Localisation of Function in the Brain

Localisation of function in the brain is the concept that some brain regions are responsible for particular functions. They carry out and oversee the functions of behaviours and processes due to their structure and position. Examples include moving a limb or being able to speak.

This idea contrasts with the holistic view of the brain, which says that the brain's functions spread across large areas, if not the entire brain itself.

Therefore, when a particular brain area is damaged, this affects that area's associated function.

The brain has a left and right half called hemispheres which operate contralaterally.

The corpus callosum connects the hemispheres.

Each hemisphere specialises in performing certain functions, which is called hemispheric lateralisation.

The cerebral cortex is critical. It is a thin outer layer of neural tissue, usually 2-4mm thick, of the brain. It is folded for the extra space (the bumps and ridges are known as the gyri, and the sulci are the furrows) and contains mainly cell bodies. Often referred to as grey matter, higher cognitive processes occur there, such as consciousness, reasoning, emotions, and language, amongst other things.

What are the examples of localisation of function in the brain?

Particular areas are well-known for their relation to specific functions in the brain. Evidence for localisation of function can be seen in multiple aspects of the brain.

The prefrontal cortex

The prefrontal cortex is part of complex cognitive processes and higher-level functions. It is involved in executive functions (sorting through complicated and conflicting thoughts etc.) and working memory, orchestrating thoughts and personality expression and performing cognitive functions whilst inhibiting impulsive thoughts and actions.It is said to be directly involved with consciousness and sentience, so it is incredibly influential on cognitive functions and processes as a whole!

The motor and somatosensory cortex

The motor and somatosensory cortex are areas of the cerebral cortex that run along a fold in the brain called the central sulcus. The motor cortex, responsible for muscle control, is located at the back of the frontal lobe, and the somatosensory cortex, responsible for processing sensations, is located in the parietal lobe.

The functions are carried out contralaterally. If you were to move your left hand, neurones in the right motor cortex would activate. Similarly, if you feel something tickle your left hand, the right sensory cortex would activate.

The motor cortex is responsible for:

Damage to these areas can result in the loss of the above functions. Paralysis may occur, as there's a loss of voluntary control over muscles. It is difficult to sequence movements or actions. For example, it would be harder to get dressed. Dexterity is affected; when the motor cortex is damaged, the more complex the movements are, the harder it is to carry them out. Eventually, with damage and muscle control loss, the muscles will become weaker, resulting in atrophy (loss) of the muscles.

The somatosensory cortex is situated in the postcentral gyrus (the bumps and ridges that make up the cerebral cortex), found in the parietal lobe, as seen in the above diagram.

The somatosensory cortex is contralaterally responsible for:

• Receiving and processing sensations, such as touch and temperature.

Damage to the somatosensory cortex causes a loss of sensation from the opposite side of the body. It can result in one area being completely ignored, or there may be a loss of ability to recognise an object by its feel, known as agnosia.

The visual cortex

The visual cortex is located in the occipital lobe at the back of the brain.

Light enters the eye and is converted into nerve impulses by the retina (located at the back of the eye). The optic nerve transmits these impulses to the thalamus and the visual cortex.

The right hemisphere processes visual information of the left eye, and vice versa for the left hemisphere. It operates contralaterally.

It is responsible for:

• Visual processing functions – it receives visual information and integrates it with various other functions, processing the data and sending it to be utilised elsewhere (such as by the motor cortex to navigate the body to pick something you have seen). Therefore, the brain can recognise things in the environment more efficiently.

Damage to the visual cortex can result in partial or complete contralateral blindness, known as cortical blindness.

The auditory cortex

The auditory cortex is located at the top of the temporal lobe. The primary auditory cortex receives auditory information from the thalamus, which receives sound impulses (sensations) from the cochlea.

The cochlea converts sound waves into nerve impulses, carried by the auditory nerve for the auditory cortex to process and interpret.

The auditory cortex is responsible for:

• Perceiving sound includes pitch, tone, frequency, and what type of sound it is.
• Determining where the origin of the sound.
• Determining what produced the sound.

Damage to this area can cause issues such as being unable to detect changes in a person's pitch or tone of voice, making conversing with someone and interpreting their intentions extremely difficult. If someone is serious or sarcastic, it would be hard to tell by pitch or tone alone. They wouldn't be able to determine where or what produced the sound.

Broca's area

Broca's area is located in the left frontal lobe, as illustrated below:

The exact location of Broca's area is up for debate, but researchers agree that it is located in the left frontal lobe.

