Molecular correlates of hypothalamic development in songbird ontogeny in comparison with the telencephalon.

Development of the songbird brain provides an excellent experimental model for understanding the regulation of sex differences in ontogeny. Considering the regulatory role of the hypothalamus in endocrine, in particular reproductive, physiology, we measured the structural (volume) and molecular correlates of hypothalamic development during ontogeny of male and female zebra finches. We quantified by relative quantitative polymerase chain reaction (rqPCR) the expression of 14 genes related to thyroid and steroid hormones actions as well as 12 genes related to brain plasticity at four specific time points during ontogeny and compared these expression patterns with the expression of the same genes as detected by transcriptomics in the telencephalon. These two different methodological approaches detected specific changes with age and demonstrated that in a substantial number of cases changes observed in both brain regions are nearly identical. Other genes however had a tissue-specific developmental pattern. Sex differences or interactions of sex by age were detected in the expression of a subset of genes, more in hypothalamus than telencephalon. These results correlate with multiple known aspects of the developmental and reproductive physiology but also raise a number of new functional questions.

Neuroplasticity in the cerebello-thalamo-basal ganglia pathway: A longitudinal in vivo MRI study in male songbirds.

Similar to human speech, bird song is controlled by several pathways including a cortico-basal ganglia-thalamo-cortical (C-BG-T-C) loop. Neurotoxic disengagement of the basal ganglia component, i.e. Area X, induces long-term changes in song performance, while most of the lesioned area regenerates within the first months. Importantly however, the timing and spatial extent of structural neuroplastic events potentially affecting other constituents of the C-BG-T-C loop is not clear. We designed a longitudinal MRI study where changes in brain structure were evaluated relative to the time after neurotoxic lesioning or to vocal performance. By acquiring both Diffusion Tensor Imaging and 3-dimensional anatomical scans, we were able to track alterations in respectively intrinsic tissue properties and local volume. Voxel-based statistical analyses revealed structural remodeling remote to the lesion, i.e. in the thalamus and, surprisingly, the cerebellum, both peaking within the first two months after lesioning Area X. Voxel-wise correlations between song performance and MRI parameters uncovered intriguing brain-behavior relationships in several brain areas pertaining to the C-BG-T-C loop supervising vocal motor control. Our results clearly point to structural neuroplasticity in the cerebellum induced by basal ganglia (striatal) damage and might point to the existence of a human-like cerebello-thalamic-basal ganglia pathway capable of modifying vocal motor output.

Functional magnetic resonance imaging (FMRI) with auditory stimulation in songbirds.

The neurobiology of birdsong, as a model for human speech, is a pronounced area of research in behavioral neuroscience. Whereas electrophysiology and molecular approaches allow the investigation of either different stimuli on few neurons, or one stimulus in large parts of the brain, blood oxygenation level dependent (BOLD) functional Magnetic Resonance Imaging (fMRI) allows combining both advantages, i.e. compare the neural activation induced by different stimuli in the entire brain at once. fMRI in songbirds is challenging because of the small size of their brains and because their bones and especially their skull comprise numerous air cavities, inducing important susceptibility artifacts. Gradient-echo (GE) BOLD fMRI has been successfully applied to songbirds (1-5) (for a review, see (6)). These studies focused on the primary and secondary auditory brain areas, which are regions free of susceptibility artifacts. However, because processes of interest may occur beyond these regions, whole brain BOLD fMRI is required using an MRI sequence less susceptible to these artifacts. This can be achieved by using spin-echo (SE) BOLD fMRI (7,8) . In this article, we describe how to use this technique in zebra finches (Taeniopygia guttata), which are small songbirds with a bodyweight of 15-25 g extensively studied in behavioral neurosciences of birdsong. The main topic of fMRI studies on songbirds is song perception and song learning. The auditory nature of the stimuli combined with the weak BOLD sensitivity of SE (compared to GE) based fMRI sequences makes the implementation of this technique very challenging.

Own-song recognition in the songbird auditory pathway: selectivity and lateralization.

The songbird brain is able to discriminate between the bird's own song and other conspecific songs. Determining where in the brain own- song selectivity emerges is of great importance because experience-dependent mechanisms are necessarily involved and because brain regions sensitive to self-generated vocalizations could mediate auditory feedback that is necessary for song learning and maintenance. Using functional MRI, here we show that this selectivity is present at the midbrain level. Surprisingly, the selectivity was found to be lateralized toward the right side, a finding reminiscent of the potential right lateralization of song production in zebra finches but also of own-face and own-voice recognition in human beings. These results indicate that a midbrain structure can process subtle information about the identity of a subject through experience-dependent mechanisms, challenging the classical perception of subcortical regions as primitive and nonplastic structures. They also open questions about the evolution of the cognitive skills and lateralization in vertebrates.

