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The heart rate variability (HRV) signal has been employed as a measure of sympathovagal balance in the human autonomic nervous system (ANS). It is known that aging affects the functional characteristics of the ANS. It has been suggested that complexity as measured by nonlinear dynamical indices, decays with age. We developed several algorithms and test protocols to characterize nonlinear dynamics in the HRV signal and to test the hypothesis that aging reduces the complexity within the HRV signal. Continuous HRV signal was obtained from 93 healthy subjects (41 males and 52 females) ranging in age between 5 and 78 years under controlled laboratory conditions in supine state. Subjects were from pediatric (PED, 5-12 years, n=15, 9 male, 6 female), adolescent (ADO, 13-17 years, n=16, 6 male, 10 female), adult (ADL, 18-30 years, n=22, 12 male, 10 female), middle aged (MDA, 31-60 years, n=21, 8 male, 13 female) and elderly (ELD, 61+ years, n=19, 6 male, 13 female) age groups. The length of data was 1000 or more R-R intervals for adequate computation. Stationary Holter HRV data from these controls were also used for the present study. Our results are as follows: There is a continuous systematic decay in the power-law scaling (beta), which decreases from -1.162 ± 0.388 for the PED group to -1.95 ± 0.6 for the ELD group (F = 6.649, p < 0.001; R = 0.475, p < 0.001. Approximate entropy (ApEn) decreases with age from 1.456 ± 0.093 for the PED group to 1.272 ± 0.135 for the ELD group (F = 7.82, p < 0.001; R = 0.519, p < 0.001. The detrended fluctuation analysis (DFA) of short-term data yielded an increase in short-range DFA scaling exponent (alpha)1 from 0.774 ± 0.204 for the PED group to 1.138 ± 0.289 for the ELD group (F = 7.535, p < 0.001), and in long-range DFA scaling exponent (alpha)2 increased from 0.667 ± 0.082 for the PED group to 0.86 ± 0.172 for the ELD group (F= 4.841,p < 0.001). The detrended fluctuation analysis (DFA) of long-term data yielded an increase in short-range DFA scaling exponent (alpha)1 from 1.052 ± 0.218 for the PED group to 1.204 ± 0.205 for the ELD group (F = 1.922), and in long-range DFA scaling exponent (alpha)2 increased from 0.961 ± 0.081 for the PED group to 1.076 ± 0.102 for the ELD group (F = 4.06, p < 0.01). Surrogate data analysis demonstrated that the hypothesis that the HRV signal is generated by a linear stochastic process is not always rejected. In summary, the HRV signal lends itself to an analysis using nonlinear dynamical methods and studies in patients may yield useful clinical information in the future. / Thesis / Master of Engineering (ME)

One of the main goals of ecological research is to understand the factors that determine how communities are structured over both space and time. However, our understanding of any system is largely a function of the scale at which we make our observations. Thus, the mechanisms that determine patterns in community structure are likely to change depending on the scale of observation. This thesis explores how environmental variability affects community structure and species performance, and how the resulting patterns change as a function of scale. Specifically, I asses the role of variability in temperature, oxygen, pH, and chloride, on species richness, abundance, diversity, and species performance, at three observational scales: micro-spatial, local temporal, and landscape-temporal scales, in 49 natural erosional rock pool microcosms, located on the northern coast of Jamaica. I found that while environmental variability was not a primary determinant of species richness or abundance, it did play a role in determining species compositions in the pools. I also show that community patterns are strongly affected by the scale of observation. Recognizing scale-dependent changes in community patterns is a prerequisite for predicting the consequences of changes in ecological systems induced by variability in abiotic factors. / Thesis / Master of Science (MS)

