In neuroscience and neuropsychology, functional specialization is a theory which suggests that different areas in the brain are specialized for different functions. It is opposed to the anti-localizationist theories and brain holism and equipotentialism.
Historical origins
Phrenology, created by Franz Joseph Gall (1758–1828) and Johann Gaspar Spurzheim (1776–1832) and best known for the idea that one's personality could be determined by the variation of bumps on their skull, proposed that different regions in one's brain have different functions and may very well be associated with different behaviours. Gall and Spurzheim were the first to observe the crossing of pyramidal tracts, thus explaining why lesions in one hemisphere are manifested in the opposite side of the body. However, Gall and Spurzheim did not attempt to justify phrenology on anatomical grounds. It has been argued that phrenology was fundamentally a science of race. Gall considered the most compelling argument in favor of phrenology the differences in skull shape found in sub-Saharan Africans and the anecdotal evidence (due to scientific travelers and colonists) of their intellectual inferiority and emotional volatility. In Italy, Luigi Rolando carried out lesion experiments and performed electrical stimulation of the brain, including the Rolandic area.
Phineas Gage became one of the first lesion case studies in 1848 when an explosion drove a large iron rod completely through his head, destroying his left frontal lobe. He recovered with no apparent sensory, motor, or gross cognitive deficits, but with behaviour so altered that friends described him as "no longer being Gage," suggesting that the damaged areas are involved in "higher functions" such as personality. However, Gage's mental changes are usually grossly exaggerated in modern presentations. Subsequent cases (such as Broca's patient Tan) gave further support to the doctrine of specialization. In the twentieth century, in the process of treating epilepsy, Wilder Penfield produced maps of the location of various functions (motor, sensory, memory, vision) in the brain.
Major theories of the brain Currently, there are two major theories of the brain's cognitive function. The first is the theory of modularity. Stemming from phrenology, this theory supports functional specialization, suggesting the brain has different modules that are domain specific in function. The second theory, distributive processing, proposes that the brain is more interactive and its regions are functionally interconnected rather than specialized. Each orientation plays a role within certain aims and tend to complement each other (see below section `Collaboration´).
Modularity The theory of modularity suggests that there are functionally specialized regions in the brain that are domain specific for different cognitive processes. Jerry Fodor expanded the initial notion of phrenology by creating his Modularity of the Mind theory. The Modularity of the Mind theory indicates that distinct neurological regions called modules are defined by their functional roles in cognition. He also rooted many of his concepts on modularity back to philosophers like Descartes, who wrote about the mind being composed of "organs" or "psychological faculties". An example of Fodor's concept of modules is seen in cognitive processes such as vision, which have many separate mechanisms for colour, shape and spatial perception. One of the fundamental beliefs of domain specificity and the theory of modularity suggests that it is a consequence of natural selection and is a feature of our cognitive architecture. Researchers Hirschfeld and Gelman propose that because the human mind has evolved by natural selection, it implies that enhanced functionality would develop if it produced an increase in "fit" behaviour. Research on this evolutionary perspective suggests that domain specificity is involved in the development of cognition because it allows one to pinpoint adaptive problems. An issue for the modular theory of cognitive neuroscience is that there are cortical anatomical differences from person to person. Although many studies of modularity are undertaken from very specific lesion case studies, the idea is to create a neurological function map that applies to people in general. To extrapolate from lesion studies and other case studies this requires adherence to the universality assumption, that there is no difference, in a qualitative sense, between subjects who are intact neurologically. For example, two subjects would fundamentally be the same neurologically before their lesions, and after have distinctly different cognitive deficits. Subject 1 with a lesion in the "A" region of the brain may show impaired functioning in cognitive ability "X" but not "Y", while subject 2 with a lesion in area "B" demonstrates reduced "Y" ability but "X" is unaffected; results like these allow inferences to be made about brain specialization and localization, also known as using a double dissociation. The difficulty with this theory is that in typical non-lesioned subjects, locations within the brain anatomy are similar but not completely identical. There is a strong defense for this inherent deficit in our ability to generalize when using functional localizing techniques (fMRI, PET etc.). To account for this problem, the coordinate-based Talairach and Tournoux stereotaxic system is widely used to compare subjects' results to a standard brain using an algorithm. Another solution using coordinates involves comparing brains using sulcal reference points. A slightly newer technique is to use functional landmarks, which combines sulcal and gyral landmarks (the groves and folds of the cortex) and then finding an area well known for its modularity such as the fusiform face area. This landmark area then serves to orient the researcher to the neighboring cortex. Future developments for modular theories of neuropsychology may lie in "modular psychiatry". The concept is that a modular understanding of the brain and advanced neuro-imaging techniques will allow for a more empirical diagnosis of mental and emotional disorders. There has been some work done towards this extension of the modularity theory with regards to the physical neurological differences in subjects with depression and schizophrenia, for example. Zielasek and Gaeble have set out a list of requirements in the field of neuropsychology in order to move towards neuropsychiatry:
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