Auditory neurons selectively respond to frequency and amplitude of sound. In the auditory midbrain, duration-tuned neurons (DTNs) are subsets of neurons that selectively respond to the duration of sound. DTNs may help further understand the neural mechanism underlying temporal processing in the central nervous system. Temporal processing has been shown to play important roles in speech, discriminating species-specific signals, and echolocation. The goal of this thesis is to explore the role of DTNs through single-unit electrophysiological recordings in the auditory midbrain of the big brown bat (Eptesicus fuscus). Monotic and dichotic paired-tone stimulation was used to evoke excitatory and inhibitory responses from DTNs. Two stimuli consisted of best duration (BD) excitatory and non-excitatory (NE) tones. In the monotic condition, both tones were presented to the contralateral ear, and when they were close in time, the NE tone always suppressed spikes evoked by the BD tone. In the dichotic condition, the BD tone was presented to the contralateral ear. The NE tone was presented to the ipsilateral ear and suppressed BD tone evoked spiking in ~50% of cells. Properties of the ipsilaterally-evoked inhibition were investigated by varying the frequency of the NE tone from the best excitatory frequency (BEF), throughout a cell’s excitatory bandwidth (eBW). We measured the inhibitory frequency response area, best inhibitory frequency (BIF), and inhibitory bandwidth (iBW) of each cell. We found inhibition became weaker as the frequency of the NE tone moved further from the middle of the eBW. We found that a DTN’s BEF and BIF closely matched, but the eBW was broader than the iBW and overlapped the iBW measured from the same cell. This suggests temporal selectivity of midbrain DTNs are created by monaural inputs, with binaural inputs playing a lesser role in shaping duration selectivity. / Thesis / Master of Science (MSc)

A method is presented for analysing a 2 x 2 factorial experiment in which the data consist cf relative rankings in pairwise comparisons. Maximum likelihood estimates are developed for the ratings of the various levels of each factor und for the treatment combinations. Likelihood ratio tests of the most important hypotheses likely to arise are derived in detail. The large sample approximations are used. In addition, the method is presented in a manner such that tests of other hypotheses in which the experimenter might be interested can easily be derived. The equations for the analysis of a factorial design of arbitrary size are presented, It can be seen, however, that the complexity of these equations render an attempt at their solution impractical in most cases and more work must be done if a useful method of analysing experiments of this, type is to be found. / Master of Science

Three watersheds located in Westmoreland County, Virginia were monitored to evaluate effectiveness of forestry best management practices (BMPs) for minimizing hydrologic and water quality impacts associated with timber harvesting. One watershed was clear-cut without implementation of BMPs, one watershed was clear-cut with the implementation of BMPs and the third watershed was left undisturbed as a control. The 27 months of pre-harvest monitoring data and 17 months of post-harvest monitoring data were compared using the paired watershed regression analysis and the minimum detectable change (MDC) statistic. Analysis of the hydrologic data showed that peak discharge rates were not impacted by harvesting, regardless of whether BMPs were implemented. Harvesting with or without the implementation of BMPs resulted in no statistically significant change in stormflow volume. BMP implementation was found to be effective in minimizing harvesting impacts on stormflow total suspended solids (TSS), ammonia, total Kjeldahl nitrogen (TKN), total nitrogen (TN), sediment-bound nitrogen and total phosphorous (TP) concentrations and total flow TSS and TP concentrations. BMP implementation was also effective in minimizing harvesting impacts on stormflow TSS, TKN, sediment-bound nitrogen, TP and sediment-bound phosphorous loadings and total ammonia loading. The BMPs were not effective in reducing nitrate concentrations and loadings. The BROOK90 forest hydrology model was used to predict the site hydrology. Model predictions compared favorably with streamflow measurements from watersheds QN3 and QN4 until the time of harvest, but did not predict streamflows as accurately for the control watershed, QN5. / Master of Science