The purpose of this study was to assess heart rate variability (HRV) in patients with coronary artery disease (CAD): reproducibility, circadian variability and the effects of stress (coronary angiogram). Sixty-one patients who had a coronary angiogram underwent 48-hour Holter monitoring during a period of high stress beginning 4-hours post-angiography (Day 1 and 2), and again two weeks later during a period of low stress (Day 3 and 4); both 24-hour time domain and power spectral measures were computed. To determine reproducibility, intraclass correlation coefficients were calculated for both time and frequency domain indices on Days 3 and 4. The intraclass correlation coefficient for the standard deviation of normal RR-intervals over 24-hours (SDNN) was 0.91, while the standard deviation of the mean of all 5-minute segments of normal RR-intervals for 24-hours (SDANN) was 0.85. The most reproducible time domain measure was pNN50 (defined as the percentage of differences between adjacent normal RR-intervals that are greater than 50 ms computed over 24-hours) with an intraclass correlation coefficient of 0.95. As for the frequency domain measures including low frequency (LF) area, high frequency (HF) area, low frequency to high frequency area ratio (LF:HF area), LF central frequency (cf), and total area (TA), intraclass correlations were found to be the best at 0300-hours and the worst at 1500-hours. Circadian pattern was determined on Day 4 of Holter monitoring. A main effect for time was found for heart rate (HR), LF area, HF area, and TA of the power spectra. Over a 24-hour period, HR, LF area, HF area and TA were the lowest at 0300-hours compared to all other times. There was also a main effect for myocardial infarction (MI) for the frequency domain indices LF area, HF area, LF:HF area ratio, and LFcf. Low frequency area and LF:HF area ratio were significantly elevated (both p<0.05), while both HF area (p<0.05) and LFcf (p<0.01) were reduced in patients with a prior Ml compared to those with no MI. As well, a significant interaction between time of day and Ml, and time of day and beta-blocker therapy was observed. Patients with coronary artery disease and a prior Ml demonstrated a reduced circadian pattern over 24-hours for HR. Likewise, the circadian pattern of HR for those on beta-blockers was also attenuated. The effects of stress was determined by comparing Day 1 to Day 4 of ambulatory Holter monitoring. The time domain measure SDNN was found to be significantly reduced during Day 1 (mean ± SEM; 111.67 ± 6.13 ms) compared to Day 4 (121.54 ± 6.94 ms; p<0.05). Patients with normal left ventricular function (LVF) showed a significant increase from Day 1 to 4 for both SDNN (p<0.01) and SDANN (p<0.05). In contrast, those with LV dysfunction had an attenuated response. Similarly, those CAD patients on beta-blockers demonstrated a significant increase for the time domain measure SDNN (p<0.05), unlike those not on beta-blockers. In the frequency domain, LF:HF area was significantly greater on Day 1 (1.74 ± 0.09) compared to Day 4 (1.64 ± 0.09; p<0.05). These findings suggest that HRV measures are reproducible, that a circadian pattern for HRV exists in patients with CAD, and that stress induced by an invasive procedure such as a coronary angiogram enhances sympathetic input to the SA node in the heart and thereby alters the sympathovagal balance, which is restored two weeks later. / Thesis / Master of Science (MSc)

Exposure to loud sounds can cause damage to the inner ear, leading to degradation of the neural response to speech and to formant frequencies in particular. This may result in decreased intelligibility of speech. An amplification scheme for hearing aids, called Contrast Enhanced Frequency Shaping (CEFS), may improve speech perception for ears with sound-induced hearing damage. CEFS takes into account across-frequency distortions introduced by the impaired ear and requires accurate and robust formant frequency estimates to allow dynamic, speech-spectrum-dependent amplification of speech in hearing aids. Several algorithms have been developed for extracting the formant information from speech signals, however most of these algorithms are either not robust in real-life noise environments or are not suitable for real-time implementation. The algorithm proposed in this thesis achieves formant extraction from continuous speech by using a time-varying adaptive filterbank to track and estimate individual formant frequencies. The formant tracker incorporates an adaptive voicing detector and a gender detector for robust formant extraction from continuous speech, for both male and female speakers in the presence of background noise. Thorough testing of the algorithm using various speech sentences has shown promising results over a wide range of SNRs for various types of background noises, such as AWGN, single and multiple competing background speakers and various other environmental sounds. / Thesis / Master of Applied Science (MASc)

Anxiety and its cognitive component of worry have been related to exaggerated cardiovascular reactivity and delayed recovery to laboratory stressors, and to increased risk of cardiovascular disease. Previous research on the anxiety-cardiovascular system relationship, including data from Knepp and Friedman (2008), are included to support this project. Two experiments were completed during the course of this study. The first consisted of two peripheral-based body positioning tasks. The second experiment used an active versus passive sympathetic stress task paradigm (mental arithmetic, hand cold pressor). Subjects were nonsmokers free of cardiovascular and neurological disease. Trait worry was examined through the Penn State Worry Questionnaire (PSWQ). Blood pressure recordings and cardiac recordings through ECG and ICG were done in each experiment during seven epochs: an anticipatory baseline with three baselines preceding and three recovery periods following each task. Repeated measures analysis was run on all cardiovascular measures. In the first experiment, high worriers had worsened blood pressure reactivity to task. The second experiment found that high worriers had increased stroke volume across all epochs. There were mixed findings in the studies relating to subjects acclimated to the laboratory experience. Future directions of research relating anxiety, worry, and cardiovascular risk factors are discussed. / Ph. D.

