A Role for the Superior Colliculus in the Control of Sonar Vocal Production in the Echolocating Bat, Eptesicus fuscus

Microchiroptera have evolved a biological sonar system that enables aerial foraging in total darkness. These echolocating bat species emit sequences of ultrasonic vocalizations and use the returning echoes to create acoustic images of the environment. Bats orient their gaze in space by adjusting their sonar vocalizations, flight dynamics, and head aim in a coordinated manner when approaching targets. Insectivorous species of echolocating bats have been shown to actively modulate the features of sonar vocalizations with changing target distance. Therefore, variations in the time-frequency structure and temporal patterning of sonar calls produced by foraging bats reflect adaptive goal directed behaviors. The bat's heavy reliance on sound production and processing is reflected in neural specializations of auditory and motor structures. The experiments described in this dissertation probe the midbrain superior colliculus (SC), a vertebrate sensorimotor nucleus mediating orienting behaviors, and they specifically explore adaptations in the SC of the insectivorous bat, Eptesicus fuscus, for acoustic orienting. The anatomical experiments conducted demonstrate that the bat SC has projections to pre-vocal motor control regions in the brainstem: paralemniscal tegmentum area, cuneiform nucleus, and midbrain reticular formation. Further insights were gained by developing chronic neural recording techniques to study SC neuronal activity in actively echolocating bats. These are the first chronic recordings in unrestrained, freely behaving bats. The physiological experiments reveal two bouts of neural activity prior to each sonar vocalization, and suggest a relationship between the timing of pre-vocal activity and sonar call duration. Based on the anatomical findings and the functional pre-motor activity identified here, along with previous electrical and chemical microstimulation studies in the bat midbrain, a conceptual model is proposed for the SC of bats that suggests its role in orienting acoustic gaze along the range axis. This role of the bat SC is similar to that proposed for primate and feline SC in controlling the visual depth of focus via vergence eye movements. The parallel between the visuomotor and echolocation systems for orienting gaze to objects at different distances suggests that the computations performed by the SC serve common functions across modalities and effort organs

Influences of behavioral state and developmental vocal learning on neural coding in the songbird auditory system

Vocal communicators such as humans and songbirds rely on their auditory systems to learn, recognize, and encode acoustic features of communication vocalizations. Yet it remains unclear how varying behavioral, experimental, and developmental contexts impact neural coding in the songbird auditory system. In this dissertation I demonstrate that experimental and behavioral contexts relating to arousal are sufficient to alter neural excitability in a way that has implications for neural coding in the songbird auditory system. First I show that urethane, a common anesthetic used in neurophysiological studies of songbird and mammalian auditory neurons, suppresses neural excitability but does not alter spectrotemporal tuning or neural discrimination in single auditory midbrain neurons. Next, I demonstrate that neurons in the songbird primary auditory cortical region Field L are sensitive to local concentrations of norepinephrine, a neurotransmitter involved mediating changes in arousal and behavioral state. Lastly, I report the results of a developmental study that demonstrates experience-dependent changes in temporal and spectral tuning in songbird auditory cortical neurons during vocal learning. These developmental effects were found to have region and cell-type specificity, and highlight potential functional roles for dorsal and ventral auditory cortical neurons in the songbird auditory cortex. The findings reported here have important implications for future studies into the neurophysiology of vocal learning

Neural circuit analysis of the dorsal nucleus of the lateral lemniscus and new viral approaches to neural circuit analysis in Mongolian gerbils

