Hardware behind the software: A neurophysiological look at aphasias, speech, and language

İsmail Uzun¹, Ali Çağan Kaya^{2, 3
1}Atatürk University, Faculty of Medicine, Erzurum, Türkiye
²Hacettepe University, Department of English Linguistics, Ankara, Türkiye
³Ankara University Brain Research Center, Ankara, Türkiye

artwork: Cihan Hasan

Hardware behind the software: A neurophysiological look at aphasias, speech, and language by İsmail Uzun & Ali Çağan Kaya

As linguists, we often focus on the communicative aspects of language. As shown brilliantly in the article titled Brief Notes on Intrapersonal Communication by Zişan Ada Çayır, language is a pretty neat tool that allows us to communicate ideas, thoughts, and emotions. However, we, more empirically inclined linguists, often think that the physiological and neurological systems that enable these are sometimes overlooked. Some of us think that the ‘language use’ enabled by those systems is so interesting that we hardly ever think about the mechanics.

And frankly, why would you?

As the saying goes, don’t change it if it’s not broken. Then, how about if these systems get corrupted? Ironically, more often than not, this allows us to understand the mechanics that enable language use better.

Aphasias could be thought of as an intersection between linguistics and medicine, and are often of particular interest to linguists with an eye for clinical studies or vice versa. In this post, we are hoping to help enthusiasts from both side of the coin to get a better grasp at these linguistic and medical pathologies, how they arise, and how a healthy brain forms and understands language. Hopefully, we will do that in an accessible way for people from both medical and linguistic backgrounds.

The term aphasia originates from the Greek word ἄφατος (áphatos), meaning “mute” or “speechless.” In medical terminology, it refers to speech disorders that arise due to damage in speech-related areas of the cerebral cortex. Aphasias are typically categorized as fluent or non-fluent (also known as expressive or motor aphasias), depending on the location and extent of the brain damage. To understand these speech disorders, we must first understand how speech functions in the brain, and that requires a general understanding of brain structure. For the more curious reader, medical details will be provided in parentheses.

The Basic Anatomy of the Cerebral Cortex

The human brain is anatomically complex. The regions responsible for speech and comprehension are located in the cerebral cortex, the outermost layer of the cerebrum, excluding the cerebellum. The cortex is what gives the brain its signature folded appearance. These folds increase the surface area and allow for a higher density of neurons. The ridges are called gyri, and the grooves are called sulci. You’ll encounter these terms frequently as we proceed.

As a side note, that the parts of the brain responsible for language are located in the outermost layers may suggest that language became a part of the human species in the later stages of its evolution. We find this to be an interesting topic of discussion, but we did not include it in the article as it does not directly relate to our main question and falls outside the scope of this work. Let us know what you think!

The cerebral cortex is responsible for higher cognitive functions such as thinking, memory, perception, attention, awareness, and information processing. In simple terms, human intellectual activity, like thinking and speaking, is driven by neural activity within the cortex. The cortex contains both afferent (incoming) and efferent (outgoing) neural pathways; medically speaking, it is the endpoint of sensory input and the origin point of motor commands. The motor cortex, frequently mentioned in this article, controls muscle movements, including those necessary for speech.

Anatomically speaking, the cortex is divided into two hemispheres, each containing four lobes: frontal, temporal, parietal, and occipital. These names correspond to the skull bones beneath which the lobes are located. (See Figure 1.)

Each lobe is separated by sulci. The frontal and parietal lobes are divided by the central sulcus (Rolando fissure), the parietal and occipital lobes by the parieto-occipital sulcus, and the temporal lobe is separated from the frontal and parietal lobes by the lateral sulcus (Sylvian fissure). The boundary between the temporal and occipital lobes is not clearly marked anatomically, but is generally considered to lie between the incisura preoccipitalis and the parieto-occipital sulcus.

The Anatomy of Language

Speech function is controlled by three primary cortical regions in the left hemisphere: Broca’s area, Wernicke’s area, and the angular gyrus.

Broca’s Area is located near the primary motor cortex, which controls muscles used in speech—lips, tongue, soft palate, and throat. Broca’s area is where speech plans and motor templates are created. This region is responsible for producing words and structuring speech. Once a response is planned here, it is sent to the adjacent motor cortex for execution. “The ability to speak words.”

