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The nervous system is the body's primary communication network, and at its centre sits the central nervous system (CNS) — the brain and the spinal cord. Within the brain, one of the most enduring questions in biological psychology is whether specific regions perform specific jobs (localisation of function) or whether complex behaviours are shared across the whole cortex (holism). This lesson examines the organisation of the CNS, the evidence that particular cortical areas are specialised for movement, sensation, vision, hearing and language, the way the two hemispheres divide their labour (hemispheric lateralisation), and the striking split-brain research of Roger Sperry that revealed what happens when the link between the hemispheres is cut. Together these ideas underpin the biological approach's core claim: that psychological functions have identifiable physical substrates in the brain.
Key Definition: Localisation of function is the principle that specific areas of the cerebral cortex are specialised for particular physical and psychological functions, in contrast to the holist view that complex functions are distributed across the whole cortex.
This lesson addresses the Edexcel 9PS0 — Paper 1, Topic 3: Biological Psychology content on the structure and function of the brain: the central nervous system, the organisation of the cerebral cortex into lobes, the localisation of function (motor, somatosensory, visual, auditory and language centres, including Broca's and Wernicke's areas), hemispheric lateralisation, and split-brain research (the work of Sperry). It introduces the case-study, stimulation-mapping and split-brain evidence the specification expects for evaluation, and sets up the neuron-level mechanisms and brain-scanning techniques covered elsewhere in the topic. In assessment-objective terms, you should be able to describe the CNS, the cortical centres and the split-brain findings (AO1), apply this knowledge to scenarios such as the effects of localised damage or a described patient profile (AO2), and evaluate localisation and lateralisation against holist, connectionist and methodological counter-arguments (AO3).
Connects to…
The human nervous system divides into two parts: the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system (PNS), which carries information between the CNS and the rest of the body.
| Division | Components | Primary role |
|---|---|---|
| Central nervous system (CNS) | Brain and spinal cord | Processing, integration and coordination of all neural information; the seat of conscious thought, decision-making and memory |
| Peripheral nervous system (PNS) | All nerves outside the CNS (somatic and autonomic) | Relays sensory information to the CNS and motor commands from it |
The spinal cord is a bundle of nerve fibres running from the base of the brain down the vertebral column. It has two main functions: it relays information between the brain and the body, and it mediates simple reflexes (such as the withdrawal reflex) without the signal needing to travel all the way to the brain. The brain sits at the top of this hierarchy, integrating incoming information and issuing coordinated responses. Damage to the CNS is typically serious and often permanent, because — as later lessons on plasticity qualify — mature CNS neurons have limited capacity for regeneration compared with those of the PNS.
Key Definition: The central nervous system (CNS) consists of the brain and spinal cord; it is the site at which sensory information is processed and from which motor commands originate.
The cerebral cortex is the outer layer of the brain — a thin, folded sheet of neural tissue roughly 2–4 mm thick. It is divided into two cerebral hemispheres (left and right), connected by the corpus callosum, a thick band of approximately 200 million nerve fibres that allows the hemispheres to communicate. Each hemisphere is divided into four lobes:
| Lobe | Location | Key functions |
|---|---|---|
| Frontal | Front of the brain | Motor function, planning, decision-making, personality, Broca's area |
| Parietal | Top and rear-centre | Somatosensory processing, spatial awareness |
| Temporal | Sides (above the ears) | Auditory processing, memory, Wernicke's area |
| Occipital | Back of the brain | Visual processing |
The division into lobes is only the first level of localisation. At the broadest scale, vision is at the back, hearing and memory at the sides, bodily sensation toward the top, and movement and executive control at the front. Within each lobe, however, there are further specialised areas — the primary motor cortex and Broca's area within the frontal lobe, the primary visual cortex within the occipital lobe, and so on. Localisation therefore operates at several scales simultaneously: lobes, then primary sensory and motor strips within lobes, then specialised regions such as the language areas. This nested organisation matters, because it means the localisation-versus-holism debate is rarely all-or-nothing: basic sensory and motor functions are tightly localised, whereas complex functions such as memory and decision-making draw on networks spanning several regions.
