When compared to their non-musician peers, adult musicians demonstrate enhanced speech-in-noise perception (Parberry-Clark et al, 2013), verbal memory (Chan et al, 1998), phonological skills (Deg & Schwarzer, 2011), and reading (Tierney & Kraus, 2013). Recent evidence from auditory brainstem responses suggests that early musical training primes neural plasticity that facilitates listening in degraded environments (Strait et al., 2012). Our study examined the hypothesis that neural enhancements, subsequent to preadolescent musical training, improves binaural speech processing and temporal processing that are known to contribute significantly to speech understanding in complex listening environments (Hirsh, 1950; Snell & Frisina, 2000). We tested middle-school aged (10 - 14 years) children with and without musical training based on years of experience (< 6 months = non-musician; > 1 year = musician) and musical aptitude (Intermediate Measures of Music Audiation; rhythm subtest) on tests of spatial listening (Listening in Spatialized Noise-Sentences Test, Dillon, 2007) and auditory temporal processing (Adaptive Test of Temporal Resolution, Lister et al., 2011). We also measured working memory and visual processing efficiency (picture pattern memory and processing speed; NIH Tool Box) to understand the potential influence of multimodality higher-order cognitive skills over modality-specific enhancements in auditory perceptual processing secondary to musical training.

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Naming speed, motor skill, and auditory temporal processing (ATP) are constructs that are important to reading and language. These variables require processing timing information inherent in the stimulus or processing stimuli rapidly. ATP deficits are found in individuals with reading impairments, but studies are conflicting regarding the relationship between reading and ATP. This study examined relationships between naming speed, motor speed, and ATP, and centered on possible factors why inconsistencies have occurred across studies examining the association between reading and ATP. If the timing element of naming speed (rapid automatized naming-RAN) and of motor speed is common to ATP, then RAN and motor speed should predict thresholds for three auditory tasks (CMR, backward masking, and the precedence effect with TOJ) known to require temporal processing. Tasks were administered to adult participants in order to examine the effects with skilled readers. Many of the variables were skewed and there were multiple outliers that altered the analyses. Ultimately, 75 participants were included in the final data set. Results indicated that RAN did not predict thresholds for any of the masking tasks given. However, motor speed predicted thresholds for one CMR and two backward masking tasks, suggesting that motor speed should be controlled for in research assessing the contribution of ATP to reading or language. Neither naming speed nor motor speed predicted localization performance. Non-verbal intelligence predicted performance on several of the masking tasks, consistent with previous research. Performance on all three auditory tasks was similar to that reported in the literature assessing smaller samples of participants. Although the suggestion of a general timing component is not supported, the relationships found between motor speed and several auditory temporal measures indicate that the underlying timing elements are not independent.

The experiments described in this thesis use psychophysical techniques and human observers to investigate temporal processing in the millisecond range. The thesis contains five main sections. Introductory chapters provide a brief overview of the visual and auditory systems, before detailing our current understanding of duration processing. During the course of this review, several important questions are highlighted. The experiments detailed in Chapters 8-11 seek to address these questions using the psychophysical techniques outlined in Chapter 7. The results of these experiments increase our understanding of duration perception in several areas. Firstly, Experiments 1 and 2 (Chapter 8) highlight the role of low level stimulus features: even when equated for visibility stimuli of differing spatial frequency have different perceived durations. Secondly, a psychophysical hypothesis arising from the 'duration channels' or 'labelled lines' model of duration perception is given strong support by the adaptation experiments detailed in Chapter 9 and 10. Specifically, adaptation to durations of a fixed temporal extent induces repulsive duration aftereffects that are sensory specific and bandwidth limited around the adapted duration. Finally Chapter 11 describes the results of experiments designed to probe the processing hierarchy within duration perception by measuring the interdependency of illusions generated via duration adaptation and via multisensory cue combination. The results of these experiments demonstrate that duration adaptation is a relatively early component of temporal processing and is likely to be sub served by duration selective neurons situated in early sections of the visual and auditory systems.

Encoding time is one of the most important features of the mammalian brain. The visual system, comprising almost half of the brain is of no exception. Time processing enables us to make goal-directed behavior in the optimum “time window” and launch a ballistic eye movement, reach/grasp an object or direct our processing resources (attention) from one point of interest to another. In addition, encoding time is critical for higher cognitive functions, enabling us to make causal inferences. The limitations of temporal individuation in the visual stream seem to vary across the visual field: the resolution gradually drops as objects become farther away from the center of gaze, where little differences were found in terms of resolution for objects in the upper versus lower visual field. This resolution of temporal individuation is vastly different from the resolution ascribed to spatial individuation. If individuation is mediated through attention, as some researchers have proposed, the general term ”attention” seems to possess different properties, at least regarding temporal and spatial processing. Next we looked at another aspect of encoding time: Temporal Order Judgments (TOJ), where animals had to judge the relative timing onset of two visual events. After training two monkeys on the task, we recorded from neurons in the lateral intraparietal area (LIP), while the animals reported the perceived order of two visual stimuli. We found that LIP neurons show differential activity based on the animal’s perceptual choice: when the animal reports the stimulus inside the receptive field of the neuron as first, the cells show an increased level of activity compared to when the animal reports he same stimulus as second. This differential activity was most reliable in the tonic period of the response \((\sim100 ms\) after stimulus onset). However, no difference in visual response latencies was observed between the different perceptual choices. The parietal cortex has previously been implicated in temporal processing based on patient studies as well as neuroimaging investigations. Physiological studies have also suggested the involvement of parietal area in encoding elapsed time. However, our study is the first to demonstrate parietal neurons encoding relative timing. / Psychology

The principal neurons of the medial superior olive (MSO) are considered to be responsible for transforming the temporal information present in the binaural acoustic stimulus into an output encoding sound location along the horizontal axis. Spatial resolution of sound localization depends critically on the time resolution with which MSO neurons can detect microsecond differences in the timing of inputs from the two ears. This fast temporal processing is contingent on voltage gated ion channels. The work presented in this thesis demonstrates that two currents, namely a hyperpolarization activated cationic current and low voltage activated potassium current dynamically interact to regulate the intrinsic time resolution of MSO neurons. We observe that the ability of MSO neurons to perform sub-millisecond temporal processing matures after birth, especially around the time of the clearing of the auditory canal. Hyperpolarization activated cationic current was found to be one of the underlying mechanisms transforming slow immature MSO neurons into temporally precise adult MSO neurons. / text

Auditory processing disorder (APD) is characterized by deficits in the auditory modality that are not due to a global processing problem or to deficiencies in the peripheral auditory system. Diagnosis of APD is time-consuming and could benefit from an objective test. Temporal processing ability is often impaired in cases of APD, which includes deficits on gap detection tasks. Previous attempts to correlate gap detection thresholds to electrophysiological responses have used the P1-N1-P2 response, mismatch negativity (MMN), and 40 Hz auditory steady-state response (ASSR), but these attempts have not been successful, especially using between-channel gap detection tasks. The current study used a modulation gap detection task and recorded the above responses to supra- and subthreshold gaps and stimuli with no gap. A significant P2 response and a later positive peak distinguished perception of a suprathreshold gap. Improvements over previous studies, the relation to auditory training, and limitations and directions for future research are discussed.