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

Segmented spin-echo echo-planar imaging improves whole-brain BOLD functional MRI in awake pigeon brains

Functional magnetic resonance imaging (fMRI) in awake small animals such as pigeons or songbirds opens a new window into the neural fundaments of cognitive behavior. However, high-field fMRI in the avian brain is challenging due to strong local magnetic field inhomogeneities caused by air cavities in the skull. A spoiled gradient-echo fMRI sequence has already been used to map the auditory network in songbirds, but due to susceptibility artifacts only 50% of the whole brain could be recorded. Since whole-brain fMRI coverage is vital to reveal whole-brain networks, an MRI sequence that is less susceptible to these artifacts was required. This was recently achieved in various bird species by using a rapid acquisition with relaxation enhancement (RARE) sequence. Weak blood oxygen level-dependent (BOLD) sensitivity, low temporal resolution, and heat caused by the long train of RF refocusing pulses are the main limits of RARE fMRI at high magnetic fields. To go beyond some of these limitations, we here describe the implementation of a two-segmented spin-echo echo-planar imaging (SE-EPI). The proposed sequence covers the whole brain of awake pigeons. The sequence was applied to investigate the auditory network in awake pigeons and assessed the relative merits of this method in comparison with the single-shot RARE sequence. At the same imaging resolution but with a volume acquisition of 3 s versus 4 s for RARE, the two-segmented SE-EPI provided twice the strength of BOLD activity compared with the single-shot RARE sequence, while the image signal-to-noise ratio (SNR) and in particular the temporal SNR were very similar for the two sequences. In addition, the activation patterns in two-segmented SE-EPI data are more symmetric and larger than single-shot RARE results. Two-segmented SE-EPI represents a valid alternative to the RARE sequence in avian fMRI research since it yields more than twice the BOLD sensitivity per unit of time with much less energy deposition and better temporal resolution, particularly for event-related experiments.

Convergent and divergent neural circuit architectures that support acoustic communication

Vocal communication is used across extant vertebrates, is evolutionarily ancient, and been maintained, in many lineages. Here I review the neural circuit architectures that support intraspecific acoustic signaling in representative anuran, mammalian and avian species as well as two invertebrates, fruit flies and Hawaiian crickets. I focus on hindbrain motor control motifs and their ties to respiratory circuits, expression of receptors for gonadal steroids in motor, sensory, and limbic neurons as well as divergent modalities that evoke vocal responses. Hindbrain and limbic participants in acoustic communication are highly conserved, while forebrain participants have diverged between anurans and mammals, as well as songbirds and rodents. I discuss the roles of natural and sexual selection in driving speciation, as well as exaptation of circuit elements with ancestral roles in respiration, for producing sounds and driving rhythmic vocal features. Recent technical advances in whole brain fMRI across species will enable real time imaging of acoustic signaling partners, tying auditory perception to vocal production.

A three-dimensional digital atlas of the Nile crocodile (Crocodylus niloticus) forebrain

The phylogenetic position of crocodilians in relation to birds and mammals makes them an interesting animal model for investigating the evolution of the nervous system in amniote vertebrates. A few neuroanatomical atlases are available for reptiles, but with a growing interest in these animals within the comparative neurosciences, a need for these anatomical reference templates is becoming apparent. With the advent of MRI being used more frequently in comparative neuroscience, the aim of this study was to create a three-dimensional MRI-based atlas of the Nile crocodile (Crocodylus niloticus) brain to provide a common reference template for the interpretation of the crocodilian, and more broadly reptilian, brain. Ex vivo MRI acquisitions in combination with histological data were used to delineate crocodilian brain areas at telencephalic, diencephalic, mesencephalic, and rhombencephalic levels. A total of 50 anatomical structures were successfully identified and outlined to create a 3-D model of the Nile crocodile brain. The majority of structures were more readily discerned within the forebrain of the crocodile with the methods used to produce this atlas. The anatomy outlined herein corresponds with both classical and recent crocodilian anatomical analyses, barring a few areas of contention predominantly related to a lack of functional data and conflicting nomenclature.

Interhemispheric functional connectivity in the zebra finch brain, absent the corpus callosum in normal ontogeny

Bilaterally symmetric intrinsic brain activity (homotopic functional connectivity; FC) is a fundamental feature of the mammalian brain's functional architecture. In mammals, homotopic FC is primarily mediated by the corpus callosum (CC), a large interhemispheric white matter tract thought to balance the bilateral coordination and hemispheric specialization critical for many complex brain functions, including human language. The CC first emerged with the Eutherian (placental) mammals ∼160 MYA and is not found among other vertebrates. Despite this, other vertebrates also exhibit complex brain functions requiring hemispheric specialization and coordination. For example, the zebra finch (Taeniopygia guttata) songbird learns to sing from tutors much as humans acquire speech and must balance hemispheric specialization and coordination to successfully learn and produce song. We therefore tested whether the zebra finch also exhibits homotopic FC, despite lacking the CC. Resting-state fMRI analyses demonstrated widespread homotopic FC throughout the zebra finch brain across development, including within a network required for learned song that lacks direct interhemispheric structural connectivity. The presence of homotopic FC in a non-Eutherian suggests that ancestral pathways, potentially including indirect connectivity via the anterior commissure, are sufficient for maintaining a homotopic functional architecture, an insight with broad implications for understanding interhemispheric coordination across phylogeny.

