Feature extraction and tonotopic organization in the avian auditory forebrain.

In a neurophysiological study within the auditory centers of the mediocaudal telencephalon of the starling, 601 neurons were tested for auditory responses. 369 of these units responded to pure tones, noise bands, amplitude modulations (AM), or species-specific sounds. Of all the auditory neurons, 16.8% did not respond to pure tones but only to more complex stimuli (tone-unresponsive-, TU-units). The remaining auditory units were classified as tone-responsive (TR-units). In 44.3% of TR-units (i.e. 36.9% of all auditory units) differing responses to tones versus more complex stimuli were observed. Responses as they occur in TU-units and in the differing responses of TR-units can be explained by neuronal extraction of features in the time (108 out of 198 neurons) and in the spectral domain (82 out of 198 neurons). Responses to species-specific sounds usually can be explained in terms of extraction of these features. Among neurons sensitive to temporal features, exclusive responses to a narrow range of AM frequencies were observed. In those TU-units that represent spectral features some restrict their responses to noise bands with distinct bandwidths centered around a specific midfrequency. These units reject both wider and narrower noise bands. A tonotopic arrangement of auditory units is found in field L, the surrounding neostriatum (NCM), and the Hyperstriatum ventrale (HV). Isofrequency lines run as a continuum through NCM, field L, and the caudal part of HV. TU-units are integrated into the tonotopic gradient according to the midfrequency of effective stimuli (e.g. noise bands or AM). The anatomical position of auditory units is correlated to their response properties. Within one isofrequency contour an increase in response selectivity is seen from field L to the postsynaptic areas in the NCM and the HV. The results are discussed in terms of possible mechanisms of feature extraction in the avian auditory system.