Neuronal encoding of vocal signals in a high-level auditory area

Sensitivity to the sequential structure and acoustic variations of communication sounds is fundamental not only for language comprehension in humans but also for song recognition in songbirds. Whether and how the auditory system processes fine variations in the temporal and acoustic features remains poorly understood. By quantifying single-unit responses, we investigate whether neurons in a high‑ level auditory area in zebra finches, a songbird species, are sensitive to the ordering of birdsong elements and to their acoustic variations. Taking advantage of the stimulus-specific adaptation property of these auditory neurons, we first show that reordering elements within conspecific songs reinstated robust responses. Neurons also detected changes in the structure of artificial songs, and this detection depended on local transitions between song elements. We then recorded neuronal responses to repeated exposures to identical and variant sound sequences by using the songs of male birds which tend to be highly repetitive with only subtle variations between renditions. We found that variations between songs did not affect the neuron firing rate but the temporal reliability of responses. This suggests that auditory processing operates on a range of distinct timescales supporting both sensitivity (short timescales) and tolerance (longer timescales) to variations in vocal signals.

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