Localisation and Lateralisation of Brain Function

Localisation of function is the theory that specific areas of the brain are responsible for specific behaviours, processes or activities. On this view, if a particular region is damaged, the function controlled by that region is lost or impaired.

The opposing idea is the holistic theory, associated with early researchers such as Flourens, which argues that all parts of the brain are involved in the processing of thought and action, so no single area is uniquely responsible for one function.

The evidence tilted towards localisation in the nineteenth century. Broca and Wernicke each identified patients whose language difficulties could be traced to damage in one specific region, showing that at least some functions have a definite home in the brain rather than being spread evenly across it.

Localisation of function = specific areas control specific functions. The holistic view = the whole brain is involved in each function. The debate between these two positions is a key evaluation point.

The brain is divided into two hemispheres, and its outer layer, the cerebral cortex, is split into four lobes: the frontal, parietal, occipital and temporal lobes. Different functions are localised across these lobes, as the next slide sets out.

Four sensory and motor centres are localised to particular lobes. Learn the area, its lobe and its function together, because AQA can ask you to link any one to the others.

Area	Lobe / location	Function
Motor area	Back of the frontal lobe	Controls voluntary movement; each side controls the opposite body side
Somatosensory area	Front of the parietal lobe	Processes sensory information such as touch, temperature and pain
Visual area	Occipital lobe	Processes visual information from the eyes
Auditory area	Temporal lobe	Processes sound and speech-based information

Two features are worth memorising precisely:

• The motor area is contralateral: the motor area in the left hemisphere controls movement on the right side of the body, and vice versa. Damage to the motor area causes a loss of control over fine movements.
• The visual area also works contralaterally by visual field. Each visual cortex receives information from the opposite visual field, so the left visual cortex processes the right visual field of both eyes.

Contralateral means "opposite side". The motor and somatosensory areas map the body on the opposite side, and each visual cortex receives from the opposite visual field. This contralateral rule is central to understanding split-brain research later.

The somatosensory area lies right behind the motor area, separated from it by a groove called the central sulcus, so movement (frontal) and body sensation (parietal) are localised next to each other.

Language is one of the most clearly localised functions, and unusually it is lateralised to the left hemisphere for most people. Two separate areas control two separate parts of language.

Broca's area sits in the left frontal lobe and is responsible for speech production. Paul Broca studied a patient known as "Tan" (real name Louis Leborgne), who could understand language but could only say the single syllable "tan". After his death, a post-mortem revealed a lesion in this left frontal region. Damage here causes Broca's aphasia: speech that is slow, laboured and lacking in fluency.

Wernicke's area sits in the left temporal lobe and is responsible for language comprehension (understanding). Damage here causes Wernicke's aphasia: speech that is fluent and grammatically structured but meaningless, often containing invented or nonsense words, because the person cannot monitor whether what they are saying makes sense.

Feature	Broca's area	Wernicke's area
Location	Left frontal lobe	Left temporal lobe
Function	Speech production	Language comprehension
Effect of damage	Broca's aphasia	Wernicke's aphasia
Nature of speech	Slow, laboured, non-fluent	Fluent but meaningless

The exam killer here is swapping the two. Broca = production (the "B" for "broken, halting speech"); Wernicke = comprehension (understanding). Both areas are in the left hemisphere, which is why language is described as lateralised.

Hemispheric lateralisation is the idea that the two halves of the brain are functionally different, and that certain processes and behaviours are dominated by one hemisphere rather than shared equally between them.

The clearest example is language, which is lateralised to the left hemisphere for the vast majority of people. This is why both Broca's and Wernicke's areas are found on the left. The left hemisphere is sometimes described as the "analyser" and the right as the "synthesiser", with the right hemisphere more involved in spatial and visual tasks such as recognising faces and drawing.

Lateralisation depends on the contralateral wiring of the brain:

• Each hemisphere controls the opposite (contralateral) side of the body.
• Each hemisphere receives visual information from the opposite visual field.

So an image in your right visual field is sent to your left hemisphere, and an image in your left visual field is sent to your right hemisphere. In a normal brain this does not matter, because the corpus callosum — the band of fibres connecting the two hemispheres — passes information across so both halves share what each one sees.

