Over the last decades, imaging methods in animal models underwent revolutionary developments. Yet the potential of novel and accurate techniques for the imaging of neural substrates realizes fully only through interaction with human research and its system-level understanding of brain function. For instance, cross-species investigation is fundamental for auditory neuroscience, in particular in the study of sound source localization processing. The translation of auditory spatial cues into their neural representation has been addressed in multiple studies across species, allowing the development of theoretical and functional models of auditory space. However, auditory localization within the vertical plane remains less explored, with few animal studies addressing the neuroscience of elevation perception in the cortex. The study presented here aims to set the basis to bridge this knowledge gap, leveraging the interaction of human and animal neuroscience. Recent human studies identified an inverse linear relationship between sound source elevation and cortical activity and revealed remarkable plasticity in auditory cortical tuning for elevation. Building on these results, our laboratory conducted an Electroencephalography (EEG) experiment with human subjects. That study confirmed that sound source elevation is represented in a systematic manner in the human auditory cortex, but did not elucidate how the cortical substrate supports this representation. In fact, EEG lacks the spatial resolution to fully investigate the generators of the signals it measures, the circuital components of the auditory cortex. To bypass this challenge, we can assess if the same experimental protocol can yield similar results in a mouse model, the substrates of which can then be interrogated with molecular imaging tools. The results of such circuital dissection do not necessarily translate back to human research but can inform and guide its explorations provided solid theoretical basis and supporting computational models. Thus, in this dissertation we develop a comprehensive experimental platform for mouse EEG, aiming to translate protocols from human cognitive neuroscience to animal models. This translation, and its validation, lays the groundwork for further interrogation of the neural substrates of auditory perception and is the purpose of two experiments we present at the end of this thesis. We dedicate Chapter 1 to highlighting the necessity of integrating human and animal models to comprehend cortical functions and their implications for complex behavior. To further demonstrate the potential of this approach, in Chapter 2 we highlight the importance of omission responses, corollary discharge, and mismatch negativity (MMN) research from an interactionist standpoint, further showcasing how animal models can elucidate circuit-level substrates and contribute to multisensory integration theories. This investigation requires a deep understanding of spatial audition, and to this end in Chapter 3, we provide such detailed exploration, focusing on the auditory system's ability to localize sound within a three-dimensional space. In Chapter 4 we detail the modular setup for mouse EEG and imaging that we developed from scratch as part of this doctoral work. This setup is designed to facilitate the precise delivery of auditory stimuli and the accurate recording of EEG and optical imaging data under controlled conditions. The modular design philosophy centers on the integration of a robotic surgery station, anesthesia system, stimulus delivery system, optical imaging, and EEG systems in an integrated station, ensuring seamless transfer between different stations depending on the experiment requirements. We overview these components in the hardware section, which also describes the auditory stimulation system with its speaker arch that can be employed in a horizontal or vertical position. We also describe the surgical station, highlighting the modified stereotaxic apparatus and the surgical robot that allows for automated skull drilling and electrode array placement with micrometer-level precision. In the EEG systems section, we delineate the two types of EEG apparatus used in the experiments: subcutaneous needle electrodes (SNE) and multielectrode array (MEA). We discuss the advantages and drawbacks of SNE, the electrode positioning, and the importance of the reference and ground electrodes. We also describe the MEA system, emphasizing its high-density recordings and reduced movement artifacts. Finally, in the workflow section, we outline the sequence of operations for the experiments, from electrode implantation to processor initialization and stimulus presentation. We detail the electrode implantation procedures for both SNE and MEA, the initialization of processors and software for managing the EEG and stimulation systems, and the Python experimental platform that integrates all these components into a cohesive experimental protocol. We first employed this setup for the experiment detailed in Chapter 5 to explore the processing of sound source elevation in mice employing an adapter-probe paradigm. The aim was to assess whether it would yield comparable results to its application in humans. This paradigm is designed to induce short-term auditory adaptation, which leads to a decrease in neural responses to stimuli. By utilizing an adapter stimulus without local cues, we prevent suppression of location-specific processing, while silencing other sound-responsive neurons. We then present probe stimuli from different elevations, the responses to which should be dependent on the elevation modulation rather than the auditory processing suppressed by the adapter. This strategy allows us to record elevation-specific EEG activity with a better signal-to-noise ratio than would be otherwise possible. With this approach, we measured ERP components that align with those documented in humans, with a typical latency shift. Among these components, we identified a novel ERP correlate of sound source elevation processing in mice. This neural signature consists of a slow-rising mid-latency ERP component, which parallels the one elicited by the same protocol in humans. However, the effect of elevation was small, and limited to a contrast between the response to central stimuli and those above and below the animal. Our results reinforce the notion that mice ERPs can be used to investigate sound source elevation, highlighting similarities between human and mouse auditory processing. However, these conclusions hinge on an additional exploration into whether the auditory system of anesthetized mice can reliably produce responses specific to sound elevation. We address this critical aspect in the experiment presented in Chapter 6. In this second experiment, we employed a mismatch paradigm to discern whether anesthetized mice could differentiate between high and low sound sources. This involved alternating each sound source elevation as a deviant within a regular sequence of stimuli at the same elevation. We hypothesized that if the mice's auditory system could distinguish these elevations, we would observe an MMN effect, indicated by more negative responses to deviant stimuli compared to standard ones. This effect would be more pronounced for deviant stimuli from elevations further from the standard than for those closer. To enhance our experimental setup, we utilized a proprietary MEA for improved standardization and spatial resolution. With this setup we observed a biphasic MMN, with two distinct negative deflections, confirming the auditory system's capability to process stimuli from different elevations. This finding was intriguing, also considering the importance of head movements in auditory spatial perception, as discussed earlier. The biphasic nature of MMN might reflect different stages of cortical processing, with the late MMN suggesting complex spectral comparison as a possible analog of the human late discriminative negativity. We also found that deviant stimuli at -30 and 90° elevation did not elicit mismatch responses when presented in experimental blocks where the standard was at a 60° distance, but did when the standard was at a 120° distance. This finding confirmed our initial hypothesis However, our results also highlighted the unique status of the 30° elevation stimulus. In contrast to other elevations, the 30° stimulus showed a more pronounced early adaptation, and elicited a strong MMN as a deviant in the 60° proximity scenario. This suggests a possible bias in auditory processing towards this elevation range, potentially influenced by top-down modulation. The distinct adaptation behavior of the 30° stimulus could be a consequence of such modulation, aligning with behavioral studies and electrophysiological findings in other species. Further, we proposed a model where MMN elicitation in mice depends on the proximity of the deviant to a preferred elevation angle, near 30°, and the distance of the standard from the deviant. Such model could capture the dynamics of elevation representation mismatch. To explore these effects, further experiments with additional conditions are needed, potentially leading to a quantitative model of elevation deviance. Finally, in Chapter 7 we further explore possible research directions that could follow the work presented here, beyond what was already introduced in the experimental chapters
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:92095 |
| Date | 14 June 2024 |
| Creators | Braga, Alessandro |
| Contributors | Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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