Molecular and cellular studies have begun to unravel a neurobiological basis of olfactory processing, which appears conserved
among vertebrate and invertebrate species. Studies have shown clearly that experience-dependent coding of odor identity occurs in associative olfactory centers (the piriform cortex in mammals and the mushroom body MB in insects). What remains unclear, however, is whether associative centers also mediate innate (
spontaneous) odor
discrimination and how ongoing experience modifies odor discrimination. Here we show in nave flies that Gq-mediated signaling in MB modulates spontaneous discrimination of odor identity but not odor intensity (concentration). In contrast, experience-dependent modification (conditioning) of both odor identity and intensity occurs in MB exclusively via Gs-mediated signaling. Our data suggest that spontaneous responses to odor identity and odor intensity discrimination are segregated at the MB level, and neural activity from MB further modulates olfactory processing by experience-independent Gq-dependent encoding of odor identity and by experience-induced Gs-dependent encoding of odor intensity and identity.