Network mechanisms underlying stable motor actions

While we can learn to produce stereotyped movements and maintain this ability for years, it is unclear how populations of individual neurons change their firing properties to coordinate these skills. This has been difficult to address because there is a lack of tools that can monitor populations of single neurons in freely behaving animals for the durations required to remark on their tuning. This thesis is divided into two main directions- device engineering and systems neuroscience. The first section describes the development of an electrode array comprised of tiny self-splaying carbon fibers that are small and flexible enough to avoid the immune response that typically limits electrophysiological recordings. I also describe the refinement of a head-mounted miniature microscope system, optimized for multi-month monitoring of cells expressing genetically encoded calcium indicators in freely behaving animals. In the second section, these tools are used to answer basic systems neuroscience questions in an animal with one of the most stable, complex learned behaviors in the animal kingdom: songbirds. This section explores the functional organization and long-term network stability of HVC, the songbird premotor cortical microcircuit that controls song. Our results reveal that neural activity in HVC is correlated with a length scale of 100um. At this mesocopic scale, basal-ganglia projecting excitatory neurons, on average, fire at a specific phase of a local 30Hz network rhythm. These results show that premotor cortical activity is inhomogeneous in time and space, and that a mesoscopic dynamical pattern underlies the generation of the neural sequences controlling song. At this mesoscopic level, neural coding is stable for weeks and months. These ensemble patterns persist after peripheral nerve damage, revealing that sensory-motor correspondence is not required to maintain the stability of the underlying neural ensemble. However, closer examination of individual excitatory neurons reveals that the participation of cells can change over the timescale of days- with particularly large shifts occurring over instances of sleep. Our findings suggest that fine-scale drift of projection neurons, stabilized by mesoscopic level dynamics dominated by inhibition, forms the mechanistic basis of memory maintenance and and motor stability

Linear and Nonlinear Auditory Response Properties Of Interneurons In A High Order Avian Vocal Motor Nucleus During Wakefulness

Motor-related forebrain areas in higher vertebrates also show responses to passively presented sensory stimuli. Sensory tuning properties in these areas, especially during wakefulness, and their relation to perception, however, are poorly understood. In the avian song system, HVC (proper name) is a vocal-motor structure with auditory responses well defined under anesthesia but poorly characterized during wakefulness. I used a large set of song stimuli including the bird‟s own song (BOS) and many conspecific stimuli (CON) to characterize auditory tuning properties in putative interneurons (HVCIN) during wakefulness. My findings suggest that HVC contains a heterogeneity of response types; a third of neurons are either suppressed or show no response to any stimuli and two thirds show excitatory responses to one or more stimuli. A subset of excitatory neurons are tuned exclusively to BOS and show very low linearity as measured by spectrotemporal receptive field analysis (STRF), but many respond well to both BOS and CON stimuli and show response linearity comparable to that previously measured in structures of the ascending auditory pathway. Fourier analysis of the STRFs of linear HVCIN reveals a range of peak spectrotemporal tuning properties, with approximately half of these neurons showing peak sensitivity to modulations occurring with high power in zebra finch song. Previous work has established that HVC lesioned birds are impaired in operant contingency reversals involving CON stimuli and birds with lesions to song nuclei receiving auditory input from HVC are impaired in discriminations between BOS and CON stimuli. The findings of the present study are consistent with these results and suggest a possible role for HVC in species-relevant auditory tasks

Network dynamics in the neural control of birdsong

Sequences of stereotyped actions are central to the everyday lives of humans and animals, from the kingfisher's dive to the performance of a piano concerto. Lashley asked how neural circuits managed this feat nearly 6 decades ago, and to this day it remains a fundamental question in neuroscience. Toward answering this question, vocal performance in the songbird was used as a model to study the performance of learned, stereotyped motor sequences. The first component of this work considers the song motor cortical zone HVC in the zebra finch, an area that sends precise timing signals to both the descending motor pathway, responsible for stereotyped vocal performance in the adult, and the basal ganglia, which is responsible for both motor variability and song learning. Despite intense interest in HVC, previous research has exclusively focused on describing the activity of small numbers of neurons recorded serially as the bird sings. To better understand HVC network dynamics, both single units and local field potentials were sampled across multiple electrodes simultaneously in awake behaving zebra finches. The local field potential and spiking data reveal a stereotyped spatio-temporal pattern of inhibition operating on a 30 ms time-scale that coordinates the neural sequences in principal cells underlying song. The second component addresses the resilience of the song circuit through cutting the motor cortical zone HVC in half along one axis. Despite this large-scale perturbation, the finch quickly recovers and sings a near-perfect song within a single day. These first two studies suggest that HVC is functionally organized to robustly generate neural dynamics that enable vocal performance. The final component concerns a statistical study of the complex, flexible songs of the domesticated canary. This study revealed that canary song is characterized by specific long-range correlations up to 7 seconds long-a time-scale more typical of human music than animal vocalizations. Thus, the neural sequences underlying birdsong must be capable of generating more structure and complexity than previously thought

