Corpus callosum

The corpus callosum is a structure of the mammalian brain in the longitudal fissure that connects the left and right cerebral hemispheres. It is the largest white matter structure in the brain, consisting of 200-250 million contralateral axonal projections. It is a wide, flat bundle of axons beneath the cortex. Much of the inter-hemispheric communication in the brain is conducted across the corpus callosum.

Monotremes and marsupials do not have a corpus callosum.

Regions
The posterior portion of the corpus callosum is called the splenium; the anterior is called the genu (or "knee"); between the two is the body.

The most anterior part is the rostrum.

Sexual dimorphism
In humans, disputed claims have been made about the importance for gender difference of a difference in size between the corpus callosum in males and females, and analogous racial claims. RB Bean, a Philadelphia anatomist, suggested in 1906 that the "exceptional size of the corpus callosum may mean exceptional intellectual activity" and claimed gender differences which were refuted by Franklin Mall, the director of his own laboratory.

Of much more substantial popular impact was a 1982 Science article claiming to be the first report of a reliable sex difference in human brain morphology and arguing for relevance to cognitive gender differences. This paper appears to be the source of a large number of lay explanations of perceived male-female difference in behaviour: for example Time magazine was reported to state in 1992 that the corpus callosum was "Often wider in the brains of women than in those of men, it may allow for greater cross talk between the hemispheres&mdash;possibly the basis for woman’s intution". It has also been used, for example, as the explanation of an increased single-task orientation of male, relative to female, learners; a smaller male corpus is said to make it harder for the left and right sides of the brain to work together and to explain a greater feminine ability to multitask.

The relationship between known gender-specific biology (such as males having, in general, higher testosterone levels than females) and claims about behaviour (such as human males being more competitive) remains a highly contested one. Unusually, the scientific dispute in the case of the corpus callosum is not about the implications of biological difference, but whether such a difference actually exists. A substantial review paper performed a meta-analysis of 49 studies and found, contrary to de Lacoste-Utamsing and Holloway, that males have a larger corpus callosum, a relationship that is true whether or not account is taken of larger male brain size. Bishop and Wahlstein found that "the widespread belief that women have a larger splenium than men and consequently think differently is untenable." However, more recent studies using new techniques revealed morphological sex differences in human corpus callosum. Whether, and to what extent, these morphological differences are associated with behavioural and cognitive differences between males and females is unclear.

Other correlations
The corpus callosum has been reported to be significantly larger in musicians than non-musicians, and to be slightly larger in left-handed people than right-handed people.

Pathology

 * Alien hand syndrome
 * A complete or partial absence of it in humans is called agenesis of the corpus callosum.
 * Split-brain
 * Septo-optic dysplasia (deMorsier syndrome)