Summary

The webpage discusses the concepts of 'small worlds' and 'rich clubs' as key to understanding the structure and function of the brain, highlighting their roles in neural efficiency and resource optimization.

Abstract

The article delves into the intricate organization of the brain, emphasizing two pivotal concepts: 'small worldness' and 'rich clubs'. Small worldness refers to the brain's network structure that balances local processing with long-range connections, enabling efficient communication with minimal wiring. This architecture is crucial for maximizing functional efficiency within the confined space of the skull. Rich clubs, a more recently discovered subnetwork, consist of a minority of neurons that are highly active and interconnected, suggesting a specialized role in brain function. These neurons, despite their small number, are responsible for a significant portion of the brain's activity and energy consumption. The rich-club organization is hypothesized to optimize resource use and may be critical for rapid responses and survival. Although the exact purpose of rich clubs remains largely unknown, their potential implications for understanding and treating mental disorders are being explored. The article concludes by underscoring the importance of ongoing research into these concepts for unlocking the mysteries of the brain and improving human health and longevity.

Opinions

The author suggests that small worldness is an evolutionary solution to the brain's need for both localized processing and rapid global communication.
It is implied that the brain's structure is influenced by environmental pressures and the types of information it needs to process for survival.
The discovery of rich clubs in the brain is presented as a significant advancement in neuroscience, with the potential to revolutionize our understanding of brain function and dysfunction.
The article posits that the brain's architecture, including both small worldness and rich clubs, is a testament to its ability to adapt and optimize within strict spatial and metabolic constraints.
There is an optimistic view that further research into rich-club networks could lead to breakthroughs in the diagnosis and treatment of mental disorders.
The author expresses that as technology advances and research tools become more sophisticated and affordable, our understanding of the brain will continue to grow, leading to improved health outcomes.

NEUROSCIENCE AND NETWORK SCIENCE

Why Efficiency in Your Brain Depends on Small Worlds and Rich Clubs

How just two concepts explain a lot about the structure and function of your brain

Image made using the AI-based image generator, Gencraft

Brains are complicated organs. They have many interacting parts, which range from the nanoscopic to the macro scale. And despite decades of intense study, brains are so complex that we’re still not sure if we even have the right vocabulary and concepts to describe them.

Fortunately, however, research over the last 30 years has identified two concepts that teach us much about brains. They are ‘rich clubs’ and ‘small worlds’.

Small worldness describes the way neurons are connected to each other across the brain as a network, while rich clubs refer to an important subnetwork that was recently discovered in the brain.

To unpack what these concepts teach us about the brain, we’ll start by describing what we mean by ‘small worldness’. I’ll then outline the recent discovery of ‘rich-club networks’ in the brain, and see how important they are for brain function.

What exactly do we mean by ‘small worlds’?

Diagram of a small-world network. Author: Keiono. Source: Wikimedia Commons

“Small world, eh?” We’ve probably all heard this before. It means that there’s an unlikely connection between two people who you’d expect to be separated by several intermediate people.

This speaks to an intuition most of us have about how social networks ought to be organised. For example, we feel like there should be many social connections separating us from celebrities. And yet the evidence suggests that we’re all no more than 6–7 degrees of separation from any person on Earth, including your favourite celebs.

How’s that possible?

The key ingredient is that many of us have ‘long-range’ connections to people in distant parts of the world. These long-range connections enable us to ‘jump’ from one point of the global social network to many other far-flung points in just a handful of steps.

Small worldness in the brain

We’re probably all familiar with this principle in social networks, but how does it relate to the brain? It turns out that the brain adopts small-world organisation as a way to balance efficiency and constraints on space inside our skulls.

On average, the human brain contains around 90 billion neurons in a skull with a volume of roughly 1.5 liters. As the fraction of neurons with long-range connections increases, so does the cost of extra wiring. This takes up precious real estate that could otherwise be spent on more neurons, thus reducing our brain’s processing power.

Diagram of brain networks. Source: Wikimedia Commons

For this reason, it makes sense to minimise long-range connections and focus on the local processing of brain signals, as local connections require much less wiring. But if the neural network is too focused on local connections, signals will propagate slowly across the brain, making us unable to function effectively.

Fortunately, as networks increase in size, the fraction of long-range connections necessary to maintain processing speed decreases. This property of networks means that our brain can prioritise local processing to minimise wiring costs and also maintain overall efficiency with a relatively small subpopulation of long-range neurons.

In this way, small worldness minimises wasted space on surplus wiring while still maximising the brain’s functional efficiency. This is how brains operate successfully despite hard constraints on space and limited time for making highly complex decisions, often with poor and/or incomplete information.

Rich clubs and inequality in the brain

You’re probably thinking “Okay, small worldness is one thing, but what the hell do rich clubs have to do with the brain?” This is a very fair question. After all, even to people who study the brain, rich clubs are a fairly new concept, as they were only discovered in the last decade or so.

While small worldness describes how the brain is structured as a whole network, rich clubs describe a subnetwork that also makes a crucial contribution to brain structure and function. For example, until recently, very little was known about how the brain’s activity is distributed among its ~90 billion neurons.

But as research tools became more sophisticated, neuroscientists made an unexpected discovery. Much to their surprise, studies in humans and our mammalian cousins found that a minority of neurons accounts for almost half of the brain’s activity.

Brains are metabolically expensive, taking up around 20% of our daily energy budget while representing only about 2% of our total body mass. And since these cells take up so much of the brain’s resources, they were described as ‘rich’.

Diagram of rich-club organisation, with the rich club represented in red. Author: Mdippel. Source: Wikimedia Commons

This minority of neurons also receives information from many parts of the brain, but prefers to share that information with each other, forming a type of exclusive ‘club’. Hence, the concept of rich-club organisation in the brain (aka rich-club networks) was born.

Why rich-club organisation?

While the value of small worldness for the brain is fairly obvious, the significance of rich-club organisation mostly remains to be determined. Currently, it seems that rich-club organisation may optimise how efficiently the brain uses its resources.

The details quickly get quite dense and confusing, but the take home message is reasonably straightforward. The brain is an amazing organ, but there are limits to how efficiently it can use energy to process and store information.

These limits are partly related to the type of information we’re likely to encounter in the world. It seems that brain evolution has taken cues from the environment, and adopted a functional architecture that is best suited to the lessons we must learn in order to survive.

Rich-club networks may also provide an efficient system for responding quickly, with clear advantages for survival and reproduction. But beyond these speculations, the purpose of rich-club organisation is essentially unknown.

Whatever the reason for rich clubs in the brain, people are now investigating how rich-club networks may be related to health and disease. In a lucky scenario, we may find that dysfunction in rich-club organisation provides a useful way to understand and treat mental disorders, but only time will tell.

Key points

The brain has to do a difficult job with little space, a limited energy budget, and often poor information. It does its best, finding clever ways to get the most out of its territory (i.e., small worldness). It also uses a subnetwork that seems to optimise efficiency (i.e., the rich-club network), although the details of how this works remain unclear.

Understanding how the brain overcomes constraints is at the core of neuroscience. Discoveries are always being made, especially as our tools become more clever and affordable. And as we keep learning, a better understanding of our most complex and important organ is sure to provide new and better ways to live happier, longer, and healthier lives.