Study design is among the most fundamental factors influencing collection and interpretation of data. The purpose of this study is to understand the effect of design choices by evaluating gait mechanics in healthy control participants using three primary objectives: 1) determine the repeatability of marker placement, 2) determine the effect of set versus self-selected walking speed, and 3) examine the correlation between gait asymmetry and motor variability. Ten and fifty-one healthy control participants were recruited for aim 1 and aims 2/3, respectively. Reflective markers were placed on lower-extremity bony landmarks and participants walked on an instrumented treadmill while 3D motion capture data was collected. For aim 1, this procedure was repeated at two time points 30 minutes apart. For aims 2 and 3, participants completed set and self-selected speed trials. JMP Pro 13 was used to compare joint kinetics and gait kinematics for all aims. Marker placement was repeatable between time points. Participants walked slower in the self-selected walking speed trial, which resulted in both kinematic and kinetic gait mechanics alterations. Gait asymmetry was significantly correlated with motor variability for both spatial and temporal measures. Current study findings reiterated the importance of walking speed when evaluating gait symmetry, joint kinetics, and kinematics. The decision regarding whether to utilize a set or self-selected speed condition within a study design should be made based on whether the measures of interest are independent of walking speed. Gait asymmetry and motor variability are related and should not be treated as independent components of gait. / Master of Science / This study aims to evaluate gait mechanics in healthy young adults by evaluating the impact of multiple study design choices and relationships between different aspects of gait (walking). Loading and movement walking data was collected from a total of sixty-one participants. This data was then used to calculate several measures of gait including symmetry between limbs, joint ranges of motion, and variability of movement. The potential impact of study design choices including setting walking speed for all participants and evaluating loading asymmetry and movement variability independently are discussed based on the findings of the current study.

Falls are the most frequent cause of unintentional injuries among older adults; afflicting 30 percent of persons aged 65 and older and more than 50 percent of persons aged 85 and older. There is a serious need for strategies to prevent falls in elderly individuals, but an important challenge in fall prevention is the paucity of objective evidence regarding the mechanisms that lead directly to falls. There exists no mechanisms about how to predict and manage elderly falls, which has multifactorial risk factors associated with its occurrence in the elderly. As the U.S. population continues to age, both the number of falls as well as the cost of treatment of fall injuries will continue to grow. Decades of research in fall prevention has not led to a decrease in the fall incidence; thus new strategies need to be introduced to understand and prevent falls. Aging reduces the adaptability of various physical and environmental stressors that hinder stability and balance maintenance and may therefore result in a fall. Movement variability in an individual's task performance can be used to assess the limitations of the movement control system. Maintaining variation in movement engenders flexible and adaptable modalities for elderly individuals to prevent falls in an unpredictable and ever changing external environment. Conversely, excessive variability of movement may drive the control system closer to its stability limits during balance and walking tasks. Accordingly, inertial sensors are an emerging wearable technology that can facilitate noninvasive monitoring of fall prone individuals in clinical settings. This research examined the potential of inertial sensors for use in clinical settings, and evaluated their effectiveness in comparison to mature laboratory systems (i.e., force platform and camera system). Study findings showed a relationship between movement variability and fall risk among healthy young and older adults. Further, the outcomes of this work translates to the clinical environment to better understand the health status (leading to frailty) of cardiac patients; reflected by the underlying adaptability of the control system, but requires further improvements if to be used as robust clinical tool. This research provides the groundwork for rapid clinical assessments in which its validity and robustness should be investigated in future efforts. / Ph. D.

