Since Brocas 1861 discovery, the left and right cerebral hemispheres have been characterised as being verbal and nonverbal respectively. Though this dichotomy serves adequately as a rule of thumb, an increasing body of research has highlight

Since Broca’s 1861 discovery, the left and right cerebral hemispheres have been characterised as being verbal and nonverbal respectively. Though this dichotomy serves adequately as a rule of thumb, an increasing body of research has highlighted linguistic capacity in the supposedly ‘nonverbal’ right hemisphere. Compare and contrast the language processing abilities of the left and right hemispheres, paying particular attention to their phonological processing abilities. Make sure you draw on evidence from visual half field research and either clinical/split brain or imaging research to support your arguments.

The right and left hemispheres of the brain are similar in appearance; being mirror images of each other. However, on closer neuropsychological examination the hemispheres differ, particularly in the planum temporal of the cortical region. This area is involved in the processing of language and it is seen that it is larger in the left hemisphere than the right hemisphere in 65% of subjects (Haralambos, 2002).

The major differences between the two hemispheres are their functions. Each hemisphere controls the movements made by the opposite side of the body. The same applies to visual processing but with more complicating factors. So the left visual field is represented in the right hemisphere and visa versa. Therefore damage to the right hemisphere will cause loss of vision in the left visual field. Broca (1861) found that some information processing only occurs in one hemisphere making certain functions lateralised to each a specific hemisphere.

The brain hemispheres are connected by the corpus callosum. The question which was studied in the 1960s was what happens when the corpus callosum is cut; do the two hemispheres act independently? Vargha-Khadem (1997) found that when one hemisphere is removed from a child at an early age due to injury, the other hemisphere adjusts to compensate for the loss, enabling the child to learn language relatively well. It has been suggested the two hemispheres possess similar potentials then later develop complementary functions (Beeman & Chiarello, 1998). This means that from early age the hemisphere are not complete in what their functions are and so can automatically take over the redundant hemisphere more easily than when the patient is older.

This is a preview of the whole essay

In split brain patients those whose hemispheres have been separated by commissurotomy, the left hemisphere seems to control most everyday activity. However in control patients the right hemisphere takes a more active role in controlling behaviour. A commissurotomy operation has been used on patients with epilepsy and those with conditions like partial seizures which involved the separating of the hemisphere. In epilepsy the surgery was aimed at reduce the severity of the attacks. However, after the surgery some of these split brain patients started expressing unusual behaviour. The two hemispheres were acting independently and it was if the left hand did not know what the right hand was doing, (Haralambos, 2002). Many of these split brain patients remained having unusual behaviour so it seems that the functions of the hemisphere are not able to be re adapted (as happens with children). Therefore it seems that age may play a part in the patient’s recovery of language abilities (Vargha-Khadem, 1997).

One way of researching the differences between the hemispheres is by using visual-half field studies, (Haralambos, 2002). Information (e.g a picture of an apple) being presented briefly in the right or left visual field. The subject had to visualise on a central visual point whilst the information was presented. The subject was then asked what object was on the picture presented. Difficulties in accuracy and response can be highlighted and compared with controlled and brain damaged patients. Unfortunately with subjects with an intact corpus callosum it is hard to determine if both hemispheres or just one hemisphere is involved in each task as information from one hemisphere to the other travels extremely quickly. Also individual differences such as dyslexia may affect their responses.

Functioning differences may be more clearly seen in split brain patients because the cutting of the corpus callosum affects the information being transferred from one hemisphere to the other so showing what functions are associated with which hemisphere. If split brain patients are shown the words “jump * start” they will be able to read aloud the word “start” as it was presented in the right visual field as it is processed by the left hemisphere which is specified for verbal processing. The word “jump” is not spoken as it was presented in the left visual field and is processed by the right hemisphere which does not speak as it is nonverbal. Split brain patients cannot access the other hemispheres functions via the corpus callosum.

Sperry (1964) found that split brain patients could name objects placed in their right hand but not in their left hand. It was found that they could report verbally the picture when using the left hemisphere yet when reporting what was in the left visual field they would guess or say that nothing appeared. It seems that the left hemisphere is dominant in split brain patients. However when using the same experiment but asking the subject instead of giving a verbal response to choose an object by touch, which is controlled by the right hemisphere, the left hand would choose the correct object but the right hand could not.