The motor cortex lies in the frontal lobe, immediately in front of the central sulcus, and generates voluntary movement. Different regions of the strip control different body parts, and the amount of cortex devoted to a body part reflects the precision of movement required, not the size of the part. Crucially, the motor cortex in each hemisphere controls the muscles on the opposite (contralateral) side of the body, because the descending pathways cross over in the brainstem — which is why a stroke damaging the left motor cortex weakens the right side of the body. Because the strip is orderly, damage to a specific part produces weakness in a specific body part rather than global paralysis — a clear illustration of localisation.
The somatosensory cortex lies in the parietal lobe, immediately behind the central sulcus, and processes bodily sensation — touch, pressure, temperature and pain. Like the motor cortex, different regions correspond to different body parts.
Wilder Penfield (1950) mapped the motor and somatosensory cortices during surgery on conscious epileptic patients (under local anaesthetic) by applying mild electrical currents and recording which body part moved or where a sensation was felt. This produced the famous motor and sensory homunculi — distorted human figures in which the size of each body part represents the amount of cortex devoted to it. The hands, lips and tongue are disproportionately large because they demand fine control or carry the densest sensory receptors. The homunculus is one of the clearest pieces of evidence for localisation, because it shows the cortex is topographically organised: adjacent areas of cortex map onto adjacent regions of the body, and cortical "real estate" is allocated by functional importance rather than physical size.
Exam Tip: Penfield's participants were conscious and could report their experiences, which gives direct evidence for localisation — but they were epilepsy patients whose brains may not be typical, which limits generalisation.
Two areas are especially critical for language, and both lie in the left hemisphere in most people.
Paul Broca (1861) studied a patient known as "Tan" (Louis Victor Leborgne), who could say almost nothing but "tan" despite seeming to understand speech. A post-mortem revealed damage to the left frontal lobe, leading Broca to conclude that this region governs speech production.
Broca's aphasia (non-fluent aphasia) results from damage here. Patients speak slowly and effortfully, use short, telegraphic sentences ("want… drink… water"), understand language relatively well, and know what they want to say but cannot articulate it.
Carl Wernicke (1874) identified a region in the left temporal lobe responsible for language comprehension. Wernicke's aphasia (fluent aphasia) produces speech that is fluent and grammatically correct but often meaningless ("word salad"), with severe difficulty understanding others and frequent unawareness that anything is wrong.
| Feature | Broca's aphasia | Wernicke's aphasia |
|---|---|---|
| Damaged area | Left frontal lobe | Left temporal lobe |
| Speech fluency | Non-fluent, effortful, telegraphic | Fluent, normal rhythm |
| Speech meaning | Meaningful but sparse | Often meaningless ("word salad") |
| Comprehension | Largely intact | Severely impaired |
| Awareness of deficit | Usually aware and frustrated | Often unaware |
The contrast between these patterns is theoretically powerful because it is a double dissociation: one region can be damaged while sparing the function served by the other, and vice versa. A single dissociation could be dismissed by arguing one task is simply harder; a double dissociation cannot, and strongly implies that production and comprehension depend on separate, localised systems.
The visual cortex (V1) lies in the occipital lobe and processes information from the eyes (via the optic nerve and thalamus). Damage can cause cortical blindness — the eyes work normally but the brain cannot process what they see. The auditory cortex lies in the temporal lobe and processes information from the ears; damage impairs the processing of sound even though the ears function.
Both centres are contralaterally and topographically organised. The visual cortex maps the visual field in an orderly way, so damage to one part produces a blind spot (scotoma) in a corresponding part of the field rather than total blindness. The auditory cortex is tonotopically organised — different sound frequencies are processed at different, orderly positions, much as the keys of a piano are laid out by pitch. This precise, map-like organisation is itself strong evidence for localisation, and cortical blindness after occipital damage — where the eye and optic nerve are intact but visual experience is lost — shows that conscious perception depends on the integrity of the relevant cortical area, not merely the sense organ.
