Individual Differences in the Anatomy of Broca’s Area Christiana M. Leonard 1, Stephen D. Towler 1, Suzanne E. Welcome 2 & Christine Chiarello 3 University of Florida, Gainesville, 1 University of Western Ontario 2 & University of California, Riverside 3 Introduction Method How language functions are divided among anatomical compartments in Broca’s area is a subject of enduring controversy. Oddly, there appear to have been few attempts to determine how much of the variability in experimental results is due to individual variation in sulcal anatomy. In the human inferior frontal gyrus, the v-shaped pars triangularis (ptr) is bordered by the anterior horizontal ramus (AHR) and the anterior ascending ramus (AAR). Pars opercularis (operc), which sometimes contains the diagonal sulcus (Diag), is found more posteriorly. Pars triangularis is roughly associated with Brodmann’s Area (BA) 45, while pars opercularis is roughly associated with BA 44. Costafreda et al. (2006) have analyzed 17 fmri studies of phonological and semantic fluency and determined the Talairach coordinates for peak BOLD activation. In this study we report the sulcal locations of these consensus coordinates in left hemisphere structural MRI images of 200 healthy college students. REFERENCES Amunts K, Schleicher A, Burgel U, Mohlberg H, Uylings HB, Zilles K. Broca's region revisited: Cytoarchitecture and intersubject variability. J Comp Neurol. 1999;412: Chiarello C, Lombardino LJ, Kacinik NA, Otto R, Leonard CM. Neuroanatomical and behavioral asymmetry in an adult compensated dyslexic. Brain Lang 2006; 98: Costafreda SG, Fu CH, Lee L, Everitt B, Brammer MJ, David AS. A systematic review and quantitative appraisal of fmri studies of verbal fluency: Role of the left inferior frontal gyrus. Hum Brain Mapp. 2006;27: Hickok G, Poeppel D. The cortical organization of speech processing. Nat Rev Neurosci May;8: Keller SS, Crow T, Foundas A, Amunts K, Roberts N. Broca's area: Nomenclature, anatomy, typology and asymmetry. Brain Lang. 2009; 109:29-48.Leonard CM, Towler S, Welcome S, Halderman LK, Otto R, Eckert MA, Chiarello, C. Size matters: Cerebral volume influences sex differences in neuroanatomy. Cereb Cortex. 2008;18: Petrides M, Pandya DN. Distinct parietal and temporal pathways to the homologues of Broca's area in the monkey. PLoS Biol Aug;7:e ACKNOWLEDGMENTS This research was supported by NIDCD grant 5R01DC6957 and the McKnight Brain Institute. Great thanks are due to Emily Jonczak and Kaitlyn Schmutz for their painstaking work. PARTICIPANTS: 100 male (86 right handers), 100 female (88 right handers) native English speakers years of age PROCEDURES: Volumetric MRI scans (1.2 mm thick sagittal images) on 1.5T GE Scanner Brain tissue extracted, reoriented, and segmented into isometric voxels with FSL software Surface and volume measurements made with scripts written in PVWave (Leonard, et al., 2008). Three observers blind to hemisphere and subject characteristics viewed a screen on which 6 evenly spaced sagittal images of the inferior frontal gyrus (Talairach x = 39 – 47) were displayed. White circles appeared at the consensus coordinates for phonological and semantic fluency (Costafreda et al., 2006) (see table). They were instructed to identify the structure at 7 o’clock on the circle (see arrow on Figure A). Results were tabulated in PC-SAS (SAS Institute, Cary NC). Results: Quantitative Small pars triangularis Three individual scans where the consensus coordinates for both phonological and semantic fluency fell in pars opercularis. These three individuals have a small pars triangularis with anterior ascending and anterior horizontal rami that originate in a common stem rather than directly from the Sylvian fissure. In the Costafreda et al study only the superior-inferior z coordinate reliably distinguished the phonological and semantic sites. Rather than concluding that phonological and semantic sites are intermingled we propose that the wide range of AP (y) coordinates stems from variable sulcal anatomy in this region. Interpretation In the monkey (below left), Petrides and Pandya (2009) have shown that temporal lobe connections to BA 45 and 44 (defined by ther cytoarchitectural similarity to human homologues) take somewhat different routes. Some connections to BA 45 come directly through the external capsule, rather than, as traditionally thought, through the arcuate fasciculus. BA 44 (which can not be seen in the image below because it is hidden in the bank of the arcuate sulcus) receives parietotemporal connections through a dorsal pathway. These findings fit with the dual pathway model of Hickok and Poeppel (below right). There was very little overlap between the sulcal locations of the consensus sites for phonological and semantic fluency. Eighty percent of the phonological sites were located in Diag and other sites in pars opercularis, while 68% of the semantic sites were located in AAR other sites in pars triangularis. The distributions were not affected by sex, hand preference, or consistency of hand preference. (Orb: pars orbitalis, located anterior to pars triangularis). A Enlargement of left hand images in B Results: Examples Well developed pars triangularis Three individual scans where the consensus coordinates for phonological fluency (top) fell in pars opercularis and semantic fluency (bottom) fell in the anterior ascending ramus of pars triangularis. Each of these individuals has a well developed pars triangularis. The anterior ascending ramus has been outlined in red to show that the circle that marks the phonological consensus site in the upper images is clearly in pars opercularis. The circle that marks the semantic consensus site in the bottom images is clearly in the anterior ascending ramus of pars triangularis. B Phonological site Semantic site C Phonological site Semantic site From Petrides and Pandya (2009) In the Hickok and Poeppel schematic at right, a strongly lateralized articulatory speech production system (pictured in blue) is distinguished from a higher level (and more bilateral) combinatorial semantic and syntactic system (pictured in pink). The parietotemporal component of the articulatory network connects with posterior regions of Broca’s area through a dorsal pathway (pictured in orange in the Petrides and Pandya image). The anterior temporal component of the combinatorial network connects with anterior regions of Broca’s area through a ventral pathway (pictured in yellow in the Petrides and Pandya image). These findings fit well with the results presented here. Phonological fluency sites are much more likely to be found in pars opercularis (a target of the dorsal articulatory pathway), while semantic fluency sites are found in pars triangularis (a target of the ventral semantic pathway). Even though the boundaries of pars triangularis and pars opercularis do not necessarily coincide with the cytoarchitectonic boundaries of BA 45 and BA 44 (Amunts et al., 1999), the general locations are similar. Individual differences in sulcal shape and location could indicate the results of developmental competition between the dorsal and ventral pathways, and, possibly, the relative dominance of ‘bottom up’ phonological and ‘top down’ semantic networks (Chiarello et al. 2006). Incorporating information on sulcal anatomy and connectivity into computational models might enable predictions about individual differences in linguistic processing. From Hickok and Poeppel (2007) x = x = In figures A and B, pars opercularis and pars triangularis are roughly equal in size and the semantic fluency consensus site lies in pars triangularis. In figure C, pars opercularis is much larger than pars triangularis and the semantic fluency consensus site lies in pars opercularis. x = x = -42.6