Exotic invasive plants have become a significant issue in the Southeastern United States for private landowners. These plants possess characteristics that allow for rapid growth and easy adaptation to many growing conditions, often outcompeting native vegetation and altering wildlife habitat, especially in disturbed areas. Disturbance, including access roads, trails, harvest sites, powerline corridors, and fence rows, is common on private land. Private landowners are often left to combat these problems without many monetary or expertise resources that are available to federal lands. Three field sites, each in a different physiographic province in Virginia, were surveyed for exotic invasive populations and sampled with nested overstory, understory, and regeneration plots and wildlife point intercept transects using paired plots during the summers of 2006 and 2007. Species richness of the overstory and understory did differ, but native percent understory cover and sapling density remained unchanged. Tree density and forest basal area were reduced with presence of exotic invasive plants. Regeneration diversity and density decreased in areas of exotic plant invasion. Eastern cottontail habitat suitability increased with the presence of exotic invasive plants. Suitability of habitat for the gray squirrel, downy woodpecker food, black-capped chickadee reproduction, and eastern wild turkey cover declined with the occurrence of exotic invasive plants. Twenty three of thirty seven total invasive plots were within twenty feet of a disturbance area. Continual assessment of impacts will help provide a better understanding of the nature of exotic invasive plants to landowners and may help them to manage and prevent plant invasions. / Master of Science

The experiment tested the hypothesis that the acetylcholine (ACh) release in the rat auditory cortex is greater in rats undergoing auditory classical conditioning compared to rats in a truly random control paradigm where no associative learning takes place and that this is mediated by prefrontal afferent projections influencing the nucleus basalis magnocellularis (NBM), which in turn modulates ACh release in neocortex. Rats with bilateral ibotenic acid lesions of medial prefrontal and agranular insular cortices were tested in an auditory classical conditioning task while ACh was collected from the primary auditory cortex. It was hypothesized that lesions of these prefrontal areas would prevent learning-related increases of ACh release in the primary auditory cortex. The hypothesized results were supported. Results from this experiment provide unique evidence that medial prefrontal cortex projections to the NBM are important for mediating cortical ACh release during associative learning.

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A few aspects of aircraft noise were evaluated. These were (i) methods of subjective evaluations, (ii) effects of equalization and (iii) the effects of cognitive aspects. In the first paper, sorting algorithms were used instead of conventional paired comparison method in order to reduce the number of pairs in the evaluation of subjective judgments. The quick sorting algorithm method revealed more than 99% correlation coefficient with paired comparison method although the method used N*log(N) evaluations instead of N(N-1)/2. In the second paper, equalization effects on perception were evaluated in two steps, first with stationary aircraft sounds and second with non-stationary aircraft sounds. The first experiment examined the effects of stationary sound segments respect to three different angle positions of the aircrafts relative to the observer (78.7°, 90° & 101.3°), two different SNR conditions (sounds having original broadband plus tonal components versus control broadband sounds having no tonal components) and two different flight conditions (arrival and take-off). Subjects were asked to scale five perceptual attributes (loudness, annoyance, hardness, power and pitch) using Borg CR100 scale. The angle condition showed highly significant effects on annoyance and hardness. Maximal effects were found at an angle of 78.7°. The SNR revealed a significant impact on loudness, power and pitch. The second experiment analyzed the effects of tonal components and the problem of appropriate equalization. The spectrum of the signals was modified in two steps (buzz-saw, isolated BPF tone). Further EPNL-equalization, A-, B-, C-, D- and spectral broadband equalizations were applied to the synthesized sounds. Annoyance, loudness, hardness and pitch in the isolated tone conditions showed significantly stronger effects than the buzz-conditions on the perceived judgments. The EPNL-equalization led to a lower degree of differentiation between the spectral conditions compared to B- and C-level equalization. In the third paper, the effects of aircraft sounds on children’s cognitive performance were investigated. Impact of aircraft noise on children cognition was found significantly higher in reading comprehensions than in basic mathematics and problem solving tests. It seems children are very sensitive to the modifications in the aircraft noise but further studies are necessary to compliment such a finding.