A three-dimensional digital neurological atlas of the mustached bat (Pteronotus parnellii)

Substantial knowledge of auditory processing within mammalian nervous systems emerged from neurophysiological studies of the mustached bat (Pteronotus parnellii). This highly social and vocal species retrieves precise information about the velocity and range of its targets through echolocation. Such high acoustic processing demands were likely the evolutionary pressures driving the over-development at peripheral (cochlea), metencephalic (cochlear nucleus), mesencephalic (inferior colliculus), diencephalic (medial geniculate body of the thalamus), and telencephalic (auditory cortex) auditory processing levels in this species. Auditory researchers stand to benefit from a three dimensional brain atlas of this species, due to its considerable contribution to auditory neuroscience. Our MRI-based atlas was generated from 2 sets of image data of an ex-vivo male mustached bat's brain: a detailed 3D-T2-weighted-RARE scan [(59 × 63 x 85) μm³] and track density images based on super resolution diffusion tensor images [(78) μm³] reconstructed from a set of low resolution diffusion weighted images using Super-Resolution-Reconstruction (SRR). By surface-rendering these delineations and extrapolating from cortical landmarks and data from previous studies, we generated overlays that estimate the locations of classic functional subregions within mustached bat auditory cortex. This atlas is freely available from our website and can simplify future electrophysiological, microinjection, and neuroimaging studies in this and related species.

3D functional ultrasound imaging of pigeons

Recent advances in ultrasound Doppler imaging have facilitated the technique of functional ultrasound (fUS) which enables visualization of brain-activity due to neurovascular coupling. As of yet, this technique has been applied to rodents as well as to human subjects during awake craniotomy surgery and human newborns. Here we demonstrate the first successful fUS studies on awake pigeons subjected to auditory and visual stimulation. To allow successful fUS on pigeons we improved the temporal resolution of fUS up to 20,000 frames per second with real-time visualization and continuous recording. We show that this gain in temporal resolution significantly increases the sensitivity for detecting small fluctuations in cerebral blood flow and volume which may reflect increased local neural activity. Through this increased sensitivity we were able to capture the elaborate 3D neural activity pattern evoked by a complex stimulation pattern, such as a moving light source. By pushing the limits of fUS further, we have reaffirmed the enormous potential of this technique as a new standard in functional brain imaging with the capacity to unravel unknown, stimulus related hemodynamics with excellent spatiotemporal resolution with a wide field of view.

Rapid changes in auditory processing in songbirds following acute aromatase inhibition as assessed by fMRI

Contribution to Special Issue on Fast effects of steroids. This review introduces functional MRI (fMRI) as an outstanding tool to assess rapid effects of sex steroids on auditory processing in seasonal songbirds. We emphasize specific advantages of this method as compared to other more conventional and invasive methods used for this purpose and summarize an exemplary auditory fMRI study performed on male starlings exposed to different types of starling song before and immediately after the inhibition of aromatase activity by an i.p. injection of Vorozole™. We describe how most challenges that relate to the necessity to anesthetize subjects and minimize image- and sound-artifacts can be overcome in order to obtain a voxel-based 3D-representation of changes in auditory brain activity to various sound stimuli before and immediately after a pharmacologically-induced depletion of endogenous estrogens. Analysis of the fMRI data by assumption-free statistical methods identified fast specific changes in activity in the auditory brain regions that were stimulus-specific, varying over different seasons, and in several instances lateralized to the left side of the brain. This set of results illustrates the unique features of fMRI that provides opportunities to localize and quantify the brain responses to rapid changes in hormonal status. fMRI offers a new image-guided research strategy in which the spatio-temporal profile of fast neuromodulations can be identified and linked to specific behavioral inputs or outputs. This approach can also be combined with more localized invasive methods to investigate the mechanisms underlying the observed neural changes.

fMRI Reveals a Novel Region for Evaluating Acoustic Information for Mate Choice in a Female Songbird

