The Role of the Corpus Callosum in Transcranial Magnetic Stimulation Induced Interhemispheric Signal Propagation Aristotle N. Voineskos, Faranak Farzan, Mera S. Barr, Nancy J. Lobaugh, Benoit H. Mulsant, Robert Chen, Paul B. Fitzgerald, and Zafiris J. Daskalakis Background: The corpus callosum, the main interhemispheric connection in the brain, may serve to preserve functional asymmetry between homologous cortical regions. Methods: To test this hypothesis, 30 healthy adult subjects underwent combined transcranial magnetic stimulation (TMS)– electroen- cephalography procedures. Nineteen of these subjects also completed diffusion tensor imaging and tractography procedures. We exam- ined the relationship between microstructural integrity of subdivisions of the corpus callosum with TMS-induced interhemispheric signal propagation. Results: We found a significant inverse relationship between microstructural integrity of callosal motor fibers with TMS-induced interhemi- spheric signal propagation from left to right motor cortex. We also found a significant inverse relationship between microstructural integrity of genu fibers of the corpus callosum and TMS-induced interhemispheric signal propagation from left to right dorsolateral prefrontal cortex (DLPFC). We then demonstrated neuroanatomic specificity of these relationships. Conclusions: Taken together, our findings suggest that TMS-induced interhemispheric signal propagation is transcallosally mediated and neuroanatomically specific and support a role for the corpus callosum in preservation of functional asymmetry between homologous cortical regions. Delineation of the relationship between corpus callosum microstructure and interhemispheric signal propagation in neuropsychiatric disorders, such as schizophrenia, may reveal novel mechanisms of pathophysiology. Key Words: Corpus callosum, diffusion tensor imaging (DTI), EEG, interhemispheric, tractography, transcranial magnetic stimulation (TMS) T he biological axiom that “structure determines function” has proved difficult to demonstrate when studying the brains of healthy humans in vivo. In particular, our capability of mea- suring the functional significance of white matter tracts is limited. Various functional roles of the largest and most studied white mat- ter tract in the brain, the corpus callosum, have been primarily inferred from lesion, surgical transection, and agenesis studies (1). In some instances, such as in interhemispheric transfer of visual information, facilitation of communication between hemispheres is essential for optimal function, verified by lesion studies of the sple- nium of the corpus callosum (2). However, handedness and linguis- tic function are examples of operations that are highly lateralized and require hemispheric specialization. Furthermore, performance on higher demand cognitive tasks of executive function can be highly lateralized, with functional imaging demonstrating asym- metry of frontal cortical activation (3,4). Therefore, one important function of neurons projecting through the corpus callosum that connect motor and particularly frontal cortical regions may be to ensure that excessive “crosstalk” does not occur between hemi- spheres (5) during asymmetrical or highly lateralized tasks. Recent advances in neuroimaging now permit sophisticated visualization and measurement of corpus callosum microstructure. Using DTI, microstructure of white matter fibers can be measured by indexing fractional anisotropy (FA) (6). FA reflects the degree to which diffusion of water molecules is restricted by microstructural elements such as cell bodies, axons, myelin, and other constituents of cytoskeleton (7). DTI tractography permits anatomically plausi- ble visualization of white matter tracts in the brain (8) and has led to neuroanatomic advances (9) and reliable quantification (10) of seg- mentation of the corpus callosum. Transcranial magnetic stimulation (TMS) can be used to study potential transcallosal processes noninvasively (11), which may provide insights into the mechanisms leading to hemispheric spe- cialization and corpus callosum function. TMS can be combined with electroencephalography (EEG) to measure changes in cortical- evoked activity, including signal propagation to the contralateral hemisphere, which should be primarily mediated by the corpus callosum. A major advantage of TMS-EEG is that cortical activity can be measured in motor cortex and dorsolateral prefrontal cortex (DLPFC) (12). TMS-EEG indexing from the DLPFC is more broadly relevant to human cognition and neuropsychiatric disorders (13). The purpose of this study, therefore, was to examine the rela- tionship between microstructural integrity of callosal fibers with TMS-induced interhemispheric signal propagation (ISP; the ratio of right to left cortical evoked activity) between the homologous cor- tical regions that these fibers connect. Consistent with a role for the corpus callosum in maintaining laterality and hemispheric special- ization in motor and frontal regions, we hypothesized that 1) micro- structural integrity of callosal motor fibers would be inversely cor- From the Centre for Addiction and Mental Health (ANV, FF, MSB, BHM, ZJD), Department of Psychiatry, University of Toronto, Toronto, Ontario; Cog- nitive Neurology (NJL), Sunnybrook Health Sciences Centre, Depart- ment of Medicine, University of Toronto, Toronto; Division of Neurology (RC), Toronto Western Research Institute, University of Toronto, Toronto, Ontario, Canada; and Alfred Psychiatry Research Centre (PBF), The Alfred and Monash University School of Psychology, Psychiatry and Psycholog- ical Medicine, Commercial Rd. Melbourne, Victoria, Australia. Authors ANV and FF contributed equally to this article. Address Correspondence to: Z. Jeff Daskalakis, M.D., Ph.D., FRCP, Associate Professor of Psychiatry and Director of Brain Stimulation Treatment and Research Program, University of Toronto, Centre for Addiction and Men- tal Health, 250 College Street, Room 713, Toronto, Ontario, Canada M5T1R8; E-mail: Jeff_Daskalakis@camh.net. Received Mar 22, 2010; revised Jun 23, 2010; accepted Jun 24, 2010. BIOL PSYCHIATRY 2010;68:825– 831 0006-3223/$36.00 doi:10.1016/j.biopsych.2010.06.021 © 2010 Society of Biological Psychiatry