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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.
Key Definition: Localisation of function is the principle that certain areas of the cerebral cortex are specialised for particular physical and psychological functions.
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 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).
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.
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:
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.
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.
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.
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