Lateralisation

Language centres of the brain! Speech production: Left frontal lobe Language comprehension: Left temporal lobe

Hemispheres & Lateralisation The brain is divided into two hemispheres: the left and the right Each hemisphere (side of the brain) is responsible for specific functions Lateralisation: the dominance of one hemisphere of the brain for particular physical and psychological functions

Hemispheric Lateralisation The term lateralisation refers to the fact that two halves of the human brain are not entirely alike. Each hemisphere has functional specialisations i.e. research has found that the left hemisphere is dominant in language and speech, whereas the right hemisphere excels at visual motor tasks. In 1861, Paul Broca established that damage in one particular area of the left hemisphere resulted in language deficits, whereas damage to the same area of the right brain hemisphere did not have the same consequence. Which functions do you know that differ between the left and right hemisphere? Left hemisphere Right Hemisphere   Work in pairs and write your ideas (if you need support then use the sheet from last lesson- but try without first)

However, this raises an important question: if language is located in the left hemisphere, how can we talk about things that are experienced in the right hemisphere e.g. face recognition? The answer is that the two hemispheres are connected by something called the corpus callosum (a bundle of nerve fibres that connect the two halves). This allows information received from one hemisphere to be sent to the other hemisphere. The chance to investigate the different abilities of the two hemispheres came about when, in a treatment for severe epilepsy, surgeons cut through the bundle of nerve fibres that formed the corpus callosum. The aim of this procedure was to prevent the violent electrical activity that accompanies epileptic seizures crossing from one hemisphere to the other. These patients are referred to as ‘split-brain’ patients.

Split brain research

split brain research Roger Sperry was amongst the first to study the capabilities of the separated hemisphere. His participants were people who’s corpus callosum had been severed in a surgical procedure commissurotomy which had been designed to stop epileptic fits. They were able to send visual information to just one hemisphere at a time in order to study what is known as hemisphere lateralisation. Sperry took advantage of the fact that information from the left visual field goes to the right hemisphere and information from the right visual field goes to the left hemisphere. Sperry’s procedure involved blind folding one of the patients eyes and presenting an image to the other eye for a split second. This ensured that he knew which hemisphere of the brain was being used (the opposite). By doing a number of variations of tasks using this procedure he was able to draw conclusions about the functions of the differing hemispheres.

Methodological evaluation Methodological evaluation. Sperry’s research was very well designed and employed well thought through standardised procedures. As he made use of presenting information to one eye whilst the other was blindfolded and he flashed the image up extremely quickly he had properly controlled which hemisphere was being exposed. He also designed a procedure which could be replicated so his findings could be validated. The split-brain procedure is rarely carried out these days. Therefore patients who have had this procedure are rarely encountered in sufficient numbers to be useful for research. Andrewes (2001) argues that many studies have only 3 (sometimes even just 1 ppt). He claims conclusions have been drawn from individuals who have either a confounding physical disorder that made the procedure necessary or who had a less complete sectioning of the 2 hemispheres than was originally believed. Andrewes claims that these ‘rogue’ cases are often only identified when the results of a study have failed to be replicated.