n psycholinguistic tasks such as dichotic listening); it is not the case that language is "localized" in any one hemisphere laterally.
Brain function lateralization is evident in the phenomena of right- or left-handedness and of right or left ear preference, but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere dominance for language, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of the left-handed have bilateral language functions. Even within various language functions (e.g., semantics, syntax, prosody), degree (and even hemisphere) of dominance may differ.
Methods of study
There are ways of determining whether particular cognitive functions tend to be lateralized to one cerebral hemisphere. The Wada Test introduces an anesthetic to one hemisphere of the brain via one of the two carotid arteries. Once the hemisphere is anesthetized, a neuropsychological examination is effected to determine whether cognitive functions such as language production, language comprehension, verbal memory, or visual memory are retained. Another common way to study neural deficits is to identify the deficits a person exhibits in relation to lesions in different areas of the brain.
Less invasive techniques, such as functional magnetic resonance imaging and transcranial magnetic stimulation may also be used to investigate the role of a particular cerebral hemisphere in a particular task, although these methods may be costly. The divided visual field paradigm is another technique that has contributed to the study of hemispheric specialization. CAT scans, PET scans and EEG are also used to study the brain. CAT scans use tomography to create a 3D image of the brain, which provides insights about neural anatomy, but it is unable to show the brain functioning in real time. PET scans image areas of high metabolic activity and neural activity by scanning for an active substance that has been tagged with positron emitting isotopes, that has been ingested by the patient. Finally, EEGs collect data from the electric fields that are produced by the brain.
Damage to either the right or left hemisphere, and its resulting deficits provide insight into the function of the damaged area. Right hemisphere damage has many effects on language production and perception. Damage or lesions to the right hemisphere can result in a lack of emotional prosody or intonation when speaking. Right hemisphere damage also has monumental effects on understanding discourse. People with damage to the right hemisphere have a reduced ability to generate inferences, comprehend and produce main concepts and a reduced ability to manage alternative meanings. Furthermore, when engaging in discourse people with right hemisphere damage, their discourse is often abrupt and perfunctory or verbose and excessive. They can also have pragmatic deficits in situations of turn taking, topic maintenance and shared knowledge.
Lateral brain damage can also have effects on spatial frequency. People with left hemisphere damage are only able to see low frequency, or big picture, parts of an image. Right hemisphere damage causes damage to low spatial frequency, so people with right hemisphere damage can only see the details of an image, or the high frequency parts of an image.
If a specific region of the brain, or even an entire hemisphere, is injured or destroyed, its functions can sometimes be assumed by a neighboring region in the same hemisphere or the corresponding region in the other hemisphere, depending upon the area damaged and the patient's age. When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.
Broca’s aphasia is a specific type of expressive aphasia and is so named due to the aphasia that results from damage or lesions to the Broca’s area of the brain, that exists most commonly in the left inferior frontal hemisp