at can be studied to examine and determine the basis and interaction of brain areas in language processes.
The production of language and language comprehension require the coordination of different subprocesses in time. Though there is debate on how these subprocesses work together and how thinking and comprehending can change, the anatomical basis and role of a loop involving Wernicke’s and Broca’s area is usually agreed upon.
Neuroscientists generally agree that around the lateral sulcus (or Sylvian Fissure) in the left hemisphere of the brain, there is a neural loop involved both in understanding and producing spoken language. At the front end or beginning of this loop lies Broca's area, which is usually associated with the production of language, or language outputs. At the other end, or specifically in the superior posterior temporal lobe, lies Wernicke's area, which is associated with the processing of words that we hear being spoken, or language inputs. Broca's area and Wernicke's area are connected by a large bundle of nerve fibres called the arcuate fasciculus.
Handedness and language
Broca's area and Wernicke's area are linked by a white matter fiber tract, the arcuate fasciculus. This axonal tract allows the neurons in the two areas to work together in creating vocal language. In more than 95% of right-handed men, and more than 90% of right-handed women, the left hemisphere is dominant in certain aspects of language and speech processing. In left-handed people, the incidence of left-hemisphere language dominance has been reported as 73% and 61%, suggesting left handed people tend to be less lateralized than right-handed people in language function. In general neuroimaging methods, such as functional magnetic resonance imaging and magnetoencephalography, involvement of both hemispheres in many aspects of language processing has been show. The "dominance" discussed in many of these studies simply refers to more brain activation relative to the other hemisphere (or better performance by that hemisphere on 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 tag