dominantly in the RHS, most notably semantic choice. These results hold for those with RHS damage who show a similar amount of functionality in specific motor skills via new neural connections developed in the LHS after RHS damage. Amazingly these shifts continue to maximize brain potential even after extensive damage. In studies where patients incur LHS damage and functionality is shifted to RHS, damage of the same magnitude to the RHS causes RHS functionality to become split once more. This effect has been shown on a neuronal level as the stimulation of specific neurons in the RHS causes similar responses in both hemispheres as neuron clusters on each side enlarge to compensate for the initial stimulation. This points to a sort of equilibrium observed by the hemispheres.
The shift of info and neuron functionality between hemispheres should not be surprising, however, as it has been observed in individuals who have lost a sense. In these individuals, neurons, even those that are specialized, are semi-repurposed to compensate for the loss. For example: those who become blind after years of vision are able to repurpose specialized sections of their brain to have increased visualization and nonspecialization to aid in the efficiency of their other senses.
History of research on lateralization
One of the first indications of brain function lateralization resulted from the research of French physician Pierre Paul Broca, in 1861. His research involved the male patient nicknamed "Tan", who suffered a speech deficit (aphasia); "tan" was one of the few words he could articulate, hence his nickname. In Tan's autopsy, Broca determined he had a syphilitic lesion in the left cerebral hemisphere. This left frontal lobe brain area (Broca's area) is an important speech production region. The motor aspects of speech production deficits caused by damage to Broca’s area are known as expressive aphasia. In clinical assessment of this aphasia, it is noted that the patient cannot clearly articulate the language being employed.
German physician Karl Wernicke continued in the vein of Broca's research by studying language deficits unlike expressive aphasia. Wernicke noted that not every deficit was in speech production; some were linguistic. He found that damage to the left posterior, superior temporal gyrus (Wernicke's area) caused language comprehension deficits rather than speech production deficits, a syndrome known as receptive aphasia.
Advance in imaging technique
These seminal works on hemispheric specialization were done on patients or postmortem brains, raising questions about the potential impact of pathology on the research findings. New methods permit the in vivo comparison of the hemispheres in healthy subjects. Particularly, magnetic resonance imaging (MRI) and positron emission tomography (PET) are important because of their high spatial resolution and ability to image subcortical brain structures.
Movement and sensation
In the 1940s, neurosurgeon Wilder Penfield and his neurologist colleague Herbert Jasper developed a technique of brain mapping to help reduce side effects caused by surgery to treat epilepsy. They stimulated motor and somatosensory cortices of the brain with small electrical currents to activate discrete brain regions. They found that stimulation of one hemisphere's motor cortex produces muscle contraction on the opposite side of the body. Furthermore, the functional map of the motor and sensory cortices is fairly consistent from person to person; Penfield and Jasper's famous pictures of the motor and sensory homunculi were the result.
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 a large part of the corpus callosum. The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a r