Auditory stimuli are processed by several parallel and serial neural circuits in the auditory brainstem. In the first part of this PhD thesis, synaptic integration of excitatory inputs in the neural network of the dorsal nucleus of the lateral lemniscus (DNLL) in Mongolian gerbils is investigated. The second part of this study analyses the feasibility of the use of viral vectors in Mongolian gerbils. This work aims to add to the available methods for neural circuit analysis in these animals by establishing tools for genetic manipulation. The DNLL receives excitatory inputs from the superior olivary complex (SOC) and provides GABAergic inhibition to its contralateral counterpart and both inferior colliculi (ICs). This GABAergic inhibition can outlast the triggering auditory stimulus by tens of milliseconds and thus differs substantially from the fast glycinergic inhibition prevailing in the SOC. It is thought that this persistent inhibition (PI) suppresses further processing of sound source information cues of lagging sounds, thereby providing the neuronal basis for sound localisation in reverberant environments. The mechanisms which PI is generated are still under debate. One hypothesized mechanism focuses on the output mechanism in DNLL neurons, favouring transmitter spillover or asynchronous release to evoke PI. A second mechanism states that integration of excitatory inputs leads to temporally extended activity in DNLL neurons, thereby prolonging the GABAergic output. Here, we tested in vitro the feasibility of the integration based mechanism in Mongolian gerbils. We analyzed the integration of excitatory inputs to DNLL neurons and found that five simultaneously stimulated excitatory fibres, each releasing on average ~18 vesicles are sufficient to trigger a single action potential (AP) in a DNLL neuron. A strong presynaptic stimulation pulse could trigger multiple APs. The input-output functions (IO-Fs) of DNLL neurons were dependent on NMDA receptor (NMDAR) mediated currents, which temporally extended the neuron's activity. The synaptic IO-Fs of DNLL neurons could also be modulated by voltage gated potassium, but not by calcium conductances.The NMDAR dependent activity amplification, which is maintained into adult stages, is shown to prolong the GABAergic output of DNLL neurons, thus contributing to PI generation. Viral vectors are widely used to alter the genetic content of a host organism. In Mongolian gerbils this approach may be suitable to compensate for the lack of genetic strategies in neural circuit analysis such as transgenic animal lines. Lentiviral and Semliki forest viral vectors were stereotactically injected into the IC or the medial nucleus of the trapezoid body (MNTB) in Mongolian gerbils. The lentiviral constructs were able to induce expression of the transgenic protein in the IC but not in MNTB principal neurons. The Semliki forest viral vector induced expression in both nuclei but also caused strong cytotoxic effects in the infected cells. In a further experiment, an eGFP expressing pseudorabies virus based on the attenuated Bartha strain (PRV-152) was stereotactically injected into the IC and was able to retrogradely infect the nuclei of the auditory brain stem in juvenile and adult Mongolian gerbils. PRV-152 spread synaptically to 2nd order neurons by ~20 hours after injection. Infection could also be started in the DNLL and showed a strongly pronounced neurotropism. The virus induced eGFP expression was high and allowed for a detailed visualization of the infected neurons, establishing PRV-152 as an effective tool for anatomical circuit analysis. The feasibility of using this virus in conjunction with electrophysiological investigations was also tested. 37% of 1st and 78% of 2nd order infected neurons show a significant decrease of excitability, which impedes the use of PRV-152 in combination with electrophysiological recordings for physiological analysis of neural circuits.Auditorische Stimuli werden in verschiedenen parallel und in Serie angeordneten neuralen Netzwerken des auditorischen Hirnstamms verarbeitet. Im ersten Teil dieser Dissertation wird die synaptische Integration exzitatorischer Eingänge zu Neuronen des dorsalen Nukleus des lateralen Lemniscus (DNLL) der mongolischen Wüstenrennmaus untersucht. Der zweite Teil der Arbeit betrachtet die Möglichkeit des Einsatzes viraler Vektoren in der mongolischen Wüstenrennmaus. Das Ziel war es, die Palette der verfügbaren Methoden zur Analyse neuronaler Netzwerke in diesen Tieren um einen genetischen Ansatz zu erweitern. Der DNLL erhält exzitatorische Eingänge vom superioren Olivenkernkomplex (SOC) und sendet GABAerge inhibitorische Projektionen zum kontralateralen DNLL und zu beiden inferioren Colliculi (ICs). Diese GABAerge Inhibition kann den auslösenden auditorischen Reiz für mehrere Millisekunden überdauern und unterscheidet sich damit grundsätzlich von der im SOC vorherrschenden schnellen, glycinergen Inhibition. Es wird vermutet, daß diese persistierende Inhibition (PI) die weitere Verarbeitung räumlicher Information von Echos unterdrückt und damit eine neuronale Grundlage zur Schallquellenlokalisation in nachhallenden Umgebungen bildet. Die Mechanismen zur Generierung der PI sind nicht vollständig erklärt. Eine mögliche Erklärung zielt auf den Mechanismus der Neurotrnasmitterausschüttung in DNLL Neuronen. Demzufolge könnten Neurotransmitter "spill over" oder asynchrone Transmitterausschüttung die GABAerge Inhibition der DNLL Neurone zeitlich verlängern. Ein zweiter Mechanismus argumentiert, daß die Aktivität in DNLL Neuronen durch die Integration exzitatorischer synaptischer Eingänge zeitlich ausgedehnt wird und somit auch die Inhibition die die DNLL Neurone auf ihre Zielzellen ausüben. In dieser Arbeit wurde mit Hilfe der "patch-clamp" Methode die Integration exzitatorischer Eingänge in DNLL Neuronen untersucht mit dem Ziel die mögliche Existenz dieses zweiten Mechanismus zu zeigen. Die Ergebnisse zeigen, daß fünf simultan erregte exzitatorische Fasern benötigt werden, die im Durchschnitt ~18 Vesikel ausschütten, um ein Aktionspotential (AP) in einem DNLL Neuron auszulösen. Ein einzelner starker präsynaptischer Stimulationspuls ist außerdem ausreichend mehrere APs auszulösen. Die Input-Output Funktionen (IO-Fs) von DNLL Neuronen sind abhängig von NMDA Rezeptorströmen, welche die Aktivität von DNLL Neuronen zeitlich verlängern. Anders als Kalziumleitfähigkeiten sind auch Kaliumleitfähigkeiten in der Lage die IO-Fs von DNLL Neuronen zu beeinflussen. Die NMDA Rezeptorstrom abhängige Aktivitätsverlängerung in DNLL Neuronen ist sowohl in juvenilen, als auch in adulten Tieren vorhanden und gipfelt in einer Verlängerung der GABAergen Inhibition die von DNLL Neuronen generiert wird. Somit ist die Integration exzitatorischer Eingänge in DNLL Neuronen grundsätzlich geeignet zum Enstehen der PI beizutragen. Virale Vektoren werden benutzt um den genetischen Inhalt eines Organismus zu verändern. In mongolischen Wüstenrennmäusen, von denen es derzeit keine transgenen Tierlinien gibt, können virale Vektoren benutzt werden diesen Nachteil auszugleichen. Wir haben lentivirale Vektoren und Vektoren basierend auf dem Semliki Forest Virus (SFV) stereotaktisch in den IC und den medialen Nukleus des Trapezkörpers (MNTB) von mongolischen Wüstenrennmäusen injiziert. Die lentiviralen Konstrukte induzieren die Expression des transgenen Proteins im IC, nicht aber in MNTB Neuronen. Der SFV-Vektor ist in der Lage in beiden Nuklei Expression auszulösen, entfaltet aber zusätzlich eine stark zytotoxische Wirkungen. In einer weiteren Experimentreihe wurde ein eGFP exprimierender attenuierter Pseudorabiesstamm (PRV-152) in den IC injiziert. Dieser Vektor ist in der Lage alle Nuklei des auditorischen Hirnstamms retrograd der Injektionsstelle in juvenilen und adulten mongolischen Wüstenrennmäusen zu infizieren. Die PRV-152 Infektion breitet sich nach etwa 20 Stunden über die erste Synapse zu infizierten Zellen zweiter Ordnung aus. Die PRV-152 Infektion kann ebenfalls vom DNLL ausgehend ausgelöst werden und zeigt einen ausgeprägten Neurotropismus. Die induzierte Expression des eGFPs ist hoch und ermöglicht eine deutliche Darstellung der infizierten Neurone, so daß PRV-152 ein vielversprechendes Werkzeug zur anatomischen Untersuchung neuronaler Netzwerke darstellt. Ebenfalls wurde untersucht, ob die PRV-152 Infektion zusätzlich die elektrophysiologische Untersuchung der infizierten Neurone erlaubt. 37% der infizierten Neurone erster Ordnung und 78% der infizierten Neurone zweiter Ordnung zeigen eine signifikant heruntergesetzte Erregbarkeit. Diese Ergebnisse zeigen deutlich, daß PRV-152 eine anatomische, nicht aber eine elektrophysiologische Untersuchung neuronale Netzwerke ermöglicht

