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Friday, February 5, 2021



Association fibers are predominantly located in the cerebral white matter and connect intrahemispheric cortical regions. There are two main types of association fibers, and they are differentiated by size and function. Short association fibers known as arcuate fibers or “U fibers” connect adjacent gyri, thus allowing for communication between neighboring cortical regions. Long association fibers provide the architectural basis for large-scale neurocognitive networks. These networks connect more widespread cortical regions and are visualized as “bundles of fibers” that allow communication between primary and association cortical regions. For instance, the superior longitudinal fasciculus (SLF) (which has three major bundles, I, II, and III) allows communication between the parietal and frontal lobes. In particular, the SLF I allows information from the superior parietal lobe, or motor cortex, to be relayed to the supplementary motor cortex. SLF II connects the caudal parietal region with the prefrontal lobes, thus allowing an individual to have a visual perception of space. SLF III connects rostral parietal areas with the frontal opercular region (the region that controls facial movements), thus enabling an individual to imitate an action. Other long association fibers include the frontooccipital fasciculus, which links the posterior and medial parietal and occipital areas; the uncinate fasciculus (or the anterior limbic fiber bundle), which connects the temporal lobe and frontal lobes; the inferior longitudinal fasciculus, which connects the temporal lobe to the occipital and parietal regions, and the cingulum bundle (or the posterior limbic fiber bundle), which stretches from the frontal lobe to the parahippocampal gyrus.

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The cingulum bundle enables monoamines (dopamine, norepinephrine, and serotonin), along with cholinergic projections, to travel to widespread cortical targets.

Lesions to cortical association bundles can provide clinical relevance to fiber pathway tracts and cortical origins and destinations. For instance, a patient who develops acute damage to the uncinate fasciculus and right anterior frontal cortex (e.g., from a stroke) will have a “disconnection” between the temporal and frontal lobes. This individual may develop amnesia for experiences predating the stroke, along with impairment of self-awareness of personal experiences across time (this clinical finding is also known as a disruption of autonoetic consciousness).

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