Abstract
Structure shapes function. Understanding what is structurally special about the brain that allows it to generate consciousness remains a fundamental scientific challenge. Recently, advances in brain imaging techniques have made it possible to measure the structure of human brain, from the morphology of neurons and neuronal connections to the gross anatomy of brain regions, in-vivo and non-invasively. Using advanced brain imaging techniques, it was discovered that the structural diversity between neurons and the topology of neuronal connections, as opposed to the sheer number of neurons or neuronal connections, are key to consciousness. When the structural diversity is high and the connections follow a modular topology, neurons will become functionally differentiable and functionally integrable with one another. The high levels of differentiation and integration, in turn, enable the brain to produce the richest conscious experiences from the smallest number of neurons and neuronal connections. Consequently, across individuals, those with a smaller brain volume but a higher structural diversity tend to have richer conscious experiences than those with a larger brain volume but a lower structural diversity. Moreover, within individuals, a reduction in neuronal connections, if accompanied by an increase in structural diversity, will result in richer conscious experiences, and vice versa. These findings suggest that having a larger number of neurons and neuronal connections is not necessarily beneficial for consciousness; in contrast, an optimal brain architecture for consciousness is one where the richest conscious experiences are generated from the smallest number of neurons and neuronal connections, at the minimal cost of biological material, physical space, and metabolic energy.
Highlights
A central idea in biology is that structure determines function, as Jean Fernel (Tubbs, 2015), the founder of modern anatomy and physiology, famously said, “anatomy is to physiology as geography is to history; it describes the theatre of events.” In emphasizing the close relationship between structure and function, Jean Fernel followed the footstep of Herophilus (Pearce, 2013), the ancient Greek who performed the first dissection of human body and recognized the importance of body structure in determining body function
The technique of single cell sequencing may be applied to map the neuronal diversity in different species (Bail et al, 2021; Bakken et al, 2020) and investigate how that relates with the emergence of consciousness
Summary
A central idea in biology is that structure determines function, as Jean Fernel (Tubbs, 2015), the founder of modern anatomy and physiology, famously said, “anatomy (the study of body structure) is to physiology (the study of body function) as geography is to history; it describes the theatre of events.” In emphasizing the close relationship between structure and function, Jean Fernel followed the footstep of Herophilus (Pearce, 2013), the ancient Greek who performed the first dissection of human body and recognized the importance of body structure in determining body function. Since the introduction of brain imaging techniques a few decades ago, researches have been focused on the functional rather than the structural basis of consciousness, with studies searching for brain regions whose activities correlate with the level or the contents of consciousness (Crick & Koch, 2003; Koch et al, 2016). This approach overlooks the important fact that the brain is an interconnected entity where the activity of one region would influence the activities of other regions via direct or indirect connections. We will provide an overview of advanced brain imaging techniques and discuss how these techniques can unveil brain structural complexity (section 2) and its relationship to consciousness (section 3)
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