Plasticity of synapses is not static across the lifespan. As the brain matures and ages, the ability of neurons to undergo structural and functional change becomes more limited. Further, there are a number of modulatory factors that influence the expression of synaptic plasticity. Here, three approaches were taken to examine and manipulate plasticity in the auditory thalamocortical system of rats. Using an in vivo preparation, long-term potentiation (LTP) and paired pulse (PP) responses were used as measures of long- and short-term plasticity, respectively. First, the effect of intracortical zinc application in the primary auditory cortex (A1) on LTP was examined. Following theta burst stimulation (TBS) of the medial geniculate nucleus (MGN), juvenile and middle-age rats, but not young adults, showed greater levels of LTP with zinc application relative to age-matched control animals. Next, PP responses were examined between rats reared in unaltered acoustic conditions and those reared in continuous white noise (WN) from postnatal day (PD) 5 to PD 50-60 (i.e., subjected to patterned sound deprivation). Rats reared in WN demonstrated less PP depression relative to controls, indicating that WN rearing alters short-term thalamocortical synaptic responses. Furthermore, control males showed no change in PP response following LTP induction, indicating a postsynaptic locus of LTP, whereas increased PP depression following LTP induction was seen in WN animals, suggestive of a presynaptic involvement in LTP. Finally, differences in plasticity between male and female rats were investigated, and the result of early WN exposure on both sexes was examined. Males and females did not show consistent differences in LTP expression; however WN exposure appeared to affect LTP of females less than their male counterparts. PP responses were then compared between WN-reared males and females, and no difference was found. This indicates that short-term plastic properties of auditory thalamocortical synapses between the sexes do not differ, even though plasticity on a longer time scale following sensory deprivation does indicate some difference. Together, the experiments summarized here identify some of the important factors that contribute to the regulation of short- and long-term synaptic plasticity in the central auditory system of the mammalian brain. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2012-10-30 16:01:28.796

No description available.

This work includes several studies exploring functional specialization of human primary and anterior secondary auditory cortex (AC) in the two hemispheres. It was hypothesized that the left hemisphere prepotency for linguistic processing is based, in part, on specialized mechanisms within this hemisphere for the processing of acoustic transients relevant to speech. Evidence supporting this idea was obtained in a positron emission tomography study (PET). Since nonlinguistic stimuli were used, the observed left-hemispheric activation cannot be specific to the speech system, but must reflect a more general processing mechanism. A reanalysis of data from six auditory PET studies revealed that the peak focus of auditory activation was significantly posterior to Heschl's gyrus (HG) in the right hemisphere, while it encompassed HG in the left. The cause of this asymmetry is unknown, but it appears to hold true for other auditory functional imaging studies in the literature. Functional specialization in auditory regions was also explored by testing patients with anterior temporal-lobe resections from either the left (LT) or right (RT) hemisphere and normal volunteers on several linguistic and nonlinguistic auditory tasks. The excisions in these patients always included some secondary AC, and sometimes included primary AC (HG). I had speculated that anterior. secondary AC in the left hemisphere was specialized for processing word-sounds, but patients with excisions in this area were unimpaired in their ability to use such information to retrieve items from the mental lexicon. Furthermore these patients did not show a disproportionate impairment on a lexical decision task, when items were presented aurally instead of visually, compared to normal subjects. With few exceptions, a battery of psychophysical tests of auditory processes were performed normally by all groups. The notable exception was an impairment in the group of RT patients with lesions that included HG on a ta

Thesis (Ph.D.)--University of Texas at Dallas, 2007. / Includes vita. Includes bibliographical references.

Thesis (Ph.D.)--University of Texas at Dallas, 2008. / Includes vita. Includes bibliographical references (leaves 90-91)

Thesis (Ph.D.)--University of Texas at Dallas, 2008. / Includes vita. Includes bibliographical references (leaves 194-202)

