The uncinate fasciculus is a hook-shaped bundle of axons that links anterior portions of the temporal lobe with the inferior frontal gyrus and the lower surfaces of the frontal lobe. It arises in the anteriorior temporal lobe and amygdala,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.. It does not appear to have cell bodies in the hippocampus proper. 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.
Function
The function of the uncinate fasciculus is not known, though it is traditionally considered to be part of the limbic system.. It has been proposed that the uncinate fasciclus allows mnemonic representations stored in the temporal lobe to interact with and guide decision making in the frontal lobe. 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 uncinate fasciculus appears to have a role in some, but not all, types of learning and memory. Reversal learning, in which a stimulus-reward association is learned over many trials, then reversed such that the initial stimulus is no longer is associated with a reward, is associated with individual variation in the uncinate fasciculus . In addition, the ability to learn associations through trial and error, such as the pairing of a name with a face, correlates with uncinate fasciculus microstructure. This relates to early work showing that surgical damage to the uncinate fasciculus is reliably associated with deficits in proper name retrieval.
Development
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 until the age of 30. Developmental disorders with core problems relating to memory retrieval, reward and valuation computation, and impulsive decision making may be linked to aberrations in uncinate fasciculus microstructure, e.g. temporal lobe epilepsy and conduct disorder. Moreover 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.
Clinical significance
Abnormalities within the fiber bundles of the uncinate fasciculus have been inconsistently associated with a host of neuropsychiatric disorders including social anxiety, depression, and schizophrenia. More research using larger sample sizes and superior diffusion imaging methods is needed. More consistent are findings linking abnormalities in the uncinate fasciculus to Alzheimer's disease, semantic dementia, and temporal lobe epilepsy. In 2009 a study implicated this tract in psychopathy—individuals with a high score in the Psychopathy Checklist and an associated history of violent behavior appeared to have abnormalities in it. This relates to the findings noted earlier about conduct disorder. In addition this relates to a classic case study, that of Phineas Gage who had damage to the frontal white matter including the uncinate fasciculus. After the accident, his personality was radically transformed, becoming impulsive, making poor decisions, and failing to follow social norms and conventions.
