Paul Broca and Modularity of Mind

What evidence is there that our brains are comprised of specialized modules with distinct functions? A case study more than a century ago laid a cornerstone in this theory.

Time Time

One day in 1839, a 30-year-old Frenchman named Louis Victor Leborgne walked into Bicêtre Hospital in the southern suburbs of Paris.[i] Leborgne had struggled with epilepsy since his youth, but the latest complication was an alarming one. Leborgne had completely lost the ability to speak.

In its stead, Leborgne voiced a single syllable that he uttered with different pitches and intonations, but always with the same sound: “Tan.” Usually he would use the syllable in pairings: “Tan, tan.” In French, this syllable sounds most like the word temps, or time, so this unfortunate man – condemned to inarticulation – might still have seemed to the staff to have some poetry in him. They started using his preferred sound as his nickname.

The remarkable fact of his ailment was that the rest of his intellectual functioning seemed largely unaffected. Leborgne was described by those who dealt with him as intelligent.

Sadly, his father – a schoolteacher – died shortly after his admission to the hospital. His mother had died when he was three, and he was unmarried. Severely handicapped and with no caretakers available, he would remain in Bicêtre for the remaining 21 years of his life.

Toward the end of his life, Leborgne’s condition deteriorated. He became bed-bound because of a paralysis. Bed sores on his right leg developed into gangrene.

Attending him was a brilliant young surgeon named Paul Broca.

More than just a surgeon, Broca’s broad-ranging curiosity led him to make important discoveries regarding rickets, arthritis, and muscular degeneration, as well as to venture into far-off fields such as anthropology.[1] However, Broca’s skills were not enough to save Leborgne’s life. In 1861, Leborgne succumbed to the blood poisoning that gangrene causes.

It would be uncharitable (and unfounded) to say that Broca was hoping for Leborgne’s death, but it’s hard to imagine that there wasn’t some anticipation. Not long before, Broca had seen a presentation by Ernest Aubertin at the Anthropological Society of Paris. Aubertin’s story was both gruesome and fascinating. A young man had come to Aubertin’s hospital with an open gunshot wound caused by a failed suicide attempt. The exposed brain contains no nerves for feeling pain, and the history of neurosurgery is filled with tales of doctors poking at the brains of their waking subjects and observing the results. This was one of those cases.

Aubertin applied light pressure to an area of the left frontal lobe that some theorists had suggested might control speech. His action cut off blood supply to that part of the brain. The patient, who had been talking freely suddenly fell silent. Upon releasing the pressure, his ability to speak returned; it was as if an on/off switch for speech had been flipped. Aubertin saw this as compelling evidence that this part of the brain was dedicated to speech.

Impressed by this presentation, Broca had asked himself: Could Leborgne’s malady be caused by a lesion in that same area? He now had an opportunity to test that hypothesis. Broca performed Leborgne’s autopsy and was able to confirm that his hunch was correct. There was indeed a lesion in the third frontal convolution of the left hemisphere, which we now recognize as the center of speech.[2] Broca did not tarry in reporting his results: he made a presentation to the Anthropological Society the very day after Leborgne’s death.

That area of the brain is now known as Broca’s Area, and Leborgne’s malady is referred to as Broca’s Aphasia. Two years later, Aubertin wrote a paper documenting the theory and evidence that had prefigured Broca’s discovery,[ii] but alas, the glory had already gone to someone else. And so began a long line of inquiry that continues to this day: the effort to localize behavioral functions in specific parts of the brain.

Leborgne has had many cousins-in-science. Phineas Gage’s personality changed when an accidental explosion drove a metal rod through his frontal lobe. Henry Molaison lost his ability to form new short-term memories when a surgeon removed about two-thirds of his hippocampus. (We shall return to Molaison and his surgeon – after which I am named – in Chapter 5.) Oliver Sacks wrote about the case of an anonymous male who developed hyper-sexuality after a part of his brain was removed to treat his seizures (the same clinical motivation that led to Molaison’s surgery.)

Modularity

Cases like these, where specific mental functions are altered when the brain is altered, have ravaged the idea that mind is ruled by an ether or spirit, as Descartes thought. And they buttress the case for modularity.

Indeed, the concept of modularity has gained broad acceptance among scientists and philosophers, although it is a house with rickety foundations. Steven Pinker can write a book titled How the Mind Works and Jerry Fodor can reply with The Mind Doesn’t Work That Way; evolutionary psychologists Cosmides and Tooby can hypothesize about massive modularity, and many researchers since can respond with carefully considered rebuttals; but they all usually believe in some sort of modularity, broadly defined. It is difficult to controvert that cognitive functions exhibit some level of independence. The real debates lie in defining how much independence exists, what form it takes, and how hierarchical structures fit into this framework.

Are horrible accidents and unfortunate surgeries the only evidence we have of modularity? Far from it. Since the 1990s Functional Magnetic Resonance Imaging (fMRI) has allowed us to identify what parts of the brain are working by capturing images of blood flow. Using this technology, scientists have observed brain activity during specific tasks and demonstrated that certain tasks activate consistent, specific networks (with some variability) across a wide range of individuals.

This doesn’t mean that there are specialized areas in your brain for each task you perform; the mind is far too complicated for that. Instead, fMRI shows that nearly any task is going to spark a network that spans many parts of the brain. Crucially, these networks are consistent and repeatable: the same ones activate when the same task is performed. Whether you’re experiencing lust for a sexual partner, recognizing a loved one’s face, or feeling fear in the face of potential danger, specific neural networks –assigned specific functions – are at work in your brain.

The case for modularity does not depend on physical examination alone; it has been supported by behavioral tests as well. The following is called the Müller-Lyer illusion. Which of these lines is longer?

The Muller-Lyon illusion.

You have probably replied that the bottom line seems longer. If you are enterprising and brought a ruler to the page, you would see that the lines are actually the same length. But does that discovery (from either the ruler, or reading the last sentence, or both) make the bottom line stop seeming longer? No, the illusion prevails. It’s as if there were a department of visual perception in our brain that has been tricked up, while a department of knowledge in our brain that has gotten an email saying “don’t trust what the visual perception department says; both lines are the same length.” If there were no wall between the two departments, visual perception might correct its mistake upon learning of it, but instead it continues in error.

[1] Alas, his curiosity also led him to work up many now disproven theories, such as the idea that there is a relationship between brain shape and intelligence.

[2] The exact location of Broca’s area can vary depending on which side of the subject’s brain is dominant, and other factors.

[i] Pascale Tremblay and Simona M. Brambati, “A Historical Perspective on the Neurobiology of Speech and Language: From the 19th Century to the Present,” Frontier Psycholology 15 (2024), doi: 10.3389/fpsyg.2024.1420133. See also, Nasser Mohammed et al., “Louis Victor Leborgne (‘Tan’),” World Neurosurgery 114 (2018): 121–125.

[ii] Society of Automotive Engineers, Aubertin, Considérations sur les localisations cérébrales et en particulier sur le siège de la faculté du language articulé (Paris: Masson, 1863).