In regards to the amount of linguistic ability in the right hemisphere in split brain patients, the ability level is very varied. Phonological processing is one of the four major components of language. Phonological processing deals with producing and processing speech sounds. In extreme cases, the right hemisphere does not seem to understand language at all whilst other patients have a grasp of concrete nouns e.g. table and are able to understand some adjectives e.g. smooth.

Patients with aphasia have been the prime study into the dysfunctions of speech caused by neurological damage (lesions). A patient with Broca’s aphasia will speak very little and when it is attempted it is agrammatical with nouns intact but function words no intact causing non fluency in speech. Broca’s aphasia is due to damage to the frontal region in the left hemisphere. Another type of aphasia is Wernicke’s aphasia which is when a patient has damage to the posterior region of the first temporal gyrus which impairs the comprehension of speech. The language is more fluent than Broca’s aphasia but can consist of inappropriate or neologisms.

Split brain studies have their limitations. Few patients received a commissurotomy so evidence is based on a tiny proportion. Also there is no evidence that the epileptic fits had no influence on the outcome of the surgery as the fits could have already caused severe brain damage. With present psychological guidelines it would be unethical to use split brain surgery so all the evidence is based on previous studies when less was known about the brain.

The right hemisphere role in terms of language has been a topic of considerable research. Beeman (1993) examined language specific deficits resulting from right hemisphere damage. It was found that damage to the right hemisphere caused an isolated impairment in one specific language component involving comprehension, perception, speech etc (Beeman & Chiarello, 1998). These patients were missing pieces to language like a jigsaw, showing the right hemisphere to have a significant role in overall verbal processing so causing problems for split brain patients and right hemisphere damaged patients.

Faust (1998) shows that the left hemisphere is better at using sentence structure to gain message-level interpretation whilst the right hemisphere is less concerned in the overall sentence structure. Faust (1998) also found that patients with right hemisphere damage had difficulty following complex dialogue during story discourse. This shows that the right hemisphere only distantly relates input words together, which may be inefficient on its own. So the left hemisphere is more involved in fine coding information and quickly selecting the most plausible interpretation given by a context. Therefore the hemispheres do not differ in semantics but in how knowledge is activated by words. Koivisto (1997) concluded that initial processing in the left hemisphere may resemble later semantic processing in the right hemisphere but the right hemisphere has a broader range of word meanings. Yet Posner and Raichle (1988) found confounding results using a Position Emission Tomography scan (PET) on language. One trial found activation blood flow in the left hemisphere whilst the second trial showed activation in both hemispheres. Therefore it is shown that the right hemisphere aids the left hemisphere in language as they interact via the corpus callosum. Split brain patients can still retain speech without the full use of the two hemispheres.

The comprehension of the meaning of words is severely disrupted when a patient has Wernicke’s aphasia. No comparison can be made to demonstrate this with right hemisphere lesions. However as seen with split brain patients it has shown that the right hemisphere does comprehend certain words especially nouns. So it seems that comprehension of concrete words like nouns can be comprehended by the right hemisphere but more abstract words like love are comprehended by the left hemisphere. So the right hemisphere does not seem to contribute much to our verbal production of speech, as the left hemisphere can do the same and more.

In conclusion, the two hemispheres appear to be mirror images of each other. The aspect which distinguishes them apart is their functions. The left hemisphere is verbal and is important for speech whereas the right hemisphere is primarily nonverbal. However, via the connection of the corpus callosum, the two hemispheres are able to interact so allowing the right hemisphere to possess some less literal verbal abilities. Evidence for the hemispheres differences comes from split brain patients who have had their hemispheres separated during a commissurotomy. It has been found that after separation the two hemispheres can work independently. However, these split brain patients they have been found to lose the ability of the interaction of the hemispheres as the corpus callosum has been cut. This means their left side does not know what the right side is doing and vice versa. Yet it has been shown that young children who have one hemisphere removed can retain normal language abilities with just one hemisphere which compensates for the other. Overall it seems that one needs the corpus callosum for interaction of both hemispheres to allow normal functioning of the body but it does depend on age.

References

Beeman, M,J., & Chiarello,C. (1998). Complementary right –and left-hemisphere

language comprehension. Current Directions in Psychological Science, 7 (1),

p.2-8.

Bishop, D.V.(1990). Handedness and developmental disorder. Mac Keith Press.

London. UK.

Code, C., Wallesch, C., Joanette,Y. (1996) Classical cases in neuropsychology.

Psychology press. UK

Haralambos, M., Rice, D. (2002) Psychology In Focus. Causeway Press. Lancashire.