What epilepsy teaches us about diversity and resilience

The importance of equity, diversity and inclusion in society and its institutions is increasingly recognised. The most progressive apex organizations consider the diversity of people essential to the success, growth, innovation and development of a society.

However, the benefits of diversity are by no means exclusive to human organizations; Heterogeneity and variability are central design principles of all complex natural systems, be they ecological, cellular or genetic networks.

Whether we are talking about an ecosystem, a society, or the brain, how does this diversity relate to the functioning and stability of a complex system?

As neuroscientists, our interdisciplinary research and clinical work has drawn us to the incredible complexity and richness of the human brain and natural systems. Not only do we want to better understand how the brain circuits work, but we also want to develop new therapies for neurological diseases such as epilepsy.

Diversity means resilience

First developed by Darwin, the idea that diversity leads to stability and survival has been debated by scientists from many disciplines for over a century. The ability of natural systems to withstand change is a property known as resilience. This fundamental property arises from interactions between members of the same system – such as species in an ecosystem, individuals in a group, or cells in an organism – and allows it to maintain its functions over time.

Resilience is tested through change. Some ecosystems can adapt to the extinction of certain species or to droughts. Some virtual communities or social networks can withstand cyber attacks. Some organizations can continue to operate after a conflict, war, political revolution or… a pandemic.

Given these common examples—and many others from the social sciences or natural sciences—it is now more important than ever to understand the role that diversity plays in maintaining the resilience of complex systems.

What if clues to the answer reside in the circuitry of the brain, particularly in a brain with epilepsy?

tipping over during a thunderstorm

Our interdisciplinary team has been dealing with epilepsy, the most common serious neurological disease, for several years. Epilepsy is primarily characterized by the seemingly spontaneous and recurrent occurrence of seizures, often triggered by stress or visual stimuli (such as flashes of light or certain images). Recent research has also shown that the frequency of these seizures can vary with the time of day or month, for example depending on the person’s sleep-wake cycle.

In this light, a brain with epilepsy can be seen as fragile and not resilient, and regularly topples into a thunderstorm. So instead of adapting normally to changes, neurons become disproportionately active and out of sync, and the resulting intense electrical activity propagates and disrupts brain function.

Because of the significant impact these seizures have on patients and their families, our team has tirelessly studied the circuitry responsible for triggering them and ways to prevent them.

What does diversity have to do with epilepsy? Our team recently measured the activity of neurons in people with epilepsy. We found that neurons in the brain regions responsible for triggering seizures were much less diverse than those in regions not responsible for seizures. These neurons were oddly similar to each other, displaying very similar properties and responses.

Could this lack of diversity explain why seizure-prone brains are less resilient?

Mathematical models to the rescue

To answer this complex question, we turned to mathematics. What if, through mathematical models of brain circuitry, we could understand how neural diversity (or lack thereof) predicts seizure resistance? Could we determine if neural diversity promotes brain resilience?

Using our equations, we found that when diversity was too low, seizure-like activity spontaneously occurred: the activity of the neurons would be vulnerable to sudden changes in synchrony, reminiscent of what we observe during seizures. These results are unequivocal: the low diversity rendered these neural circuits fragile, not resilient, and unable to sustain the type of activity needed to maintain brain function.

What do these results mean? They provide important insights into the role that different types of neurons play in maintaining brain function.

These results help us to look at neurological diseases like epilepsy differently than before and may open up new avenues to treat them. Our approach of using interdisciplinary methods and mathematics allows us to go further and better understand how diversity increases resilience by providing invaluable clues and answering difficult questions such as: Is there an optimal level of diversity? What are the different types of diversity and do they all promote stability in equal measure? Could we increase resilience by promoting neuronal diversity through targeted therapeutic interventions?

Most importantly, our results also provide a strong reminder of the original role that diversity plays in the robustness of systems in the face of change: this is true not only for neurons and circuits, but also for humans and collectives. Variety really is the spice of life.