Abstract
Different stimuli intensities elicit distinct perceptions or responses, implying that input signals are either conveyed through an overlapping but unique sub-population of sensory neurons or channeled into divergent brain circuits according to intensity. Carbon dioxide (CO2) is detected by a single type of olfactory sensory neuron (OSN) but is conveyed to higher brain centers through a diverse assortment of second-order projection neurons (PNs). We identified the circuitry that mediates Drosophila avoidance to different CO2 concentrations. Two distinct pathways, PNv-1 and PNv-2, are necessary and sufficient for avoidance responses to low and high CO2 concentrations, respectively. Low concentrations only activate PNv-1, but high concentrations activate both PNvs. However, high CO2 concentrations also activate a third PNv class, the γ-aminobutyric acid (GABA)ergic releasing PNv-3 neurons, which may inhibit PNv-1 pathway-mediated avoidance behavior. A circuit configuration that channels a common sensory input into distinct neural pathways would allow the perception of or the response elicited by a given odor to be further modulated by both stimulus intensity and context.