2. Brain Organisation

On the model presented here, the brain is organised as a supervenience hierarchy, such that higher levels of organisation emerge from interactions at lower levels of organisation during development and experience. There is no interaction between levels; for example, higher levels of organisation do not control lower levels — ‘downward causation’ — any more than a clock controls the molecules on which it supervenes as a level of organisation, because levels of organisation are complementary perspectives on the same phenomenon. On the TNGS model, the notion of ‘control’ is better reinterpreted in terms of selectional interaction between systems at the same level of organisation, if only because it is a ‘category error’ to map a hierarchy of degrees of control onto a hierarchy of organisational levels. 

In considering interactions between systems at the same level of organisation, each system is necessary but not sufficient for the function it performs, just as a specific gene, as “for” iris colour, is necessary but not sufficient for its function (phenotypic expression); a gene only functions in the context of (the functions of) other genes, and its function is distinguished by contrast with the functions of other genes in the genome. Similarly, neurological functions are carried out in the context of other functions and each function is distinguished by contrast with those other functions. Absence or disruption of a specific function results from the absence or disruption of a necessary condition for its performance, as the absence or mutation of a gene results in the absence or variation of its phenotypic expression. By identifying loss of brain function with localised anatomical damage, some have argued that those functions are carried out in those areas, as if such areas are sufficient for the function. However, as brain imaging shows, even for something as “simple” as reciting digits, neural activity is distributed over many regions the brain, and the precise locations of activity vary from one individual to the next.

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The Brain as a Supervenience Hierarchy: A Reinterpretation of 'Control' and Function

In the model presented here, the brain is organised as a supervenience hierarchy, where higher levels of organisation emerge from the interactions at lower levels during development and experience. Crucially, there is no direct interaction between these levels. Higher levels of organisation do not "control" lower levels—a concept often referred to as "downward causation"—any more than a clock controls the molecules it supervenes upon as a level of organisation. Both higher and lower levels of organisation provide complementary perspectives on the same underlying phenomenon, rather than representing a relationship of control.

This interpretation challenges traditional notions of control in neurological systems. On the Theory of Neuronal Group Selection (TNGS) model, the idea of control is better understood in terms of selectional interaction between systems within the same level of organisation, rather than imposing a hierarchy of control across organisational levels. The suggestion that higher levels exert downward control over lower levels represents a category error: applying a hierarchy of control to a hierarchy of organisational levels distorts the nature of how systems interact within those levels.

When we consider interactions at the same level of organisation, each system is necessary for the function it performs, but not sufficient in isolation. For example, a gene associated with iris colour is necessary but not sufficient for the expression of that colour in a phenotype. Its function is dependent on the context provided by other genes and their own functions within the broader genetic system. The same principle applies to neurological functions: they are carried out within the context of other functions, and each function is distinguished by its relationship to other functions within the network. The absence or disruption of a specific function is due to the absence or disruption of a necessary condition for its performance, much like the absence or mutation of a gene alters its phenotypic expression.

Some have argued that specific brain functions are localised to certain anatomical areas, based on the idea that the presence or absence of brain activity in those areas directly corresponds to the presence or absence of function. However, neuro-imaging studies demonstrate that even simple tasks, such as reciting digits, involve the activation of multiple brain regions. The precise locations of this neural activity can vary between individuals, challenging the notion that specific brain functions are exclusively confined to distinct anatomical regions. The brain's functioning is thus distributed and dynamic, and its organisation should be understood not in terms of rigid localisation, but as a complex interplay of systems interacting at multiple levels.