|Brain: Anterior commissure|
|Coronal section of brain through anterior commissure. (Label for "anterior commissure" is on left, third from bottom.)|
|The hypophysis cerebri in position. Shown in sagittal section. (Caption for anterior commissure is at center top.)|
|Gray's||subject #189 840|
The anterior commissure (also known as the precommissure) is a bundle of nerve fibers (white matter), connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix. The great majority of fibers connecting the two hemispheres travel through the corpus callosum, which is over 10 times larger than the anterior commissure, and other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals. The anterior commissure plays a key role in pain and pain sensation, more specifically sharp, acute pain. It also contains decussating fibers from the olfactory tracts, vital for the sense of smell and chemoreception. The anterior commissure works with the posterior commissure to link the two cerebral hemispheres of the brain and also interconnects the amygdalas and temporal lobes, contributing to the role of memory, emotion, speech and hearing. It also is involved in olfaction, instinct, and sexual behavior.
In a sagittal section, the anterior commissure is oval in shape, having a long vertical axis that measures about 5 mm.
The fibers of the anterior commissure can be traced laterally and posteriorly on either side beneath the corpus striatum into the substance of the temporal lobe.
It serves in this way to connect the two temporal lobes, but it also contains decussating fibers from the olfactory tracts, and is a part of the neospinothalamic tract for pain. The anterior commissure also serves to connect the two amygdala.
The corpus callosum allows for communication between the two hemispheres and is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish. The anterior commissure serves as the primary mode of interhemispheric communication in marsupials, and which carries all the commissural fibers arising from the neocortex (also known as the neopallium), whereas in placental mammals the anterior commissure carries only some of these fibers).
In 1992 Laura Allen and Roger Gorski of UCLA measured the anterior commissures of 30 homosexual men, 30 heterosexual men, and 30 heterosexual women. They found that all three groups' commissures were significantly different from one another, with homosexual males having the largest anterior commissure, followed by heterosexual women, and then heterosexual men, who had the smallest anterior commissures.
In 1993, a review by Byne and Parsons criticized this research, noting that 27 of the 33 homosexual males fell within the range of heterosexual males in the study. However, because range is defined only by the two most extreme data points in a group, the existence of a single heterosexual male with an exceptionally large anterior commisure for his group (an outlier ) would cause this large range irrespective of the data from the rest of the individuals in the group. This individual's existence would not change the fact that the groups on average were quite different from one another, and that these differences were statistically significant.
A later report by Byne et al. (2001) noted that
We also measured the anterior commissure in the same blocks of tissue used for the present hypothalamic study (data not shown) and were unable to replicate a report [by Allen and Gorski] that its cross-sectional area is larger in women than in men.
Also, a study by Lasco et al. (2002) said:
We examined the cross-sectional area of the AC in postmortem material from 120 individuals, and found no variation in the size of the AC with age, HIV status, sex, or sexual orientation.
See also 
- Ashwell, Ken (2010). The Neurobiology of Australian Marsupials: Brain Evolution in the Other Mammalian Radiation, p. 50
- Armati, Patricia J., Chris R. Dickman, and Ian D. Hume (2006). Marsupials, p. 175
- Butler, Ann B., and William Hodos (2005). Comparative Vertebrate Neuroanatomy: Evolution and Adaptation, p. 361
- Allen, LS; Gorski, RA (1992 Aug 1). "Sexual orientation and the size of the anterior commissure in the human brain.". Proceedings of the National Academy of Sciences of the United States of America 89 (15): 7199–202. doi:10.1073/pnas.89.15.7199. PMC 49673. PMID 1496013.
- Byne W., Parsons B. (1993). "Human sexual orientation: The biological theories reappraised". Archives of General Psychiatry 50: 228–239.
- William Byne, Stuart Tobet, Linda A. Mattiace, Mitchell S. Lasco, Eileen Kemether, Mark A. Edgar, Susan Morgello, Monte S. Buchsbaum, and Liesl B. Jones, The Interstitial Nuclei of the Human Anterior Hypothalamus: An Investigation of Variation with Sex, Sexual Orientation, and HIV Status. Hormones and Behavior 40, 86–92 (2001)
- Lasco MS, Jordan TJ, Edgar MA, Petito CK, Byne W., A lack of dimorphism of sex or sexual orientation in the human anterior commissure. Brain Res. 2002 May 17;936(1-2):95-8.
Additional images 
|Wikimedia Commons has media related to: Anterior commissure|
- Overview at University of Cambridge
- Roche Lexicon - illustrated navigator, at Elsevier 13048.000-3
- Sexual orientation and the size of the anterior commissure in the human brain
- NIF Search - Anterior Commissure via the Neuroscience Information Framework