AQA Psychology for A Level Year 2 - Student Bk

ring The specification says… Split-brain research into hemispheric lateralisation Hemispheric lateralisation; split-brain research Neuroscientists are interested in whether certain activities and behaviours are controlled or dominated by one hemisphere rather than the other (known as lateralisation – as distinct from localisation). Broadly speaking, the right hemisphere controls activity on the left side of the body and vice versa. Here, we discuss this question with the help of some ground-breaking experiments involving so-called split- brain patients. Key terms Hemispheric lateralisation – The idea that the two halves (hemispheres) of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other, as in the example of language (which is localised as well as lateralised). Split-brain research – A series of studies which began in the 1960s (and are still ongoing) involving epileptic patients who had experienced a surgical separation of the hemispheres of the brain. This allowed researchers to investigate the extent to which brain function is lateralised. Testing a split-brain patient. Methods: Lateralisation, language and handedness An investigation of 400 right-handed people revealed that 380 of them had left-hemisphere dominance for language, i.e. language was lateralised on the left. This reflects the trend in the general population. Questions 1. What percentage of right-handed people in the investigation above were left hemisphere dominant for language? ( 1 mark ) 2. Express your answer to question 1 as a fraction. ( 1 mark ) In left-handed people the situation was more complex. In a further study there were 400 left-handed people tested. In 80 cases language was located on the right hemisphere, and for a further 80, language functions were arranged bilaterally , that is, across both hemispheres. 3. What percentage of left-handed people’s language functions were arranged bilaterally? ( 1 mark ) 4. How might the results of Sperry’s studies have been different if some of his patients were left-handed? ( 3 marks ) @ookx hs @ookx hs Split-brain research Hemispheric lateralisation As we have already seen, the ability to produce and understand language, for most people, is controlled by the left hemisphere. This suggests that for the majority of us, language is subject to hemispheric lateralisation . In other words, the specialised areas associated with language are found in one of the brain’s hemispheres rather than both. The question of whether other neural processes may be organised in this way was investigated in a series of ingenious experiments conducted by Roger Sperry and his colleagues (known as split-brain research ) for which Sperry was awarded the Nobel Prize in 1981. Split-brain studies Sperry’s (1968) studies involved a unique group of individuals, all of whom had undergone the same surgical procedure – an operation called a commissurotomy – in which the corpus callosum and other tissues which connect the two hemispheres were cut down the middle in order to separate the two hemispheres and control frequent and severe epileptic seizures. This meant that for these split-brain patients the main communication line between the two hemispheres was removed. This allowed Sperry and his colleagues to see the extent to which the two hemispheres were specialised for certain functions, and whether the hemispheres performed tasks independently of one another. Procedure Sperry devised a general procedure (see picture, left) in which an image or word could be projected to a patient’s right visual field (processed by the left hemisphere) and the same, or different, image could be projected to the left visual field (processed by the right hemisphere). In the ‘normal’ brain, the corpus callosum would immediately share the information between both hemispheres giving a complete picture of the visual world. However, presenting the image to one hemisphere of a split-brain patient meant that the information could not be conveyed from that hemisphere to the other. Key findings Describing what you see When a picture of an object was shown to a patient’s right visual field, the patient could easily describe what was seen. If, however, the same object was shown to the left visual field, the patient could not describe what was seen, and typically reported that there was nothing there. You may recall that for most people (and this was true of all of Sperry’s patients) language is processed in the left hemisphere. Thus, the patient’s inability to describe objects in the left visual field (processed in the right hemisphere) was because of the lack of language centres in the right hemisphere! In the normal brain, messages from the right hemisphere would be relayed to the language centres in the left hemisphere. Recognition by touch Although patients could not attach verbal labels to objects projected in the left visual field, they were able to select a matching object from a grab-bag of different objects using their left hand (linked to right hemisphere). The objects were placed behind a screen so as not to be seen (see picture). The left hand was also able to select an object that was most closely associated with an object presented to the left visual field (for instance, an ashtray was selected in response to a picture of a cigarette). In each case the patient was not able to verbally identify what they had seen but could nevertheless ‘understand’ what the object was using the right hemisphere and select the corresponding object accordingly. Composite words If two words were presented simultaneously, one on either side of the visual field (for example, a ‘key’ on the left and ‘ring’ on the right as in the picture), the patient would write with their left hand the word ‘key’ (left hand goes to right hemisphere linked to left visual field) and say the word ‘ring’. The superiority of the right hemisphere in terms of drawing tasks is illustrated in the ‘Apply-it’ on drawing abilities on the facing page. Matching faces The right hemisphere also appeared dominant in terms of recognising faces. When asked to match a face from a series of other faces, the picture processed by the right hemisphere (left visual field) was consistently selected, whilst the picture presented to the left hemisphere was consistently ignored. When a composite picture made up of two different halves of a face was presented – one half to each hemisphere – the left hemisphere dominated in terms of verbal description whereas the right hemisphere dominated in terms of selecting a matching picture. Chapter 2 Biopsychology 42 •