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What is localisation of brain function (LOF)?

Localisation of brain function (LOF) is the theory that specific functions/processes are localised to/can be found in specific locations in the brain, e.g. the hippocampus has been strongly implicated in the processing of memory; the pre-frontal cortex has been linked to impulse control and executive functioning. The theory challenges and refutes previous assumptions that the brain is unimodal. LOF can be measured using brain-imaging techniques such as MRI, fMRI, PET.

What is the frontal lobe responsible for?

The brain is divided into two hemispheres, left and right, and is covered by a thin layer known as the cerebral cortex. Each hemisphere is responsible for different functions, so each lobe is responsible for specific functions located within that hemisphere.

Frontal lobe:

· The motor cortex is responsible for movement

· The prefrontal cortex is responsible for executive functioning

· Broca's area is responsible for speech production

What is the temporal lobe responsible for?

Temporal lobe:

· The auditory area is responsible for auditory processing e.g. hearing

· Wernicke's area is responsible for language comprehension

· The hippocampus is responsible for memory and spatial navigation

What is the parietal lobe responsible for?

Parietal lobe:

· The somatosensory cortex is responsible for touch sensation e.g. heat, pain

· The parietal lobe governs body orientation e.g. understanding whether an object is on the right or the left of you

· The parietal lobe aids learning in the practice of complex, precise movements e.g. writing

What is the occipital lobe responsible for, and what are the cerebellum and brain stem?

Occipital lobe:

· The visual area is responsible for the processing of visual information e.g. sight

· The visual area aids in understanding distance and depth perception

· The visual area aids in the recognition of objects and faces

The brain also includes:

· The cerebellum, which controls balance and coordination

· The brain stem, which controls involuntary responses

What are the strengths of localisation of function?

There is robust evidence to support the idea of LOF:

· Maguire et al. (2000): Spatial navigation is localised to the posterior hippocampus via analysis of MRI scans

· The case of Phineas Gage (Harlow, 1848): Linked personality changes to damage in the frontal lobe caused by an iron bar shooting through his eye socket and out through the top of his skull

· Petersen et al. (1988): PET scans showed that Broca's area is active when reading; Wernicke's area is active when listening

The techniques used to find supporting evidence for LOF are objective and clinical. This means that the procedure can be replicated to check for consistency. Therefore these techniques are high in reliability.

What are the limitations of localisation of function?

The biggest criticism of LOF was presented by Lashley (1950), who researched rats which had parts of their cortex removed. The rats' ability to navigate and memorize routes through a maze did not appear to correspond with any specific brain region. Lashley concluded that cognitive functions are shared across the cortex holistically rather than being localised to one structure/region.

Lashley also suggested the law of equipotentiality as a challenge to LOF. This law states that when one part of the brain (specifically the cerebral cortex) is damaged, another part or region can take over. Research into neuroplasticity supports this.

What is the examiner's tip for distinguishing localisation and lateralisation?

Be very careful not to confuse localisation with lateralisation in the exam as the two theories are different (and students often make this mistake, sometimes on high-value questions).

Remember that lateralisation refers to an overall sharing of skills and functions across one larger region (e.g. the left hemisphere is more associated with language than the right).

Localisation, however, refers to the specific areas that specialise in that function. So, if lateralisation is like a department store, localisation is like the departments within it (e.g. you would go to the make-up department for eyeliner; you would find memory in the hippocampus).

What is hemispheric lateralisation?

The brain is split into two hemispheres: the right hemisphere and the left hemisphere. The two hemispheres are connected by the corpus callosum, which acts as a bridge between the two hemispheres and allows them to send messages and work together.

The brain is contralateral: the left side of the body is controlled by the right side of the brain and vice versa. A stroke in the left hemisphere will be physically evident on the right side of the body e.g. the right side of the mouth will droop or there may be weakness in the limbs on the right side of the body.

Hemispheric lateralisation refers to the division of functions between the two hemispheres.

What functions are lateralised to the left and right hemispheres?

Left hemisphere: Generally lateralised (in most people) to the processing of language e.g. Broca's area (speech production) and Wernicke's area (language comprehension) are found in the left hemisphere. A stroke in the left hemisphere is likely to result in impaired speech.

Right hemisphere: Generally lateralised (in most people) to the processing of visual information, imagination and creativity. Spatial information is lateralised to the right hemisphere e.g. one case study of a woman with right hemisphere damage found that she would often get lost even in familiar locations if she wasn't given verbal instructions.

What is Broca's area and how was it discovered?

Paul Broca (1824-1880) was a surgeon and neurologist particularly interested in speech disorders.

One of Broca's patients was a man named Louis Leborgne, who had suffered from epilepsy throughout childhood. Leborgne was hospitalised at the age of 30 and stayed until he died in 1861 at the age of 51. He was transferred to Broca's care toward the end of his life. Leborgne became known by the nickname 'Tan' because his neurological condition meant that he could only produce one spoken word: "Tan". Tan suffered from complete paralysis down the right side of his body but he was able to move his left arm and leg with ease.

