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Superior and Inferior Longitudinal Fasciculi

Surgical Correlation

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Superior and Inferior Longitudinal Fasciculi. A, Removal of the cortical gray matter and adjacent superficial short fibers of the frontal, temporal, and parietal opercula, the middle frontal, superior temporal, and middle temporal gyri, and the inferior parietal lobule exposes the superior longitudinal fasciculus arching around the outer edges of the insula. The superior parietal lobule and the pars orbicularis of the inferior frontal gyrus are intact. Long fibers are observed descending from the frontal opercula, precentral and postcentral gyri, inferior parietal lobule, and transverse superior and middle temporal gyri. B, Tractographic reconstruction of the superior longitudinal fasciculus. The location of the superior parietal lobule is displayed to facilitate the comparison with Figure 3A. The ROI is selected in the color-coded DTI axial map at the most lateral fasciculus with an anteroposterior orientation (green) and at the deep level of the inferior parietal lobule (inset). C, Further step in the dissection of the superior longitudinal fasciculus. Many of the frontal fibers of the superior longitudinal fasciculus end at the region of the inferior parietal lobule, forming the frontoparietal or horizontal segment of the superior longitudinal fasciculus. At the level of the temporoparietal junction area, and at approximately 20 to 25 mm from the cortical surface, a group of vertically oriented fibers travels between the posterior part of the middle and superior temporal gyri and the inferior parietal lobule region to form the temporoparietal or vertical segment of the superior longitudinal fasciculus. At a deeper level in the temporoparietal area, a group of fibers arches around the posterosuperior insular border connecting the posterior temporal region and the prefrontal area to form the frontotemporal or arcuate segment of the superior longitudinal fasciculus. D, Tractographic reconstruction of the superior longitudinal fasciculus after selection of different ROIs (inset) at the deep level of the middle frontal (green), inferior parietal (yellow), and posterior temporal regions (red). A segmentation pattern in frontoparietal (green), frontotemporal (yellow), and temporoparietal (red) parts, as described in C, is observed. The supramarginal gyrus, a high order association cortical area, is the intermediate station between the frontal and temporal cortices connected by the superior longitudinal fasciculus. E, Left hemisphere. Activation map of functional MRI during verb generation in a healthy volunteer. The task involves different aspects of language: audition, comprehension, selection of an appropriate response, and motor speech response. Cortical activations are observed in the inferior frontal gyrus (pars triangularis and opercularis), pre- and postcentral gyrus, supramarginal gyrus, and posterior superior temporal gyrus and sulcus. The superior longitudinal fasciculus provides anatomic interconnection between these cortical areas, as shown in A–D. F, Left hemisphere. Activation map of functional MRI during verb generation in a different healthy volunteer. The anatomic image shows the white matter deep to the inferior frontal gyrus, inferior parietal lobule, and superior temporal gyrus. This white matter arches around the outer edges of the insula and corresponds to the superior longitudinal fasciculus. Cortical activations are observed in the inferior frontal gyrus (pars orbicularis, triangularis, and opercularis); precentral, supramarginal, and angular gyrus; anterior and posterior superior temporal gyrus and sulcus; and insular apex. The superior longitudinal fasciculus is the anatomic substrate of a high-order multisensory associative system that coordinates various inputs as required in higher human brain functions such as language in the dominant hemisphere. G, The superior longitudinal fasciculus courses superficial to the corona radiata and external capsule. The central insular sulcus separates the insula into larger anterior and smaller posterior portions. The anterior portion consists of three short gyri (anterior, middle, and posterior) arranged in a radiating pattern that converges at the insular pole located at the anteroinferior edge of the short insular gyri. The anterior and posterior long gyri extend backward and upward from the limen insulae. The distal portion of the auditory radiation courses inside Heschl’s gyrus. The gray matter and superficial short fibers of the inferotemporal and temporo-occipital gyrus have been removed to expose the inferior longitudinal fasciculus, which runs from the anterobasal temporal region to the occipital lobe. H, Tractographic reconstruction of the inferior and superior longitudinal fasciculi. For the reconstruction of the inferior longitudinal fasciculus, an ROI (blue) slightly lateral to the inferolateral wall of the temporal horn and in the deep white matter of the inferior temporal and fusiform gyri, is selected (inset). The inferior longitudinal fasciculus (blue) courses deep to the superior longitudinal fasciculus (green), and runs from the anterobasal temporal region to the occipital lobe. (Images courtesy of E de Oliveira)

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