Phineas Gage was a railway foreman who survived an accident in which an iron tamping rod was driven through his left frontal lobe. He could still walk and talk, but his physician John Harlow reported a dramatic personality change — from responsible and capable to impulsive and unreliable. The case provided early evidence that the frontal lobes are involved in personality, decision-making and social behaviour, pointing to the role of the prefrontal cortex in regulating impulses — functions now grouped under "executive function." This foreshadows modern neural explanations of impulse control and aggression, in which reduced prefrontal activity is associated with poorer regulation of behaviour.
The Gage case must be handled critically, however. Much of what is "known" about his change comes from a few contemporary reports, some written years later, and the story has been embellished. Macmillan (2000), reviewing the original sources, argued that Gage may have recovered far more than the legend suggests, possibly returning to skilled work as a stagecoach driver — itself evidence of functional recovery. The case therefore illustrates both the promise and the pitfalls of case-study evidence: vivid and suggestive, but anecdotal, unreplicable and vulnerable to distortion.
A more modern study links structure to function. Maguire et al. (2000) used structural MRI to compare London taxi drivers — who must memorise the city's street layout — with controls, and found the drivers had a significantly larger posterior hippocampus, the region associated with spatial navigation, with volume correlating with years of experience. This supports localisation by tying the hippocampus to spatial memory, and simultaneously demonstrates plasticity, showing that localisation and plasticity are complementary rather than contradictory.
Although the two hemispheres look structurally similar, they differ in function — a phenomenon known as lateralisation.
Key Definition: Hemispheric lateralisation is the idea that the two hemispheres of the brain are functionally different, with certain cognitive processes and behaviours dominated by one hemisphere more than the other.
| Function | Left hemisphere | Right hemisphere |
|---|---|---|
| Language | Dominant for production (Broca's) and comprehension (Wernicke's) | Limited; some understanding of simple words |
| Processing style | Analytic, sequential, mathematical calculation | Holistic, parallel |
| Motor control | Controls the right side of the body | Controls the left side of the body |
| Spatial awareness | Limited | Dominant — navigation, face recognition |
| Emotional tone | Limited | Dominant for prosody (emotional tone of speech) |
Two caveats are vital. First, lateralisation is a matter of relative dominance: both hemispheres usually contribute to most tasks, with one taking the lead. Second, it is more pronounced for some functions (notably language) than others and varies between individuals. The popular leap from "language is lateralised" to labelling whole people as "left-brained" or "right-brained" is unwarranted — a myth unsupported by imaging (Nielsen et al., 2013, who analysed over 1,000 scans and found no evidence that individuals preferentially use one hemisphere).
Each hemisphere primarily controls, and receives sensory input from, the opposite (contralateral) side of the body. The visual system deserves special care, because it is the route Sperry exploited. It is not the case that the left eye sends everything to the right hemisphere. Rather, the right visual field of both eyes is processed by the left hemisphere, and the left visual field of both eyes by the right hemisphere. The crossing happens at the optic chiasm. In an intact brain this matters little, because the corpus callosum immediately shares the two halves of the scene — but in a split-brain patient, information presented to one visual field is trapped in the contralateral hemisphere with no way to cross over.
Exam Tip: Before writing about Sperry, always explain contralateral processing. A frequent error is confusing eyes with visual fields — it is the field, not the eye, that maps to a hemisphere. The left visual field projects to the right hemisphere.
In the 1960s, Roger Sperry and his student Michael Gazzaniga studied patients who had undergone a commissurotomy — surgical severing of the corpus callosum — to treat severe, intractable epilepsy. The surgery confined seizures to one hemisphere, but it also meant the hemispheres could no longer communicate directly, giving Sperry an unprecedented natural experiment. It is essential to remember the surgery was performed for clinical reasons; Sperry studied an existing patient group rather than creating it, which is why split-brain research is a quasi-experiment using a naturally occurring independent variable.
