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The [[human brain]] is separated by a [[longitudinal fissure]], separating the [[brain]] into two distinct [[cerebral hemisphere]]s. BothThe two sides of the brain are similar in appearance, and every structure in each hemisphere is mirrored on boththe sidesother side, yet despite these gross similarities, the functions of each [[cerebral cortex | cortical]] hemisphere are somewhat different. Human [[neopallium | neocortex]]--the outer, visible portions of the brain--is the source of all [[cognition | cognitive]], [[sense | sensory]], [[language | linguistic]], voluntary movement, and [[attention | attentive]] processes. These higher-level functions--as opposed to [[reflex action | reflexes]], [[human position| posture maintenance]], etc.--that seem to preferentially exist in one half of the brain or the other. Deeper (subcortical) structures such as the [[basal ganglia]] and [[thalamus | thalami]] seem to play the same role inon each halfside of the brain, regardless of their hemisphere.
It is important to note that--while functions are indeed lateralized--these lateralizations are trends and do not apply to every person in every case. Short of having undergone a [[hemispherectomy]] (the removal of an entire cerebral hemisphere) there are no "left-brained" or "right-brained" people.
====Wernicke====
[[Germany | German]] [[physician]] [[Karl Wernicke]] followed up on the work done by Broca by studying language deficits unlike those shown by Broca's aphasics. Wernicke noticed that not all deficits were in speech production, but rather linguistic. He found that damage to the left [[Anatomical terms of ___location | posterior]], superior [[temporal lobe | temporal]] [[gyrus]] resulted in deficits in language comprehension rather than speech production. This region is now referred to as [[Wernicke's area]], and the associated syndrome is known as [[Wernicke's aphasia]], for his discovery.
====Advance in imaging technique====
All theseThese seminal works on hemispheric specialization were done on patients and/or postmortem brains, raising questions about the potential impact of pathology on the research findings. New methods permit the ''[[in vivo]]'' comparision of the hemispheres in healthy subjects. Particularly, [[magnetic resonance imaging]] (MRI) and [[positron emission tomography]] (PET) are of importanceimportant because of their high spatial resolution and ability to image subcortical brain structures.
===Handedness and language===
Broca's area and Wernicke’s area are linked by a [[white matter]] fiber tract called the [[arcuate fasciculus]]. This [[axon]]al tract allows the [[neuron]]s in these two areas to work together to create vocal language. Further research indicates that, inIn about 98% of [[right-handed]] [[male]]s and 90-95% of right-handed [[female]]s, language and speech are subserved by the left hemisphere of the brain. In [[left-handed]] people, language is subserved fairly equally by the left, right, or both hemispheres.
There are several methodsways of determining hemisphere dominance in a living [[human]]. The [[Wada test]] involves introducing an [[anesthetic]] into one hemisphere of the brain through one of the two [[carotid artery | carotid arteries]]. Once one hemisphere is anesthetized, and a [[neuropsychology | neuropsychological]] exam is performed to determine dominance for such functions as language production and comprehension, verbal memory, and visual memory. More modern, less invasive, and in some cases costlier techniques, such as [[functional magnetic resonance imaging]] and [[transcranial magnetic stimulation]] can also be used to determine dominance, but their use is controversial and still considered experimental.
===Movement and sensation===
In the [[1940s]], [[Canada | Canadian]] [[neurosurgery | neurosurgeon]] [[Wilder Penfield]] and his [[neurologist]] colleague [[Herbert Jasper]] developed a technique of brain mapping outto help reduce [[Adverse effect (medicine) | side effect]]s caused by [[surgery]] to treat [[epilepsy]]. They stimulated [[motor cortex | motor]] and [[somatosensory cortex | somatosensory cortices]] of the brain with small electrical currents to activate discrete brain regions. They found that stimulation of one hemisphere's motor cortex could produce [[muscle]] contraction on the opposite side of the body. Furthermore, the functional map of the motor and [[Somatosensory system|sensory]] cortices is fairly consistent from person to person; Penfield and Jasper's famous pictures of the motor and sensory [[homunculus|homunculi]] were the result.
===Split brain patients===
Research by [[Michael Gazzaniga]] and [[Roger Wolcott Sperry]] in the [[1960s]] on [[split-brain]] patients led to an even greater understanding of functional laterality. Split-brain patients are patients who have undergone corpus callosotomy (usually as a treatment for severe epilepsy), a severing of the [[corpus callosum]]. The corpus callosum connects the two hemispheres of the brain and allows them to communicate. By severingWhen these connections are cut, the two halves of the brain act independently of one another. This process leadled to many interesting [[behavior]]al phenomena that allowed Gazzaniga and Sperry to researchstudy the contributions of each hemisphere into various cognitive and perceptual processes. One of their main findings was that the right hemisphere was capable of rudimentary lanugage processing, but often has no lexical or grammatical abilities{{an|Kandel}}.
==References==
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