Corpus Callosum: Does size really matter?

Menstuff® has compiled the following information on the Corpus Callosum, the massive formation of nerve fibers bridging the two cerebral hemispheres. Most (but certainly not all) communication between regions in different halves of the brain are carried over the corpus callosum.

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 (Bishop and Wahlsten, 1997).

Of much more substantial popular impact was a 1982 Science article (de Lacoste-Utamsing and Holloway) 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 Newsweek stated 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—possibly the basis for woman’s intuition”. 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 organ is said to make it harder for the left and right sides of the brain to work together and to explain a 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 (Bishop and Wahlsten, 1997) 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 "(t)he widespread belief that women have a larger splenium than men and consequently think differently is untenable."

There is no current evidence that difference in male and female cognitive behaviour can be explained by differences in the size of the corpus callosum.


The corpus callosum is a very thick bundle of nerve fibers containing both myelinated and unmyelinated axons. In the 18th century, the corpus callosum was considered the site of the soul (Maurice Ptito), and in the early 20th it was assigned the mere role of preventing the cerebral hemispheres from collapsing onto each other. It was only in the 1950's that the corpus callosum, in the pioneering work of Myers and Sperry, was attributed the function of transferal of information between the two hemispheres. This was followed by the development, in the early 1960's, of a surgical intervention aimed at reducing the interhemispheric transmission of abnormal electrical discharges in epileptic patients. This involved the sectioning of the corpus callosum and other commissural structures of severe epileptic patients in which drug treatment was ineffective. The study of these patients by Sperry, Gazzaniga, and collaborators have greatly contributed to our knowledge of the functions of this midline structure. Even today the corpus callosum is still the focus interest for many neuroscientists who study interhemispheric communication.


Each hemisphere contains neurons which project callosal axons not only to homologous (homotopic) areas in the contralateral hemisphere but also to heterologous (heterotopic) areas. There are approximately 200,000,000 callosal axons in humans! The corpus callosum is undoubtedly the most important commissure to connect the two hemispheres, not only by virtue of its size, but also due to the wealth of its neural connections. It is through these projections that information is shared between the two halves of the brain. Very little is known about the neural signals that pass between the hemispheres, but recent studies have used modern tract tracing techniques to determine precisely the sites of origin and termination of neurons which project across the corpus callosum. Using a retrograde tracer (horseradish peroxidase), Lomber et al. (1994) were able to link the functional divisions of the cerebral cortex to fiber trajectory through the corpus callosum. The motor cortex sends fibers through the rostrum and genu of the corpus callosum. The adjacent somatosensory cortex projects fibers through the anterior half of the corpus callosum whereas axons arising from auditory regions pass through the posterior two-thirds of the corpus callosum and the dorsal splenium. Axons from the limbic cortex also help to form these regions of the corpus callosum. Finally, axons from visual cortices which occupy the greatest single fraction of the cortical mantle pass through the largest portion of the corpus callosum; the fibers are present throughout the splenium and extend well into the body and the anterior portion. Ramifications of the axonal breakdown of the corpus callosum are discussed in out section on new developements.

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