Selection of sexual partners is among the most critical decisions that individuals make and is therefore strongly shaped by evolution. In social species, where communication signals can convey substantial information about the identity, state, or quality of the signaler, accurate interpretation of communication signals for mate choice is crucial. Despite the importance of social information processing, to date, relatively little is known about the neurobiological mechanisms that contribute to sexual decision making and preferences. In this study, we used a combination of whole-brain functional magnetic resonance imaging (fMRI), immediate early gene expression, and behavior tests to identify the circuits that are important for the perception and evaluation of courtship songs in a female songbird, the zebra finch (Taeniopygia guttata). Female zebra finches are sensitive to subtle differences in male song performance and strongly prefer the longer, faster, and more stereotyped courtship songs to non-courtship renditions. Using BOLD fMRI and EGR1 expression assays, we uncovered a novel region involved in auditory perceptual decision making located in a sensory integrative region of the avian central nidopallium outside the traditionally studied auditory forebrain pathways. Changes in activity in this region in response to acoustically similar but categorically divergent stimuli showed stronger parallels to behavioral responses than an auditory sensory region. These data highlight a potential role for the caudocentral nidopallium (NCC) as a novel node in the avian circuitry underlying the evaluation of acoustic signals and their use in mate choice.

Developmental song learning as a model to understand neural mechanisms that limit and promote the ability to learn

Songbirds famously learn their vocalizations. Some species can learn continuously, others seasonally, and still others just once. The zebra finch (Taeniopygia guttata) learns to sing during a single developmental "Critical Period," a restricted phase during which a specific experience has profound and permanent effects on brain function and behavioral patterns. The zebra finch can therefore provide fundamental insight into features that promote and limit the ability to acquire complex learned behaviors. For example, what properties permit the brain to come "on-line" for learning? How does experience become encoded to prevent future learning? What features define the brain in receptive compared to closed learning states? This piece will focus on epigenomic, genomic, and molecular levels of analysis that operate on the timescales of development and complex behavioral learning. Existing data will be discussed as they relate to Critical Period learning, and strategies for future studies to more directly address these questions will be considered. Birdsong learning is a powerful model for advancing knowledge of the biological intersections of maturation and experience. Lessons from its study not only have implications for understanding developmental song learning, but also broader questions of learning potential and the enduring effects of early life experience on neural systems and behavior.

Auditory evoked BOLD responses in awake compared to lightly anaesthetized zebra finches

Functional magnetic resonance imaging (fMRI) is increasingly used in cognitive neuroscience and has become a valuable tool in the study of auditory processing in zebra finches, a well-established model of learned vocal communication. Due to its sensitivity to head motion, most fMRI studies in animals are performed in anaesthetized conditions, which might significantly impact neural activity evoked by stimuli and cognitive tasks. In this study, we (1) demonstrate the feasibility of fMRI in awake zebra finches and (2) explore how light anaesthesia regimes affect auditory-evoked BOLD responses to biologically relevant songs. After an acclimation procedure, we show that fMRI can be successfully performed during wakefulness, enabling the detection of reproducible BOLD responses to sound. Additionally, two light anaesthesia protocols were tested (isoflurane and a combination of medetomidine and isoflurane), of which isoflurane alone appeared to be the most promising given the high success rate, non-invasive induction, and quick recovery. By comparing auditory evoked BOLD responses in awake versus lightly anaesthetized conditions, we observed overall effects of anaesthetics on cerebrovascular reactivity as reflected in the extent of positive and negative BOLD responses. Further, our results indicate that light anaesthesia has limited effects on selective BOLD responses to natural versus synthetic sounds.

Mapping brain structure and function: cellular resolution, global perspective

A comprehensive understanding of the brain requires analysis, although from a global perspective, with cellular, and even subcellular, resolution. An important step towards this goal involves the establishment of three-dimensional high-resolution brain maps, incorporating brain-wide information about the cells and their connections, as well as the chemical architecture. The progress made in such anatomical brain mapping in recent years has been paralleled by the development of physiological techniques that enable investigators to generate global neural activity maps, also with cellular resolution, while simultaneously recording the organism's behavioral activity. Combination of the high-resolution anatomical and physiological maps, followed by theoretical systems analysis of the deduced network, will offer unprecedented opportunities for a better understanding of how the brain, as a whole, processes sensory information and generates behavior.

The genomics of memory and learning in songbirds

Songbirds have unique value as a model for memory and learning. In their natural social life, they communicate through vocalizations that they must learn to produce and recognize. Song communication elicits abrupt changes in gene expression in regions of the forebrain responsible for song perception and production--what is the functional significance of this genomic response? For 20 years, the focus of research was on just a few genes [primarily ZENK, now known as egr1 (early gene response 1)]. Recently, however, DNA microarrays have been developed and applied to songbird behavioral research, and in 2010 the initial draft assembly of the zebra finch genome was published. Together, these new data reveal that the genomic involvement in song processing is far more complex than anticipated. The concepts of neurogenomic computation and biological embedding are introduced as frameworks for future research.