Localisation of Function in the Brain (AQA A-Level Psychology): Revision Notes

Localisation of Function in the Brain

What is localisation of function?

Localisation of function refers to the theory that specific regions of the brain are responsible for particular behaviours, processes, or activities. This concept emerged during the 19th century through the work of scientists like Paul Broca and Karl Wernicke, who discovered that certain brain areas were associated with specific physical and psychological functions.

This theory contrasts with the earlier holistic theory of the brain, which suggested that all parts of the brain were involved in processing thought and action. The localisation approach, sometimes called cortical specialisation, argues that if a particular brain region becomes damaged through illness or injury, the associated function will also be affected.

Brain structure and organisation

The human brain is divided into two symmetrical halves called hemispheres (left and right). These hemispheres show lateralisation, meaning some functions are controlled by one side more than the other. Generally, the left hemisphere controls the right side of the body, whilst the right hemisphere controls the left side.

The outer layer of both hemispheres is the cerebral cortex, which appears grey due to the concentration of cell bodies (hence 'grey matter'). This cortex is much more developed in humans compared to other animals and is divided into four main sections called lobes: frontal, parietal, occipital, and temporal lobes.

The main functional areas

Motor area

Located at the back of the frontal lobe, the motor area controls voluntary movement throughout the body. Different parts of this area correspond to different body parts, with areas requiring fine motor control (like hands and face) taking up proportionally more space. Damage to the motor area results in loss of control over fine movements.

Somatosensory area

Found at the front of both parietal lobes, the somatosensory area processes sensory information from the skin, including touch, heat, pressure, and pain. It is separated from the motor area by a fold called the central sulcus. Like the motor area, different body parts are represented in proportion to their sensitivity - hands and face occupy large areas of the somatosensory cortex.

Visual area

The visual area (or visual cortex) is located in the occipital lobe at the back of the brain. Each eye sends information from the right visual field to the left visual cortex and from the left visual field to the right visual cortex. Damage to this area can cause partial or complete blindness in parts of the visual field.

Auditory area

Located in the temporal lobes, the auditory area analyses speech-based information and other sounds. Damage to this region can result in partial hearing loss, and more extensive damage can affect language comprehension abilities.

Language areas of the brain

Unlike the sensory and motor areas which exist in both hemispheres, language functions are typically restricted to the left hemisphere in most people (approximately 95% of right-handed individuals and 70% of left-handed individuals).

Broca's area

In the 1880s, surgeon Paul Broca identified a small region in the left frontal lobe responsible for speech production. Damage to Broca's area causes Broca's aphasia, characterised by slow, laborious speech that lacks fluency. Patients with this condition understand language well but struggle to produce coherent speech. Broca famously studied a patient nicknamed 'Tan' because this was the only word he could say.

Wernicke's area

Around the same time, Karl Wernicke was studying patients who had no difficulty producing speech but severe problems understanding it. He identified Wernicke's area in the left temporal lobe, surrounding the auditory cortex, as being responsible for language comprehension. When damaged, this area causes Wernicke's aphasia, where patients produce fluent but meaningless speech, often including made-up words called neologisms.

Evidence supporting localisation

Modern research provides compelling evidence for the localisation theory through multiple approaches and methodologies.

Brain scan evidence

Modern neuroimaging techniques provide strong support for localisation theory. Petersen et al. (1988) used brain scans to demonstrate that Wernicke's area was active during listening tasks whilst Broca's area was active during reading tasks, showing these areas have different functions. Similarly, Tulving et al. (1994) found that semantic and episodic memories are stored in different parts of the prefrontal cortex, supporting the idea that different types of memory are localised to specific regions.

Neurosurgical evidence

The practice of surgically removing or destroying brain areas to control behaviour, though crude in early attempts, provides evidence for localisation. Walter Freeman pioneered the lobotomy in the 1950s, severing connections in the frontal lobe to control aggressive behaviour. Modern neurosurgery is still used sparingly for severe cases. Dougherty et al. (2002) reported on 44 people with obsessive-compulsive disorder who underwent cingulotomy (lesioning of the cingulate gyrus). After 32 weeks, one-third showed successful response and 14% showed partial response, suggesting that symptoms of mental disorders can be localised to specific brain regions.

Case study evidence

Evidence challenging localisation

While localisation theory has strong support, several findings challenge the idea that brain functions are strictly localised to specific regions.

Lashley's research

Karl Lashley's (1950) work challenged the strict localisation view. He trained rats to learn a maze, then removed different areas of their cortex (between 10% and 50%). Surprisingly, no single area proved more important than any other for maze learning. The rats' ability to navigate the maze seemed to depend on the amount of cortex removed rather than which specific area was damaged. This led Lashley to propose the law of equipotentiality - that learning involves the entire cortex rather than specific localised areas.

However, Lashley's work was conducted on rats, and we must be cautious about generalising these findings to humans. Humans possess consciousness and more complex cognitive abilities, making direct comparisons problematic.

Plasticity and recovery

The brain's ability to reorganise itself, known as cortical remapping or plasticity, challenges strict localisation theory. When brain damage occurs, other areas can sometimes take over lost functions. Lashley described this as surviving brain circuits 'chipping in' to achieve the same neurological action. Many stroke victims recover abilities that seemed permanently lost, suggesting the brain works more holistically than pure localisation theory would predict.

Evidence from neurosurgeon John Lorber supports this view. He examined patients with hydrocephalus (fluid-filled brain ventricles) and found cases where ventricles occupied over 50% of the brain space, yet these individuals functioned normally and even excelled academically. This suggests the brain can function effectively even when large areas are compromised.