The brain’s identity crisis
Neuronal classification of cells throughout the brain and the overall nervous system is one of the oldest and most controversial problems in neuroscience. With the notable exception of the retina, in which roughly 60 neuronal types have been identified, and the 302 neurons of a worm called Caenorhabditis elegans, scientists don’t agree on how to identify most neurons. That leaves them trying to make sense of the complex ecosystem of the central nervous system with only the most basic idea of the species making it up. Lately, this problem has attracted a surge of attention from research efforts such as the U.S. Brain Research through Advancing Innovative Neurotechnologies (BRAIN) initiative, which, among other goals, aims to create a “census” of cells from brain tissue across different species. Without such a census, many researchers believe that BRAIN’s primary goal of decoding complex cognitive processes by recording the activity of thousands to billions of neurons will be impossible. Some believe that a breakthrough in neuronal classification is imminent thanks to new methods that rapidly sample gene activity from thousands of individual neurons, revealing patterns that distinguish cell types. Others see the quest to categorize as a fool’s errand.