Deconvoluting human Brodmann area 8 based on its unique structural and functional connectivity

Abstract

Brodmann area 8 (BA8) is traditionally defined as the prefrontal region of the human cerebrum just anterior to the premotor cortices and enveloping most of the superior frontal gyrus. Early studies have suggested the frontal eye fields are situated at its most caudal aspect, causing many to consider BA8 as primarily an ocular center which controls contralateral gaze and attention. However, years of refinement in cytoarchitectural studies have challenged this traditional anatomical definition, providing a refined definition of its boundaries with neighboring cortical areas and the presence of meaningful subdivisions. Furthermore, functional imaging studies have suggested its involvement in a diverse number of higher-order functions, such as motor, cognition, and language. Thus, our traditional working definition of BA8 has likely been insufficient to truly understand the complex structural and functional significance of this area. Recently, large-scale multi-modal neuroimaging approaches have allowed for improved mapping of the neural connectivity of the human brain. Insight into the structural and functional connectivity of the brain connectome, comprised of large-scale brain networks, has allowed for greater understanding of complex neurological functioning and pathophysiological diseases states. Simultaneously, the structural and functional connectivity of BA8 has recently been highlighted in various neuroimaging studies and detailed anatomic dissections. However, while Brodmann's nomenclature is still widely used today, such as for clinical discussions and the communication of research findings, the importance of the underlying connectivity of BA8 requires further review.

Keywords: Brodmann area 8; cognition; connectivity; fMRI; network; neuroimaging.

FIGURE 1

Parcellated Human Cerebrum. Panel (A) presents Brodmann’s original atlas. Panel (B) presents the 180 cortical parcellations described by the Human Connectome Project (HCP). The color of each parcellation is based on a 3D color space, reflecting the extent to which each areas is associated in the resting state with auditory (red), somatosensory (green), visual (blue), task positive (white), or task negative (black) groups of areas.

FIGURE 2

Connectivity of BA8. Panel (A) details the anatomical location of each BA8 subdivision, projected onto a left-hemispheric model brain. Panels (B–F) display the structural connectivity of all 5 area 8 subdivisions through tractography, projected onto a sample MRI image: area 8C (B), area 8Av (C), area 8Ad (D), area 8BL (E), and area 8BM (F). Note that there may be parallax error within the projections, given the two-dimensional nature of the images.

FIGURE 3

Network Interactions of BA8. Higher-order cognitive processes like goal directed behavior and motor planning and initiation are likely supported by the connectivity of BA8 to the visual system via the inferior fronto-occipital fasciculus (IFOF) (A) (Conner et al., 2018) and the interaction of BA8 with the salience, default mode, and control networks comprising an initiation axis spanning the middle frontal lobe (Poologaindran et al., 2020) (B). Note that schematic in panel (A) shows the entire start and end points of the IFOF, which include areas 8BM and 8BL. Panel A was reproduced with permission from Conner et al. (2018) and Panel (B) with permission from Poologaindran et al. (2020).