Localisation of function is the idea that specific psychological functions are carried out by specific regions of the brain. Lateralisation is the idea that the two cerebral hemispheres are not identical — each is dominant for different functions.
The four lobes
Each hemisphere is divided into four lobes:
- Frontal lobe — planning, decision-making, personality; contains the motor cortex (precentral gyrus).
- Parietal lobe — body sensation; contains the somatosensory cortex (postcentral gyrus).
- Occipital lobe — vision; contains the visual cortex.
- Temporal lobe — hearing and language comprehension; contains the auditory cortex.
Key localised areas
- Motor cortex (frontal lobe) — controls voluntary movement on the opposite side of the body. The cortex is mapped like a "homunculus": fingers and lips have huge representations because they need fine control.
- Somatosensory cortex (parietal lobe) — receives touch, temperature, pain from the opposite side of the body, again with a homunculus map.
- Visual cortex (occipital lobe) — processes input from the eyes; damage produces blind spots in the corresponding part of the visual field.
- Auditory cortex (temporal lobe) — processes sound; damage causes hearing loss or auditory agnosia.
- Broca's area (left frontal lobe, near the motor cortex) — speech production. Damage causes Broca's aphasia: slow, halting, telegraphic speech with effort, but comprehension preserved.
- Wernicke's area (left temporal lobe) — language comprehension. Damage causes Wernicke's aphasia: fluent but meaningless speech ("word salad"), with poor comprehension.
Lateralisation
Most (about 95% of right-handers and 70% of left-handers) have left-hemisphere dominance for language. Other functions show different patterns:
- Right hemisphere: spatial awareness, face recognition, emotional processing of music.
- Left hemisphere: language, logic, sequencing.
The two hemispheres communicate via the corpus callosum, a thick band of myelinated fibres.
Evidence
- Paul Broca (1861) examined a patient ("Tan") who could only say "tan" but understood normal speech. Post-mortem revealed damage in the left frontal lobe, now called Broca's area.
- Carl Wernicke (1874) identified patients with fluent but nonsensical speech and damage in the left temporal lobe.
- Petersen et al. (1988) used PET scans to show Wernicke's area active during a listening task and Broca's area active during a reading task — modern confirmation of localisation.
- Sperry (1968) split-brain studies (severed corpus callosum to treat epilepsy) showed left hemisphere named objects shown to the right visual field but couldn't name objects in the left visual field — direct evidence for lateralised language.
Strengths of localisation
- Decades of consistent evidence from brain damage, scanning and split-brain research.
- Practical applications: planning surgery to spare functional regions; predicting recovery patterns after stroke.
Limitations
- Equipotentiality (Lashley, 1950) — in rats, removing different parts of cortex impaired maze learning by similar amounts; complex functions are distributed.
- Plasticity (P2.B.3) shows functions can shift to other regions after damage.
- Modern view: localisation is correct for primary sensory and motor functions, but higher cognition involves distributed networks, not single areas.
⚠Common mistakes— Common errors
- Saying Broca's area is for "speech" generically — it's specifically for production.
- Confusing motor cortex (output) with somatosensory cortex (input).
- Forgetting that motor and somatosensory cortices are contralateral — left hemisphere controls the right side of the body.
AI-generated · claude-opus-4-7 · v3-deep-psychology