Detailed Examination of the Sulci and Gyri Surrounding Broca's Area: A Comparative Morphological Study of Japanese and North American Populations

抄録

The inferior frontal cortex (IFC), which includes Broca's area, is a frequent target in neurosurgical procedures, yet its sulcal landmarks remain under-characterized across populations. We evaluated 7 IFC sulci-the inferior precentral sulcus (prcs-i), inferior frontal sulcus (ifs), triangular sulcus (ts), diagonal sulcus (ds), pretriangular sulcus (prts), and the ascending (aaLF) and horizontal (haLF) anterior rami of the lateral fissure-using high-resolution magnetic resonance imaging (MRI) data from 40 Japanese and 40 North American White adults, supplemented by 9 cadaveric hemispheres. Sulci were manually labeled in FreeSurfer, quantified for surface area and depth, and analyzed for interhemispheric and inter-racial differences. Maximum probability maps (MPMs) were projected against meta-analytic functional MRI maps to evaluate structure-function relationships. The prcs-i and haLF were universally present, whereas the ds showed the lowest incidence (≈55%-60% left, ≈75% right), with variability across studies partly explained by misclassification with the aaLF. The prts was also infrequent and highly variable, whereas the ts, although structurally diverse, was nearly universal. Quantitative analyses confirmed the aaLF was significantly deeper than the ds (p < 0.0001), providing an objective criterion for differentiation. Functionally, high-frequency sulci (aaLF, haLF, ifs, prcs-i, ts) aligned with reliable activation patterns for verbal fluency, semantics, and syntax, whereas the ds frequently coincided with hotspots despite inconsistent presence. These findings can offer anatomical insights that may guide future clinical research on the frontal language area.