Four-dimensional tractography animates neural propagations via distinct interhemispheric pathways

Objective To visualize and validate the dynamics of interhemispheric neural propagations induced by single-pulse electrical stimulation (SPES). Methods This methodological study included three patients with drug-resistant focal epilepsy who underwent measurement of cortico-cortical spectral responses (CCSRs) during bilateral stereo-electroencephalography recording. We delivered SPES to 83 electrode pairs and analyzed CCSRs recorded at 268 nonepileptic electrode sites. Diffusion-weighted imaging (DWI) tractography localized the interhemispheric white matter pathways as streamlines directly connecting two electrode sites. We localized and visualized the putative SPES-related fiber activation, at each 1-ms time window, based on the propagation velocity defined as the DWI-based streamline length divided by the early CCSR peak latency. Results The resulting movie, herein referred to as four-dimensional tractography, delineated the spatiotemporal dynamics of fiber activation via the corpus callosum and anterior commissure. Longer streamline length was associated with delayed peak latency and smaller amplitude of CCSRs. The cortical regions adjacent to each fiber activation site indeed exhibited CCSRs at the same time window. Conclusions Our four-dimensional tractography successfully animated neural propagations via distinct interhemispheric pathways. Significance Our novel animation method has the potential to help investigators in addressing the mechanistic significance of the interhemispheric network dynamics supporting physiological function. ### Competing Interest Statement The authors have declared no competing interest. * CCEPs : cortico-cortical evoked potentials CCSRs : cortico-cortical spectral responses DWI : diffusion-weighted imaging MNI : Montreal Neurological Institute sEEG : stereo-electroencephalography SOZ : seizure onset zone SPES : single-pulse electrical stimulation 4D : four-dimensional