Anatomically, it corresponds to Brodmann areas 44 and 45 and is located in the inferior part of the precentral gyrus (frontal lobe). It connects to the motor cortex via specialized fibers (e.g., capsula externa). (See Figure 2)

Angular Gyrus lies just behind Wernicke’s area and is essential for visual language processing (i.e., reading). It transforms written words into meaningful input and relays it to Wernicke’s area. It corresponds to Brodmann area 39 in the parietal lobe. “The ability to see words.”

Wernicke’s Area is very important for language comprehension, both auditory and visual. It transforms acoustic input into recognizable language units and decodes implied meanings and abstract ideas. It is closely tied to verbal and symbolic intelligence, and its functionality is said to reflect one’s cognitive potential. If you’re reading and understanding this text right now, you’re using your Wernicke’s area. “The ability to understand words.”

Anatomically, it is found in the posterior section of the superior temporal gyrus (Brodmann area 22), adjacent to the end of the lateral sulcus. It connects to Broca’s area via the arcuate fasciculus. (See Figure 2)

The Physiology of Speech

Speech is essentially the transformation of airflow into sound, using articulators like the tongue, lips, and larynx. While speech (or communication) might seem like a simple act, it involves an intricate biological process.

Let’s walk through an example: Imagine you’re walking outside and see a sign.

First, the visual input reaches the retina and is transmitted via the optic nerve to the primary visual cortex (Brodmann area 17), and from there to higher-order visual areas (area 18).

From here, the language-related process begins. The visual input is sent to the angular gyrus (area 39), where the brain “sees” the words and begins to process meaning.

The information is relayed to Wernicke’s area (area 22), where comprehension occurs. Once the message is understood, the brain formulates a response. This is transmitted via the arcuate fasciculus to Broca’s area, where the verbal response is structured. Broca’s area sends signals to the adjacent motor cortex, which activates the muscles needed to speak the response.

This process summarizes how meaning is constructed and expressed in the brain. Now that we understand how language works in the brain, we can explore what happens when parts of this system are impaired. (See Figures 3 and 4.)

Broca’s Aphasia (Non-Fluent / Motor Aphasia)

As we noted earlier, Broca’s area plans and organizes speech. Damage to this region, due to stroke, trauma, embolism, etc., does not prevent the patient from producing sound (articulation in linguistic terms), but it severely limits their ability to articulate meaningful words. The individual may be able to say isolated words like “yes” or “no,” but struggles to form complete sentences.

Wernicke’s Aphasia (Fluent Aphasia)

Wernicke’s area is responsible for comprehension. Damage here impairs the patient’s ability to process the meaning behind spoken or written words. Although their speech remains fluent, the sentences often lack meaning. Patients may substitute incorrect words or even invent new ones (neologisms). They are often unaware of how incoherent their speech is.

Conduction Aphasia

The arcuate fasciculus connects Broca’s and Wernicke’s areas. Damage to this tract disrupts the link between comprehension and expression. A person with conduction aphasia can understand language (Wernicke intact) and plan speech (Broca intact), but has difficulty repeating words or naming objects due to disrupted signal transmission. A typical response to a naming task might be:

“Ugh, I know this one, I was just about to say it… what was it again? Car… care… cap… cabra… ara… why can’t I say it? I know what it is!“

This example shows how conduction aphasia particularly affects word retrieval, especially for nouns.

Conclusion

As we’ve discovered, language is more about the brain’s immense capacity for processing, understanding, and producing words rather than choosing words and articulating them in a way that makes sense. By looking closer at the anatomy and physiology of speech, we could gain a better understanding of how complex and fascinating aphasias really are. Whether linguistics-tending or from a more medical direction, there is something intriguing about the way that these two areas intersect in the study of language disorders.

We were just thinking about language, and İsmail asked me a question on aphasias. This, in return, evolved into an hour-long discussion about how underrated language is in medicine. We were approaching this subject from different points of view and were curious to see our intersection. This is how we decided to do our homework and started writing. We hope this little article has been interesting and perhaps even made you want to find out more about the role of the brain in language.

Truly, what is more human than a foray into how we know and how we create?

References

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