localisation of functoon in the brain

Intro

The theory that different areas of the brain are responsible for different behaviours, processes or activities.

 

Localisation versus holistic theory

 

During the 19th century, scientists such as Paul broca and Karl Wernicke discovered that scientific areas of the brain are associated with particular physical and psychological functions.

 

Before these investigations (and before the case of Phineas Gage) scientists generally supported the holistic theory of the brain - that all parts of the brain were involved with different parts of the body.

 

It follows then, that certain area of the brain becomes damaged through illness or injury, the function associated with that area will also be affected.

Hemispheres of the brain

The main part of the brain (the cerebrum)  is divided into two symmetrical halves called the left and right hemisphere.

 

Some of our physical and psychological functions are controlled or dominated by a particular hemisphere - this is called lateralisation

 

As a general rule, activity on the left hand side of the body is controlled by the right hemisphere and activity on the right hand side of the body is controlled by the left hemisphere.

• Language is linked to the left hemisphere.

The motor, somatosensory, visual and auditory centres (1)

The cerebral cortex or ‘cortex’ os the outer layer of both hemispheres. It covers the inner parts of the brain like a ‘tea cosy’. The cortex appears grey due to the location of cell bodies, hence the phrase ’grey matter’.

 

The cortex of both hemispheres is subdivided into four centres - called the ‘lobes’ of the brain: the frontal  love, the occipital lobe and the temporal lobe.

 

• A lobe is a part of an organ that is separate in some way from the rest. Each lobe in the brain is associated with different functions.

 

At the back of the frontal lobes (in both hemispheres) is the motor centre which controls voluntary movement in the opposite side of the body. Damage to this area of  the brain may result in a loss of control over fine movements.

 

At the front of both parietal lobes is the somatosensory centre which is separated from the Motor centre by a valley called the central sulcus.

The motor, somatosensory, visual and auditory centres (2)

The somatosensory centre is where sensory information from the skin is represented. The amount of somatosensory area devoted to a particularly body part denotes its sensitivity, for instance, receptors for our face and hands occupy over half of the somatosensory centre.

 

In the occipital lobes at the back of the brain is the visual centre. Each sends information from the right visual field to the left visual cortex and from the left visual field to the right visual cortex.

• This means that damage to the left hemisphere can produce blindness in part of the right visual field of both eyes.

 

The temporal lobes house the auditory centre which analyses speech based information. Damage may produce hearing loss.

• The more extensive the damage the more excessive the loss.

• In addition, damage to a specific area of the left temporal love - Wernicke’s area - may affect the ability to comprehend langauge

The langauges of the brain (1)

Unlike the areas above which are found in both hemispheres, language is restricted to the left side of the brain in most people.

 

In 1861, Paul Broca, a surgeon, identified a small area in the left frontal lobe responsible for speech production.

 

• Damage to Broca area causes Broca aphasia which Is characterised by speech that is slow, labour ions and lacking fluency.

• Deficit in inability to produce language, but doesn’t mean you don’t understand language (comprehension generally preserved)

• Language is localised, understand but not speak

 

Broca’s most famous patient was ‘tan’ - so callled because that was the only word he could say. People with broca’s aphasia often have stilted speech, omitting words such as ‘the’ and ‘and’.

 

The langauges of the brain (2)

Around the same time as Broca, Karl Wernicke was describing people who has no problem producing language but severe difficulties understanding it, which meant that the speech they produced was effortless but meaningless.

• Located and the junction of 2 lobes

• Can produce speech but language is meaningless

 

Wernicke identified a region (Wernicke area) in the left temporal lobe as being responsible for language understanding.

 

This results in Wernicke aphasia when damaged

• People who have wenickes aphasia will often produce language but have problems understanding it, so they produce nonsen words (neologisms) as a part of the content of their speech

A03 - strengths

Evidence from neurosurgery

 

One strength of localisation theory is that damage to areas of the brain has been linked to mental health conditions.

 

Neurosurgery is a last resort method for treating some mental health conditions by targeting specific areas of the brain which may be involved.

 

For example, cingulotomy involves isolation a region called the cingulate gyrus which has gene implicated in OCD.

 

Dougherty reported on 44 people with OCD who had undergone a cingulotomy. At post surgical follow up after 32 weeks, about 30% has met the criteria for succesful response to the surgery. With 14% for partial response.

 

The success of these procedures suggests that behaviours associated with serioues mental health conditons may be localised.

A03 - strengths

Brain mapping

 

Another strength fo localisation is supportive evidence from brain mapping.

 

A study by glasses created a map of the human cerebral cortex by identifying distinct brain regions. The researchers used high resolution fMRI scans of 210 healthy young adults.

 

They identified 180 regions per hemisphere, including 97 new regions and 83 already known regions (Broca’s areas and the motor cortex)

 

Newly identified regions were linked to higher order cognitive functions including langauge, problem solving, and motor and sensory processing.

 

This evidence confirms the importance of localisation in the brain.

A03 - limitations

Language localised questioned

 

One limitation is that langauge may not just be localised to Broca’s and wernickes areas.

 

A review done by 2 psychologists reported that only 2% of modern researchers think that language in the brain is completely controlled by Broca’s and Wernick’es areas.

 

Advances in brain imaging technqiues such as fMRI, mean that neural processes in the brain can be studies with more clarity then ever before.

 

It seems that language function is distributed far more holistically in the brain than was first thought.

 

So called language streams have been identified across the cortex, including brain regions such as the right hemisphere as well as sub cortical regions such as the thalamus.

 

This suggest that, rather than being confined to a couple of key areas, language may be organised more holistically in the brain, which contradicts localisation theory.