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Codage neuronal de l'ordre des signaux acoustiques dans les chants des oiseaux

Abstract : Songs of songbirds, such as speech in humans, requires fine recognition of emitted acoustic signals. But bird's song isn’t only a succession of sound elements, called syllables. The order of syllables follows rules, and, from a behavioral point of view, birds can discriminate songs according to this order. Moreover, since they have a set of specialized brain regions in the perception, production and learning of singing, they are a major model for studying neural mechanisms involved in the processing of temporal order of acoustic signals. During this thesis, the focus was on two studies based on electrophysiological recordings (extracellular) of neurons activity during the song diffusion.The first study focused on the treatment of order in an analog area of the mammalian secondary auditory cortex, the caudomedial nidopallium (NCM), in one species, the zebra finch (Taeniopygia guttata), able to discriminate songs according to syllables order. NCM neurons responses decrease during iterations of a stimulus and increase if a new song is presented. Based on this adaptation property, we have shown that it's not necessary to change the song, a change in the order of syllables is sufficient to re-establish responses. Changing the order changes the acoustic context of the syllable (what comes before is different), so the NCM neurons would be sensitive to this context. To go further, we used as stimuli a short sequence composed of 2 different syllables, A and B, organized according to ABAB or AABB grammatical structures. The results show that the neurons detect this difference in structure, providing an additional argument to the hypothesis of a treatment depending on how the syllables follow each other in a song.The second study focused on the treatment of the order of syllables in a sensorimotor area, the HVC (proper noum), in the canary (Serinus canaria). HVC neurons have already been identified as sensitive to syllable order in the bird's own song (BOS). The canary produces complex songs composed of successive phrases, themselves containing repetitions of the same syllable. The arrangement of the phrases in the songs depends on transition probabilities and leads to the formation of stereotyped and recurrent sequences, called chunks. In addition, the position of sentences varies from one sentence to another. The activity of the HVC neurons was recorded during the diffusion of one of the BOS composed of 3 chunks and variants: BOS in which only the order of the phrases in a chunk or only the order of the chunks, is changed. The results show a stronger impact on the neuronal responses, of changes in the order of the chunks than in the order of sentences in the chunks. At the contrary to a study of another species of bird, our results don’t provide any arguments in favor of the idea that order sensitivity is based on a treatment of transition probabilities between phrases. They suggest that the sensitivity of neurons extends over several phrases, beyond the structure of the chunks themselves and depends on chunks order in the song.These studies show that responses of neurons to a given syllable or phrase depend on what precedes it, whether at the level of a sensorimotor area or a high-level auditory area. At the HVC level, this sensitivity to context can extend over several phrases, and therefore several seconds, to consider how the order of words of human language could be treated at the neuronal level.
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Aurore Cazala. Codage neuronal de l'ordre des signaux acoustiques dans les chants des oiseaux. Neurobiologie. Université Paris Saclay (COmUE), 2019. Français. ⟨NNT : 2019SACLS481⟩. ⟨tel-02496165⟩

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