Recent research suggests that visual-verbal paired associate learning (PAL) taps a crossmodal associative learning mechanism involved in reading acquisition. However, evidence from children with dyslexia indicates that deficits in visual-verbal PAL are strongly linked to the verbal demands of the task. The research presented in this thesis had two overarching aims: first, to dissociate crossmodal and verbal demands in driving the PAL-reading relationship, and second, to assess the hypothesis that visual-verbal PAL plays a causal role in reading development. To address the first aim, a series of experiments examined the relationship between reading ability and PAL tasks differing in modality (crossmodal, unimodal) and output demand (visual, verbal). The results supported a verbal account of the PAL-reading relationship. In typically developing children and children with dyslexia, only tasks with a verbal output demand (i.e., visual-verbal PAL, verbal-verbal PAL) demonstrated a relationship with reading ability. In children with dyslexia, poor performance was isolated to difficulties learning novel phonological forms, rather than difficulties specific to crossmodal associative learning. Furthermore, the ability to learn novel phonological forms was found to fully explain visual-verbal PAL performance across reading abilities. In a final experiment, the causal role of visual-verbal PAL in reading development was assessed. The results of a longitudinal study from the start to the end of kindergarten showed that visual-verbal PAL measured in pre-readers did not predict reading ability at the end of kindergarten. Instead, PAL performance was influenced by learning to read.

Reinforcement learning occurs when organisms adapt the propensities of given behaviours on the basis of associations with reward and punishment. Currently, reinforcement learning models have been validated in minimalist environments in which only 1-2 environmental stimuli are present as possible predictors of reward. The exception to this is two studies in which the responses of the dopamine system to configurations of multiple stimuli were investigated, however, in both cases the stimuli were presented simultaneously rather than in a sequence. Therefore, we set out to understand how current models of reinforcement learning would respond under more complex conditions in which sequences of events are predictors of reward. In the two experimental chapters of this thesis, we attempted to understand whether midbrain dopaminergic neurons would respond to occasion setters (Chapter 3), and to the overexpectation effect (Chapter 4). In addition, we ran simulations of the behavioural paradigms using temporal difference models of reinforcement learning (Chapter 2) and compared the predictions of the model with the behavioural and neurophysiological data. In Chapter 3, by performing single-neuron recording from VTA and SNpc dopaminergic cells, we demonstrated that our population of neurons were most responsive to the latest predictor of reward, the conditioned stimulus (CS) and not the earliest, the occasion setter (the OS). This is in stark contrast with the predictions of the model (Chapter 2), where the greatest response is seen at the OS onset. We also showed at a neural level that there was only a weak enhancement of the response to the discriminative stimulus (SD) when this was preceded by the OS. On the other hand, at a behavioural level, bar pressing was greatest when the SD was preceded by the OS, demonstrating that rats could use the information provided by the OS, but that dopamine was not controlling the conditioned response. In Chapter 4, our population of dopaminergic neurons showed that they would preferentially respond to only one of the two conditioned stimuli (CSA, CSB) in the overexpectation paradigm. The predictions of the model (Chapter 2) suggested that when the two stimuli would be presented in compound, there would be an inhibitory response if the reward magnitude was kept constant and an excitatory response if the reward magnitude was doubled. The lack of neural firing to one of the two conditioned stimuli, however, does not make for easy interpretation of the data. Perhaps, one of the conditioned stimuli acted as if it were overshadowing the other, resulting in no response to the second CS. Interestingly, at a behavioural level, we did not see increased licking frequency to the compound stimuli presentation, a result that is somewhat at odds with the previous literature. Overall, the results of our experimental chapters suggest that the role that midbrain dopaminergic neurons play in reinforcement learning is more complex than that envisaged by previous investigations.

published_or_final_version / Education / Doctoral / Doctor of Philosophy