Localisation of Function in the Brain

Localisation of Brain Function

Localisation of brain function is the idea that specific areas of the brain are responsible for a particular function; they have explicit locations within the brain. Examples include areas dedicated to memory and language, i.e., Broca's area. Evidence of localisation is seen through neuroimaging techniques and loss of function due to damage.

When a particular brain area is damaged, the associated area of function is likely to suffer, supporting the idea of localisation of function. To understand how the brain operates, however, we must first understand the structure of the brain and how it operates. The brain has two hemispheres, the left and right, operating contralaterally.

The corpus callosum connects the two hemispheres, allowing them to communicate. Each hemisphere specialises in performing certain functions, which is called hemispheric lateralisation. By communicating, they can share, integrate, and process information excellently.

Localisation of Function in the Brain: Diagram

Particular areas are well-known for their relation to specific functions in the brain. Evidence for the localisation of function can be seen in multiple parts of the brain, including the prefrontal cortex, the motor and somatosensory cortex, the visual cortex, the auditor cortex, and Broca's and Wernicke's areas.

Fig. 2 - The different brain areas are associated with different functions.

Evidence for Localisation of Function in the Brain

The prefrontal cortex, the motor and somatosensory cortex, the visual cortex, the auditor cortex, and Broca's and Wernicke's areas all show properties of localisation of function. We can find evidence for the localisation of function by identifying the functional areas associated with the aforementioned brain structures.

The Prefrontal Cortex

The prefrontal cortex is responsible for various complex cognitive processes and higher-level functions. It is involved in:

• Executive functions.
• Working memory.
• Orchestrating thoughts and personality expression.
• Performing cognitive functions whilst inhibiting impulsive thoughts and actions.

The Motor Cortex

The motor cortex is located at the back of the frontal lobe, anterior to the central sulcus, one of the more notable folds in the brain. The motor cortex is primarily responsible for voluntary muscle control.

The functions are carried out contralaterally.

More specifically, the motor cortex is responsible for:

• Planning and carrying out voluntary movements, such as choosing to move a limb or more complex movements (dexterity).
• Coordinating the two sides of the body through movements.
• Spatial and sensory guidance of movement, which lets you navigate through areas with voluntary control of your muscles.

Damage to these areas can result in the loss of the above functions. Paralysis may occur, as voluntary control over muscles may be lost. It is difficult to sequence movements or actions.

The Somatosensory Cortex

The somatosensory cortex is situated in the postcentral gyrus in the parietal lobe, and sits posterior to the central sulcus. The somatosensory cortex operates contralaterally.

Localisation of Function in the Brain Diagram: The Somatosensory Cortex

We can see where the somatosensory cortex is located in the diagram. Note how the diagram illustrates the approximate location of sensations within the body.

Fig. 3 - The primary somatosensory cortex is involved in interpreting sensations.

The somatosensory cortex is 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 retinal photoreceptors convert light into nerve impulses. The optic nerve transmits these impulses through the optic canal to the thalamus (the lateral geniculate nucleus), which then sends the information to 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.

Localisation of Function in the Brain Diagram: The Visual Cortex

We can see where the visual cortex is located in the localisation of function in the brain diagram. As discussed above, the primary visual cortex is in the occipital lobe.

Fig. 4 - The primary visual cortex is located in the occipital lobe, seen in the posterior view.

Damage to the visual cortex can result in partial or complete blindness, and this blindness can manifest in different forms.

The Auditory Cortex

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

The auditory cortex is responsible for:

• Perceiving sound, which includes pitch, tone, frequency, and 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 the inability to detect changes in a person's pitch or tone of voice, making conversing with someone and interpreting their intentions extremely difficult.

Broca's Area

Broca's area is located in the left frontal lobe in the left hemisphere. It is responsible for:

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

Localisation of Function in the Brain Diagram: Broca's Area

We can see precisely where Broca's area is in the diagram. As we discussed above, it is located in the left hemisphere, specifically in the frontal lobe.

Fig. 5 - Broca's area is involved in speech production.

Wernicke's Area

Wernicke's area is primarily said to be located in the upper temporal lobe. It resides in the left hemisphere. Wernicke's area is associated with speech comprehension.

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

Localisation of Function in the Brain Diagram: Wernicke's Area

We can see in the diagram where Wernicke's area is located. As we discussed above, it is found in the temporal lobe.

Fig. 6 - Wernicke's area is involved in speech comprehension.

Functional Localisation in the Brain: Evaluation

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

Strengths

Supporting arguments for the localisation of function include:

• 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 damage occurs to these areas, it can result in Broca's aphasia and Wernicke's aphasia, specific issues with language production and comprehension.
• Confirmation through neuroimaging techniques: Modern brain scanning techniques show activation in parts of the brain related to specified areas highlighted in modern and older research. In studies on patients performing tasks, such as speaking, Broca's and Wernicke's areas were active in fMRI scans.
• 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, suggesting speech is tied to different localised areas.
• Dominance in the hemispheres: Broca's and Wernicke's areas demonstrate how the left hemisphere is dominant in language-associated functions compared to the right hemisphere.
• Transcranial Magnetic Stimulation (TMS) studies: TMS works through electromagnetic pulses. TMS disrupts processes in the brain by passing high-voltage currents through a coil placed close to the head, which affects action potentials in neurones in the target areas. Barker et al. (1985) used TMS to produce recordable responses in the motor cortex. They sent a large but brief pulse into the motor cortex, observing movements in the opposite arm and leg (remember, the brain operates contralaterally). Boyer et al. (2005) identified through TMS how visual processing involves different areas of the brain by deactivating the primary visual cortex. Despite experiencing transient blindness, participants were able to identify the qualities of their target stimuli (orientation of an object and its colour) well above what could be ascribed to chance. This highlights how different areas of the brain are responsible for different functions, even within the primary areas associated with the chief function.

Weaknesses

Opposing arguments that criticise the localisation of function in the brain include:

• Inconsistencies: Dronkers et al. (2007) found that when re-examining the preserved brains of Broca's famous patients, including Tan, lesions extended significantly beyond the areas Broca initially highlighted (into the medial regions of the brain). There were inconsistencies in what is now called Broca's area. These inconsistencies raise significant issues with the validity of this area and the functional associations.
• 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. Stating one area is responsible for complex human behaviours detracts from human experiences and fails to acknowledge the complex nature of the brain.
• Functions aren't completely localised: Déjerine (1891) exemplifies how localisation is not as credible as first thought. 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 but are intertwined.
• Cuomo et al. (2014) found a similar result in their study, where a teacher (M.P.) was taking attendance in her class but could not understand any of the words or letters on her sheet. When checking her lesson plan, it was the same. It was discovered she had had a stroke and had alexia without agraphia, also known as word blindness. It interrupted communication in the language zone. 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. (1997) found that women had larger Broca's and Wernicke's areas than men, which could account for their differences and greater use of language.
• Plasticity of the brain: We briefly touched upon plasticity in equipotentiality theory. 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 can be seen in Danelli et al. (2013), which shows how a patient, known as EB, had their left hemisphere removed at the age of two due to a tumour and had initial issues with aphasia. 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.