Seeing sound: a new way to illustrate auditory objects and their neural correlates

This thesis develops a new method for time-frequency signal processing and examines the relevance of the new representation in studies of neural coding in songbirds. The method groups together associated regions of the time-frequency plane into objects defined by time-frequency contours. By combining information about structurally stable contour shapes over multiple time-scales and angles, a signal decomposition is produced that distributes resolution adaptively. As a result, distinct signal components are represented in their own most parsimonious forms.  Next, through neural recordings in singing birds, it was found that activity in song premotor cortex is significantly correlated with the objects defined by this new representation of sound. In this process, an automated way of finding sub-syllable acoustic transitions in birdsongs was first developed, and then increased spiking probability was found at the boundaries of these acoustic transitions. Finally, a new approach to study auditory cortical sequence processing more generally is proposed. In this approach, songbirds were trained to discriminate Morse-code-like sequences of clicks, and the neural correlates of this behavior were examined in primary and secondary auditory cortex. It was found that a distinct transformation of auditory responses to the sequences of clicks exists as information transferred from primary to secondary auditory areas. Neurons in secondary auditory areas respond asynchronously and selectively -- in a manner that depends on the temporal context of the click. This transformation from a temporal to a spatial representation of sound provides a possible basis for the songbird's natural ability to discriminate complex temporal sequences

Dynamic control of auditory activity during sleep: Correlation between song response and EEG

The song nucleus high vocal center (HVC) sends neural signals for song production and receives auditory input. By using electroencephalography (EEG) to objectively identify wake/sleep state, we show that HVC auditory responses change with physiological states. Comparison of EEG and HVC records revealed that HVC response to auditory stimuli is greatest during slow-wave sleep. During slow-wave sleep, HVC neurons responded preferentially to the bird's own song. Strikingly, both spontaneous and forced waking during sleep caused HVC auditory responses to cease within milliseconds of an EEG-measured state change. State-dependent phenomena in downstream nuclei, such as robustus archistriatalis, are likely to be derivatives of those in HVC

Doctor of Philosophy

dissertationGlycosaminoglycans are carboyhydrate side chains of proteoglycans that have a myriad of biological functions. In the brain, these molecules are implicated in everything from development to plasticity to disease. Two of the main types of glycosaminoglycans (GAGs), heparan sulfate and chondroitin sulfate, have been implicated in both plasticity and learning; however, the exact role they play has remained unclear. One of the more interesting sensorimotor systems in the brain involves the learning and production of vocalizations. The goal of this work was to investigate the role GAGs play in two different aspects of this complex behavior, the neural control of vocal ontogeny and superfast muscle involvement in song production of zebra finch. In order to fully understand the role GAGs play in complex biological behaviors, such as vocalizations, it is imperative that the proper tools be synthesized, characterized, and produced for the study of these carbohydrates. Enzymes, specifically sulfated polymers and oligosaccharides, and small molecules provide unique opportunities to examine the role of GAGs. The use of enzymes in the song-specific nucleus, HVC, allowed the validation of the functionality of these enzymes in the model system of interest. Changes in stereotyped song were observed showing that GAG modulation could lead to alteration of a learned behavior. After this confirmation that GAGs were present and involved in song, small molecules called xylosides were used to examine the role of chondroitin sulfate iv biosynthesis during vocal ontogeny. Infusion of xyloside into RA (robust nucleus of the arcopallium), a nucleus important for vocal ontogeny, led to a change in the development of song. This implies that regulated biosynthesis of chondroitin sulfate during the critical period for vocal ontogeny is important. Lastly, the role of superfast syringeal muscles in song production was examined. Heparan sulfate degradation in these muscles alters the ability of the syrinx to modulate airflow. This change in muscle kinetics was correlated with significant, but temporary, differences in acoustic structure and frequency modulation while long-term differences showed aberrant syllable production

ACUTE ESTROGEN SYNTHESIS AND ACTION IN THE AUDITORY CORTEX OF DEVELOPING MALE ZEBRA FINCHES (TAENIOPYGIA GUTTATA)