Peripheral and Central Mechanisms of Temporal Pattern Recognition

Encoding information into the timing patterns of action potentials, or spikes, is a strategy used broadly in neural circuits. This type of coding scheme requires downstream neurons to be sensitive to the temporal patterns of presynaptic inputs. Indeed, neurons with temporal filtering properties have been found in a wide range of sensory pathways. However, how such response properties arise was previously not well understood. The goal of my dissertation research has been to elucidate how temporal filtering by single neurons contributes to the behavioral ability to recognize timing patterns in communication signals. I have addressed this question using mormyrid weakly electric fish, which vary the time intervals between successive electric pulses to communicate. Fish detect these signals with sensory receptors in their skin. In the majority of species, these receptors fire a single spike in response to each electric pulse. Spiking receptors faithfully encode the interpulse intervals in communication signals into interspike intervals, which are then decoded by interval-selective midbrain neurons. Using in vivo intracellular recordings from awake fish during sensory stimulation, I found that short-term depression and temporal summation play important roles in establishing single-neuron interval selectivity. Moreover, the combination of short-term depression and temporal summation in the circuit resulted in greater diversity of interval tuning properties across the population of neurons, which would increase the population’s ability to detect temporally patterned communication signals. Indeed, I found that the responses of single interval-selective neurons were sensitive to subtle variation in the timing patterns of a specific communication display produced by different individuals. A subset of mormyrid species has sensory receptors that produce spontaneously oscillating potentials. How the electrosensory system of these species established sensitivity to temporally patterned communication signals was completely unknown. Using in vivo extracellular recordings, I demonstrated that these receptors encode sensory stimuli into phase resets, which is the first clear instance of information coding by oscillatory phase reset. Furthermore, the ongoing oscillations conferred enhanced sensitivity to fast temporal patterns that are only found in the communication signals of a large group of fish. Behavioral playback experiments provided further support for the hypothesis that oscillating receptors are specialized for detecting communication signals produced by a group of conspecifics, which is a novel role for a sensory receptor. These findings demonstrate that temporal pattern sensitivity, which was previously thought to be a central processing problem, can also arise from peripheral filtering through a novel oscillatory phase reset mechanism