Functional MRI of auditory responses in the zebra finch forebrain reveals a hierarchical organisation based on signal strength but not selectivity.

BACKGROUND: Male songbirds learn their songs from an adult tutor when they are young. A network of brain nuclei known as the 'song system' is the likely neural substrate for sensorimotor learning and production of song, but the neural networks involved in processing the auditory feedback signals necessary for song learning and maintenance remain unknown. Determining which regions show preferential responsiveness to the bird's own song (BOS) is of great importance because neurons sensitive to self-generated vocalisations could mediate this auditory feedback process. Neurons in the song nuclei and in a secondary auditory area, the caudal medial mesopallium (CMM), show selective responses to the BOS. The aim of the present study is to investigate the emergence of BOS selectivity within the network of primary auditory sub-regions in the avian pallium. METHODS AND FINDINGS: Using blood oxygen level-dependent (BOLD) fMRI, we investigated neural responsiveness to natural and manipulated self-generated vocalisations and compared the selectivity for BOS and conspecific song in different sub-regions of the thalamo-recipient area Field L. Zebra finch males were exposed to conspecific song, BOS and to synthetic variations on BOS that differed in spectro-temporal and/or modulation phase structure. We found significant differences in the strength of BOLD responses between regions L2a, L2b and CMM, but no inter-stimuli differences within regions. In particular, we have shown that the overall signal strength to song and synthetic variations thereof was different within two sub-regions of Field L2: zone L2a was significantly more activated compared to the adjacent sub-region L2b. CONCLUSIONS: Based on our results we suggest that unlike nuclei in the song system, sub-regions in the primary auditory pallium do not show selectivity for the BOS, but appear to show different levels of activity with exposure to any sound according to their place in the auditory processing stream.

Functional magnetic resonance imaging in zebra finch discerns the neural substrate involved in segregation of conspecific song from background noise.

Recently, fMRI was introduced in a well-documented animal model for vocal learning, the songbird. Using fMRI and conspecific signals mixed with different levels of broadband noise, we now demonstrate auditory-induced activation representing discriminatory properties of auditory forebrain regions in anesthetized male zebra finches (Taeniopygia guttata). Earlier behavioral tests showed comparable calling responses to the original conspecific song stimulus heard outside and inside the magnet. A significant fMRI response was elicited by conspecific song in the primary auditory thalamo-recipient subfield L2a; in neighboring subareas L2b, L3, and L; and in the rostral part of the higher-order auditory area NCM (caudomedial nidopallium). Temporal BOLD response clustering revealed rostral and caudal clusters that we defined as "cluster Field L" and "cluster NCM", respectively. However, because the actual border between caudal Field L subregions and NCM cannot be seen in the structural MR image and is not precisely reported elsewhere, the cluster NCM might also contain subregion L and the medial extremes of the subregions L2b and L3. Our results show that whereas in cluster Field L the response was not reduced by added noise, in cluster NCM the response was reduced and finally disappeared with increasing levels of noise added to the song stimulus. The activation in cluster NCM was significant for only two experimental stimuli that showed significantly more behavioral responses than the more degraded stimuli, suggesting that the first area within the auditory system where the ability to discern song from masking noise emerges is located in cluster NCM.

Spatiotemporal properties of the BOLD response in the songbirds' auditory circuit during a variety of listening tasks.

Auditory fMRI in humans has recently received increasing attention from cognitive neuroscientists as a tool to understand mental processing of learned acoustic sequences and analyzing speech recognition and development of musical skills. The present study introduces this tool in a well-documented animal model for vocal learning, the songbird, and provides fundamental insight in the main technical issues associated with auditory fMRI in these songbirds. Stimulation protocols with various listening tasks lead to appropriate activation of successive relays in the songbirds' auditory pathway. The elicited BOLD response is also region and stimulus specific, and its temporal aspects provide accurate measures of the changes in brain physiology induced by the acoustic stimuli. Extensive repetition of an identical stimulus does not lead to habituation of the response in the primary or secondary telencephalic auditory regions of anesthetized subjects. The BOLD signal intensity changes during a stimulation and subsequent rest period have a very specific time course which shows a remarkable resemblance to auditory evoked BOLD responses commonly observed in human subjects. This observation indicates that auditory fMRI in the songbird may establish a link between auditory related neuro-imaging studies done in humans and the large body of neuro-ethological research on song learning and neuro-plasticity performed in songbirds.