The brain and hemispheric specialisation: WACE Year 12 Psychology Unit 3

What this dot point is asking

SCSA expects you to label the major brain structures, state what each does, name the four lobes of the cerebral cortex and their functions, and explain hemispheric specialisation using evidence. This is high-yield because the external examination often shows a labelled diagram or describes a patient with damage to a specific region.

The three regions of the brain

The brain is conventionally divided into three regions.

• The hindbrain controls basic survival functions. It includes the medulla (heart rate and breathing), the pons (sleep and arousal) and the cerebellum (balance, posture and coordinated movement).
• The midbrain relays sensory and motor signals and helps regulate arousal, attention and sleep-wake cycles.
• The forebrain is the largest region. It contains the thalamus (the sensory relay station), the hypothalamus (homeostasis, hunger, thirst and the stress response), and the cerebrum with its outer cerebral cortex.

The lobes of the cerebral cortex

The cerebral cortex is the wrinkled outer layer responsible for higher mental processes. It has four lobes in each hemisphere.

• The frontal lobe handles reasoning, planning, voluntary movement (the motor cortex) and personality. Broca's area, usually in the left frontal lobe, controls speech production.
• The parietal lobe processes touch, temperature and spatial awareness through the somatosensory cortex.
• The temporal lobe processes hearing and is involved in memory. Wernicke's area, usually in the left temporal lobe, handles language comprehension.
• The occipital lobe processes vision.

Hemispheric specialisation

The cerebrum is split into two hemispheres connected by the corpus callosum, a thick band of nerve fibres that allows the two sides to communicate. Each hemisphere controls the opposite (contralateral) side of the body.

Although the hemispheres cooperate constantly, they show specialisation. In most people the left hemisphere is dominant for language, logic, analysis and detailed sequential processing, while the right hemisphere is more involved in spatial reasoning, face recognition, music and holistic processing. Specialisation is a tendency, not an absolute split, and the popular notion of people being purely left-brained or right-brained is a myth.

Sperry's split-brain studies

Roger Sperry studied patients whose corpus callosum had been surgically cut to control severe epilepsy. With the two hemispheres unable to communicate, he could present information to one hemisphere at a time.

When an object was shown to the right visual field (processed by the left, language-dominant hemisphere), patients could name it. When the same object was shown only to the left visual field (processed by the right hemisphere), patients could not name it aloud but could pick it out by touch with the left hand. This demonstrated that language is normally localised in the left hemisphere and provided strong evidence for hemispheric specialisation.

Why this matters for behaviour

Knowing where functions are localised lets psychologists predict the effects of brain injury, interpret brain-imaging results and understand disorders. It also underpins later topics: the hippocampus (in the temporal lobe) is central to memory, and the amygdala is central to emotion and the stress response.