Sperry used a tachistoscope to flash stimuli to one visual field for under 0.1 seconds — too fast for the eyes to move and redirect the image. Key tasks included flashing an image to one visual field and asking the patient to name it or select it by hand, and placing objects in one hand behind a screen for tactile identification.
| Stimulus presented to | Verbal response | Manual response (left hand) |
|---|---|---|
| Right visual field (left hemisphere) | Patient could name the object | N/A — right hand, same hemisphere |
| Left visual field (right hemisphere) | "Nothing" / could not name it | Could select the correct object by touch |
The deep significance of this work lies in what it reveals about consciousness. In the intact brain the corpus callosum fuses the two hemispheres into a single, unified awareness. When it is severed, each hemisphere is left with access only to its own half of the visual world. The left hemisphere, possessing language, can report what it knows and so speaks for the whole person; the right hemisphere knows things too but, lacking speech, can express them only non-verbally. Split-brain research therefore speaks not only to localisation but to one of the deepest questions in psychology — the biological basis of the unity of the self. Gazzaniga later proposed the "left-brain interpreter": the idea that the left hemisphere constantly invents explanations for behaviour, even when it lacks access to the true reasons.
Key Definition: A tachistoscope is a device used to present visual stimuli for very brief durations, ensuring information is directed to only one hemisphere.
A major strength of localisation is the convergent support from clinical case studies, which provides strong evidence that specific areas underpin specific functions. Broca's (1861) post-mortem of "Tan" linked left-frontal damage to a speech-production deficit while comprehension was largely intact, and Wernicke's cases linked left-temporal damage to a comprehension deficit. The dissociation between fluent-but-meaningless and effortful-but-meaningful speech is difficult to explain without localised language centres. The implication is that damage to a defined region produces a predictable, specific deficit — exactly what a localisation account predicts and a purely holist account struggles to accommodate.
Localisation is further supported by controlled stimulation and neuroimaging evidence, which raises validity beyond anecdotal case study. Penfield's (1950) electrical stimulation of conscious patients produced reliable, region-specific movements and sensations, allowing him to map the homunculi, and modern fMRI shows that particular tasks reliably activate particular regions. Because these findings come from replicable methods rather than single unusual patients, they corroborate the case-study evidence. The implication is that localisation rests on triangulation across stimulation, lesion and imaging methods, which collectively make the conclusion robust.
However, the case-study evidence that founds localisation has serious limitations of generalisability. Phineas Gage and Broca's patient Tan are single individuals with unique, severe and uncontrolled brain damage, and reports of Gage's personality change may have been exaggerated (Macmillan, 2000). Because case studies cannot be replicated and the participants are by definition atypical, conclusions drawn from them may not generalise to the wider population. The implication is that such cases suggest hypotheses rather than confirming firm causal claims about how the typical brain is organised.
Sperry's split-brain research has high internal validity, strengthening its conclusions about lateralisation. The tachistoscope flashed stimuli for under a tenth of a second while patients fixated centrally, ensuring information reached only one hemisphere, so differences in verbal versus manual responses could be attributed confidently to hemispheric specialisation rather than eye movements. The implication is that Sperry's cause-and-effect conclusions rest on a methodologically rigorous design, which is why the work was awarded the Nobel Prize in 1981.
Nevertheless, split-brain conclusions are limited by a small, atypical sample and an epilepsy confound. Sperry's key conclusions rest on around eleven commissurotomy patients, all with severe epilepsy, and years of seizure activity may itself have reorganised their brains (for instance producing more bilateral language). This is a confounding variable, because the observed lateralisation might reflect the epilepsy or its treatment rather than normal organisation. The implication is that differences from controls cannot be attributed solely to the severed callosum, and the findings may not describe the neurologically typical brain.
