Just some of these are :

• WADA technique.
• Getting the participant to close their eyes while testing the transference of haptic information from one hemisphere to another.
• Carrying out instructions with the left hand (right hemisphere) while getting them to sit on their right hand (so the left hand can not be copied by the right hemisphere).
• Tests requiring the coordination of the two hands e.g. doing up their shoes.
• Tests projecting information to one hemisphere (in the case of persons without disconnection between the hemispheres, information is freely passed to the other hemisphere).
• Tests of handedness.
• Surgical electrical stimulation of the cortex.
• Electroencephalogram (EEG).
• Various types of neuroimaging.
• Electro-convulsive therapy.
• Brain neurosurgery.

There are further tests in later sections e.g. see 6.4.1.

Handedness.

Penfield described the areas that interfered with language in his patients, but Ojemann provided more definitive information that showed the frequency of an area being identified as serving language.

Theoretically they will randomly show left or right handedness.

Gesture may be predominantly right-handed when accompanying language but may become bilateral when describing spatial components within the environment.

There is likely to be a stronger genetic component for language when compared to handedness.

Any answer to this question has to be speculative. The two hemispheres have a number of structural differences between the left and right hemispheres and some of these appear to favour language on the left. The size of structures on the right predict musical interest. One argument that supports this statement might invoke the view that increased specialisation of say the left hemisphere for language may at the same time inhibit the right hemisphere in this functional role. Levy (1985) suggests that the commissures exist to play a role in the development of hemispheric organisation and allow attentional control and regulation of the hemispheres. However, reduced lateral specialisation in children with language difficulties might predict less left specialisation of language and the use of the right hemisphere's language abilities as a means of compensation. This would encourage the callosal connections and could account for a thicker CC in the children with language problems. In some support of the view is the finding that structures that represent language are sometimes relatively smaller compared to the right hemosphere in such children.

Therefore in the first case the CC would encourage specialisation but in the second case specialisation would not be encouraged.

Taking into account that vision is the dominant perceptual process for humans, the splenium which is the most caudal (posterior) aspect of the corpus callosum has a crucial role in passing visual information from one hemisphere to another.

Apart from the already mentioned role of the splenium, the trunk in the mid-corpus callosum is important for passing information concerning movement from one hemisphere to another. This is demonstrated when this area is resected — the left hand being unable to copy the movements of the right hand. The anterior commissure is involved in the inter hemispheric communication between areas of the frontal cortex. Also, between the amygdalae and piriform cortex and entorhinal cortex and therefore a role in emotional learning would be expected. The most rostral area of the CC is the genu which is also important in the inter-hemispheric communication of a medial area important to emotional and social functioning. This area is discussed in the chapter on emotional dysfunction.

There is the experiment in which the left hand attached to the right hemisphere was able to pick out an object that was shown to the right hemisphere. Also, when a bird was shown to the right hemisphere, the left hand (attached to the right hemisphere) was able draw a bird. Some semantic association made by the right hemisphere has been also demonstrated by a patient drawing a saddle with their left hand being associated with a horse presented to the right hemisphere. In all these cases the right hemisphere presentations were not reported as reaching consciousness.

With gross damage to the left hemisphere in adulthood the recovery of language may take as many as 10 years in some adult patients with limited recovery of comprehension of words rather than syntax after some two years. In contrast hemispherectomy in childhood may show better results when conducted between around 5 to 10 years old. However, there is an indication that more comprehensive testing of syntax and more complex language remains to show some impairment.

The right hemisphere does not obviously support the production of langauge i.e. speech, but has some basic level of comprehension. The right hemisphere is more able to support visuo-spatial judgements and reasoning.

This is a difficult question to answer. It is clear that some form of processing remains after left hemisphere damage or as registered in an apparent mute right hemisphere. When the left language hemisphere reports that nothing was seen this would not be surprising because the object was presented to the right hemisphere. This ignorance would still be professed when the patient was asked to reach for an object because the question is addressed to the left language hemisphere and again the right hemisphere received the information not the left. Patients do show a reaction when shown an emotional image to the right hemisphere, but again the left hemisphere is asked to interpret this experience. Patients with gross left damage do show evidence of conscious processing but perhaps further studies should be carried out.

Researchers are less excited by the global right versus left local perspective largely because this depends on the task. Verbal material such as words are read holistically by the left hemisphere but as letters by the right hemisphere. In contrast the right is more obviously able to cope with spatial relationships especially when praxis is required.

The phenomena of unilateral left-handed anomia, pseudo double hemianopia, unimanual pseudo apraxia and agraphia may be described. These tests often ask the patient to carry out tasks with their left hand. The instructions fail to reach the right hemisphere which controls the left hand.

This comes from a rule proposed originally by Luria. For example, the left and right sensory areas at the primary visual cortex do not differ obviously in their function but as information travels to the final association areas the roles of the two hemispheres can be differentiated more and more easily. At the primary and secondary visual sensory area the communication between the two hemispheres is extensive but as the analysis moves anteriorly the intra-hemispheric communication is enhanced while the inter-hemispheric communication is reduced.

There is a theory that when the two hemispheres are joined in the healthy person one hemisphere inhibits the other in its speciality. So, for example, if someone is comprehending speech then the rather basic comprehension abilities of the language hemisphere are dominant and any abilities of the right hemisphere are subdued and dominated. This theory is supported in that TMS that disrupts the motor area of one hemisphere increases activity in the controlateral hemisphere. Some reservation in accepting this see-saw rivalry theory is the finding that despite this extra sensitivity the sensory differentiation attributed to the contralateral hemisphere is not necessarily enhanced.