Seasonal changes in selectivity for individual song in the songbird auditory forebrain
Résumé
Seasonal differences in song behaviour and anatomy of the song control system are well documented in songbirds. Although seasonal effects on songbird vocal production have been studied extensively, relatively less work has been done on seasonal effects on songbird auditory processing. The European starling (Sturnus vulgaris) has proven to be a remarkably interesting model to study the relationship between social/breeding behaviour plasticity and auditory processing. Starlings are highly social songbirds with songs that show different levels of variations that reflect not only species-specific but also individual-specific identity. Taking advantage of the whole-brain approach allowed by functional Magnetic Resonance Imaging (fMRI), we measured the neural responses to these songs in medetomidine-anesthetized adult male starlings (N=9) in both the breeding and the non-breeding seasons. Subjects were exposed to species-specific songs (universally-shared male songs), individual male conspecific songs (used in individual recognition) and synthetic pure tones. The direct comparison of the neural activation elicited by songs used in individual recognition, as compared to species-specific songs, revealed that perception of individual male songs induced a greater activity in the regions analogous to the auditory cortex of mammals (Field L and caudomedial nidopallium (NCM)). There were however differences between Field L and NCM according to the season. Whereas Field L showed the same selectivity for individual male songs in both seasons, NCM (more specifically the most caudal part of NCM) showed this selectivity only during the breeding season. In the non-breeding season, selectivity for individual male songs was not present anymore in caudal NCM. Responses to synthetic pure tone stimuli remained unchanged between the seasons, indicating that this seasonal change in selectivity for individual stimuli in caudal NCM was not due to a change in auditory perception as a whole. Finally, the data seems to suggest lateralization of selectivity for individual songs towards the right hemisphere, which is somewhat similar to the right-hemisphere specialization for the recognition of faces observed in humans. These results suggest that seasons impact auditory processing as well as vocal production. A differential representation of sounds with distinct biological significance is thus observed between seasons, which opens questions about a potential neural substrate for the extraction of biologically relevant information used in vocal communication.