CASE REPORT J Neurosurg Pediatr 22:214–219, 2018 F unctional connectivity describes coincident activ- ity between brain regions. This may be the result of direct synchrony between brain regions, or of a common input similarly infuencing multiple brain re- gions. It may support synaptic formation and maintenance, or provide a mechanism for cognitive integration and in- formation transfer. 11,27 The corpus callosum provides di- rect anatomical connections mostly between homologous regions, but its role in maintaining functional connectivity has not been fully established. Homologous sites in op- posite hemispheres are generally robustly synchronized, which can be observed with both functional MRI (fMRI) and electrophysiological methods. 12,13,23–25,31 Synchrony of apparently spontaneous origin exists at “rest,” independent of any stimuli. 32 Corpus callosotomy is performed to reduce the severity of drug-resistant epilepsy, particularly of drop attacks. The corpus callosum may be completely disconnected, or par- tially disconnected in the anterior or posterior portion. 16 Approximately 75% of patients experience a reduction in frequency or severity of seizures after anterior calloso- tomy. 26 After partial or complete disconnection of the cor- pus callosum, patients experience a reduction in cogni- tive integration, especially for language and visuospatial processing, which are particularly lateralized functions. 3 Complete disconnection generally results in acute disrup- tion of interhemispheric synchrony between homologous sites, both shortly after the procedure 1,6,21,32 and during the course of the procedure itself, as measured with intraop- erative scalp electroencephalography (EEG) 14 and with electrocorticography (ECoG). 21 ABBREVIATIONS amHG = amplitude modulation of the high gamma band; BOLD = blood oxygen level–dependent; DTI = diffusion tensor imaging; ECoG = electrocorti- cography; EEG = electroencephalography; fMRI = functional MRI; HG = high gamma; PLV = phase-locking value; VNS = vagus nerve stimulator. SUBMITTED October 9, 2017. ACCEPTED February 13, 2018. INCLUDE WHEN CITING Published online May 18, 2018; DOI: 10.3171/2018.2.PEDS17549. Preservation of electrophysiological functional connectivity after partial corpus callosotomy: case report Kaitlyn Casimo, BA, 1,2 Fabio Grassia, MD, 3 Sandra L. Poliachik, PhD, 3–5 Edward Novotny, MD, 6,7 Andrew Poliakov, PhD, 3,4 and Jeffrey G. Ojemann, MD 1,2,8 1 Graduate Program in Neuroscience and 2 Center for Sensorimotor Neural Engineering, University of Washington, Seattle, Washington; 3 Department of Neurosurgery, University of Milan, San Gerardo Hospital, Monza, Italy; and 4 Department of Radiology, 5 Center for Clinical and Translational Research, 6 Department of Neurology, 7 Center for Integrated Brain Research, and 8 Department of Neurosurgery, Seattle Children’s Hospital, Seattle, Washington Prior studies of functional connectivity following callosotomy have disagreed in the observed effects on interhemispheric functional connectivity. These connectivity studies, in multiple electrophysiological methods and functional MRI, have found conficting reductions in connectivity or patterns resembling typical individuals. The authors examined a case of partial anterior corpus callosum connection, where pairs of bilateral electrocorticographic electrodes had been placed over homologous regions in the left and right hemispheres. They sorted electrode pairs by whether their direct corpus callosum connection had been disconnected or preserved using diffusion tensor imaging and native anatomical MRI, and they estimated functional connectivity between pairs of electrodes over homologous regions using phase-locking value. They found no signifcant differences in any frequency band between pairs of electrodes that had their corpus callosum connection disconnected and those that had an intact connection. The authors’ results may imply that the corpus callosum is not an obligatory mediator of connectivity between homologous sites in opposite hemispheres. This interhemispheric synchronization may also be linked to disruption of seizure activity. https://thejns.org/doi/abs/10.3171/2018.2.PEDS17549 KEYWORDS functional connectivity; corpus callosum; partial corpus callosotomy; electrophysiology; electrocorticography; epilepsy J Neurosurg Pediatr Volume 22 • August 2018 214 ©AANS 2018, except where prohibited by US copyright law