Following the case study of Phineas Gage scientists, doctors and psychologists started to really investigate the idea that parts of the brain are responsible for certain behaviours and tasks and that any damage to these areas causes abnormalities, like those found in Phineas behaviour after his accident.
Article about how localisation of the brain has developed over time
Localisation of Function
Definition
Localisation of function is the theory that specific areas of the brain are responsible for particular physical and psychological functions.
According to localisation theory, damage to a particular area of the brain is likely to result in difficulties carrying out the function associated with that area.
The Cerebrum vs Cerebral Cortex
The cerebrum is the largest part of the brain. It contains:
- The left hemisphere
- The right hemisphere
- The cerebral cortex
- Deeper structures involved in functions such as memory and emotion
The cerebral cortex is the outer layer of the cerebrum and is responsible for many higher mental processes. This is where the AQA localisation content is located:
- Frontal lobe
- Parietal lobe
- Temporal lobe
- Occipital lobe
- Motor cortex
- Somatosensory cortex
- Visual cortex
- Auditory cortex
- Broca’s area
- Wernicke’s area
Think of an orange:
- Cerebrum = the whole orange 🍊
- Cerebral cortex = the peel on the outside
The cortex is part of the cerebrum, but it isn’t the entire cerebrum.
So when you’re learning about the localisation of function, you’re really focusing on the cerebral cortex.
Motor Cortex
Location: Rear of the frontal lobe in both hemispheres.
Function: Controls voluntary movement.
The motor cortex on the left side of the brain controls movement on the right side of the body, and vice versa.
Different parts of the motor cortex control different body parts. Areas requiring precise movements (e.g. hands and face) occupy larger regions of the motor cortex.
Damage to the motor cortex may result in loss of movement or paralysis.
Somatosensory Cortex
Location: Front of the parietal lobe in both hemispheres.
Function: Receives and processes sensory information from the skin and body, including:
- Touch
- Pressure
- Temperature
- Pain
Like the motor cortex, different areas correspond to different parts of the body. Areas with many sensory receptors, such as the hands and lips, occupy larger regions.
Damage may result in reduced or abnormal sensation.
Visual Cortex
Location: Occipital lobe.
Function: Receives and processes visual information.
Information from the right visual field is processed by the left hemisphere, while information from the left visual field is processed by the right hemisphere.
Different areas of the visual cortex process different aspects of vision, such as colour, shape and movement.
Damage can lead to visual impairments or blindness.
Auditory Cortex
Location: Temporal lobe.
Function: Receives and processes auditory information.
The auditory cortex receives information from both ears and allows us to recognise and interpret sounds.
Damage can cause difficulties processing sounds.
Broca’s Area
Location: Left frontal lobe (usually in the left hemisphere).
Function: Speech production.
Broca’s area was identified by Paul Broca (1861) when studying patients who experienced difficulties producing speech.
Damage to Broca’s area results in Broca’s aphasia, characterised by:
- Slow, laboured speech
- Difficulty speaking
- Relatively intact language comprehension
Speech is often meaningful but difficult to produce.
Wernicke’s Area
Location: Left temporal lobe (usually in the left hemisphere).
Function: Language comprehension.
Wernicke identified this area after studying patients who had difficulty understanding language.
Damage to Wernicke’s area results in Wernicke’s aphasia, characterised by:
- Poor understanding of language
- Fluent but often meaningless speech
- Difficulty finding the correct words
Wernicke (1874)
Wernicke studied patients who could speak fluently but were unable to understand language.
He identified an area in the left temporal lobe involved in language comprehension.
This provided further support for localisation.
Evidence for Localisation
Broca (1861)
Broca studied a patient known as “Tan”, who could understand language but could only say the word “tan”.
After the patient’s death, Broca found damage in the left frontal lobe.
This suggested that speech production is localised in Broca’s area.
Below are 2 videos which show you how different the effects of Broca Aphasia can be.
Brain Scanning Evidence
Modern brain scans such as fMRI scans provide support for localisation.
For example, Petersen et al. (1988) found different areas of the brain became active during different language tasks, suggesting specific functions are carried out by particular brain regions.
Evaluation of Localisation
Remember: *just because it’s biology doesn’t make it correct!*
Strength: Research Evidence Supports Localisation
Studies by Broca, Wernicke and modern brain-scanning research demonstrate that specific functions are associated with specific areas of the brain. This provides strong support for localisation theory.
