AQA Syllabus focus:
'Broca’s and Wernicke’s areas, split brain research and hemispheric lateralisation.'
This subtopic explains how language is mainly controlled by the left hemisphere and how split-brain studies revealed specialized functions in each side of the brain.
Hemispheric lateralization
In most people, especially right-handed people, language is mainly controlled by the left hemisphere. This is an example of hemispheric lateralization.
Hemispheric lateralization: The tendency for some psychological or biological functions to be mainly controlled by one cerebral hemisphere.
Lateralization does not mean one hemisphere works alone all the time. In an intact brain, the two hemispheres normally communicate through the corpus callosum, so behavior is usually integrated. However, each side tends to be more efficient at particular tasks. The left hemisphere is strongly associated with speech, grammar, and analytical processing, whereas the right hemisphere is more involved in some visuospatial skills, face processing, and the emotional tone of language. This pattern is important when interpreting evidence from language disorders and split-brain studies.
Broca’s and Wernicke’s areas
This labeled lateral view of the (typically) left hemisphere highlights Broca’s area in the frontal lobe and Wernicke’s area in the temporal lobe. It reinforces the idea that language is both localized (specific cortical regions) and lateralized (most strongly associated with the left hemisphere). Source
Broca’s area
Broca’s area is located in the left frontal lobe, close to regions involved in controlling the muscles used in speech. It is mainly linked to speech production. If this area is damaged, a person may develop Broca’s aphasia, a language disorder in which speech becomes slow, effortful, and lacking normal grammatical structure. Understanding can be relatively preserved, so the person often knows what they want to say but struggles to say it fluently.
This suggests that producing spoken language is not spread equally across both hemispheres. Instead, an important part of this function is localized in a specific area of the left hemisphere.
Wernicke’s area
Wernicke’s area is found in the left temporal lobe and is mainly involved in language comprehension. Damage to this area can lead to Wernicke’s aphasia, a disorder in which speech remains fluent but may be confused, meaningless, or filled with inappropriate words. The person may also have difficulty understanding spoken language.
Broca’s area and Wernicke’s area perform different but connected functions. One is mainly concerned with producing language, while the other is mainly concerned with understanding it. Together, they provide strong evidence that language functions are largely lateralized to the left hemisphere. Although language involves wider brain networks, these two areas remain central examples in the study of localization and lateralization.
Split-brain research
Researchers gained especially strong evidence for lateralization by studying split-brain patients.
Split-brain patient: A person whose corpus callosum has been severed, reducing communication between the two cerebral hemispheres.
These patients underwent surgery in which the corpus callosum was cut, usually to reduce the spread of severe epileptic seizures from one hemisphere to the other. After the operation, both hemispheres could still function, but direct communication between them was greatly reduced. This allowed researchers to test each hemisphere more independently than in a typical brain.
Procedure and tasks
In classic studies by Sperry and Gazzaniga, information was presented to only one hemisphere at a time. Visual material was flashed briefly to either the left visual field or the right visual field, so it was first processed by the opposite hemisphere. Material in the right visual field is processed by the left hemisphere, while material in the left visual field is processed by the right hemisphere.
This figure summarizes the divided-visual-field method used in classic split-brain research: stimuli shown to the left visual field are first processed by the right hemisphere, while stimuli shown to the right visual field are first processed by the left hemisphere. It also illustrates why split-brain patients may be able to point/draw (right hemisphere control) without being able to name the stimulus (left-hemisphere language dominance) when interhemispheric transfer is disrupted. Source
Researchers also used tactile tasks. An object could be placed in one hand while the participant could not see it. Because touch from the left hand is mainly processed by the right hemisphere, and touch from the right hand by the left hemisphere, this method could also isolate the abilities of each side of the brain.
Key findings
When a word or picture was presented to the right visual field, split-brain patients could usually name it. This is because the information reached the left hemisphere, which is usually dominant for language.
When a stimulus was presented to the left visual field, the information reached the right hemisphere. Patients often could not say what they had seen. However, they could often point to the item, pick out a matching object, or draw it with the left hand. This showed that the right hemisphere had processed the stimulus, even though it could not easily produce a verbal answer.
Similar findings appeared in tactile tasks. If an object was placed in the right hand, the patient could usually name it. If the same object was placed in the left hand, the patient often could not name it, but could still identify it nonverbally. Split-brain patients also tended to perform better with the right hemisphere on some visuospatial tasks, such as recognizing shapes or arranging patterns.
These findings supported the idea that the hemispheres are specialized. The left hemisphere is usually dominant for language, while the right hemisphere is stronger in some nonverbal and spatial tasks.
Interpreting split-brain evidence
Split-brain research provided unusually clear support for hemispheric lateralization because it prevented the hemispheres from sharing information in the normal way. This made their different strengths visible in a highly controlled setting.
At the same time, the evidence has limits:
Split-brain patients are unusual participants. They had severe epilepsy and major surgery, so findings may not fully generalize to people with typical brains.
Samples were very small. Because the operation was rare, some conclusions were based on only a few individuals.
The tasks were artificial. Everyday life normally involves both hemispheres working together with constant visual and sensory input.
Lateralization is not absolute. Even though language is usually left-dominant, the right hemisphere can still contribute to communication, and most behavior depends on cooperation between both hemispheres.
Practice Questions
Outline one function of Broca’s area. (2 marks)
1 mark for stating that Broca’s area is involved in speech production.
1 mark for stating that it is usually located in the left frontal lobe or that damage to it causes slow, non-fluent speech.
Explain what split-brain research has shown about hemispheric lateralization. (6 marks)
1 mark for stating that split-brain patients had a severed corpus callosum, reducing communication between hemispheres.
1 mark for explaining that information in the right visual field goes to the left hemisphere, and information in the left visual field goes to the right hemisphere.
1 mark for stating that stimuli presented to the right visual field could usually be named.
1 mark for stating that stimuli presented to the left visual field often could not be named verbally.
1 mark for stating that the right hemisphere could still identify, point to, or draw the item using the left hand.
1 mark for concluding that the hemispheres have specialized functions, with the left hemisphere usually dominant for language and the right hemisphere stronger in some visuospatial tasks.
FAQ
The images were shown very quickly so the participant could not move their eyes and bring the information into both visual fields.
If the eyes moved, both hemispheres might receive the stimulus, which would weaken the test. Brief presentation helped researchers isolate one hemisphere at a time.
This happened when the object was processed by the right hemisphere. The right hemisphere could recognize the item, but speech is usually controlled by the left hemisphere.
Because the hemispheres could not easily share information after surgery, the patient might be able to show knowledge nonverbally, such as by drawing or selecting the object, without being able to say its name.
Gazzaniga suggested that the left hemisphere often acts as an “interpreter.” This means it tries to make sense of behavior and create a verbal explanation, even when it does not have full information.
In split-brain studies, the left hemisphere sometimes gave a plausible explanation for an action produced by the right hemisphere. This suggests that conscious verbal reasoning can involve reconstructing events rather than simply reporting them.
No. The left hemisphere is the most common location for language, but not everyone shows exactly the same pattern.
Most right-handed people have left-hemisphere language dominance.
Left-handed people are more variable.
Some people show more bilateral language processing.
A small minority show stronger right-hemisphere involvement.
This means lateralization is a strong tendency, not a fixed rule for every individual.
No. They are key language areas, but modern research shows that language depends on wider networks as well.
Problems with language can also involve:
connections between language areas
surrounding cortical regions
the exact size and location of damage
differences between speaking, reading, writing, and understanding
So Broca’s and Wernicke’s areas remain important for AQA Psychology, but real language functioning is more complex than a two-area model.
