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Localisation of function refers to the theory that specific areas of the brain are responsible for specific behaviours, processes, or activities. This is one of the most intensely researched and debated topics in biopsychology. Evidence from brain damage, brain stimulation, and modern neuroimaging supports localisation, though the extent to which functions are strictly localised versus distributed across interconnected networks remains an active area of investigation. The opposing position — holism — holds that complex functions are distributed across the whole cortex rather than pinned to single areas.
Key Definition: Localisation of function is the principle that certain areas of the cerebral cortex are specialised for particular physical and psychological functions.
This lesson addresses the following points in AQA A-Level Psychology (7182), Paper 2, Section A (Biopsychology):
Assessment objectives engaged: AO1 (the cortical centres and language areas; supporting case studies; brain-scanning evidence), and AO3 (evaluation of localisation — case-study and scanning evidence versus holist counter-arguments such as Lashley's equipotentiality and distributed processing). This is one of the most frequently essay-assessed topics in the Biopsychology section.
The cerebral cortex is the outer layer of the brain — a thin, folded sheet of neural tissue approximately 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 communication between the hemispheres.
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 of the cortex into lobes is the first level of localisation: at the broadest scale, vision is at the back (occipital), hearing and memory at the sides (temporal), bodily sensation toward the top (parietal), and movement and executive control at the front (frontal). Within each lobe, however, there are further specialised areas — the primary motor cortex within the frontal lobe, Broca's area also within the frontal lobe, the primary visual cortex within the occipital lobe, and so on. Localisation therefore operates at multiple scales simultaneously: lobes, then primary sensory and motor areas within lobes, then specialised regions such as the language areas. This nested organisation is important because it means the debate between localisation and holism 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 that span several regions.
The motor cortex is located in the frontal lobe, immediately anterior to (in front of) the central sulcus. It is responsible for generating voluntary movements. Different regions of the motor cortex control different body parts — the amount of cortex devoted to a body part is proportional to the precision of movement required (not the size of the body part). Crucially, the motor cortex in each hemisphere controls the muscles on the opposite (contralateral) side of the body, because the descending motor pathways cross over in the brainstem. This is why a stroke damaging the left motor cortex produces weakness or paralysis on the right side of the body. The orderly arrangement of body parts along the motor strip means that damage to a specific part of the motor cortex produces weakness in a specific part of the body, rather than a global loss of movement — a clear illustration of localisation.
The somatosensory cortex is located in the parietal lobe, immediately posterior to (behind) the central sulcus. It processes sensory input from the body — 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 brain surgery on epileptic patients using electrical stimulation. He applied mild electrical currents to the cortex of conscious patients (under local anaesthetic) and recorded which body part moved or where the patient reported a sensation. This produced the famous motor homunculus and sensory homunculus — distorted human figures where the size of each body part represents the amount of cortex devoted to it.
In the motor homunculus, the hands, lips, and tongue are disproportionately large because they require extremely fine motor control. In the sensory homunculus, the hands, lips, and face are enlarged because they have the highest density of sensory receptors.
The homunculus is more than a curiosity; it is one of the clearest pieces of evidence for localisation, because it demonstrates that the cortex is topographically organised — adjacent areas of cortex map onto adjacent regions of the body, in an orderly sequence from toe to head. The distortion of the figure reveals a key principle: the brain allocates cortical "real estate" according to functional importance (the precision of movement or the acuity of sensation required), not according to the physical size of the body part. The lips and fingertips command vast cortical territory because dexterity and tactile discrimination are vital, whereas the trunk — though large — is represented by only a small strip of cortex. This principle of representation-by-importance recurs throughout the nervous system and reappears in the Plasticity lesson, where the cortical map can actually reorganise with experience.
Exam Tip: When discussing Penfield's work, note that it involved conscious patients who could report their experiences, providing direct evidence for localisation. However, his participants were epilepsy patients — their brains may not be typical, which limits generalisation.
Two areas of the brain are especially critical for language:
Paul Broca (1861) studied a patient known as "Tan" (real name Louis Victor Leborgne), who could only say the word "tan" despite appearing to understand language. After Tan's death, Broca performed a post-mortem and found damage to the left frontal lobe (specifically the posterior part of the inferior frontal gyrus). Broca concluded that this area was responsible for speech production.
Key Definition: Broca's area is a region in the left frontal lobe responsible for speech production and the motor aspects of language.
Broca's aphasia (non-fluent aphasia) results from damage to Broca's area. Patients:
Carl Wernicke (1874) identified a region in the left temporal lobe (posterior part of the superior temporal gyrus) that is responsible for language comprehension.
Key Definition: Wernicke's area is a region in the left temporal lobe responsible for language comprehension and the understanding of spoken and written language.
Wernicke's aphasia (fluent aphasia) results from damage to Wernicke's area. Patients:
| 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 two patterns is theoretically powerful because it constitutes a double dissociation: one region can be damaged while sparing the function served by the other, and vice versa. Broca's patients lose production but keep comprehension; Wernicke's patients lose comprehension but keep fluent production. A single dissociation could be explained away by arguing that one task is simply harder than the other, but a double dissociation cannot — it strongly implies that production and comprehension depend on separate, localised neural systems. This logic is one of the most persuasive arguments for localisation and is worth deploying explicitly in evaluation.
The visual cortex (also called the primary visual cortex or V1) is located in the occipital lobe at the back of the brain. It receives and processes visual information from the eyes (via the optic nerve and lateral geniculate nucleus of the thalamus). Damage to the visual cortex can cause cortical blindness — the eyes function normally, but the brain cannot process visual information.
