Anatomical Organisation, Auditory Processing of Communication Signals and Role of Auditory Experience in the Auditory Midbrain of the Zebra Finch

Abstract

Zebra finch males learn their vocalizations from a tutor that is usually the father. The song is biologically important for both reproduction and social interactions, making song perception a key parameter in songbird biology. Song selectivity and auditory processing in songbirds has been primarily studied in the forebrain and only a few studies have addressed the mechanisms of auditory processing in the brainstem. Since most auditory information reaching forebrain structures passes through the midbrain nucleus mesencephalicus lateralis, pars dorsalis (MLd), this nucleus is a prime candidate for early tuning to complex sounds, including conspecific signals. The focus of this thesis, therefore, was to describe the anatomical organisation of MLd with respect to auditory projections from lower brainstem nuclei and to describe the processing of both simple and complex auditory signals by MLd neurons. In contrast to non-singing species, calcium-binding protein staining patterns in the zebra finch MLd showed two distinct subdivisions that did not match the terminal fields of ascending auditory projections. Moreover, although the nuclear expression of the IEG ZENK in MLd in response to simple (tones, white noise) and complex (conspecific and heterospecific songs) stimuli was positive, the expression pattern after tone presentation did not match the known frequency representation in the nucleus, thus questioning the validity of this widely used method. Responses of single-units in MLd of birds reared by their conspecifics (ZF-ZF) to the same categories of simple and complex stimuli were recorded. MLd neurons were classified into units that responded to all types of stimuli and units that showed a higher level of selectivity. As a population, MLd units were also able to discriminate conspecific from heterospecific signals. These selective neurons could represent a first stage in the processing of complex song. Moreover, recordings of single units in MLd of birds reared by Bengalese finches showed the same general tuning properties but the units were not able to discriminate conspecific signals as well as those recorded in the MLd of ZF-ZF birds. That is, the rearing environment was found to influence the response properties of neurons even at this early stage of the auditory pathway.