Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item THE ACOUSTIC QUALITIES THAT INFLUENCE AUDITORY OBJECT AND EVENT RECOGNITION(2019) Ogg, Mattson Wallace; Slevc, L. Robert; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Throughout the course of a given day, human listeners encounter an immense variety of sounds in their environment. These are quickly transformed into mental representations of objects and events in the world, which guide more complex cognitive processes and behaviors. Through five experiments in this dissertation, I investigated the rapid formation of auditory object and event representations (i.e., shortly after sound onset) with a particular focus on understanding what acoustic information the auditory system uses to support this recognition process. The first three experiments analyzed behavioral (dissimilarity ratings in Experiment 1; duration-gated identification in Experiment 2) and neural (MEG decoding in Experiment 3) responses to a diverse array of natural sound recordings as a function of the acoustic qualities of the stimuli and their temporal development alongside participants’ concurrently developing responses. The findings from these studies highlight the importance of acoustic qualities related to noisiness, spectral envelope, spectrotemporal change over time, and change in fundamental frequency over time for sound recognition. Two additional studies further tested these results via syntheszied stimuli that explicitly manipulated these acoustic cues, interspersed among a new set of natural sounds. Findings from these acoustic manipulations as well as replications of my previous findings (with new stimuli and tasks) again revealed the importance of aperiodicity, spectral envelope, spectral variability and fundamental frequency in sound-category representations. Moreover, analyses of the synthesized stimuli suggested that aperiodicity is a particularly robust cue for some categories and that speech is difficult to characterize acoustically, at least based on this set of acoustic dimensions and synthesis approach. While the study of the perception of these acoustic cues has a long history, a fuller understanding of how these qualities contribute to natural auditory object recognition in humans has been difficult to glean. This is in part because behaviorally important categories of sound (studied together in this work) have previously been studied in isolation. By bringing these literatures together over these five experiments, this dissertation begins to outline a feature space that encapsulates many different behaviorally relevant sounds with dimensions related to aperiodicity, spectral envelope, spectral variability and fundamental frequency.Item Understanding Neuroplastic Effects of Transcranial Direct Current Stimulation through Analysis of Dynamics of Large-Scale Brain Networks(2012) Venkatakrishnan, Anusha; Contreras-Vidal, José L.; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Intrinsic adult neuroplasticity plays a critical role in learning and memory as well as mediating functional recovery from brain lesions like stroke and traumatic brain injuries. Extrinsic strategies to aid favorable modulation of neuroplasticity act as important adjunctive tools of neurorehabilitation. Transcranial direct current stimulation (tDCS) is an example of a non-invasive technique that can successfully induce neuroplastic changes in the human brain, although the underlying mechanisms are not completely understood. In this regard, characterization of neuroplastic changes in large-scale brain networks is a functional and necessary step towards non-invasively understanding neuroplastic modulation mediated by tDCS in humans. This dissertation, thus, aimed to understand the effects of tDCS, on large-scale brain network dynamics recorded through magnetoencephalography (MEG) through three specific aims that will provide novel insights into the mechanism(s) through which plastic changes are promoted by tDCS, specifically in the context motor learning. This dissertation pursued a systematic investigation of these changes in whole-head cortical dynamics using both model-free and model-based analysis techniques. Two experiments were conducted to dissociate between network changes mediated by tDCS at rest as well as when coupled with a task in order to determine optimal conditions for using tDCS for clinical purposes. Results from Study 1 using model-free analysis showed that a specific fronto-parietal network at rest was modulated up to a period of 30 minutes outlasting the duration of the stimulation. Further model-based analysis of this fronto-parietal network showed that these differences were driven by network activity primarily involving high frequency gamma band connectivity to and from the supplementary motor area to associated regions (left primary motor cortex (stimulated region), left prefrontal and parietal cortices). Results from Study 2 showed that the tDCS exerts highly polarity-specific effects on the impact of oscillatory network connectivity, within the functionally relevant fronto-parietal network, on behavioral changes associated with motor learning. These results advance our understanding of neuroplasticity mediated by tDCS and thus, have implications in the clinical use of tDCS for enhancing efficacy of neurorehabilitation in patients with stroke and traumatic brain injury.