The auditory cortex is located in the temporal lobe (superior temporal gyrus). It receives and processes auditory information from the ears (via the auditory nerve and medial geniculate nucleus of the thalamus). Damage can impair the ability to process sounds, even though the ears function normally.
A subtle but important point about both the visual and auditory centres is that they too are contralaterally and topographically organised. The visual cortex maps the visual field in an orderly way, so that damage to one part produces a blind spot (scotoma) in a corresponding part of the visual field rather than total blindness. The auditory cortex is tonotopically organised — different frequencies of sound are processed in different, orderly positions along the cortex, much as the keys of a piano are laid out by pitch. This precise, map-like organisation of the sensory cortices is itself strong evidence for localisation: not only are whole functions localised to lobes, but within a sensory area, specific features (a location in space, a pitch, a body part) are mapped to specific cortical positions. Cortical blindness following occipital damage — where the eyes and optic nerve are intact but visual experience is lost — is a particularly compelling demonstration that conscious perception depends on the integrity of the relevant cortical area, not merely the sense organ.
Phineas Gage was a railway construction foreman who survived a devastating accident in which a large iron tamping rod was driven completely through his skull, destroying much of his left frontal lobe. Remarkably, Gage survived and could walk and talk. However, his personality changed dramatically — his physician, John Harlow, described him as becoming "fitful, irreverent, indulging in the grossest profanity, manifesting but little deference for his fellows."
This case provided early evidence that the frontal lobes are involved in personality, decision-making, and social behaviour. In particular, it pointed to the role of the prefrontal cortex in regulating impulses, planning, and socially appropriate conduct — functions now grouped under the heading of "executive function." Gage's reported transformation from a responsible, capable foreman into someone impulsive and unreliable suggested that the part of the brain damaged by the rod normally acts as a kind of "brake" on impulsive behaviour. This foreshadows modern neural explanations of disorders of impulse control and aggression, in which reduced prefrontal activity is associated with poorer regulation of behaviour.
It is important, however, to handle the Gage case critically. Much of what is "known" about his personality change comes from a small number of contemporary reports, some written years after the event, and the story has been embellished in retellings. Macmillan (2000), who reviewed the original sources, argued that Gage may in fact have recovered far more than the popular legend suggests, possibly returning to skilled work as a stagecoach driver — which would itself be evidence of functional recovery and plasticity. The Gage case therefore illustrates both the promise and the pitfalls of case-study evidence: vivid and suggestive, but anecdotal, unreplicable, and vulnerable to distortion over time.
Exam Tip: The Gage case is powerful as early evidence but has significant limitations: the evidence is largely anecdotal, the reports of his personality change may be exaggerated (Macmillan, 2000), and it is a single case study — we cannot generalise from one person.
As described above, Broca's study of Tan provided direct post-mortem evidence linking the left frontal lobe to speech production. Broca went on to study additional patients with similar damage and similar deficits, strengthening his conclusion.
A more modern study links a specific structure to a specific function. Maguire et al. (2000) used structural MRI to compare the brains of London taxi drivers — who must memorise the city's complex street layout — with controls, and found that the taxi drivers had a significantly larger posterior hippocampus, the region associated with spatial navigation. The volume correlated with years of experience. This supports localisation by tying the hippocampus to spatial memory, and it simultaneously demonstrates plasticity (covered in the final lesson of this unit), showing that localisation and plasticity are complementary rather than contradictory: functions are localised, and the regions that carry them can be reshaped by experience.
Modern technology allows researchers to study brain function in living participants, providing further evidence for localisation:
| Technique | What It Measures | Strengths | Limitations |
|---|---|---|---|
| fMRI (functional magnetic resonance imaging) | Blood oxygenation (BOLD signal) as a proxy for neural activity | High spatial resolution (~1 mm); non-invasive; no radiation | Low temporal resolution (seconds); measures blood flow, not neurons directly; expensive |
| EEG (electroencephalography) | Electrical activity via electrodes on the scalp | High temporal resolution (milliseconds); inexpensive; non-invasive | Low spatial resolution; cannot identify deep brain structures |
| ERP (event-related potentials) | Averaged EEG signals time-locked to specific stimuli | Excellent temporal resolution; can isolate specific cognitive processes | Low spatial resolution; requires many trials; averaging may obscure individual differences |
| PET (positron emission tomography) | Glucose metabolism or blood flow using radioactive tracers | Can measure neurotransmitter activity; functional information | Low spatial and temporal resolution; invasive (radioactive injection); expensive |
Each technique provides a different kind of evidence for localisation, and they are complementary rather than competing. fMRI offers excellent spatial resolution, so it can pinpoint where activity occurs and is therefore ideal for mapping which region supports which function — but its poor temporal resolution means it cannot capture the rapid sequence of processing. EEG and ERP offer the opposite trade-off: superb temporal resolution that can track the millisecond-by-millisecond timing of cognitive events, but poor spatial resolution that cannot localise the source precisely. PET can additionally reveal neurochemical activity, such as where particular neurotransmitters are being used, but is invasive and has relatively poor resolution. The most convincing demonstrations of localisation therefore come from converging evidence across techniques — for example, when fMRI shows that a region is active during a task and lesion studies show that damage to that region abolishes the function. No single technique is decisive on its own; each compensates for the limitations of the others.
Exam Tip: When comparing scanning techniques, organise your answer around three criteria: spatial resolution, temporal resolution, and invasiveness. This structure ensures comprehensive evaluation and full marks.
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