A Connectomic Atlas of the Human Cerebrum-Chapter 12: Tractographic Description of the Middle Longitudinal Fasciculus

Abstract

The middle longitudinal fasciculus (MdLF) is a small and somewhat controversial white matter tract of the human cerebrum, confined to the posterior superior temporal region from which it courses posteriorly to connect at the occipital-parietal interface. The tract appears to be involved in language processing as well as auditory organization and localization, while sub-serving other higher level cognitive functions that have yet to be fully elucidated. Little is known about the specific, interparcellation connections that integrate to form the MdLF. Utilizing diffusion spectrum magnetic resonance imaging tractography coupled with the human cortex parcellation data presented earlier in this supplement, we aim to describe the macro-connectome of the MdLF in relation to the linked parcellations present within the human cortex. The purpose of this study is to present this information in an indexed, illustrated, and tractographically aided series of figures and tables for anatomic and clinical reference.

FIGURE 1.

Simplified tract map showing the structural connections that integrate within the MdLF. Connections between cortical areas are color-coded based on the parcellation of origin (eg yellow arrows indicate structural connections from origin A5 to areas V7, V6a, V6, V4, and V3). Note that arrows are not meant to imply the direction of information transmit.

FIGURE 2.

MdLF connections from superior temporal lobe parcellations A and B, Mbelt, C and D, PBelt, E and F, A4 in the left cerebral hemisphere. Connections are shown on T1-weighted magnetic resonance (MR) images in the sagittal and axial planes. All three cortical areas demonstrate structural connections to different parietal lobe parcellations, including IP0, IP1, MIP, VIP, IPS1, and 7PL. The MdLF is readily seen in this figure coursing from the superior temporal lobe posteriorly and superiorly to terminate in the parietal lobe.

FIGURE 3.

MdLF connections from area STSda to visual processing regions V1, V2, V3, V4, and V7. Connections are shown on T1-weighted MR images in the A and C, sagittal and B and D, axial planes. The MdLF is readily seen coursing from the superior temporal lobe posteriorly and superiorly to terminate in parts of the occipital lobe.

FIGURE 4.

MdLF connections from area STSva to early processing regions V1, V2, V3, V3a, V4, and V7. Connections are shown on T1-weighted MR images in the A, B, and E, sagittal and C, D, and F, axial planes. The MdLF is readily seen coursing from the superior temporal lobe posteriorly and superiorly to terminate in parts of the occipital lobe.