Viral Tracing Gone Viral: Expanded Possibilities Call for Increased Caution in Describing Cerebellar Network Connections

Abstract This review discusses the huge expansion of using cleverly constructed neurotropic viruses and viral transgenes in neuroanatomical research, with a special focus on the cerebellar field. Thanks to the near-limitless possibilities these new tracing techniques offer, vital new information on connections and functions within the brain are rapidly becoming available. However, tracing techniques in general and viral tracing techniques in particular, can harbor some pitfalls and drawbacks that need to be carefully evaluated in order to come to the correct interpretation of obtained results. After providing a general introduction, this review will briefly discuss some of the most commonly used viral tracers with special focus on their individual characteristics and point out some, potentially unexpected, limitations. Finally, we discuss recent observations concerning some newly described cerebellar connections, mostly obtained with viral tracing, that should be treated with caution and need careful replication. We conclude that only after proper evaluation of tracing results, the correct construction of complex anatomical networks and its subsequent understanding will be possible.