Since the discovery that individual neurons are basic building blocks of the nervous system (Cajal, 1892), the diversity and heterogeneity of nerve cells have remained a challenge for deciphering the organization of neural circuits (Armananzas and Ascoli, 2015). Recent technical advances in anatomical, physiological and functional studies increasingly reveal multi-modal and multi-dimensional variations of neuronal phenotypes (Huang and Zeng, 2013). A fundamental question is whether these variations are largely subjective to measurements and can only be managed by operational grouping, or whether multiple distinct and congruent cell features can be integrated to define discrete “cells types” that reflect biological reality and mechanisms. The problem of neuronal diversity is unlikely to be solved without solving the equally if not more fundamental problem of neuronal identity, the flip side of the cell type coin (Seung and Sumbul, 2014). However, the biological basis of neuronal identity is poorly understood and cell definition and classification schemes in the brain remain contentious (DeFelipe et al., 2013).
