Legendary American actor Bruce Willis recently retired from acting due to aphasia. Language is uniquely human. Two models were proposed to understand the neuroscience behind language, the classic Wernicke-Geschwind model, and the newer Dual-Stream model. Lesions in some regions of the brain, involved in language processing can lead to aphasia. There are many types of aphasia. The major ones are Broca's aphasia, Wernicke's Aphasia, Conduction aphasia, and Global Aphasia.
Language is one of humanity's most significant possessions. While other organisms too communicate among themselves in some form or the other, humans are the only organisms on the planet who have such an advanced form of communication. There is no consensus on the definition of language. Oxford dictionary defines language as "the principal method of human communication, consisting of words used in a structured and conventional way and conveyed by speech, writing, or gesture." This is in accordance with the widespread understanding that language is uniquely human. Whether communication in other species qualifies as language is debatable1.
Aphasia is a language disorder. Ever since the American actor Bruce Willis' family revealed that Bruce Willis has it and shall be retiring from acting, it has been in the limelight. Despite the sudden escalation in the buzz around this disorder, it is, as a matter of fact, a fairly common disorder. The most common cause is stroke, but there can be other causes like traumatic brain injury, brain tumours, brain surgery, brain infections (meningitis, encephalitis), and progressive neurological diseases (e.g., dementia). About 1 million people in the United States of America (as of 2015) and 2 million people in India have aphasia2. So, what exactly is it?
Image credit: Gage Skidmore
The term aphasia was derived from the Greek word phásis, which translates to utterance or statement. Thereby aphasia literally translates to 'without utterance'. However, language is not merely the utterance of words. It includes comprehension as well. In the medical context, aphasia is the impairment in expression or comprehension of spoken language or written language. It may include any one of these impairments or a combination of them, or all of them. To understand this better, it would be helpful to understand how language is processed in our brains.
Our initial understanding was primarily based on the works of 4 physicians, namely, Paul Broca (1861), Carl Wernicke (1874), Ludwig Lichtheim (1885), and Norman Geschwind (1970). Widely known as the Wernicke-Geschwind model, it states that spoken words are first processed in the brain's auditory cortex. This information is relayed to a region in the brain known as Wernicke's area, responsible for comprehension of these words. To produce a speech as a response to the spoken words, the extracted meaning is relayed to a distinct area in the brain, known as Broca's area, which is responsible for the articulation of words from where it is ultimately relayed to the motor cortex. The neuronal tract involved in relaying information from Wernicke's area to Broca's area was discovered by Lichtheim and is known as arcuate fasciculus.3 But what about reading and writing? The process is similar. The written words are processed first in the primary visual cortex, and then it is relayed to an area known as the angular gyrus, which Geschwind defined as a visual memory centre for words that turns written language into spoken language and vice versa. After the initial comprehension in the angular gyrus, it is then relayed to Wernicke's area for further processing. Once the meaning has been extracted, it is relayed to Broca's area and ultimately to the motor cortex4.
Hagoort, Peter. (2013). MUC (Memory, Unification, Control) and beyond. Frontiers in psychology. 4. 416. 10.3389/fpsyg.2013.00416.
The Wernicke-Geschwind model assumed that arcuate fasciculus is a unidirectional pathway that relays information from Wernicke's area to Broca's area to enable speech production. With further advancements in neuroscience, a better understanding of language was possible. A new model was adapted, known as the dual-stream model, proposed by Gregory Hickok and David Poeppel. It's more complicated than the previous Wernicke-Geschwind model, but let's simplify it. The name, dual-stream, suggests that two streams are involved, i.e., two pathways instead of just arcuate fasciculus as proposed in the Wernicke-Geschwind model. The spoken words reach the auditory cortex from where it is relayed to Wernicke's area for comprehension. Here, two separate pathways relay this information: the dorsal (upper, posterior) pathway and the ventral (lower, anterior) pathway. The dorsal stream relays speech sounds through the posterior superior temporal gyrus to the motor cortex. The ventral stream relays speech sounds to Broca's area through the anterior superior temporal gyrus for further auditory comprehension and transformation into a "brain-based dictionary". The arcuate fasciculus helps relay information in the dorsal stream. Language processing is now known to involve large-scale networks composed of different brain areas, each with a specialized function, and the white matter tracts that connect them.3,5
Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., Hudspeth, A. J., & Mack, S. (2013). Principles of neural science (Fifth edition.). New York, N.Y.: McGraw-Hill Education LLC.
