The psychophysics and physiology of temporal coherence in birds and humans
Neilans, Erikson G.
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Auditory scene analysis has been suggested as a universal process that exists across all animals. Relative to humans, however, little work has been devoted to how animals perceptually isolate different sound sources. Frequency separation of sounds is arguably the most common parameter studied in auditory streaming, but it is not the only factor contributing to how the auditory scene is perceived. In humans, synchronous tones are heard as a single auditory stream, whereas asynchronous tones with the same frequency separations are perceived as two distinct sounds. It is unclear how animals, such as budgerigars ( Melopsittacus undulatus ), behaviorally perceive synchronous and asynchronous sounds and how the auditory system responds to temporally overlapped signals. In this study, budgerigars and humans ( Homo sapiens ) were tested on their perception of synchronous, asynchronous, and partially overlapping pure tones and complex signals using the same psychophysical procedures. Species differences were found between budgerigars and humans in how partially overlapping sounds were perceived, with budgerigars more likely to segregate overlapping sounds and humans more apt to fuse the two sounds together. Furthermore, budgerigars required less temporal separation than humans for stream segregation. Both humans and budgerigars were able to segregate complex signals with less temporal separation compared to pure tones. Analogous results were found in the budgerigar inferior colliculus (IC). Units in the IC had lower firing rates to asynchronous sounds relative to temporally overlapped sets of sounds and were found to respond more robustly to complex signals than to harmonic pure tones. Furthermore, the firing rate thresholds were comparable to many of the behavioral thresholds observed in the two psychophysical experiments, suggesting that not only is rate based coding for temporal coherence found at the level of the IC but that the midbrain is nearly as sensitive to auditory scene analysis as an awake, behaving animal.