Localisation and lateralisation are not the same thing. Localisation = a function has a specific place. Lateralisation = a function is dominated by one hemisphere. Language is both localised (to Broca's and Wernicke's areas) and lateralised (to the left).

This contralateral, corpus-callosum-linked arrangement is exactly what split-brain research exploited to test lateralisation directly.

Roger Sperry studied a rare group of patients who had undergone commissurotomy — an operation that cuts the corpus callosum to stop severe epileptic seizures spreading between the hemispheres. In these split-brain patients the two hemispheres could no longer communicate, so a function processed in one hemisphere stayed trapped there.

Sperry presented an image or word to one visual field at a time while the patient fixed their gaze on a central point. Because vision is contralateral, this sent the information to only one hemisphere.

Image shown to…	Hemisphere reached	What the patient could do
Right visual field	Left (language)	Name / describe the object aloud
Left visual field	Right	Could not name it, but could select it by touch with the left hand or draw it

An object flashed to the right visual field reached the left, language-dominant hemisphere, so the patient could say what it was. The same object flashed to the left visual field reached the right hemisphere, which has no language output, so the patient said they saw nothing — yet their left hand (controlled by the right hemisphere) could still pick out the matching object by touch or draw it.

The patients could still see the left-field object. They simply could not name it, because the right hemisphere that received it cannot produce speech. This distinction is a common exam trap.

Sperry's findings are strong evidence for lateralisation: they show the left hemisphere's dominance for language and the right hemisphere's role in spatial and visual tasks, with each hemisphere able to work independently once the corpus callosum is cut.

A top-band answer weighs evidence for localisation and lateralisation against the challenges to it. Develop three or four points fully rather than listing many.

Supporting evidence from aphasia and brain scans. The link between damaged areas and lost functions supports localised language. Neuroimaging backs this up: Petersen used brain scans to show Wernicke's area was active during a listening task and Broca's area during a reading task, demonstrating that these functions have measurably different locations in a living brain, not just in post-mortem lesions.

Challenge from plasticity and equipotentiality. Lashley removed areas of the cortex in rats learning a maze and found no single area was more important than any other for learning; what mattered was the amount of tissue removed, suggesting higher cognitive functions are distributed rather than strictly localised. The brain's ability to reorganise and recover after damage (plasticity) also implies functions are not always rigidly fixed to one region.

Strengths and limits of split-brain research. Sperry used highly standardised, controlled procedures (fixed gaze, one visual field at a time), which makes the studies replicable and the findings robust. However, the sample was small and unusual — a handful of people who had both severe epilepsy and radical surgery — so generalising to neurotypical brains is risky, and their epilepsy may itself have caused unusual brain organisation.

Lateralisation may be overstated. The strict "left = language, right = spatial" split is an oversimplification. Many everyday tasks recruit both hemispheres working together through connectivity, and research suggests functional differences between the hemispheres reduce with age. This suggests the brain operates more as an integrated whole than a pair of specialists, favouring a more holistic view for some functions.

A balanced conclusion: localisation is well supported for basic sensory, motor and language functions, but higher cognitive functions appear more distributed, and a strict hemispheric split oversimplifies how the brain really works.

1. Swapping Broca's and Wernicke's areas

Broca's area (frontal lobe) controls speech production; Wernicke's area (temporal lobe) controls comprehension. Reversing them, or reversing the aphasias (non-fluent for Broca, fluent-but-meaningless for Wernicke), loses easy marks.

2. Forgetting the contralateral rule

The motor area and each visual cortex work on the opposite side. The left hemisphere controls the right side of the body and receives the right visual field. Students often assume same-side wiring, which breaks the entire explanation of split-brain findings.

3. Saying split-brain patients "couldn't see" left-field objects

They could see them. The right hemisphere received the image perfectly well; it simply cannot produce speech, so the patient could not name the object but could still draw it or select it by touch. Write "could not name", not "could not see".

4. Treating lateralisation as absolute

Language being lateralised to the left is a dominance, not an exclusive rule — it is left-dominant for most people, and many tasks use both hemispheres. Writing that the right hemisphere does "nothing" for language, or that the split is total, overstates the claim.

5. Confusing localisation with lateralisation

Localisation is about a function having a specific place; lateralisation is about a function being dominated by one hemisphere. Define each precisely — language happens to be both, but they are separate concepts and examiners award marks for keeping them distinct.