Finally, the localisation-versus-holism debate is best resolved by a connectionist compromise. Lashley (1930) removed varying amounts of cortex from maze-learning rats and found that the amount of tissue removed, not its location, predicted the learning deficit ("equipotentiality"), implying complex functions are distributed. Evidence of plasticity and recovery — where intact regions assume lost functions — similarly undermines a rigidly localised view. The implication is that specialised regions are real but operate within interconnected networks: localisation holds best for basic sensory, motor and language functions, while higher cognition is better understood as the product of distributed networks.
Specimen question modelled on the Edexcel 9PS0 paper format.
Evaluate the evidence for localisation and lateralisation of function in the brain, including Sperry's split-brain research. (16 marks)
This 16-mark extended-response question is marked as roughly 6 marks AO1 (accurate, detailed description of the cortical centres, language areas, lateralisation and Sperry's procedure and findings) and 10 marks AO3 (evaluation — case-study and stimulation support, the internal validity of the split-brain method, sampling and confound limitations, and the holist/connectionist counter-position). Application (AO2) marks would apply only if a scenario stem were provided. The top band requires sustained, integrated evaluation building to a reasoned judgement rather than a list of isolated points.
Localisation means different parts of the brain do different jobs. The motor cortex is in the frontal lobe and controls movement, and the somatosensory cortex is in the parietal lobe and processes touch. The visual cortex is in the occipital lobe and the auditory cortex is in the temporal lobe. Broca's area is in the left frontal lobe and controls speech, and Wernicke's area is in the left temporal lobe and controls understanding. Sperry studied split-brain patients who had their corpus callosum cut for epilepsy. He flashed pictures to one visual field. When a picture went to the right visual field the patient could name it, but when it went to the left visual field they could not, though they could pick it out with the left hand.
One strength is that case studies like Phineas Gage support localisation because his personality changed after frontal lobe damage. One weakness is that Sperry only had about eleven patients, so it is hard to generalise. Another weakness is that the patients had epilepsy, which might have changed their brains.
Examiner-style commentary: To reach the next band this answer must explain why each evaluation point matters rather than naming it — for example, that the epilepsy is a confound that undermines attributing the results to the severed callosum. The description is accurate but thin: contralateral organisation is missing, the homunculus and the two aphasias are not described, and the "KEY/RING" finding is omitted. With the 10:6 weighting towards AO3, the sparse, undeveloped evaluation caps the mark.
Localisation is the idea that specific cortical areas are responsible for specific functions. The motor cortex (frontal lobe) controls voluntary movement and the somatosensory cortex (parietal lobe) processes sensation; Penfield's (1950) homunculus shows the cortex devoted to a body part reflects its precision or sensitivity. The visual cortex is in the occipital lobe and the auditory cortex in the temporal lobe. Language is lateralised to the left hemisphere: Broca's area controls production (damage causes slow, telegraphic speech) and Wernicke's area controls comprehension (damage causes fluent but meaningless speech). Because of contralateral organisation, the left visual field projects to the right hemisphere. Sperry (1968) studied around eleven commissurotomy patients using a tachistoscope; an image in the right visual field could be named, but one in the left visual field could not, though it could be selected by the left hand. In the "KEY/RING" task the patient said "RING" but picked up a key.
A strength is the convergent support from case studies (Tan, Gage) and Penfield's stimulation mapping, which link specific regions to specific functions. Sperry's tachistoscope procedure gives high internal validity. A limitation is the small, atypical sample of epilepsy patients, and the epilepsy is a confounding variable that may have altered their lateralisation. A further challenge is Lashley's (1930) equipotentiality, which suggests complex functions are distributed rather than localised.
