Karl Zilles (1944–2020): a personal tribute

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Karl Zilles, founding co-editor-in-chief of this journal, died on April 26, 2020. He was born in Würzburg, a small city in Bavaria with famous baroque architecture that was heavily damaged by allied bombing during WWII; he often visited his parents in this town. We first met in the mid 1970s, while I was on leave of absence from Semmelweis University Hungary, and I was working in the University of Würzburg anatomy department, located in the Koellikerstrasse, named after the famous anatomist whose hand was X-rayed by Roentgen, another famous professor at that university. We also met in many of the annual deliberations of the German Anatomical Society, of which he was president in 1996/1997. He followed the best tradition of German neuroanatomical research in the cortex, but he realized the shortcomings of Brodmann’s map (Brodmann 1909) that needed to be solved to be able to interpret the burgeoning in vivo human imaging data: (i) lack of observer-independent quantification, (ii) lack of data on the intrasulcular surface that makes up 2/3 of the cortical surface, (iii) lack of knowledge on interindividual differences, and (iv) lack of conversion from 2D to 3D. At the time, a direct technique to compare postmortem and in vivo data was not available. To solve this problem, Karl invented two ingenious techniques: first, he established a method enabling statistical validation of the position of cortical borders, and second, donated brains were perfused, in situ imaged by T1-weighted MRI scanning and then processed with traditional histologic techniques. Cortical regions were identified using the quantitative technique, delineated on 2D images of the sections and overlaid in a reference brain: this statistically testable probabilistic 3D mapping became a standard technique in many laboratories around the world to correlate microstructural details with functional data. Before he embarked on a large-scale re-mapping of the human cerebral cortex (Amunts and Zilles 2006; Eickhoff et al. 2005), he published a definitive cytoarchitectonic map of the rat cerebral cortex (Zilles and Wree 1985). An important contribution towards defining cortical cellular architecture and hierarchical organization was the development of techniques to show the quantitative distributions of receptors and create so-called receptor ‘fingerprints’ that characterize each cortical area (Zilles et al. 2002; Zilles and Palomero-Gallagher 2017a; Palomero-Gallagher et al. 2020). Eventually, he published the BigBrain, an ultra-high resolution 3D atlas consisting over 7000 sections (Amunts et al. 2013). Karl also analyzed cyto- and receptor architecture as well as connectivity in non-human primate brains (Zilles and Palomero-Gallagher 2017b; Zilles et al. 1982, 2016) to better understand the development of cognitive, motor, and sensory functions during brain evolution. He considered this evolutionary perspective to be a crucial aspect of his research, since studies focused on an exclusively anthropocentric interpretation do not explain how cognitive functions have developed by changes in the underlying microstructure and encoding strategies during evolution. …