Cortical asymmetries in the precuneus and fusiform gyri are associated with visual field asymmetries in word processing: A voxel based analysis Christiana M. Leonard 1, Mark A. Eckert 2, Suzanne E. Welcome 3 & Christine Chiarello 4 University of Florida 1, Medical University of South Carolina 2, University of Western Ontario 3 University of California, Riverside 4 Introduction Method A right visual field/left hemisphere (RVF/LH) advantage is routinely obtained in verbal tasks where words are briefly presented to one or the other visual field. We predicted that individual differences in the strength of this advantage would be associated with individual differences in the asymmetry of Heschl’s gyrus, the planum temporale, pars triangularis and the paracingulate sulcus. This hypothesis found little support in a study that used manual measurements in a large sample of college students (Chiarello 2009). In the present study we used an automated whole brain analysis to revisit the question of how structural asymmetries contribute to visual field asymmetries in accuracy and reaction time. REFERENCES Ashburner J. A fast diffeomorphic image registration algorithm. Neuroimage 2007; 38: Ben-Shachar M, Dougherty RF, Deutsch GK, Wandell BA. Differential sensitivity to words and shapes in ventral occipito-temporal cortex. Cereb Cortex 2007;17: Cohen L, Lehericy S, Chochon F, Lemer C, Rivaud S, Dehaene S. Language-specific tuning of visual cortex? Functional properties of the Visual Word Form Area. Brain. 2002;125: Chiarello C, Welcome SE, Halderman LK, Towler S, Julagay J, Otto R, Leonard CM. A large-scale investigation of lateralization in cortical anatomy and word reading: are there sex differences? Neuropsychology 2009; 23: Graves WW, Desai R, Humphries C, Seidenberg MS, Binder JR. Neural systems for reading aloud: A multiparametric approach. Cereb Cortex 2009; 20: Maisog JM, Einbinder ER, Flowers DL, Turkeltaub PE, Eden GF. A meta-analysis of functional neuroimaging studies of dyslexia. Ann N Y Acad Sci 2008; 1145: Vigneau M, Jobard G, Mazoyer B, Tzourio-Mazoyer N. Word and non-word reading: what role for the Visual Word Form Area? Neuroimage. 2005; 27: Watkins KE, Paus T, Lerch JP, Zijdenbos A, Collins DL, Neelin P, et al. Structural asymmetries in the human brain: a voxel-based statistical analysis of 142 MRI scans. Cereb Cortex 2001; 11: ACKNOWLEDGMENTS This research was supported by NIDCD grant 5R01DC6957 and the McKnight Brain Institute. PARTICIPANTS: 100 male (86 right handers), 100 female (88 right handers) native English speakers, ranging in age from years. PROCEDURES: Seven divided visual field tasks: lexical decision, masked word recognition, word naming, nonword naming, semantic (manmade vs natural) decision, verb and category generation. IMAGE ANALYSIS: Two volumetric MRI scans (1.2 mm thick sagittal images) were collected from each individual on a 1.5T GE Scanner. Scans were co-registered and averaged to improve S/N. Native space and mirror image, flipped, images were segmented with SPM8 Unified Segmentation using ICBM a priori templates. Native space and flipped, segmented images were normalized into symmetrical study-specific space using DARTEL (Ashburner 2007). Normalized, modulated images were smoothed with an 8 mm kernel. Gray and white matter asymmetry images were created by subtracting each scan from its mirror image (Watkins 2001). STATISTICAL ANALYSIS: Asymmetry scores across the seven tasks were calculated separately for accuracy and reaction time. Composite asymmetries were calculated by averaging the individual z-scored task asymmetries. A positive asymmetry indicates faster RT and greater accuracy for words presented in the right visual field. A one-sample t test was conducted to identify gray matter regions with significant asymmetry in order to verify that processing the images with DARTEL produced results similar to those of Watkins (2001). Four separate regression analyses examined the relation between composite reaction time (CRTA) and accuracy asymmetries (CAA) and gray and white matter asymmetries. T values for voxel clusters reported exceed p = 0.001, uncorrected. Summary Results Neither manual nor automated analyses identified any significant associations between VF and perisylvian or paracingulate asymmetries. Reaction time asymmetry was associated with reciprocal gray and white matter asymmetries in the fusiform VWFA and precuneus gyri, areas implicated in reading and dyslexia (Maisog 2008) that have no known population asymmetry. Functional lateralization in VWFA for letters and words varies with the task and contrast (Vigneau 2005; Graves 2009) but not visual field (Cohen 2002), possibly, due to lateralized “top down” signals from language cortex (Ben Shachar 2007). Cohen (2002) found that activation in the left precuneus but not the VWFA was stronger to words presented in the RVF. They speculated that this activation “reflected the attentional component of the RVF advantage”. The present results suggest that one factor contributing to the RVF/LH advantage for reading words may be the asymmetrical development of fibers connecting nodes in a lateralized reading network. Left hemisphere regions where white matter asymmetry was associated with faster reaction times to words presented in the right visual field (p <.001, uncorrected). Clusters are displayed on the symmetrical white matter template. The fusiform region overlaps with the region called the “visual word form area” (VWFA, Cohen 2002). Precuneus Fusiform gyrus Brain regions where structural asymmetry was associated with a RVF/LH advantage for reading words The results of the analyses were remarkably consistent in identifying the fusiform (3/4 regressions) and the precuneus (4/4 regressions) gyri as regions where structural asymmetry was associated with behavioral (VF) asymmetry. The values for fusiform and precuneus asymmetry were not correlated (r =.05, p NS). Three major clusters showed significant gray matter asymmetry. In agreement with Watkins (2001), there were large clusters of significantly asymmetric (L > R) voxels in the superior temporal lobe/insula and occipital cortex (R > L). There was also a L > R cluster in the cerebellum. All three asymmetry clusters can be seen in the figure on the right. For display purposes, a threshold of t(198) = 24.0 (p (FWE-corrected) <.0001) was used. The values for peak gray matter asymmetry in fusiform and precuneus were summed and plotted against the composite RT asymmetry. There is a modest positive relationship throughout the range. Brain regions with significant gray matter asymmetry Heschl’s gyrus/planum/insula (L > R) Occipital (R > L) Cerebellum L > R) Regions with significant gray matter asymmetry in 200 college students. Clusters are thresholded at t (198) = 24.0 and displayed (MRIcron) on the symmetrical study specific gray matter template.