Lateralisation of Brain Function

Lateralisation refers to the idea that the two hemispheres of the brain are not identical in function — certain cognitive processes are specialised in one hemisphere more than the other. The most important evidence for lateralisation comes from Sperry's split-brain studies.

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What is Lateralisation?

The brain has two hemispheres (left and right), connected by the corpus callosum — a thick bundle of nerve fibres that allows the two hemispheres to communicate and share information.

Lateralisation means that some functions are predominantly controlled by one hemisphere:

Left Hemisphere	Right Hemisphere
Language production (Broca's area)	Spatial awareness
Language comprehension (Wernicke's area)	Face recognition
Logical and analytical thinking	Creativity and imagination
Mathematical calculations	Processing emotions
Right side of the body	Left side of the body

Contralateral Control

Each hemisphere controls the opposite side of the body. This is called contralateral control:

• The left hemisphere controls the right side of the body
• The right hemisphere controls the left side of the body

This also applies to vision: information from the left visual field goes to the right hemisphere, and information from the right visual field goes to the left hemisphere.

Lateralisation map

flowchart TB
    A["Cerebral cortex<br/>two hemispheres"] --> B["Corpus callosum<br/>connects hemispheres"]
    A --> L[Left hemisphere]
    A --> R[Right hemisphere]
    L --> L1["Language production<br/>Broca area"]
    L --> L2["Language comprehension<br/>Wernicke area"]
    L --> L3[Analytical / mathematical]
    L --> L4["Controls right side<br/>of body"]
    R --> R1[Spatial awareness]
    R --> R2[Face recognition]
    R --> R3[Emotion / creativity]
    R --> R4["Controls left side<br/>of body"]

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Sperry's Split-Brain Studies (1968)

Background

Some patients with severe epilepsy underwent a surgical procedure called a commissurotomy — cutting the corpus callosum to prevent epileptic seizures from spreading between hemispheres. After surgery, these patients were called split-brain patients because their two hemispheres could no longer communicate directly.

Roger Sperry studied these split-brain patients to investigate what happens when the hemispheres cannot share information.

Method

Sperry used a special apparatus to present images to only one visual field (and therefore only one hemisphere):

• The patient focused on a central point
• An image was flashed briefly in either the left visual field (processed by the right hemisphere) or the right visual field (processed by the left hemisphere)
• The patient was then asked to say what they saw or select an object by touch

Key Findings

Visual Field	Hemisphere Processing	What Happened
Right visual field → Left hemisphere	Left (language)	Patient could say what they saw (because the left hemisphere controls language)
Left visual field → Right hemisphere	Right (non-verbal)	Patient could not say what they saw (right hemisphere lacks language ability) but could select the object by touch with their left hand (controlled by the right hemisphere)

Two Objects Experiment

When different images were shown simultaneously (e.g. a dollar sign to the left visual field and a question mark to the right visual field):

• The patient said they saw a question mark (left hemisphere, language)
• But with their left hand they drew a dollar sign (right hemisphere, non-verbal)

This demonstrated that each hemisphere has its own separate awareness and abilities when they cannot communicate.

Conclusion

• The left hemisphere specialises in language and analytical thinking
• The right hemisphere specialises in spatial tasks, face recognition, and emotional processing
• When the corpus callosum is cut, the hemispheres operate independently, and each can process information without the other's awareness

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Evaluation of Sperry's Research

Strengths

• Provided groundbreaking evidence for lateralisation of brain function
• Used highly controlled experimental procedures — images were flashed briefly to prevent eye movement
• The findings have been confirmed by subsequent research using brain imaging technology
• Sperry won the Nobel Prize in 1981 for this work

Weaknesses

• Split-brain patients are a very rare and unusual group — findings may not generalise to people with intact brains
• The patients had existing brain abnormalities (severe epilepsy) that may have affected their results
• The surgery itself may have caused additional brain changes beyond simply cutting the corpus callosum
• In people with intact brains, the hemispheres work together through the corpus callosum — lateralisation does not mean one hemisphere works alone
• The extent of lateralisation may be exaggerated — both hemispheres contribute to most tasks, with one simply being more dominant

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Lateralisation in Everyday Life

In people with intact brains, lateralisation is more subtle than in split-brain patients:

• Both hemispheres are involved in most tasks — lateralisation means one hemisphere makes a greater contribution, not that it works alone
• Language, for example, primarily involves the left hemisphere, but the right hemisphere contributes to aspects such as intonation, humour, and metaphor
• Handedness is related to lateralisation — about 90% of people are right-handed (left hemisphere dominant for motor control)

Exam Tip: When describing Sperry's research, always explain the method (images flashed to one visual field), the key findings (left hemisphere = language, right hemisphere = spatial/non-verbal), and the conclusion. For evaluation, note that split-brain patients are unusual and that both hemispheres normally work together.

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Key Points

• Lateralisation means that certain functions are specialised in one hemisphere.
• Left hemisphere: language, analytical thinking; Right hemisphere: spatial, emotional, face recognition.
• Contralateral control: each hemisphere controls the opposite side of the body.
• Sperry's split-brain studies provided key evidence for lateralisation.
• In intact brains, both hemispheres work together for most tasks.

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Worked case study: Paul Broca and "Tan" (1861)

Aim: To identify the brain region responsible for speech production by studying a patient with severe speech impairment.

Procedure: Paul Broca was a French surgeon who studied a patient known as "Tan" (real name Louis Victor Leborgne), a 51-year-old man who had lost the ability to produce meaningful speech. Tan could only utter the single syllable "tan" (hence his nickname), although his comprehension of language and his intelligence remained relatively intact. Broca observed Tan clinically for several years. When Tan died in 1861, Broca performed an autopsy and examined his brain.