Horizontal portion of arcuate fasciculus fibers track to pars opercularis, not pars triangularis, in right and left hemispheres: a DTI study

Abstract

The arcuate fasciculus (AF) is a white matter pathway traditionally considered to connect left Broca's area with posterior language zones. We utilized diffusion tensor imaging (DTI) in eight healthy subjects (5 M) to track pathways in the horizontal mid-portion of the AF (hAF) to subregions of Broca's area - pars triangularis (PTr) and pars opercularis (POp); and to ventral premotor cortex (vPMC) in the right and left hemispheres (RH, LH). These pathways have previously been studied in the LH, but not in the RH. Only 1/8 subjects showed fiber tracts between PTr and hAF in the RH (also, only 1/8 in the LH). In contrast to PTr, 5/8 subjects showed fiber tracts between POp and hAF in the RH (8/8 in the LH). Fiber tracts for vPMC were similar to those of POp, where 7/8 subjects showed fiber tracts between vPMC and hAF in the RH (8/8 in the LH). Our designated hAF could have included some of the superior longitudinal fasciculus (SLF) III, because it is difficult to separate the two fiber bundles. The SLF III has been previously reported to connect supramarginal gyrus with POp and vPMC in the LH. Thus, although the present DTI study showed almost no pathways between PTr and hAF in the RH (and in the LH), robust pathways were observed between POp and/or vPMC with hAF in the RH (and in LH). These results replicate previous studies for the LH, but are new, for the RH. They could contribute to better understanding of recovery in aphasia.

Published by Elsevier Inc.

Figures

Figure 1

Aphasia case reported by Barlow (1877) showing detailed drawing of lesion in POp and vPMC following initial L hemisphere stroke. The second stroke (a few months later) occurred in identical areas in the RH; there was no recovery of speech. The L and R PTr remained intact, suggesting possibly different roles for POp and PTr in aphasia recovery. Permission to reprint would need to obtained from The British Medical Journal, for July 28, 1877, page 104.

Figure 2

(A) Sulcal landmarks used to define boundaries and location of the four cortical ROIs. The ascending vertical ramus was used to delimit PTr from POp in Broca’s area. (B) Location of the four cortical ROIs in the LH and RH are superimposed on one of several sample sagittal slices used for ROI tracings. LH sample ROIs are for subject N4; RH sample ROIs are for subject N1. Colors: red, anterior PTr; yellow, posterior PTr; light blue, POp; and dark blue, ventral premotor cortex. Abbreviations: PTr, pars triangularis; POp, pars opercularis; ROI, region of interest; LH, left hemisphere; RH, right hemisphere.

Figure 3

Illustration of method used for locating the hAF seedpoint (pink) in sagittal, coronal and axial views in the LH (subject N4). The hAF seedpoint was drawn on 5–7 coronal slices in the horizontal mid-portion of the AF fibers oriented in the anterior-posterior direction (green). The AF seedpoint was located superior to the insula, extreme capsule, claustrum, external capsule, and internal capsule. A similar hAF seedpoint was drawn in the RH (not shown). (A) Fractional Anisotropy Color Maps; (B) Structural MRI Images. Abbreviations: hAF, horizontal mid-portion of the arcuate fasciculus; LH, left hemisphere; RH, right hemisphere.

Figure 4

Fiber tracts between the horizontal mid-portion of the AF (hAF) seedpoint (pink) and the separate cortical ROI seedpoints in the LH and RH for each subject. The cortical seedpoints are anterior PTr (red); posterior PTr (yellow); POp (light blue); and vPMC (dark blue). The fiber tracts between hAF and POp are shown in green; and those between hAF and POp, orange. Fiber tracts between hAF and posterior PTr (present only in N3) are shown in white (shown on the same slice as those for vPMC, to conserve space; tractography was performed separately between hAF and each cortical ROI seedpoint). There were no fiber tracts between hAF and A-PTr in either hemisphere for any subjects. See Table 1. Additional axial views are provided for subjects N4 and N5, showing fiber tracts between the hAF and R POp, where view of R POp was obscured by presence of R vPMC on other axial views above, for these two subjects. Abbreviations: hAF, horizontal, mid-portion arcuate fasciculus; POp, pars opercularis; PTr, pars triangularis; vPMC, ventral premotor cortex.