Lateralization of brain function

This article is about specialization of function between the left and right hemispheres of the brain. For specialization of brain function generally, see Functional specialization (brain).

The human brain is divided into two hemispheres–left and right. Scientists continue to explore how some cognitive functions tend to be dominated by one side or the other; that is, how they are lateralized.

The longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The structure and function of both hemispheres is very similar. Large parts exist n both parts, and are bilaterally symmetrical. A structural example of this is the lateral sulcus which is generally is longer in the left hemisphere than in the right one. Functionally, Broca's area and Wernicke's area are located in the left cerebral hemisphere for about 95% of right-handers, but about 70% of left-handers.

Popular psychology often makes broad generalisations about one side or the other, such as "logical thinking" being in the left side, or "creativity" being in the right. These generalisations are not supported by studies on lateralization, as lateralization does not add specialized usage from either hemisphere. Both hemispheres contribute to both kinds of processes, and experimental evidence provides little support for correlating the structural differences between the sides with such broadly-defined 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 injured or destroyed, its functions can sometimes be assumed by a neighboring region in the same hemisphere or the corresponding region in the other hemisphere; this effect depends upon the area damaged and the patient's age. When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

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.

Even though some functions are lateralized, the examples above only show a tendency. The trend across many individuals may also vary significantly as to how a specific function is implemented. The areas which are being explored to explain these differences include the gross anatomy of the brain, its 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.