The Unique Fiber Anatomy of Middle Temporal Gyrus Default Mode Connectivity

Abstract

Background: The middle temporal gyrus (MTG) is understood to play a role in language-related tasks such as lexical comprehension and semantic cognition. However, a more specific understanding of its key white matter connections could promote the preservation of these functions during neurosurgery.

Objective: To provide a detailed description of the underlying white matter tracts associated with the MTG to improve semantic preservation during neurosurgery.

Methods: Tractography was performed using diffusion imaging obtained from 10 healthy adults from the Human Connectome Project. All tracts were mapped between cerebral hemispheres with a subsequent laterality index calculated based on resultant tract volumes. Ten postmortem dissections were performed for ex vivo validation of the tractography based on qualitative visual agreement.

Results: We identified 2 major white matter bundles leaving the MTG: the inferior longitudinal fasciculus and superior longitudinal fasciculus. In addition to long association fibers, a unique linear sequence of U-shaped fibers was identified, possibly representing a form of visual semantic transfer down the temporal lobe.

Conclusion: We elucidate the underlying fiber-bundle anatomy of the MTG, an area highly involved in the brain's language network. Improved understanding of the unique, underlying white matter connections in and around this area may augment our overall understanding of language processing as well as the involvement of higher order cerebral networks like the default mode network in these functions.

Keywords: Anatomy; DSI; MTG; Middle temporal gyrus; Tractography; White matter.

FIGURE 1.

Superficial anatomy of the middle temporal gyrus (MTG). The dotted lines delineate the boundaries of the MTG. It is separated dorsally from the superior temporal gyrus by the superior temporal sulcus (white dotted line), and ventrally from the inferior temporal gyrus by the inferior temporal sulcus (orange dotted line). The boundary between the MTG and the inferior parietal lobule is marked by an ill-defined line from the terminal descending limb of the lateral fissure. MTG, middle temporal gyrus; STG, superior temporal gyrus; ITG, inferior temporal gyrus; TP, temporal pole; OP, occipital pole; IPL, inferior parietal lobule; postC, postcentral gyrus; preC, precentral gyrus; IFG, inferior frontal gyrus; MFG, middle frontal gyrus; FP, frontal pole.

FIGURE 2.

The superior longitudinal fasciculus (SLF). The SLF, pink and purple tracts on tractography A-D and depicted with and without red lines on dissection E-H, arises within the middle and posterior thirds of the MTG, and to a lesser extent from the anterior temporal pole. The fibers travel craniocaudally then curve 180° at the level of the angular gyrus to course anteriorly toward the frontal lobe, curving around the operculum (*). SLF fibers then diverge at the posterior extent of the frontal lobe and terminate in the posterior portions of the superior, middle, and inferior frontal gyri. In some subjects, fibers terminated at different regions of the operculum. MTG, middle temporal gyrus; ITG, inferior temporal gyrus; STG, superior temporal gyrus; TP, temporal pole; IFG, inferior frontal gyrus; MFG, middle frontal gyrus; SFG, superior frontal gyrus.

FIGURE 3.

Inferior longitudinal fasciculus (ILF). The ILF, cyan and magenta tracts on tractography A-D and depicted with and without blue lines on dissection E-F, arises from the anterolateral aspect of the middle temporal gyrus, and courses posteriorly deep to the SLF, passing deep to the gray matter of the temporo-parieto- occipital junction and lateral to the ventricle. It terminates within the parts of the occipital lobe, including the posterior part of the lingual gyrus and the postero-superior aspect of the cuneus. MTG, middle temporal gyrus; ITG, inferior temporal gyrus; TP, temporal pole; FP, frontal pole.

FIGURE 4.

Short association fibers of the middle temporal gyrus. An abundance of local U-fibers was identified on dissection A and tractography B. These fibers connect the middle temporal gyrus to the superior (green lines) and inferior (orange lines) temporal gyri.

FIGURE 5.

The unique “U”-shaped fiber sequence of the middle temporal gyrus. A sequential arrangement of U-shaped fibers from the posterior aspect of the middle temporal gyrus to the temporal pole is highlighted on tractography A and dissection B-F. These fibers are present bilaterally and run along the lateral aspect of the gyrus, superficial to the SLF and ILF. All images A-F are similarly oriented caudal-rostral.