Deciphering Brain Function and Circuitry through Disease and Disability

By Nitya Gupta

Though the brain looks like a mere palmful of jelly, it is the most complex structure in the universe. It is capable of directing and coordinating behaviour, remembering facts and events, conjuring emotions, contemplating the vastness of infinity – and more than that is self-aware of its abilities. Damage to one region of the brain does not result in an overall attenuation of cognitive ability or brain functioning, but rather a highly selective loss of a particular function. This indicates that the brain is divided into smaller parts, each with a highly specialised function. In fact, the functions of particular brain regions are often identified by observing the loss-of-function phenotypes caused by damage or a mutation in a specific part of the brain, while all the other functions remain intact. (TED, 2007)

Aphasia is a condition in which people have trouble communicating. Even though sufferers are still intelligent and know what they intend to say, they struggle to put their thoughts into words. Paul Broca was a French doctor who cared for a patient who was only able to say the word ‘tan’. Upon performing an autopsy after the patient’s death, Broca found a large distinct lesion in the frontal lobe in the left hemisphere of his brain. Damage to this same area was then found to be common amongst other patients suffering from difficulties in producing articulate speech and finding the right words, to the extent that it was challenging to name simple everyday objects. Furthermore, they often could not produce the correct sounds and took long pauses between words. However, the patients’ ability to comprehend speech and general intelligence remained uncompromised. This area came to be known as Broca’s area and is now understood to be responsible for speech and language production, specifically coordinating the muscles involved in speech. This region is also responsible for naming objects. (LaBracio, 2020; Acharya, 2001; Keller, 1974)

Broca’s aphasia is also known as expressive aphasia; however, there is another type of aphasia called Wernicke’s aphasia. It is caused by damage to Wernicke’s area in the posterior temporal lobe of the dominant hemisphere, near the auditory cortex. Wernicke’s aphasia is characterised by the inability of the brain to comprehend the meaning behind the speech of others as well as nonsensical speech involving the use of meaningless words. Unlike Broca’s aphasia, patients still maintain the ability to speak fluently. Wernicke’s area has thus been associated with language comprehension, or the process of attaching meaning to speech sounds. (LaBracio, 2020; Alexander, 2008)

Similar loss-of-function traits are also produced when there is damage not to a region of the brain, but to a connection between two functional brain regions. An example of this is the Capgras delusion, otherwise known as imposter syndrome. In this disorder, patients are convinced that a parent, spouse, friend or pet has been replaced by an identical impostor. They are completely healthy and intelligent, but refuse to acknowledge their mother as their mother, for instance, and consider her to be a woman pretending to be their mother. (Ramachandran, 1998)

In the visual cortex of temporal lobe of the brain there is a small structure called the fusiform gyrus that is responsible for facial recognition. When this area is damaged, the ability to recognise faces are lost, including one’s own face. These individuals come to rely on other stimuli such as smell or tone of voice to recognise people. The fusiform gyrus is connected to the amygdala that is part of the limbic system and is the emotional centre of the brain. When a facial visual stimulus is received it travels down a pathway from the fusiform gyrus to the amygdala that then gauges the emotional significance of the stimulus. If it is decided that what someone is looking at is important, a signal is sent to the autonomic nervous system resulting in an accelerated heart rate, as well as sweating from the palms and general feelings of warmth. In the case of Capgras delusion, the connection between the fusiform gyrus and the amygdala is severed by a head injury, and so even though a patient is able to feel emotions and recognise faces, no emotional response or warmth is produced upon seeing the important people in their lives. To account for the inexplicable lack of emotion felt, the brain then decides that even though the woman resembles their mother, she must be an imposter. However, if they were to receive a phone call from their mother, they would feel warmth and their palms would get sweaty, because there is a separate connection between the hearing and emotional centres of the brain that is still intact. (Ramachandran, 1998)

The brain contains an elaborate complex network of billions of neurons, arranged into  various structures each with a specific function. Damage to these delicate inner regions or the connections between them, disrupts precise neural functions and (as seen in the examples above) can lead to their identification. Though it may seem as though a lot has been uncovered about the brain, the process of discovery is a work in progress and some functions remain a mystery.

References:

Acharya, A. B. & Wroten, M. (2021) Broca Aphasia. StatPearls. Available from: http://www.ncbi.nlm.nih.gov/books/NBK436010/. [Accessed Mar 7, 2021]. 

Alexander, M. P. & Hillis, A. E. (2008) Chapter 14 Aphasia. In: Anonymous Handbook of Clinical Neurology. [e-book] , Elsevier. pp. 287-309. Available from: https://www.sciencedirect.com/science/article/pii/S0072975207880146 [Accessed Mar 7, 2021].

Aphasia: The disorder that makes you lose your words. (2020) Directed by: Lisa LaBracio. Available from: https://ed.ted.com/lessons/aphasia-the-disorder-that-makes-you-lose-your-words-susan-wortman-jutt .

Keller, S. S., Crow, T., Foundas, A., Amunts, K. & Roberts, N. (1974) Brain & language. Brain & Language. 109 (1), 29-48. Available from: https://www.sciencedirect.com/science/article/pii/S0093934X08001636

Ramachandran, V.S. & Blakeslee, S. 1998, “The Unbearable Likeness of Being” in Phantoms in the Brain: Probing the Mysteries of the Human Mind HarperCollins, , pp. 158-174.

TED. 3 clues to understanding your brain. (2007) Directed by: TED. Available from: https://www.ted.com/talks/vs_ramachandran_3_clues_to_understanding_your_brain .

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