. published their brain maps (see Sect. 15.2). The various cortical lobes and areas will be briefly discussed in Sect. 15.3.

The neocortex is the end station of all sensory projections from the thalamus and has extensive corticofugal projections via the internal capsule to the basal ganglia, the thalamus, the brain stem and the spinal cord. These connections have been extensively discussed in previous chapters. In this chapter, emphasis is on corticocortical projections (Sect. 15.4), hemispheric differences (Sect. 15.5), language and the brain (Sect. 15.6) and disorders of cortical connectivity, known as disconnection syndromes (Sect. 15.7). Classic disconnection syndromes were described in the late nineteenth century by Wernicke, Lissauer, Liepmann and Dejerine and include conduction aphasia, associative visual agnosia, apraxia and alexia without agraphia. In 1965, Norman Geschwind re-introduced the disconnection paradigm and, more recently, other disorders such as visual amnesia and prosopagnosia have also been attributed to disconnection mechanisms. The hodological paradigm may be extended beyond the classic disconnection syndromes by including disorders of hyperconnectivity. The term hodological syndromes was introduced to refer to cognitive and behavioural dysfunctions arising from pathologies of white matter pathways, irrespective of whether the dysfunction is one of hypoconnection, hyperconnection or a combination of the two. The final Sect. 15.8 contains a discussion of the neuroanatomical basis of cognitive impairment in the primary degenerative dementias.