Geert De Groof

Microstructural changes observed with DKI in a transgenic Huntington rat model: evidence for abnormal neurodevelopment.

Huntington Disease (HD) is a fatal neurodegenerative disorder, caused by a mutation in the Huntington gene. Although HD is most often diagnosed in mid-life, the key to its clinical expression may be found during brain maturation. In the present work, we performed in vivo diffusion kurtosis imaging (DKI) in order to study brain microstructure alterations in developing transgenic HD rat pups. Several developing brain regions, relevant for HD pathology (caudate putamen, cortex, corpus callosum, external capsule and anterior commissure anterior), were examined at postnatal days 15 (P15) and 30 (P30), and DKI results were validated with histology. At P15, we observed higher mean (MD) and radial (RD) diffusivity values in the cortex of transgenic HD rat pups. In addition, at the age of P30, lower axial kurtosis (AK) values in the caudate putamen of transgenic HD pups were found. At the level of the external capsule, higher MD values at P15 but lower MD and AD values at P30 were detected. The observed DKI results have been confirmed by myelin basic protein immunohistochemistry, which revealed a reduced fiber staining as well as less ordered fibers in transgenic HD rat pups. These results indicate that neuronal development in young transgenic HD rat pups occurs differently compared to controls and that the presence of mutant huntingtin has an influence on postnatal brain development. In this context, various diffusivity parameters estimated by the DKI model are a powerful tool to assess changes in tissue microstructure and detect developmental changes in young transgenic HD rat pups.

Structural Changes between Seasons in the Songbird Auditory Forebrain

The song control system (SCS) of seasonal songbirds shows remarkable seasonal plasticity. Male starlings (Sturnus vulgaris) sing throughout the year, but in the breeding season, when concentrations of testosterone are elevated, the song is highly sexually motivated. The main goal of this study was to investigate structural seasonal changes in regions involved in auditory processing and in socio-sexual behavior. Using in vivo Diffusion Tensor Imaging (DTI), we measured in breeding and nonbreeding seasons volume and tissue characteristics of several brain regions of nine adult male starlings. We demonstrate that the songbird brain exhibits an extreme seasonal plasticity not merely limited to the SCS. Volumetric analysis showed seasonal telencephalon volume changes and more importantly also a volumetric change in the caudal region of the nidopallium (NCM), a region analogous to the mammalian secondary auditory cortex. Analysis of the DTI data allowed detection of seasonal changes in cellular attributes in NCM and regions involved in social behavior. This study extends our view on a seasonally dynamic avian brain which not only hones its song control system but also auditory and social systems to be prepared for the breeding season.

Seasonal rewiring of the songbird brain: an in vivo MRI study.

The song control system (SCS) of songbirds displays a remarkable plasticity in species where song output changes seasonally. The mechanisms underlying this plasticity are barely understood and research has primarily been focused on the song nuclei themselves, largely neglecting their interconnections and connections with other brain regions. We investigated seasonal changes in the entire brain, including the song nuclei and their connections, of nine male starlings (Sturnus vulgaris). At two times of the year, during the breeding (April) and nonbreeding (July) seasons, we measured in the same subjects cellular attributes of brain regions using in vivo high‐resolution diffusion tensor imaging (DTI) at 7 T. An increased fractional anisotropy in the HVC–RA pathway that correlates with an increase in axonal density (and myelination) was found during the breeding season, confirming multiple previous histological reports. Other parts of the SCS, namely the occipitomesencephalic axonal pathway, which contains fiber tracts important for song production, showed increased fractional anisotropy due to myelination during the breeding season and the connection between HVC and Area X showed an increase in axonal connectivity. Beyond the SCS we discerned fractional anisotropy changes that correlate with myelination changes in the optic chiasm and axonal organization changes in an interhemispheric connection, the posterior commissure. These results demonstrate an unexpectedly broad plasticity in the connectivity of the avian brain that might be involved in preparing subjects for the competitive and demanding behavioral tasks that are associated with successful reproduction.

