The Staggering Complexity of the Human Brain

While the Milky Way has more stars than the brain has neurons, the manifold connections and interactions in the brain make it arguably more complex.

Source: theartofsounds2001 | Pixabay

The human brain has been described as the most complex known structure in the universe (Dolan, 2007). The numbers are indeed staggering: Ingenious research methods that examined the almost 1,000 brain regions, and extrapolated the expected density from small samples, estimated the total number of nerve cells—or neurons—in the brain at 86 billion (Azevedo et al., 2009).

Staggering Numbers

On top of this, there are another 85 billion cells that serve various functions in the brain. Even more incredible is that each one of these neurons on average connects to thousands of other neurons, resulting in an estimated 100 trillion connections (Caruso, 2023; Zimmer, 2011).

These connections allow signals to travel between neurons and can function as logic gates, giving it a computing power of one exaflop—that is one quintillion mathematical operations per second (Madhavan, 2023)—that is a billion times a billion or 1 followed by 18 zeros.

And it does all of this in an incredibly efficient way, using only 20 watts of power. In comparison, one of the most advanced supercomputers in the world requires a million times more power (20 megawatts) to reach anything near a similar computing capacity (Madhavan, 2023).

The total length of this complex fiber network is estimated to be around 500,000km or over 310,000 miles long (Science Daily, 2017). Signals inside this network can reach traveling speeds of 431 kmh or 268 mph (Ross, 2011). Such complexity comes at a biological cost: Despite only accounting for around 2 percent of the body’s mass, the brain consumes around 20 percent of the body’s total energy (Herculano-Houzel, 2012).

Is it really the most complex thing?

A widespread myth is that there are more neurons in the brain than there are stars in the Milky Way. This is not actually true: There are around 200 to 400 billion stars in our galaxy and the commonly cited 100 billion neurons in the brain is probably an overestimation (Stoke & Boytek, 2013). A more accurate figure is around 86 billion (Azevedo et al., 2009).

So, is it really the most complex structure we know of? Many experts still seem to believe so, from physicist Michio Kaku (2014) who described the three pounds in our head as the most complex object in the solar system (pp.2-3), to neuroscientist Christof Koch (2013), psychiatrist Sir Robing Murray (2012), and neurobiologist Gerald Fischbach (1992), who all described it as the most complex structure in the known universe. This is because the complexity is much greater than the number of cells themselves: The many complex connections and the myriad interactions give the brain not only structural complexity but turns it into an intricately functioning whole.

While stars and galaxies interact, they cannot rival the complicated interplay, signaling, and computation going on between neurons.

It is difficult for us to grasp very large numbers, like the number of neurons or connections in the brain. Another way to highlight the complexity of the brain is through something we are more familiar with money, time, and people.

Mapping the Brain

The Human Brain Project was one of the biggest research endeavors the European Union has ever undertaken: It had a budget of €600 million, ran for 10 years, and involved 500 scientists across different institutions (Naddaf, 2023). The goal was to map and then simulate the entire human brain.

Although the project managed to create 3D maps of around 200 brain structures, this represents only a fraction of the initial goal (Naddaf, 2023). Similarly, the US BRAIN project, announced by President Obama in 2013 with an initial funding of $100 million is still ongoing but had to scale back its initial goal of mapping the entire human brain at the level of individual neurons (Mullin, 2023).

Critics have pointed out problems in the management of these projects (Mullin, 2023; Naddaf, 2023) but a key reason for the seeming failure is the underlying complexity of the brain itself.

Hundreds of millions of dollars and over a decade of research by leading scientists have not been able to fully describe the structure of the brain. And understanding the structure of the brain is much simpler than grasping its functioning. A leading neuroscientist wagered in 1998 that the neural mechanisms of consciousness would be discovered within 25 years. He recently conceded his bet lost (Lenharo, 2023).

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Remaining Mysteries

Despite the tremendous progress that has been made in understanding the brain, its staggering complexity means that we are still a long way off from fully understanding its structure and even further away from fully understanding the way it functions. Many ancient cultures saw the heart as the seat of thought, emotion, and volition and had little regard for the brain.

In ancient Egypt, for example, the brain was discarded during the mummification process, while the heart was carefully preserved (see Ancient Concepts of the Mind, Brain (and Soul), Pang, 2023a). We now know that the brain is linked to our thoughts, emotions, and experiences.

Evidence for this includes that damage to the brain impacts those processes (Vaidya et al., 2019), that these processes are congruent with measured electrical activity in the brain, as well as with images taken (for example through functional magnetic resonance images or fMRI; DeSouza et al., 2012), and that psychoactive drugs that target the brain can influence these functions (Buxton et al., 2008).

There is further evidence from developmental psychology and from brain stimulation experiments. Despite the tremendous progress that has been made in neuroscience, psychology, and neurology, many aspects of the brain remain mysterious. The Australian philosopher David Chalmers (1995) suggested that while neuroscience will likely be able to answer most conventional (or what he termed "easy") questions about the brain, the hard problem of why electrical impulses in the brain lead to conscious experience may not be solved (see What is Consciousness? and The Many Dimensions of Consciousness; Pang, 2023b, 2023c).

Conclusion

Our brains are staggeringly intricate and have been described as the most complex structures in the known universe. Despite tremendous advances in neuroscience, many aspects of the brain remain mysterious. Arguably the biggest mystery and the hardest problem to solve is understanding how neural activity gives rise to conscious experiences.