Examiner-style commentary: The move into the top band is to stop listing strengths then weaknesses in parallel and instead build an integrated argument that separates well-supported localisation (basic sensory, motor, language) from the holist challenge to higher cognition. The description and studies are accurate and the evaluation points relevant, but each is stated rather than developed through point–evidence–explanation–implication, and there is no reasoned overall judgement.
Localisation is the principle that specific cortical regions are specialised for specific functions, in contrast to the holist view that complex functions are distributed. The motor cortex (frontal lobe, anterior to the central sulcus) generates voluntary movement, and the somatosensory cortex (parietal lobe, posterior to it) processes bodily sensation; Penfield (1950) mapped both by electrical stimulation, producing the homunculi in which cortical area reflects precision or receptor density. The visual cortex lies in the occipital lobe and the auditory cortex in the temporal lobe. Language is strongly lateralised: Broca's area (left frontal) underpins production — its damage producing effortful, telegraphic aphasia — while Wernicke's area (left temporal) underpins comprehension, its damage producing fluent but meaningless speech; the double dissociation between the two aphasias is hard to explain without localised centres. Because sensory pathways are contralateral, the left visual field projects to the right hemisphere — the fact Sperry (1968) exploited. Studying around eleven commissurotomy patients with a tachistoscope, he found an image in the right visual field could be named but one in the left visual field could not, though it could be retrieved by the left hand; in the composite "KEY/RING" task the patient verbally reported "RING" yet selected a key, implying the disconnected hemispheres could respond independently.
The theory is supported by powerful converging evidence. Clinical cases (Tan, Gage) and Penfield's controlled stimulation link defined regions to defined functions, modern fMRI reliably shows task-specific activation, and Sperry's tachistoscope procedure — with central fixation ensuring each stimulus reached one hemisphere — gives high internal validity, earning the 1981 Nobel Prize.
However, the evidence must be qualified. Case studies such as Gage and Tan are single, atypical individuals, and reports of Gage's change may be exaggerated (Macmillan, 2000), so firm causal claims are unsafe. Sperry's sample of around eleven is small and atypical, and the epilepsy is a confound: years of seizures may have reorganised the participants' brains, so differences from controls cannot be attributed solely to the severed callosum. More fundamentally, Lashley's (1930) equipotentiality showed that in rats the amount rather than location of cortex removed predicted learning deficits, implying higher functions are distributed, and plasticity and recovery similarly challenge a rigid model. The most defensible position is therefore connectionist: specialised regions are real but operate within interconnected networks. Overall, localisation and lateralisation are well supported for basic sensory, motor and language functions, but higher cognition is better understood as the product of distributed networks than of isolated centres.
Examiner-style commentary: This answer is in the top band; the only refinement would be to weigh the reverse-inference limitation of imaging explicitly. It is distinguished by sustained, integrated evaluation: it frames the debate as localisation versus holism, deploys the double dissociation and the epilepsy confound precisely, develops each AO3 point through point–evidence–explanation–implication, and reaches a genuine judgement distinguishing basic from higher functions. The discriminator throughout is the connectedness of the AO3 reasoning rather than the quantity of description.
Contemporary neuroscience has largely vindicated a connectionist view of the brain, in which specialised hubs operate within extensive networks linked by white-matter tracts such as the arcuate fasciculus. The Wernicke–Geschwind model of language illustrates this by combining localised centres (Broca's, Wernicke's) with the connections between them, and the existence of conduction aphasia — where the grey-matter areas are intact but the tract joining them is damaged — shows that a function can be disrupted by damage to a connection rather than a centre. Gazzaniga's continued split-brain work on the "left-brain interpreter" suggests that the sense we all have of being a single, rational agent who knows why we act may partly be a narrative constructed after the fact by the language-dominant hemisphere, connecting biological psychology to fundamental debates about consciousness and free will. A productive further question is how far the popular "left-brained/right-brained" dichotomy survives modern imaging — the answer, on the evidence of Nielsen et al. (2013), being that it does not.
This content is aligned with the Edexcel A-Level Psychology (9PS0) specification.