Mechanisms for Complex Feature Selectivity in the Songbird Auditory Forebrain

Neurons in many secondary sensory areas exhibit high selectivity for specific stimuli. These representations are thought to underlie the ability of animals to quickly recognize complex objects under a wide range of conditions, but it is not well understood how this selectivity is built up from the simpler response properties of neurons at earlier stages of processing. The songs of European starlings (Sturnus vulgaris) incorpo-rate large repertoires of motifs (each a complex vocalization circa 1 s in duration) that are unique to each individual bird, and contribute to individual recognition by song [1–3]. We explored the receptive field properties of cells in the caudomedial mesopallium (CMM), which respond selectively to learned songs and may contribute to vocal recognition behavior [4]. In a series of awake-restrained and urethane anesthetized recordings from 5 starlings, we recorded the responses of 70 well-isolated single units in CMM to novel and familiar motifs. Out of 59 neurons that gave significant responses to at least one motif, 36 (61%) were significantly selective, responding robustly to only a small number of motifs. Selective neurons had sig-nificantly lower spontaneous firing rates, and exhibited phasic firing patterns, with one or more precisely timed peaks of excitation. Because these response properties are poorly predicted by linear spectrotempo-ral receptive field (STRF) models [4], we examined instead if CMM neurons were responding to multiple auditory features of the stimuli. Motifs were decomposed into spectrotemporal features that corresponde