In vivo diffusion tensor imaging (DTI) of brain subdivisions and vocal pathways in songbirds.

The neural substrate for song behavior in songbirds, the song control system (SCS), is thus far the best-documented brain circuit in which to study neuroplasticity and adult neurogenesis. Not only does the volume of the key song control nuclei change in size, but also the density of the connections between them changes as a function of seasonal and hormonal influences. This study explores the potentials of in vivo Diffusion-Tensor MRI (DT-MRI or DTI) to visualize the distinct, concentrated connections of the SCS in the brain of the starling (Sturnus vulgaris). In vivo DTI on starling was performed on a 7T MR system using sagittal and coronal slices. DTI was accomplished with diffusion gradients applied in seven non-collinear directions. Fractional Anisotropy (FA)-maps allowed us to distinguish most of the grey matter and white matter-tracts, including the laminae subdividing the avian telencephalon and the tracts connecting the major song control nuclei (e.g., HVC with RA and X). The FA-maps also allowed us to discern a number of song control, auditory and visual nuclei. Fiber tracking was implemented to illustrate the discrimination of all tracts running from and to RA. Because of the remarkable plasticity inherent to the songbird brain, the successful implementation of DTI in this model could represent a useful tool for the in vivo exploration of fiber degeneration and regeneration and the biological mechanisms involved in brain plasticity.

Functional MRI and functional connectivity of the visual system of awake pigeons.

At present, functional MRI (fMRI) is increasingly used in animal research but the disadvantage is that the majority of the imaging is applied in anaesthetized animals. Only a few articles present results obtained in awake rodents. In this study both traditional fMRI and resting state (rsfMRI) were applied to four pigeons, that were trained to remain still while being imaged, removing the need for anesthesia. This is the first time functional connectivity measurements are performed in a non-mammalian species. Since the visual system of pigeons is a well-known model for brain asymmetry, the focus of the study was on the neural substrate of the visual system. For fMRI a visual stimulus was used and functional connectivity measurements were done with the entopallium (E; analog for the primary visual cortex) as a seed region. Interestingly in awake pigeons the left E was significantly functionally connected to the right E. Moreover we compared connectivity maps for a seed region in both hemispheres resulting in a stronger bilateral connectivity starting from left E then from right E. These results could be used as a starting point for further imaging studies in awake birds and also provide a new window into the analysis of hemispheric dominance in the pigeon.

A 3-dimensional digital atlas of the ascending sensory and the descending motor systems in the pigeon brain

Pigeons are classic animal models for learning, memory, and cognition. The majority of the current understanding about avian neurobiology outside of the domain of the song system has been established using pigeons. Since MRI represents an increasingly relevant tool for comparative neuroscience, a 3-dimensional MRI-based atlas of the pigeon brain becomes essential. Using multiple imaging protocols, we delineated diverse ascending sensory and descending motor systems as well as the hippocampal formation. This pigeon brain atlas can easily be used to determine the stereotactic location of identified neural structures at any angle of the head. In addition, the atlas is useful to find the optimal angle of sectioning for slice experiments, stereotactic injections and electrophysiological recordings. This pigeon brain atlas is freely available for the scientific community.

Functional changes between seasons in the male songbird auditory forebrain.

Songbirds are an excellent model for investigating the perception of learned complex acoustic communication signals. Male European starlings (Sturnus vulgaris) sing throughout the year distinct types of song that bear either social or individual information. Although the relative importance of social and individual information changes seasonally, evidence of functional seasonal changes in neural response to these songs remains elusive. We thus decided to use in vivo functional magnetic resonance imaging (fMRI) to examine auditory responses of male starlings that were exposed to songs that convey different levels of information (species-specific and group identity or individual identity), both during (when mate recognition is particularly important) and outside the breeding season (when group recognition is particularly important). We report three main findings: (1) the auditory area caudomedial nidopallium (NCM), an auditory region that is analogous to the mammalian auditory cortex, is clearly involved in the processing/categorization of conspecific songs; (2) season-related change in differential song processing is limited to a caudal part of NCM; in the more rostral parts, songs bearing individual information induce higher BOLD responses than songs bearing species and group information, regardless of the season; (3) the differentiation between songs bearing species and group information and songs bearing individual information seems to be biased toward the right hemisphere. This study provides evidence that auditory processing of behaviorally-relevant (conspecific) communication signals changes seasonally, even when the spectro-temporal properties of these signals do not change.

