Intelligence Uses the Whole Brain

In a study of brain imaging (fMRI) changes in over 800 people, both while they were active and while they were resting, University of Würzburg researchers found that the whole brain is active more often than we may think. They distinguished three types of intelligence that we use:

Fluid intelligence refers to the ability to solve logical problems, recognize patterns and process new information, independent of existing knowledge or learned skills.

Crystallized intelligence encompasses the knowledge and skills that a person acquires over the course of their life. This includes general knowledge, experience and understanding of language and concepts. It arises through education and experience.

Together, these two forms make up general intelligence. The best predictive performance was achieved with general intelligence, followed by crystallized and fluid intelligence. 

University of Würzburg. “Intelligence requires “the whole brain,” Science Daily. 11 December 2024. The paper is open access.

Thinking as a Whole

That shouldn’t be too surprising if the brain does its thinking as a whole.

One crucial observation: the distribution of connections across the entire brain as well as the pure number of connections were most important for predictive performance, more important than between which exact brain regions the individual connections were located.

“The interchangeability of the selected connections suggests that intelligence is a global property of the whole brain. We were able to predict intelligence not just from a specific set of brain connections, but from different combinations of connections distributed throughout the brain,” says Kirsten Hilger.

““The whole brain”

From the paper:

The choice of the predicted intelligence component as well as the task during which connectivity was measured proved crucial for better understanding intelligence at the neural level. Further, intelligence could be predicted not solely from one specific set of brain connections, but from various combinations of connections with system-wide locations. Such partially redundant, brain-wide functional connectivity characteristics complement intelligence-relevant connectivity of brain regions proposed by established intelligence theories. 

Jonas A Thiele, Joshua Faskowitz, Olaf Sporns, Kirsten Hilger, Choosing explanation over performance: Insights from machine learning-based prediction of human intelligence from brain connectivity, PNAS Nexus, Volume 3, Issue 12, December 2024, page 519

In short, when assessing intelligence, a focus on specific regions like the prefrontal cortex can mislead. When we are thinking, we are using brain-wide connections between many parts of the brain at once.

It’s only natural that many researchers would want to pinpoint the source of human intelligence in a single organ in the brain. But that entails a failure to understand the nature of our intelligence. It is the sum total of a mind’s interaction with an environment, not a box labeled “Intelligence Unit” in the brain.

Can a Brainwide Approach Help Us Understand Brain Damage Better?

That may help answer the question of why people with split brains, half a brain, or no cerebellum can think reasonably well and do daily tasks.

We should not generally think of the brain as a machine where, if a part is missing, the machine simply won’t work. Sometimes that does happen. But often, the human mind attempts to work with whatever brain is available, even with a dying brain.

While working with neurosurgeon Michael Egnor on our upcoming book, The Immortal Mind (Worthy June 3, 2025), I was struck by something he wrote. He had had to remove half of a patient’s cerebellum (a part of the brain that supervises movement):

Every neuroanatomy and neurophysiology textbook I studied as a medical student described the function of the cerebellum in terms of circuits, neural networks, and computation. And so according to these textbooks, it’s highly unlikely — if not impossible — that anyone could sustain even a tiny injury in such a complex “computer” and still retain normal coordination. Let alone the superb coordination that allowed him to play a sport at a high collegiate level. Yet it happens. Neurosurgeons routinely remove large parts of the cerebellum to treat strokes and tumors, and most patients retain normal coordination.

Of course we should want an intact cerebellum! But the relationship of the parts of the brain to each other is more flexible than many neuroscientists have traditionally thought. And the mind surely plays a role in promoting whatever arrangements will work in a given situation.

It’s not that the textbooks are wrong; they are just not telling the whole story. For that, we must also look to, among other sources, the life stories of people who cope with various brain losses or damages.