After Tan died in 1861, Broca performed a brain autopsy. Broca found a lesion on the left frontal lobe of Tan's brain. This was the only damage to the brain; all other areas were intact. As the left frontal lobe was the only visible area of damage to Tan's brain, Broca concluded that this was the area that is responsible for speech production. The term Broca's aphasia is now used as a diagnosis for people who have difficulty producing speech.

What is Wernicke's area and how was it discovered?

Around 10 years after the discovery of Broca's area, Carl Wernicke identified a similar language-related problem in which patients were able to speak but were not able to comprehend language.

Wernicke examined the brains of patients who exhibited this specific problem. His analysis of their brains revealed that damage to the temporal lobe in the left hemisphere resulted in patients producing speech that was fluent but devoid of meaning. The damage in the temporal lobe was close to the auditory area.

The term Wernicke's aphasia is now used as a diagnosis for people who have difficulty understanding language and accessing the correct words.

What was Sperry's split-brain research procedure?

The corpus callosum connects the left and right hemispheres. This connection allows for rapid communication between the two hemispheres. A type of epilepsy called tonic-clonic seizures can be treated by severing the corpus callosum in a procedure called a corpus callosotomy. The surgery aims to stop erratic electric messages bouncing between hemispheres, reducing the number of seizures. Patients report very few side effects, however, some have reported that they feel like they are two people inside one body.

Sperry (1968) studied 11 patients who had a corpus callosotomy as a treatment for their epileptic seizures. Sperry's procedure was based on the brain being contralateral:

· Information from the left visual field goes to the right hemisphere

· Information from the right visual field goes to the left hemisphere

Sperry projected information into each visual field, controlling which information each visual field accessed. Each participant was placed in front of a screen and asked to fixate on a central point. A tachistoscope was used to present visual stimuli to either the left or right visual field for 1/10th of a second - too brief for eye movements to transfer the information to the wrong visual field.

What did Sperry find in the "describe what you see" and "recognise by touch" tasks?

Describe what you see:

· When an image was shown to the participant's right visual field they could easily describe what was seen (the information had gone to their left hemisphere)

· When a picture was presented to their left visual field they could not describe it in words; in fact, they often reported that there was nothing there (the information had gone to their right hemisphere)

Recognise by touch:

· The participants' hands were covered and they could not see the objects that were then presented to them

· Sperry placed a series of objects in the participants' hands (objects placed in the right hand are processed in the left hemisphere and vice-versa)

· When an object was placed in the right hand the participant could describe it using speech or writing

· When an object was placed in the left hand the participant made wild guesses as to what it was and often seemed unaware that they were holding anything, but they could select a matching object from a 'grab bag' using their left hand

What did Sperry find in the composite words and drawing tasks?

Composite words:

· When two words were presented simultaneously (e.g. key and ring, one word to each visual field), the participant would write the word 'key' with their left hand (left-hand goes to right hemisphere linked to the left visual field) and say the word 'ring' (right visual field links to left hemisphere)

Drawing tasks:

· When participants were asked to draw a picture presented to the right visual field (left hemisphere), the right hand performed worse than the left hand

· When participants were asked to draw a picture presented to the left visual field (right hemisphere), the drawings from the left hand were much better than from the right hand (all the participants were right-handed)

What are the strengths of hemispheric lateralisation research?

This is a well-controlled experiment which involved a range of tasks designed to test hemispheric lateralisation. The standardised procedures of each task mean that the research is replicable. A replicable procedure can be tested for consistency. This means that Sperry's research is high in reliability.

Sperry won the Nobel prize for this research as it provided huge insight into the workings of the brain. The findings led many researchers to conclude that severing the corpus callosum causes each hemisphere to acquire its own consciousness. This is a strength because it has contributed significantly to understanding the complex and detailed interplay between brain function, cognition and behaviour

What are the limitations of hemispheric lateralisation research?

Some researchers argue that even in split-brain patients the brain can only produce one consciousness. They suggest that the two hemispheres are not separately conscious after surgery. Visual perception remains separate, but consciousness is shared across the brain. This means that more research should be conducted with split-brain patients to determine what is and is not separate per hemisphere.

The issue with trying to replicate Sperry's research and conduct more studies in this field is that Sperry was working with a small, highly specific sample. People who have had a corpus callosotomy represent a restricted population. This means that Sperry's findings are difficult to generalise beyond this very limited demographic.

How would you evaluate split-brain research? (4 marks)

Strength with explanation: Research conducted in the field of split-brain patients is scientific e.g. Sperry's (1968) use of standardised procedures in controlled conditions successfully demonstrates the differences in function per hemisphere.

Limitation with explanation: However, the quasi nature of his experiment (the IV was naturally-occurring) means that Sperry could not exert 100% control as his sample comprised the IV, plus the sample itself is small (11 participants) which limits the reliability of the data.

Another strength with explanation: The findings from split-brain research have been extremely useful in understanding how the brain is organised which in turn can inform post-surgery/injury therapy for people with brain damage e.g. the finding that the language centre of the brain is in the left hemisphere.

Another limitation with explanation: Some research however (e.g. Gazzaniga, 1998) casts doubt as the idea that a split brain operates independently and has two consciousnesses: plasticity and functional recovery allow the brain to compensate for loss in one hemisphere and to unify the brain into one, intact consciousness.