Birdsong, as with human speech, is learned during an age- and experience-dependent sensitive period early in life. Songbirds must first memorize their parents’ song during a sensory phase, then refine their own burgeoning vocalizations to match the auditory memory of their parents’ song during a sensorimotor phase. While the error-correction aspect of the sensorimotor phase of song learning is comparatively well understood, it is largely unknown how auditory memories are formed and how auditory processing may change across development to facilitate song memorization. The songbird caudomedial nidopallium (NCM) is a brain region that encodes complex communication signals like song and is rich in aromatase (enzyme necessary for converting precursor androgens to estrogens) and estrogen receptors. In adults, acute estrogen signaling enhances auditory encoding, suggesting that one role for 17β-estradiol (E2) in NCM during development may be to enhance auditory processing and facilitate auditory memorization. Moreover, in the hippocampus of rodents, birds, and nonhuman primates, local E2 acts to enhance post-training memory consolidation. As such, I set out to determine whether this role for E2 in auditory processing and memorization occurs within the auditory cortex of juvenile songbirds. I tested this hypothesis across several experiments: I first tested how local E2 administration in NCM modulated auditory processing in developing songbirds. Next, I explored how changes in developing neural architecture and aromatase expression are aligned with distinct song learning phases. I then tested how global and local aromatase inhibition following song learning sessions impacted motor production, vocal learning, and neurophysiology in developing songbirds. Finally, using a stimulus-specific adaptation paradigm, I determined whether findings in juvenile songbirds extended to adults. Specifically, I locally blocked local E2 synthesis in NCM immediately following song exposure and subsequently measured neural recognition of the exposed song. My results showed that sensory coding is substantially enhanced in the NCM of sensory-aged birds compared to song-producing (sensorimotor-aged) juvenile birds, and that E2 exerts an age- and hemisphere-dependent effect on modulation of auditory processing. I also found that cell density in NCM peaks in sensory-aged birds, and is overall higher in dorsal vs. ventral NCM, but that aromatase and parvalbumin expression remain high and constant across development; no hemispheric differences for cell density or expression were found. Further, I found that neither circulating nor locally-derived E2 are required for tutor song memorization in development and adulthood; however, estrogen synthesis blockade can impair song production in developing birds and can also transform the lasting neural representations of autogenous and tutor song in adulthood. Taken together, this dissertation provides new insights into the pleiotropic effects of rapid steroid signaling and synthesis within the auditory cortex of developing male songbirds with implications for communication processing and sensorimotor learning

Acetylcholine acts on songbird premotor circuitry to invigorate vocal output

The neuromodulator acetylcholine has a well-establi¬shed role in enhancing sensory perception in states of heightened arousal, but whether acetylcholine acts centrally to exert an analogous influence on behavioral outputs is largely unknown. Here we use the quantifiable nature of birdsong to investigate how cholinergic tone modulates the cortical song premotor nucleus HVC, and influences vocal output. We found that dialysis of the cholinergic agonist carbachol into HVC enhanced the vigor of vocal output by increasing the pitch, tempo, amplitude and stereotypy of song. These effects did not require input from basal-ganglia circuitry, indicating direct cholinergic modulation of song premotor circuitry. Moreover, blockade of muscarinic acetylcholine receptors in HVC attenuated natural increases in vigor observed when song is directed at females in a courtship context. Neural recordings revealed that both dialysis of carbachol and courtship song were associated with higher firing rates in HVC, with conspicuous enhancement of low-frequency activity locked to the underlying rhythm of song. Further, neural activity in HVC predicted behavioral variability on a trial-by-trial basis, consistent with the possibility that natural variation in cholinergic tone influences acoustic output. Our findings establish that acetylcholine exerts a potent influence on forebrain premotor circuitry that acts to invigorate motor output, and indicates that such modulation contributes to the natural invigoration of song during courtship

Neural correlates of vocal learning in songbirds and humans

Animal models, songbirds particularly, are increasingly used to study the human capacity for speech and language. In the light of understanding both language evolution and individual language acquisition these models are highly valuable, provided that they are studied within a valid comparative framework. In the past few decades, non-invasive methods such as functional Magnetic Resonance Imaging (fMRI) and Near-InfraRed Spectroscopy (NIRS) have become available for human as well as animal brain research. In the studies discussed in this thesis, fMRI is employed to unravel the neural correlates of vocal learning in the human and songbird brain. Specifically, fMRI in both songbirds and humans is used to study the neural mechanisms underlying birdsong learning and human artificial grammar learning. In a series of fMRI studies investigating these neural mechanisms in adult and juvenile zebra finches and human adults, this thesis compares the neural substrates of song learning in birds with those of language learning in humans. Studies in both species show correlations between behavioral learning of song or speech and neural activity. These results contribute to the songbird model for human vocal learning.Language Use in Past and Presen

Large deviations for template matching between point processes

We study the asymptotics related to the following matching criteria for two independent realizations of point processes X\sim X and Y\sim Y. Given l>0, X\cap [0,l) serves as a template. For each t>0, the matching score between the template and Y\cap [t,t+l) is a weighted sum of the Euclidean distances from y-t to the template over all y\in Y\cap [t,t+l). The template matching criteria are used in neuroscience to detect neural activity with certain patterns. We first consider W_l(\theta), the waiting time until the matching score is above a given threshold \theta. We show that whether the score is scalar- or vector-valued, (1/l)\log W_l(\theta) converges almost surely to a constant whose explicit form is available, when X is a stationary ergodic process and Y is a homogeneous Poisson point process. Second, as l\to\infty, a strong approximation for -\log [\Pr{W_l(\theta)=0}] by its rate function is established, and in the case where X is sufficiently mixing, the rates, after being centered and normalized by \sqrtl, satisfy a central limit theorem and almost sure invariance principle. The explicit form of the variance of the normal distribution is given for the case where X is a homogeneous Poisson process as well.Comment: Published at http://dx.doi.org/10.1214/105051604000000576 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org