Localisation of function in the brain and hemispheric lateralisation: motor, somatosensory, visual, auditory and language centres; Broca's and Wernicke's areas, split-brain research.

What this dot point is asking

AQA wants you to describe localisation of function and hemispheric lateralisation, including the main cortical areas, Broca's and Wernicke's areas and split-brain research. The exam skill is to locate each area and its function precisely, to use aphasia and split-brain evidence, and to evaluate localisation against plasticity.

The cortical areas

Localisation theory holds that specific functions have specific locations, an idea that grew from nineteenth-century clinical cases. The motor cortex at the back of the frontal lobe controls voluntary movement, and it is organised contralaterally, so the left motor cortex controls the right side of the body and vice versa. The somatosensory cortex in the parietal lobe processes sensory information such as touch and temperature, with the amount of cortex devoted to a body part reflecting its sensitivity. The visual cortex sits in the occipital lobe, and the auditory cortex in the temporal lobe. The two language centres are the most heavily tested. Broca's area in the left frontal lobe controls speech production; damage produces Broca's aphasia (slow, halting, effortful speech with relatively preserved understanding). Wernicke's area in the left temporal lobe controls language comprehension; damage produces Wernicke's aphasia (fluent but meaningless speech and poor comprehension). These contrasting aphasias are the classic clinical evidence for localisation.

Lateralisation and split-brain research

Hemispheric lateralisation means that some functions are dominant in one hemisphere, the clearest example being language, which is typically controlled by the left hemisphere. Sperry's split-brain studies exploited a rare group of patients whose corpus callosum (the band of fibres connecting the hemispheres) had been surgically severed to control intractable epilepsy. With the hemispheres disconnected, Sperry could present information to one hemisphere alone by flashing an image to one visual field while the patient fixated centrally. When an object was shown to the right visual field (left hemisphere), the patient could name it, because language is left-lateralised; when shown to the left visual field (right hemisphere), the patient could not name it, but could pick out the matching object using the left hand. This elegantly demonstrated lateralised function. The research must be evaluated, though: the sample was tiny and unusual (the patients had a history of severe epilepsy and surgery), so generalising to typical brains is risky, and modern accounts stress that in the intact brain the hemispheres work together rather than as wholly separate units.