The primary auditory cortex (area AI) plays an important role in the localization of static sound sources. However, little is known concerning how it processes information about sound source motion. This study was undertaken to investigate the responses of single neurons in the primary auditory cortex of the cat to correlates of auditory motion in space. Diotic and dichotic changes in sound intensity presented through earphones simulated auditory motion in four directions: toward and away from the receiver along the midline, into the ipsilateral hemifield and into the contralateral hemifield. Different rates of intensity change simulated sound source velocity. Results indicate that AI neurons can be highly selective to intensity correlates of auditory motion. Three major classes of neurons were encountered: neurons sensitive to motion toward or away from the receiver, neurons sensitive to ipsilateral- or contralateral-directed motion, and monaural-like neurons. The different classes of direction-selective neurons were spatially segregated from each other and appeared to occur in clusters or columns in the cortex. In addition to their selectivity for different directions of simulated sound source motion, AI neurons also responded selectively to the rate and excursion of intensity changes, a correlate of sound source velocity. The major determinants of direction and velocity selectivity were interactions between the following response properties of AI neurons: binaural interaction type, ear dominance, on/off responses, and monotonicity of rate/intensity function. These findings suggest that neural processing of auditory motion may involve neural mechanisms distinct from those involved in static sound localization, and indicate that some neurons in the primary auditory cortex may be part of a specialized motion-detecting mechanism in the auditory system. / Arts, Faculty of / Psychology, Department of / Graduate

No description available.

This work examines the link between low-level cortical acoustic processing and higher-level cortical phonemic processing. Specifically, using functional magnetic resonance imaging, it looks at 1) whether or not the vowels [ɑ] and [i] are distinguishable in regions of interest defined by the first two resonant frequencies (formants) of those vowels and 2) whether or not that neural discrimination ability changes based on anatomical region. The formant-frequency based regions of interest are found to respond differentially to [ɑ] and [i] with the response to [ɑ] statistically significantly greater than the response to [i] in the averaged [ɑ] formant-frequency based region. Unexpectedly, the response to [i] is numerically but not statistically significantly greater than the response to [ɑ] in the averaged [i] formant-frequency based region. Additionally, there is not a significant interaction of this pattern with anatomical region, although early cortical auditory regions appear to show the pattern while later ones do not. Further investigation into the results leads to the hypotheses that they could be due to task-specific neural processing strategies and that the link between lower and higher-level cortical auditory processing is more complex than originally hypothesized.

Synaptic plasticity reflects the capacity of synapses to undergo changes in synaptic strength and connectivity, and is highly regulated by age and sensory experience. This thesis focuses on the characterization of synaptic plasticity in the primary auditory cortex (A1) of rats throughout development and following sensory deprivation. Initial experiments revealed an age-dependent decline in plasticity, as indicated by reductions in long-term potentiation (LTP). The enhanced plasticity of juvenile rats appeared to be mediated by NR2B subunits of the N-methyl-d-aspartate receptor (NMDAR), as NR2B antagonist application reduced LTP to adult-like levels in juveniles, yet had no effect in adults. The importance of sensory experience in mediating plasticity was revealed in experiments using white noise exposure, which is a sensory deprivation technique known to arrest cortical development in A1. Notably, adult rats reared in continuous white noise maintained more juvenile-like levels of LTP, which normalized upon subsequent exposure to an unaltered acoustic environment. The white noise-induced LTP enhancements also appeared to be mediated by NR2B subunits, as NR2B antagonists reversed these LTP enhancements in white noise-reared rats. Given the strong influence that sensory experience exerts on plasticity, additional experiments examined the effect of shorter episodes of white noise exposure on LTP in adult rats. Exposure to white noise during early postnatal life appeared to “prime” A1 for subsequent exposure in adulthood, resulting in enhanced LTP. The necessity of early-life exposure was evident, as repeated episodes of white noise in adulthood did not enhance plasticity. In older rats that typically no longer express LTP in A1, pharmacological methods to enhance plasticity were explored. Moderate LTP was observed in older rats with cortical zinc application, which may act through its antagonism of NR2A subunits of the NMDAR. Additionally, current source density and cortical silencing analyses were conducted to characterize the distinct peaks of field postsynaptic potentials recorded in A1, with the earlier and later peaks likely representing thalamocortical and intracortical synapses, respectively. Together, this thesis emphasizes the critical role of sensory experience in determining levels of cortical plasticity, and demonstrates strategies to enhance plasticity in the mature auditory cortex. / Thesis (Ph.D, Neuroscience Studies) -- Queen's University, 2011-01-11 14:53:57.677