Auditory Processing of Sequences and Song Syllables in Vocal Learning Birds

Abstract

The ability to use speech and language is at the core of what it means to be human. How our brains manage this cognitive feat and how it evolved in our species remain mysterious, in part because of how unique speech and language seem to be. We are the only primates who can produce learned vocalizations, but vocal learning is widespread among songbirds and parrots. Just like us, those birds rely on auditory perception to learn their songs and extract information used for communication. Studies using vocal learning birds can thereby help us understand how the brain processes vocal signals and why species differ in vocal communication abilities. But while the melodic patterns of birdsong are striking to the human ear, we cannot assume that song sequences are perceived that way by the birds, nor that the features birds hear are detectable by us.In this dissertation, I investigate how songbirds (focusing on the zebra finch (Taeniopygia guttata)) and parrots (focusing on the budgerigar (Melopsittacus undulatus)) process the sequential patterns and syllable-level details of birdsong, using behavioral auditory discrimination experiments and neurophysiological recordings in the central auditory system. The results show the following: 1) zebra finches and other songbirds are much more sensitive to changes in individual elements than changes in sequence, 2) budgerigars are better than zebra finches at hearing sequence changes but are also limited in their abilities compared to humans, 3) zebra finches are highly sensitive to the acoustic differences in utterances of the same motif syllables, 4) the budgerigar central auditory system encodes sequence more strongly in some respects than the zebra finch central auditory system, and 5) both the zebra finch and budgerigar central auditory systems can encode the rapid acoustic details of sounds well beyond human hearing abilities. Together, these findings indicate that vocal learning birds may communicate more at the level of syllable details than through sequential patterns, in contrast to human speech. The results also show neurophysiological species differences in sequence processing that could help us understand the differences between humans and other primates in vocal communication.