Localisation of function in the brain
The brain has two hemispheres (halves) that are linked by the corpus callosum, a bundle of nerve fibres that allows the two hemispheres to communicate.
Most people’s brains are contralateral, where the right hemisphere deals with the left side of the body and vice versa. For example, information in an individual’s right visual field is processed by their left hemisphere. When a function is dealt with by one hemisphere it is said to be lateralised, with the division of functions between the two hemispheres known as hemispheric lateralisation:
• Left hemisphere — for most people, language processing occurs in the left hemisphere. A stroke on the left side of the brain can therefore affect speech. The left hemisphere is also seen to focus on detail in visual information.
• Right hemisphere — recognising emotions in others occurs in the right hemisphere, as does dealing with spatial information (knowing where things are in relationship to each other in the visual field). The right hemisphere is also thought to process overall patterns in visual information.
• Motor cortex — movement is centred on the primary motor cortex, which sends information to the muscles via the brain stem and spinal cord. This brain area is important for complex movements and non-basic actions, such as coughing and crying. The spinal cord and brain co-ordinate movements, while the premotor cortex helps plan movements and the prefrontal cortex stores sensory information prior to a movement and assesses the probability of outcomes of movements.
• Somatosensory centres — touch is perceived in the somatosensory cortex, with processing of some body areas, like the face, involving larger parts of this brain area than others.
• Visual centres — the primary visual cortex is the main visual centre, with visual information conveyed along two pathways. One contains components of the visual field and the other the location of the components within the visual field.
• Auditory centres — the primary auditory cortex receives information from both ears via two pathways that transmit information about what a sound is and where it is.
• Language centres — Broca’s area, located in the left temporal lobe, is responsible for production of speech, while Wernicke’s area, close to the auditory cortex, is important for understanding language and accessing words.
Domanski (2013) reported on neurologist Paul Broca, who conducted a post mortem on Louis Leborgne, who through epilepsy had lost the ability to speak (apart from the word ’Tan’) and died aged 51. Broca found damage to the left temporal lobe, which suggests this brain area (now known as Broca’s area — see Figure 6.5) has a localised responsibility for production of speech.
• Wernicke (1874) found that damage to an area next to the auditory cortex (now known as Wernicke’s area) produced an impairment to the ability to comprehend language and anomia, a difficulty in finding a desired word. This suggests this brain area has a localised function in understanding and accessing language.
• Clarke et al. (1993) reported a case study of a woman who had damage to her right hemisphere, which resulted in her getting lost even in familiar surroundings. This suggests the right hemisphere processes spatial information.
• MacMillan (2002) reported on the case study of Phineas Gage, who had a much altered personality due to damage to a brain area that catered for the planning, reasoning and control of an individual. This supports the idea of localisation of function of brain areas.
Fig 6.5 Broca’s area and other localisation of cortical function
There is a wealth of research evidence to support the idea of localisation of function within specific brain areas.
Modern brain scanning methods, such as PET and MRI scans, have backed up the idea of localisation of function by finding evidence of activation within specific brain areas when certain tasks are being performed.
Some people seem able to recover functions after specific brain damage, which suggests that localisation of function may not exist universally.
Much research in this area relies on case studies, which present problems of generalisation, as the results from single individuals cannot be said to be representative of all.
The holistic theory of brain function argues against localisation of function. Lashley (1950) reported that work on rats’ brains indicated no one specific brain area was responsible for memory, which suggests the idea of there being certain brain areas for specific tasks is wrong.
One practical application of research into brain functioning is a corpus callosotomy, an operation performed on epileptics where the nerve fibres at the corpus callosum are severed to separate the two hemispheres of the brain. This reduces the intensity of seizures, increasing the quality of life.