There Is No General Model of Hemisphere Asymmetries (lack of Equality or Equivalence Between Parts or Aspects of Something; Lack of Symmetry) as They Are Affected by Both Gender and Handedness

I would put forward my theory that there is some evidence of hemisphere asymmetries as each side of the brain do not function similarly. Although there are some differences in the right and left hemisphere in both genders, why and where these come from is quite debatable. I would also suggest that there is a predominance of language in the left hemisphere with both right and left hander's though there is stronger evidence of it in right handed people and it is quite difficult to determine from current research if handedness is the determining cause of differences in hemisphere function.

Hemisphere asymmetries is about the lack of equality in the two hemispheres in the brain, i.e. the right and left hemisphere do not have the same cognitive and behavioural functions in the same way. It would be fair to put forward a suggestion that hemisphere is affected by both handedness and gender. Fossilised brains are interesting to look at but unfortunately cannot tell us how brain functioning occurred million and millions of years ago, if they did we could determine how the brain may have developed over time. However we do know more about how brain functioning occurs today thanks to research by Broca (1861) , Wernicke (1871) and Roger Sperry (1950's) leading the way. I will next have a look at Sperry and the split-brain studies.

Sperry devised a method to test split brain patients in the 1950's . Following treatment resistant epileptic patients inability to lead normal lives an operation was carried out to cut the corpus collasum, known as the commissurotomy or 'split brain' operation cutting the information 'bridge' between the right and left hemisphere. Sperry devised the divided field technique, which involved presenting words and pictures to split brain patients to the separate hemispheres. This provided some research to show that most of our language processing is in the left hemisphere but not all as the right hemisphere does recognise concrete nouns (names of objects) and that our visuo-spatial stimuli is processed in the right hemisphere.

Further studies by Bever and Chiarello (1974) suggests that the left hemisphere processes words and deals with sequential, information distributed over time breaking it down into separate components. When Bever and Chiarello tested musicians and non-musicians in a music recognition test, they found their left hemisphere superiority for musicians and right hemisphere superiority for non-musicians. This was because non-musicians will process music as a whole and musicians will break it down into different compartments looking at musical phrases cords and rhythms using the left hemisphere. Below, I will look at the possible affects of gender on the brain.

Gender on Hemisphere Asymmetries

Moving onto early development, there is no strict assignment to the left or right side of the brain at the start of conception by gender however there are hypothesis by Geschwind (1980) that exposure to the male hormone testosterone whilst in the womb, can slow down the rate of development to the left hemisphere of the brain by longer exposure higher levels of prenatal androgens than females. The slowing down of development in the left hemisphere from exposure to this prenatal androgens, it is said leads to the right hemisphere in the male brain becoming more dominant. For example Levy (Levy & Reid 1978) and McGlone (1980) indicated from their research that female language is less lateralised to the left hemisphere and the fact language may be bilaterally represented in the female brain.

Levy (1969) suggested that the females deficiencies in visuo-spatial skills is a result of competition between the left and right hemispheres in females hemispheres as they tend to use both hemispheres for language whilst men tend to use just the left hemisphere. A sound argument as from Levy (1978) is that it may come from evolutionary human needs. Going back in time, millions of years, when men were the hunter gatherers having to search in the woods for prey thus their development of visuo-spatial skills, is more lateralised. Whilst the women were more involved in the social activities back in the dwellings, such as child-rearing. However the problem with Levy's arguments is that there is no evidence to suggest that looking for berries might not require equal amount of communication skills as hunting as looking out for danger and communicating this.

However more recent studies by Bolle and Hellige (1992) as cited in Eviatar & Zaidel (1997) who look at hemisphere lateralisation suggest that differences in one component may be independent of differences in other dimensions which contrasts with Geschwind's model suggestion that hemisphere lateralisation is much more functionally systematic.

Though further studies have shown that men may undergo more left hemisphere structural decline there is no evidence to prove that there is any more decline in function in the left hemisphere any more than females.