Language (symbolic representation and communication) in a general sense, includes sign language used by deaf persons (Ursula Bellugi, 1989).
(See motor and sensory aphasia and location of the areas described in sec. )
Anterior speech cortex (in left frontal cortex - Broca's area): Production, i.e. representation of motor commands for speech (and probably signing).
Posterior speech cortex (in left temporoparietal cortex - includes Wernicke's area): Comprehension, i.e. sensory representations of words and symbols.
Superior speech cortex (supplementary motor): Physical production of speech (different for sign language?)
See Geshwind's classification of Aphasias in Neuroscience p. 486.
Roger Sperry, 1960s and '70s: Confirmation of language lateralisation by split-brain studies - patients with corpus callosum and anterior commissure severed for treatment of severe epilepsy. (Several other minor ``commissures'' also connect the two hemispheres and their effect has since been studied - see Gazzaniga in Scientific American, July 1998.)
For normal subject, tachistoscopic presentation of stimulus (either visual or tactile) selectively to one hemisphere - when the (left eye or hand, i.e.) right hemisphere receives the instruction, verbal response is delayed (? this was done not by Sperry but later by Bryden, 1982).
Split-brain subjects could not carry out this task at all. Using the right hemisphere they could give only an indirect description of the object - ``a round thing'' instead of ``a ball''. For the right hemisphere written commands were more effective than oral but response could be given only nonverbally. (check)
The right hemisphere outperformed the left in the following experiment: picture shown in left visual field and felt with the left hand (i.e. synthesising of visual and tactile stimulus by the right hemisphere was better than the same task done by the left hemisphere.) (Also performance on Gestalt tasks?)
(Michael Gazzaniga was a student of Roger Sperry. See his two articles in the Scientific American: The Split Brain, ... and The Split Brain Revisited, July 1998, pp. 51-55. Also, Gazzaniga, Michael S. (1985) The Social Brain: Discovering the Networks of the Mind Basic Books, New York.)
Plasticity: Patients who were younger at the time of the operation, by adulthood developed the missing capacities in both hemispheres. In fact even with the loss of the entire left hemisphere in early life language abilities do develop, though it may be at the cost of some spatial abilities and some syntactic features of language may still be missing.
Juhn Wada (1960s, Montreal Neurological Institute): With normal subjects - inject sodium amytal, a short-acting anaesthetic, into one carotid artery, eg. the left one, to anaesthetise the left cerebral hemisphere. In this case, the subject becomes transiently aphasic.
PET and fMRI scans have confirmed these results.
George Ojemann and group (University of Washington, 1989) By electrical stimulation and mapping confirmed earlier conclusions from lesion research - that a large region of the (perisylvian) cortex of the left hemisphere is involved in language production and comprehension. But they also found large unexplained variation between individuals. Sampler - bilingual patients did not necessarily use the same cortical area for storing the names of the same object in two different languages! Aphasias in bilingual persons have been found to be restricted to only one or other of the languages, or different syndromes might be seen in the two languages.
Marcus Raichle, Steve Peterson and others at the University of Washington have found (1994) from PET scans that large areas of both hemispheres are activated in word recognition or production tasks (scans of listening to words, passively viewing words, speaking words and generating words - see Science article, also Neuroscience p. 491, fig. 25.6)
The major language functions are carried out by the left hemisphere, though the right hemisphere does produce rudimentary language and also contributes emotional context to language. + visuo-spatial abilities.
Damage to the areas in the right hemisphere, corresponding to Broca's and Wernicke's areas in the left, leads to language deficits called ``aprosodias'': absense of the emotional and tonal i.e. ``prosodic'' elements in language.
Lateralisation of brain function: See Neuroscience p. 487 fig. 28.3 B (This division holds in 97% of the population, ``including the majority of left-handers'' i.e. same incidence? Evidence of relation between handedness and lateralisation, and specifically between left-handedness and neuropathology, reading disorders etc. seems as yet inconclusive?)
The popular view:
Left hemisphere: verbal, sequential and analytic skills; digital processing
Right hemisphere: spatial, holistic and synthetic skills; analog processing
Actually, most everyday activities are complex enough that regions within each hemisphere contribute to it; the interaction is dynamical.
For Memory See Neuroscience Chapter 29; summary diagram p. 559, fig. 29.8. More in sec. .
See also Brain development and Plasticity in the adult nervous system in Neuroscience.