Abstract
Different stimulus intensities elicit distinct perceptions, implying that input signals are either conveyed through an overlapping but distinct subpopulation of sensory neurons or channeled into divergent brain circuits according to intensity. In Drosophila, carbon dioxide (CO 2 ) is detected by a single type of olfactory sensory neuron, but information is conveyed to higher brain centers through second-order projection neurons (PNs). Two distinct pathways, PN v -1 and PN v -2, are necessary and sufficient for avoidance responses to low and high CO 2 concentrations, respectively. Whereas low concentrations activate PN v -1, high concentrations activate both PN v s and GABAergic PN v -3, which may inhibit PN v -1 pathway-mediated avoidance behavior. Channeling a sensory input into distinct neural pathways allows the perception of an odor to be further modulated by both stimulus intensity and context.