lateralisation and split brain research

• Hemispheric lateralisation refers to the fact that the 2 halves of the brain are not exactly alike. Each hemisphere has functional mechanisms for some functions that are localised to one side of the brain. For example, Paul Broca established that damage in one particular area on the left side of the brain led to language deficits, but on the right side, it didn’t. The two hemispheres are connected which is how we can talk about things presented to the right hemisphere. They are connected by a bundle of nerve fibres called the corpus collosum.
• This chance to investigate different abilities of the two came due to the treatment of severe epilepsy where surgeons cut the corpus collosum. The aim was to prevent the violent electrical activity that accompanies the seizers. These patients are often referred to as ‘split brain’ patients. Sperry and Gazzaniga did research into split brain patients. They sent visual information to just one hemisphere at a time to study hemispheric lateralisation. Because the corpus collosum has been cut, the two hemispheres cant travel to the other one and only processed by the hemisphere it was presented to.
• In a typical study, the split brain patients would fixate on a dot in the middle of the screen while info was presented to either left or right visual field. They were then asked to respond with either their left hand (right hemisphere), right (left hemisphere) or verbally. If the patient was flashed a picture of a dog to the right visual field, they would answer ‘dog’. If flashed to the left they would say they see nothing. The right hemisphere has no language centre, so cannot respond verbally. The left has a language centre but doesn’t receive the info and so cannot say they have seen it.
• Lateralisation of function appears not to stay exactly the same throughout a lifetime, but changes with ageing.

  Lateralised patterns found in younger individuals tend to switch to bilateral patterns in healthy older adults. A researcher found that language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after 25, lateralisation decreased with each decade. Its difficult to know why, but one possibility is that using the extra processing resources of the other hemisphere may in some way compensate for age-related declines in function.
• The split-brain procedure is rarely carried out nowadays and there are as few as 3 pt or even just 1 single pt. 

  Andrewes 2001 claims that sometimes conclusions have been drawn from individuals who either have a confounding physical disorder that made the split-brain procedure necessary or have had less complete sectioning of the 2 hemispheres. 

  As a result, patients who have had this procedure without confounding variables, rarely are encountered in sufficient numbers to be useful for research.
• Gazzangia suggests that some of the early discoveries from split-brain research have been disconfirmed by more recent discoveries.

  Damage to the left hemisphere was found to be more detrimental to language function than to the right. However, case studies have demonstrated that this wasn't necessarily the case. A patient, JW, developed the capacity to speak out of the right hemisphere, with the result that he can now speak about info presented to the right or left side of the brain. This challenges the claim that the right hemisphere is unable to handle language.
• There are a number of disadvantages associated with lateralisation. For example, the mathematically gifted tend to have superior right-hemispheric skills but more likely to be left-handed and suffer higher rates of allergies and problems with immune system. A researcher found a small but significant relationship between handedness and immune system disorders, suggesting that genetic processes that lead to lateralisation may also affect the development of immune system. Support for this, found that lefties had a higher immune system disorders in their immediate families than did right.