PROJECT SUMMARY Acoustic communication is critical for normal social interactions in many species, including humans, yet our knowledge of how the auditory system functions in more naturalistic social communication settings is limited. In particular, we need a more detailed understanding for how the vocabulary of sounds that carry socially commu- nicative meaning (e.g. speech sounds for humans) becomes established and grows through experience. Hu- mans are especially good at learning a diversity of sounds that convey similar meanings – a prerequisite for rich semantic categories in languages – but how and where the brain learns to fuse new spectrotemporal signals with behaviorally synonymous ones into one “schema” is rarely investigated at the cellular, circuit or network level. There is thus a gap in knowledge about detailed mechanisms supporting learning the social meaning of new communication sounds. The basic associative processes that must link distinct stimuli to the same pheno- typic behavior also exist in experimentally accessible mammals like mice, for whom such learning can also have adaptive value, especially for hearing these socially important sounds in complex soundscapes where there can be noise or distractors (e.g. cocktail party). Our long-term goal is to understand the cellular, circuit and network mechanisms by which mammalian auditory systems encode and learn sounds that mediate acoustic communi- cation and hears them in complex soundscapes, so that causes underlying deficits in communication processing and learning can be inferred. Our objective here is to uncover how specific forms of noncanonical auditory cortical plasticity (observed as adult female mice learn the communicative significance of sounds predictive of pups requiring care) impact detection, discrimination and grouping of these sounds both behaviorally and neurally within the auditory cortical core and secondary fields. We will use a combination of naturalistic behavior and operant conditioning, electrophysiology in head-fixed and freely moving mice, and cell type-specific targeting for optogenetics to pursue three Aims. First, we will determine what impact learning a new communicative sound stream has on its auditory cortical detectability in a complex soundscape. Second, we will determine how auditory cortex is altered while learning in a complex soundscape to discriminate or group new sound streams that com- municate a social behavioral function. Third, we will determine the contribution of specific responses and ACx circuits in causally mediating recognition of a new communicative sound stream in a complex soundscape and the associated ACx plasticity. This research’s significance lies in its unique ability to bridge the scientific gap between sensory and social neuroscience for learning communicative sounds, in an animal model where studies of a high-level auditory function (communication) can be conducted from a systems down to a molecular level.