We train mice to perform simple perceptual tasks. By using quantitative behavior, optogenetic and chemical-genetic gain- and loss-of-function perturbations, in vivo two-photon imaging, and electrophysiology, we assemble a description of the relationship between neural circuit function and perception. We work in the mouse tactile system to capitalize on an accessible mammalian circuit with a precise mapping between the sensory periphery and multiple brain areas. We investigate neural coding and circuit dynamics at levels ranging from sensory receptors to cortex.