Own song selectivity in the songbird auditory pathway: Suppression by norepinephrine

Background Like human speech, birdsong is a learned behavior that supports species and individual recognition. Norepinephrine is a catecholamine suspected to play a role in song learning. The goal of this study was to investigate the role of norepinephrine in birds own song selectivity, a property thought to be important for auditory feedback processes required for song learning and maintenance. Methodology/Principal Findings Using functional magnetic resonance imaging, we show that injection of DSP-4, a specific noradrenergic toxin, unmasks own song selectivity in the dorsal part of NCM, a secondary auditory region. Conclusions/Significance The level of norepinephrine throughout the telencephalon is known to be high in alert birds and low in sleeping birds. Our results suggest that norepinephrine activity can be further decreased, giving rise to a strong own song selective signal in dorsal NCM. This latent own song selective signal, which is only revealed under conditions of very low noradrenergic activity, might play a role in the auditory feedback and/or the integration of this feedback with the motor circuitry for vocal learning and maintenance.

Current state-of-the-art of auditory functional MRI (fMRI) on zebra finches: Technique and scientific achievements

Songbirds provide an excellent model system exhibiting vocal learning associated with an extreme brain plasticity linked to quantifiable behavioral changes. This animal model has thus far been intensively studied using electrophysiological, histological and molecular mapping techniques. However, these approaches do not provide a global view of the brain and/or do not allow repeated measures, which are necessary to establish correlations between alterations in neural substrate and behavior. In contrast, functional Magnetic Resonance Imaging (fMRI) is a non-invasive in vivo technique which allows one (i) to study brain function in the same subject over time, and (ii) to address the entire brain at once. During the last decades, fMRI has become one of the most popular neuroimaging techniques in cognitive neuroscience for the study of brain activity during various tasks ranging from simple sensory-motor to highly cognitive tasks. By alternating various stimulation periods with resting periods during scanning, resting and task-specific regional brain activity can be determined with this technique. Despite its obvious benefits, fMRI has, until now, only been sparsely used to study cognition in non-human species such as songbirds. The Bio-Imaging Lab (University of Antwerp, Belgium) was the first to implement Blood Oxygen Level Dependent (BOLD) fMRI in songbirds - and in particular zebra finches - for the visualization of sound perception and processing in auditory and song control brain regions. The present article provides an overview of the establishment and optimization of this technique in our laboratory and of the resulting scientific findings. The introduction of fMRI in songbirds has opened new research avenues that permit experimental analysis of complex sensorimotor and cognitive processes underlying vocal communication in this animal model.

Network structure of functional hippocampal lateralization in birds

Functional hemispheric asymmetry is a common feature of vertebrate brain organization, yet little is known about how hemispheric dominance is implemented at the neural level. One notable example of hemispheric dominance in birds is the leading role of the left hippocampal formation in controlling navigational processes that support homing in pigeons. Relying on resting state fMRI analyses (where Functional connectivity (FC) can be determined by placing a reference ‘seed’ for connectivity in one hemisphere), we show that following seeding in either an anterior or posterior region of the hippocampal formation of homing pigeons and starlings, the emergent FC maps are consistently larger following seeding of the left hippocampus. Left seedings are also more likely to result in FC maps that extend to the contralateral hippocampus and outside the boundaries of the hippocampus. The data support the hypothesis that broader FC is one neural‐organizational property that confers, with respect to navigation, functional dominance to the left hippocampus of birds.