With the understanding of how language is processed in our brains, we can attempt to understand aphasias. A defect anywhere in the pathway, either the processing centres or relay tracts, will lead to impairment in either comprehension or speech production, depending on where the lesion is. There are many types of aphasias; the major ones include Broca's aphasia, Wernicke's aphasia, Conduction aphasia and Global aphasia.
Broca's aphasia involves lesions in the left posterior frontal cortex (where Broca's area is located). This results in non-fluent, effortful speech. However, since Wernicke's area and conduction pathways are intact, speech comprehension is preserved. Wernicke's aphasia involves lesions in the left posterior superior temporal cortex (where Wernicke's area is located). Thereby, the person is capable of a fluent, well-articulated speech, but it doesn't make sense since comprehension is impaired. Conduction aphasia involves lesions in the relay pathways (dorsal and ventral streams). Such individuals can speak fluently, but there are few articulatory defects. This is because comprehension is intact, but the relay of this meaning is impaired. Last but not least, global aphasia involves lesions in both the frontal and temporal cortex. This leads to a non-fluent, scanty speech with impaired comprehension as well.3,6
Aphasia is a disabling disorder. Thankfully, we do have some treatment options. Treatment can be restorative (i.e., aimed at improving or restoring impaired function) and/or compensatory (i.e., aimed at compensating for deficits not amenable to retraining). This involves therapies like Constraint-Induced Language Therapy (CILT), Melodic Intonation Therapy (MIT), etc.6 The details are beyond the scope of this article. So, let's come back to what's happening with the legendary American actor Bruce Willis. His family has not revealed the details of his aphasia or the condition that led to it. Whatever be the reason, once we understand the mechanism behind aphasia, we can start realizing the disastrous impact it can have on a person's life. Language is an integral part of our existence, and a disorder like aphasia can vastly limit that.
1. Hedeager U. Is a language unique to the human species. Retrieved from Columbia University: http://www. columbia. edu/~ rmk7/HC/HC_Readings/AnimalComm. pdf. 2019.
2. Pauranik A, George A, Sahu A, Nehra A, Paplikar A, Bhat C, Krishnan G, Kaur H. Expert group meeting on aphasia: A report. Annals of Indian Academy of Neurology. 2019 Apr;22(2):137.
3. Kandel ER, Schwartz JH, Jessell TM, Siegelbaum S, Hudspeth AJ, Mack S, editors. Principles of neural science. New York: McGraw-hill; 2000 Jan.
4. Seghier ML. The angular gyrus: multiple functions and multiple subdivisions. The Neuroscientist. 2013 Feb;19(1):43-61.
5. Nasios G, Dardiotis E, Messinis L. From Broca and Wernicke to the neuromodulation era: insights of brain language networks for neurorehabilitation. Behavioural neurology. 2019 Oct;2019.
6. American Speech-Language-Hearing Association (n.d.). Aphasia (Practice Portal). Retrieved April 6th, 2022, from www.asha.org/Practice-Portal/Clinical-Topics/Aphasia/.
About the author
Dr. Harsh Srivastava is an MBBS intern at Uttar Pradesh University of Medical Sciences, India. Currently, he is an Executive Editor at Project Encephalon. His academic interests include behavioral and cognitive neuroscience, public health, and sociology. He likes drawing comics and pondering about his existence in his free time.