Lateralization of Brain Function

Lateralization Of Brain Function

A longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each cortical hemisphere are managed differently. For example, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere.

Broad generalizations are often made in popular psychology about one side or the other having characteristic labels such as "logical" for the left side or "creative" for the right. These labels need to be treated carefully; although a lateral dominance is measurable, these characteristics are in fact existent in both sides, and experimental evidence provides little support for correlating the structural differences between the sides with functional differences.

The extent of any modularity, or specialization of brain function by area, remains under investigation. If a specific region of the brain or even an entire hemisphere is either injured or destroyed, its functions can sometimes be assumed by a neighboring region, even in the opposite 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 come to exist to communicate information with detached areas, despite the inefficiencies.

While functions are lateralized, these are only a tendency. The trend across many individuals may also vary significantly as to how any specific function is implemented. The areas of exploration of this causal or effectual difference of a particular brain function includes its gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere), no one is a "left-brain only" or "right-brain only" person.

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.