Auditory Representation of Vocal Signals in a Pallial Cortical Circuit

Knowledge of how vocal communication signals are represented in the auditory system is crucial for understanding the perceptual basis of vocal communication. Using male and female zebra finches, we identified differentially expressed molecular markers that helped define distinct (caudal, rostral, dorsal, and ventral) domains within the caudomedial nidopallium (NCM), a high-order cortical auditory area known for its song-selective responses. Using expression analysis of the activity-inducible gene zenk, we found that the number of activated neurons is more stimulus dependent in NCM than in the auditory midbrain or the caudomedial mesopallium and that information on the density and spatial distribution of responsive neurons in NCM is sufficient to discriminate responses to conspecific song from other stimuli. We observed stronger activation of dorsal NCM, higher selectivity of caudal NCM toward conspecific song, and strong activation of the inhibitory network of rostral NCM by nonconspecific song stimuli. The spatial organization of responsive cells was particularly sensitive to both spectral and temporal components of song. We also obtained evidence of broadly distributed song-selective neuronal ensembles and that individual NCM neurons participate in the representation of different conspecific songs, implying independent activation and molecular induction responses. We conclude that some basic aspects of the cortical response to complex auditory stimuli are topographically organized, a finding that has been elusive in other systems. These findings advance our knowledge of the functional organization of a key song-processing cortical area, providing novel insights into the auditory representation of vocal communication signals.