The Wada test further provides evidence for the lateralization of language. In the Wada test, anesthetic is injected into the carotid artery to anesthetize one hemisphere. When the left hemisphere is anesthetized, reading aloud is disrupted in more than 90 per cent of people (Parker & Riley, 2010). .
Apart from being lateralized, the brain is also contralateralized whereby the right hemisphere is responsible for the functions of the left side of the body and the converse. This is apparent in the dichotic listening test (Aitchison,2008) in which when auditory signals are transmitted to both ears, the right ear (LH) is better at processing language-like signals compared to the left (RH).
Roger Sperry's study of split-brain patients, whose corpus callosum was severed due to epilepsy, also confirmed hemispherical specialization. In the study, the patients could name and retrieve the object of the word presented to their right visual field (left-hemisphere-LH) but when presented to the left (right-hemisphere-RH), they were able to retrieve but unable to name the object. This suggested that the LH can comprehend and produce linguistic stimuli. Sperry's study concluded that the LH is the dominant hemisphere for active language processing (Parker & Riley,2010). .
Results from brain scans and imaging have also established that there was typically more activity in the LH than RH during language-related activities. These tests and observations are clear indications that language processing is predominantly a left-hemispheric lateralized function. However, the RH does contribute to some aspects of language processing such as the production and comprehension of intonational patterns and understanding of humor and metaphors (Parker&Riley;,2010). .
Paul Broca's (1861) correlation of the location of the damage in the left frontal lobe of the brain with patients who had expressive aphasia, the inability to produce fluent speech, is evidence of the localization of brain function (Parker & Riley, 2010).