It is responsible for:

• Speech production, featuring heavily in language.
• Breathing during vocalisation.

Broca's area is in the left hemisphere specifically. The right hemisphere has a similar area, but it's not as pertinent.

Wernicke's areas

Wernicke's area is primarily said to be located in the upper temporal lobe, shown below:

It resides in the left hemisphere. The left hemisphere is becoming increasingly prominent in its dominance of language production functions.

Damaging Broca's and Wernicke's areas can result in global aphasia, where patients struggle to produce and understand speech.

Evaluation of localisation of function in the brain

There are strengths and weaknesses for the argument of localisation of function within the brain.

Strengths

• Confirmation by loss of function: Broca's and Wernicke's Areas are direct examples of where damage to a specific, localised area can cause a loss of function and ability. When this occurs, damage in these areas causes speech production and memory issues. Broca's aphasia and Wernicke's aphasia are explicit examples of localised functions the damage affects. The function is thus tied to a specific area, supporting the localisation of function in the brain.
• Confirmation through neuroimaging techniques: Modern brain scanning techniques show activation in areas of the brain related to specified areas highlighted in older research. In studies on patients performing tasks, such as speaking, Broca's and Wernicke's areas were active in fMRI. These were in healthy patients, too.
• Double Dissociation: Where dissociation (damage to the brain that affects one function, but not another) occurs in one patient, the opposite can occur in another patient. So, if one person has a damaged Broca's area and their speech production is affected, they are compared to someone with a damaged Wernicke's area, where their speech comprehension is affected.This comparison allows for functional associations to be made, as Broca's area affected patient can understand language, and Wernicke's area patient can produce language. Damage has affected one function but not the other.
• Transcranial Magnetic Stimulation: Magnetic pulses stimulate the brain (a coil is placed over the head). Barker et al. were able to use TMS to produce recordable responses in the motor cortex.¹ Bolognini and Ro found that a single pulse can disrupt visual perception when we apply TMS to the visual cortex.² A single pulse to the motor cortex can disrupt hand muscle use.

Weaknesses

• Dronkers et al.³ found that when re-examining the brain of Broca's famous patient, Tan, lesions extended significantly beyond the areas Broca initially highlighted. There were inconsistencies in what is now called Broca's area. How can we confidently say that this specific area is responsible for speech production? These inconsistencies raise significant issues with the validity of this area and the functional associations.
• The equipotentiality theory suggests that basic functions, such as motor and sensory (moving your arm, speaking, feeling something and tasting something), are localised, but higher cognitive functions are not.
• Reductionist: Some argue it is important to discuss how these brain areas communicate with each other, rather than focusing on the reductionist argument of localised functions. By stating one area is responsible for complex human behaviours, it detracts from human experiences.
• Functions aren't completely localised: Déjerine exemplifies how localisation is not as credible as first thought. (4) In this study, a patient suffered damage after having a stroke between the connection of the visual cortex to Wernicke's area. He lost his ability to read as a result, but not his writing ability. The lesions isolated the left angular gyrus, or 'language zone', from the visual cortex. Damage to the connection of the two areas was similar to the damage had it occurred to the localised area, which suggests that functions are not completely localised.
• Cuomo et al. found a similar result in their study, where a teacher was taking attendance in her class but could not understand any of the words or letters on her sheet. (5) When checking her lesson plan, it was the same.It was discovered she had had a stroke and had alexia without agraphia. It interrupted communication to the language zone. Unfortunately, the teacher could not relearn the ability to read, but by tracing the letter specifically in a word, she could begin to recognise and decipher words this way. It is a much lengthier process but a fascinating reimagination of what it is to read. This incident supports the idea that functions are not tied locally to one specific area. The interruption of communication to the language zone caused the disorder, not particular damage to the language zone.
• Localisation doesn't acknowledge individual differences. Men and women have different sized brain regions, and Harasty et al. found that women had larger Broca's and Wernicke's areas than men, which could account for their differences and greater use of language. (6) This causes issues with beta bias, where these studies largely ignore sex differences.
• Plasticity of the brain: functions can be adopted by other brain areas when trauma occurs. If the function is localised, another, unrelated portion of the brain should not take over this function. An example in Danelli et al. shows how the patient, known as EB, had their left hemisphere removed at the age of two due to a tumour. (7) Despite this, they regained full use of their language abilities with only mild language comprehension disorders (dyslexia). The right hemisphere had adapted to deal with the damage.