Cytoarchitectural Modules: Functional Specialisation and Disruption in Neuropsychiatric Disorders

Abstract

Central to the concept of a modular brain is the notion that ‘units’ of function can be identified. Although structural units corresponding to these functions are not a necessary consequence of the model, the presence of modular structures suggests a neural basis. Three levels of structural modular organisation–cortical region, macrocolumn, minicolumn–are considered here, with a focus on the smallest units, the minicolumns, which provide an index of the specialisation and functional integrity of the larger modules. The issue of processing specialisation is discussed with regard to functions that may themselves be considered to be modular: face processing and language. The issue of functional integrity is approached through the association between pathology, minicolumn disruption and functional abnormality in Alzheimer’s disease and schizophrenia. In such disorders there is evidence of pervasive functional disruption across multiple domains which suggests commonalties across modules. In dementia and old age a process of ‘dedifferentiation’ has been observed whereby functions become simplified and less distinct. This appears to be the opposite of ‘emergent modularisation’ that is observed during development. In this context, the degree of modularity of the brain may be considered to change over time, initially increasing and then decreasing across the lifespan. The widespread structural motif of the (mini)column is a simple modular component, common to most brain regions but varying in relation to regional processing biases. Systematic regional variations may develop to support emergent modular function and acquisition of expertise, or may be eroded, reflecting vulnerability to disease and loss of specialisation.