Swayed by sound: sonic guidance as a neurorehabilitation strategy in the cerebellar ataxias

Cerebellar disease leads to problems in controlling movement. The most common difficulties are dysmetria and instability when standing. Recent understanding of cerebellar function has expanded to include non -motor aspects such as emotional, cognitive and sensory processing. Deficits in the acquisition and processing of sensory information are one explanation for the movement problems observed in cerebellar ataxia. Sensory deficits result in an inability to make predictions about future events; a primary function of the cerebellum. A question therefore, is whether augmenting or replacing sensory information can improve motor performance in cerebellar disease. This question is tested in this thesis by augmenting sensory information through the provision of an auditory movement guide.A variable described in motor control theory (tau) was used to develop auditory guides that were continuous and dynamic. A reaching experiment using healthy individuals showed that the timing of peak velocity, audiomotor coordination accuracy, and velocity of approach, could be altered in line with the movement parameters embedded in the auditory guides. The thesis then investigated the use of these sonic guides in a clinical population with cerebellar disease. Performance on neurorehabilitation exercises for balance control was tested in twenty people with cerebellar atrophy, with and without auditory guides. Results suggested that continuous, predictive, dynamic auditory guidance is an effective way of improving iii movement smoothness in ataxia (as measured by jerk). In addition, generating and swaying with imaginary auditory guides was also found to increase movement smoothness in cerebellar disease.Following the tests of instantaneous effects, the thesis then investigated the longterm consequences on motor behaviour of following a two -month exercise with auditory guide programme. Seven people with cerebellar atrophy were assessed pre - and post -intervention using two measures, weight -shifting and walking. The results of the weight -shifting test indicated that the sonic -guide exercise programme does not initiate long -term changes in motor behaviour. Whilst there were minor, improvements in walking, because of the weight -shifting results, these could not be attributed to the sonic guides. This finding confirms the difficulties of motor rehabilitation in people with cerebellar disease.This thesis contributes original findings to the field of neurorehabilitation by first showing that on -going and predictive stimuli are an appropriate tool for improving motor behaviour. In addition, the thesis is the first of its kind to apply externally presented guides that convey continuous meaningful information within a clinical population. Finally, findings show that sensory augmentation using the auditory domain is an effective way of improving motor coordination in some forms of cerebellar disease

Neural plasticity and the limits of scientific knowledge

Western science claims to provide unique, objective information about the world. This is supported by the observation that peoples across cultures will agree upon a common description of the physical world. Further, the use of scientific instruments and mathematics is claimed to enable the objectification of science. In this work, carried out by reviewing the scientific literature, the above claims are disputed systematically by evaluating the definition of physical reality and the scientific method, showing that empiricism relies ultimately upon the human senses for the evaluation of scientific theories and that measuring instruments cannot replace the human sensory system. Nativist and constructivist theories of human sensory development are reviewed, and it is shown that nativist claims of core conceptual knowledge cannot be supported by the findings in the literature, which shows that perception does not simply arise from a process of maturation. Instead, sensory function requires a long process of learning through interactions with the environment. To more rigorously define physical reality and systematically evaluate the stability of perception, and thus the basis of empiricism, the development of the method of dimension analysis is reviewed. It is shown that this methodology, relied upon for the mathematical analysis of physical quantities, is itself based upon empiricism, and that all of physical reality can be described in terms of the three fundamental dimensions of mass, length and time. Hereafter the sensory modalities that inform us about these three dimensions are systematically evaluated. The following careful analysis of neuronal plasticity in these modalities shows that all the relevant senses acquire from the environment the capacity to apprehend physical reality. It is concluded that physical reality is acquired rather than given innately, and leads to the position that science cannot provide unique results. Rather, those it can provide are sufficient for a particular environmental setting