Strength: Neurosurgical Evidence
Localisation has practical applications in neurosurgery. For example, Dougherty et al. (2002) found that cingulotomy surgery reduced symptoms in patients with severe OCD. This suggests that behaviour can be linked to specific brain areas.
Limitation: Lashley’s Research
Lashley (1950) removed areas of the cerebral cortex in rats learning a maze. He found that learning appeared to depend more on the amount of cortex damaged than the location of the damage. Lashley proposed equipotentiality, suggesting that higher cognitive functions may involve many brain regions working together. This challenges strict localisation.
Limitation: Plasticity
Research into brain plasticity shows that functions can sometimes be transferred to undamaged areas following injury. This suggests that localisation is not always fixed and that the brain can adapt after damage. Therefore, strict localisation may oversimplify brain functioning.
Hemispheric Lateralisation
Definition
Hemispheric lateralisation refers to the dominance of one hemisphere for particular psychological or physical functions.
Although the two hemispheres work together, some functions are more strongly associated with one side.
Left Hemisphere
Usually dominant for:
- Language
- Speech production
- Language comprehension
This hemisphere contains Broca’s area and Wernicke’s area in most people.
Right Hemisphere
Usually dominant for:
- Spatial awareness
- Face recognition
- Some visual-spatial tasks
The Corpus Callosum
The corpus callosum is a bundle of nerve fibres connecting the two hemispheres.
It allows information to pass between the left and right sides of the brain so that they can work together.
Split-Brain Research (Sperry, 1968)
Aim
To investigate the extent to which the two hemispheres are specialised for different functions.
Participants
Eleven split-brain patients whose corpus callosum had been severed to reduce severe epilepsy.
Procedure
Participants focused on a fixed point in the centre of a screen. Stimuli were flashed for less than 0.1 seconds, preventing eye movements.
- Information presented to the left visual field was received only by the right hemisphere.
Information presented to the right visual field was received only by the left hemisphere.
Because the corpus callosum had been cut, information could not pass between the hemispheres.
Tasks included:
Recognising words and pictures.
Identifying objects by touch.
Drawing images with either hand.
Findings
Visual Tasks
When an object was presented to the right visual field (left hemisphere), participants could describe it verbally.
Information presented to the left visual field (right hemisphere) could not be described verbally. Participants often reported seeing nothing. However, they could point to or select the correct object with their left hand. For example, if the word “KEY” was shown to the left visual field: Participant could not say “key”. But could select a key from a collection of objects with the left hand.
Tactile Tasks
When an object was placed in the left hand (right hemisphere) where the participant could not see the object, participants could not name it but could identify it by touch or pointing it out from a selection of images.
Drawing Tasks
Participants could draw objects presented to the right hemisphere with their left hand. They could also draw objects presented to their left hemisphere with their right hand.
Conclusion
Sperry demonstrated that the cerebral hemispheres are functionally specialised. The left hemisphere appears dominant for language production and verbal processing, whereas the right hemisphere is dominant for visuospatial skills and certain non-verbal functions. The findings provide strong evidence for hemispheric lateralisation.
Evaluation of Split-Brain Research
Strength: Controlled Scientific Research
Sperry used highly controlled laboratory procedures. This increased internal validity because researchers could be confident that differences in performance were due to hemisphere specialisation.
Strength: Useful Evidence for Lateralisation
The research demonstrated that the two hemispheres have specialised functions. It significantly improved our understanding of brain organisation.
Limitation: Small and Unusual Sample
Only 11 split-brain patients were studied. These individuals had severe epilepsy and had undergone major surgery, making them unrepresentative of the general population. This limits generalisability.
Limitation: Low Ecological Validity
The tasks involved artificial laboratory procedures, such as briefly presenting images to one visual field. These tasks may not reflect how the hemispheres operate in everyday life.
Limitation: Plasticity May Affect Findings
The brains of split-brain patients may adapt after surgery. Therefore, the findings may not fully represent the functioning of a normal brain.
Split Brain Research
Introducing split brain patients! These are people who, due to extreme epileptic seizures in one side of the brain, have had their corpus callosum cut to stop the electrical activity from one side spreading into the other side of the brain. Your probably now thinking, surely this person now has some form of brain damage and cannot function as a ‘normal’ human being?! Who would be mad enough to 1. suggest it to a person and 2. agree to it?!?!
The research Sperry conducted on his split brain patients is one you need to know really well, as you could be given a 16 or 8 marker on this particular research.
