localisation of function in the brain Flashcards
localisation of function versus holistic theory
During the 19th century, scientists such as Paul Broca and Karl Wernicke discovered that specific areas of the brain are associated with particular physical and psychological functions. Before these investigations (and before the case of Phineas Gage - see facing page), scientists generally supported the holistic theory of the brain - that all parts of the brain were involved in the processing of thought and action.
In contrast, Broca and Wernicke argued for localisation of function (sometimes referred to as cortical specialisation). This is the idea that different parts of the brain perform different tasks and are involved with different parts of the body. It follows then, that if a certain area of the brain becomes damaged through illness or injury, the function associated with that area will also be affected.
hemispheres
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 (see next spread). 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 by the left hemisphere. Language, as we will see below, is linked to the left hemisphere.
the centres of the cerebral cortex
The cerebral cortex (or cortex ) is the outer layer of both hemispheres - as described on page 34. The cortex of both hemispheres is subdivided into four centres - called the lobes’ of the brain: the frontal lobe, the parietal lobe, the occipital lobe and the temporal lobe (see diagram on facing page). 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 lobe (in both hemispheres) is the motor area 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 area which is separated from the motor area by a valley’ called the central sulcus. The somatosensory area is where sensory information from the skin (e.g. related to touch, heat, pressure, etc.) is represented. The amount of somatosensory area devoted to a particular body part denotes its sensitivity, for instance, receptors for our face and hands occupy over half of the somatosensory area.
In the occipital lobe at the back of the brain is the visual area (or visual cortex). Each eye 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, for example, can produce blindness in part of the right visual field of both eyes.
Finally, the temporal lobes house the auditory area, which analyses speech-based information. Damage may produce partial hearing loss. The more extensive the damage, the more extensive the loss. In addition, damage to a specific area of the temporal lobe-Wernicke’s area (discussed below) - may affect the ability to comprehend language.
the language area of the brain
Unlike the areas above which are found in both hemispheres, language is restricted to the left side of the brain in most people. In the 18805, Paul Broca, a surgeon, identified a small area in the left frontal lobe responsible for speech production. Damage to Broca’s area causes Broca’s aphasia which is characterised by speech that is slow, laborious and lacking in fluency. Broca’s most famous patient was ‘Tan’ - so-called because that was the only word he could say. People with Broca’s aphasia have difficulty with prepositions and conjunctions (e.g. a, the, and).
Around the same time as Broca, Karl Wernicke was describing people who had no problem producing language but severe difficulties understanding it, such that the speech they produced was fluent but meaningless. Wernicke identified a region (Wernicke’s area) in the left temporal lobe as being responsible for language understanding. This results in Wernicke’s aphasia when damaged. People who have Wernicke’s aphasia will often produce nonsense words (neologisms) as part of the content of their speech.
strength-evidence from neurosurgery
One strength of localisation theory is that damage to areas of the brain has been linked to mental disorders.
Neurosurgery (surgery on the brain) is a last resort method for treating some mental disorders, targeting specific areas of the brain which may be involved. For example, cingulotomy involves isolating a region called the cingulate gyrus which has been implicated in OCD. Darin Dougherty et al.
(2002) reported on 44 people with OCD who had undergone a cingulotomy.
At post-surgical follow-up after 32 weeks, about 30% had met the criteria for successful response to the surgery and 14% for partial response.
The success of these procedures suggests that behaviours associated
with serious mental disorders may be localised.
strength-evidence from brain scans
Another strength is evidence from brain scans that supports the idea that many everyday brain functions are localised.
For instance, Steven Petersen et al. (1988) used brain scans to demonstrate how Wernicke’s area was active during a listening task and Broca’s area was active during a reading task. Also, a review of long-term memory studies by Buckner and Petersen (1996) revealed that semantic and episodic memories reside in different parts of the prefrontal cortex.
These studies confirm localised areas for everyday behaviours.
Therefore objective methods for measuring brain activity have provided sound scientific evidence that many brain functions are localised.
Counterpoint A challenge to localisation theory comes from the work of Karl Lashley (1950). Lashley removed areas of the cortex (between 10% and 50%) in rats that were learning the route through a maze. No area was proven to be more important than any other area in terms of the rats’ ability to learn the route. The process of learning seemed to require every part of the cortex rather than being confined to a particular area.
This suggests that higher cognitive processes, such as learning, are not localised but distributed in a more holistic way in the brain.
strength-case study evidence
One limitation is that language may not be localised just to Broca’s and Wernicke’s areas.
A recent review by Anthony Dick and Pascale Tremblay (2016) found that only 2% of modern researchers think that language in the brain is completely controlled by Broca’s and Wernicke’s areas. Advances in brain imaging techniques, such as fMRI, mean that neural processes in the brain can be studied with more clarity than 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 in the right hemisphere, as well as subcortical regions such as the thalamus.
This suggests that, rather than being confined to a couple of key areas, language may be organised more holistically in the brain, which contradicts localisation theory.
