Uncinate fasciculus

The uncinate fasciculus is a white matter association tract in the human brain that connects parts of the limbic system such as the hippocampus and amygdala in the temporal lobe with frontal ones such as the orbitofrontal cortex. Its function is unknown though it is affected in several psychiatric conditions. It is the last white matter tract to mature in the human brain.Diagram showing principal systems of association fibers in the cerebrum. (Uncinate fasc. visible at lower left, in red.)Tractography showing uncinate fasciculus The uncinate fasciculus is a white matter association tract in the human brain that connects parts of the limbic system such as the hippocampus and amygdala in the temporal lobe with frontal ones such as the orbitofrontal cortex. Its function is unknown though it is affected in several psychiatric conditions. It is the last white matter tract to mature in the human brain. The uncinate fasciculus is a hook-shaped bundle that links the anterior portions of the temporal lobe with the inferior frontal gyrus and the lower surfaces of the frontal lobe. It does this by arising lateral to the amygdala and hippocampus in the temporal lobe curving in an upward pathway behind the external capsule inward of the insular cortex and continuing up into the posterior part of the orbital gyrus. The average length of the uncinate fasciculus is 45 mm with a range 40–49 mm. Its volume in adults is 1425.9±138.6 mm3, being slightly larger in men, at 1504.3±150.4, than women 1378.5±107.4. It has three parts: a ventral or frontal extension, an intermediary segment called the isthmus or insular segment and a temporal or dorsal segment. The function of the uncinate fasciculus is not known, though it is traditionally considered to be part of the limbic system. Diffusion tensor imaging, a reconstruction model available from a diffusion MRI scan, shows a greater fractional anisotropy on the left side than on the right. The difference in this measure of anisotropy has been linked to the left hemispheric specialization for language. However, the use of electrical brain stimulation upon it fails to disrupt language, suggesting it might not be involved in language, though it is possible that this disruption failed to happen because it was functionally compensated by alternative pathways. The capacity for autonoetic self-awareness that is re-experiencing previous events as part of one's past as a continuous entity across time has been linked to the right uncinate fasciculus as has proficiency in auditory-verbal memory and declarative memory to the integrity of the left uncinate fasciculus. The uncinate fasciculus has the longest period of development in terms of fractional anisotropy as it alone amongst the major white fibre tracks continues to develop beyond the age of 30. It seems to be developmentally vulnerable. In 12-year-old males that were preterm, abnormalities measured by fractional anisotropy in the left anterior uncinate correlated with verbal IQ, full-scale IQ, and Peabody Picture Vocabulary Test-Revised scores. In 10-year-old children who have suffered socioemotional deprivation, the left uncinate fasciculus shows reduced fractional anisotropy compared to that in other children, and this might underlie their cognitive, socioemotional, and behavioral difficulties.