Although the negative consequences of traumatic exposure across various domains of functioning have been well-documented, gaps and discrepancies continue to exist in the understanding of the impact of complex trauma, such as interpersonal violence (IPV), and how outcomes may vary across diverse populations and identities. In this cross-sectional study investigating the impact of traumatic exposure on physiological domains of functioning, a sample of female-identifying college students completed a number of self-reported measures (assessing past and present trauma exposure, complex posttraumatic stress disorder [CPTSD] symptoms, racial-ethnic minority status, and age of onset of first traumatic exposure) and provided resting high-frequency heart rate variability (hfHRV) data, which served as a biomarker for the potential impact of trauma exposure on physiological domains. Correlational and multiple regression analyses were conducted to determine the strength of relationships between variables and the predictive value of the models. Results indicated endorsement of IPV trauma was significantly associated with earlier age of onset, more severe levels of CPTSD symptoms, and higher hfHRV, but not racial-ethnic minority status. Racial-ethnic minority status was significantly related to more severe CPTSD symptoms. Type of trauma exposure was the only variable that emerged as having predictive value for changes in hfHRV. These findings suggest that experiencing IPV may have unique implications for trauma symptomatology and functioning above and beyond other forms of traumatic exposure, but that continued research must be conducted in order to draw more robust conclusions about the effects of exposure on physiological regulation across various racial-ethnic identities. / M.S. / Research has highlighted the consequences that extremely negative, stressful experiences, also called traumatic events, can have on the way humans think, emote, behave, and physically react. It can be more difficult to draw conclusions about the effects of interpersonal violence (IPV), or violence that occurs at the hands of another (i.e., family, partner, or community violence), due to the complex, severe, and long-term nature of symptoms that survivors experience. There is also limited research about what complex trauma looks like across diverse populations. This study aimed to investigate the impact of traumatic exposure on physiology, which falls under biology and broadly includes the functions of living things. A sample of female-identifying college students completed a number of self-reported measures (assessing trauma exposure, complex posttraumatic stress disorder [CPTSD] symptoms, racial-ethnic minority status, and age of onset of first traumatic exposure) and provided resting high-frequency heart rate variability (hfHRV) data, which measures variation in time between beats and served as a measure for the potential impact of trauma exposure on physiology. Results suggested that IPV exposure was associated with earlier age of first traumatic experience, more severe levels of CPTSD symptoms, and higher hfHRV. Racial-minority status was significantly related to more severe CPTSD symptoms. Type of trauma exposure significantly predicted changes in hfHRV. Findings suggest that experiencing IPV may uniquely influence trauma symptoms and functioning above and beyond other forms of traumatic exposure. Continued research will allow for stronger conclusions about the effects of traumatic exposure on physiology across various racial-ethnic identities.

Heart rate variability (HRV) has been used to evaluate cardiac autonomic function by measuring variations in electrocardiographic R-R intervals between cardiac cycles. HRV was first used to associate decreases in autonomic nervous system (ANS) control with an increased risk of mortality in coronary heart disease and in the diagnosis of diabetes (1). Current clinical research interest has extended to investigate uses of HRV to evaluate changes in the cardiovascular system due to disease, aging, physical activity, and cardiac rehabilitation treatment (2, 5). HRV scores are derivatives of R-R intervals and these may be represented as a function of either time or frequency domain parameters. Time domain analysis is the simplest and includes: the standard deviation of R-R intervals and the number of adjacent RR intervals that differ by >50ms (dRR50). Frequency domain measures involve more elaborate calculation and have been applied in studies to evaluate sympathetic and parasympathetic autonomic balance. The latter include: Low Frequency Power (LF), High Frequency Power (HF), and LF/HF ratio. HRV has been measured in a variety of ways, the most common being a continuous 24-hour collection of R-R data. In recent years, several investigators have sought to assess HRV by utilizing brief collection periods. Controversy exists about the potential of these short-term sampling intervals to yield reproducible and meaningful measurements of HRV. Many confounders such as respiration, stress, and body positioning can influence HRV, which is why a longer collection period has been accepted as the standard for providing a stable index of ANS function. However, short sampling periods would be useful to evaluate HRV when faced with time constraints. The purpose of the current study was to evaluate the reproducibility of HRV using 8-hour daytime measures with the Polar R-R RecorderTM (Polar Electro Oy, Kempele, Finland) and with short sampling duration of 512 cardiac cycles, using the Schiller AT-10TM device (Schiller AG, Baar, Switzerland). Methods: 10 apparently healthy adult volunteers participated in the study, which was conducted at the Sleep Disorders Clinic in Christiansburg, VA. Each subject performed two HRV trials with the Cardiovit AT-10TM device using recordings of 512 cardiac cycles. Within one or two days following the Schiller, the same subjects wore a Polar R-R RecorderTM device to obtain an 8-hour recording of HRV during waking hours; 24-hour urine samples were collected on the same day. Urine was analyzed for catecholamine levels, including norepinephrine and epinephrine in order to evaluate sympathetic nervous system globally. Each subject recorded their personal impressions of unavoidable physical activity and daytime stress demands on the day of the 8-hour recording and urine collection. This entire protocol was repeated one week later. On one of the days of the short sampling recording, VO2pk also was evaluated for each subject using a ramp protocol on the cycle ergometer and a metabolic cart. Results: The correlation analysis for the HRV response variables using the Schiller method indicated a high-to-very high correlation between trials within a day for the time domain measures (r = 0.75-0.99). The frequency domain measures, however, were low-to-moderately correlated (r = 0.24-0.66) between trials within a day for the Schiller method. Correlations between days for HRV response variables using the Schiller method were similarly low for both time (r < 0.5) and (r < 0.4) frequency domain measures. Correlation coefficients between days for the HRV response variables using the Polar method were moderate (r = 0.59-0.67) for the time domain and only low-moderate for the frequency domain measures (r = 0.37-0.69). However, an important finding was that Polar R-R data for two of the subjects contained excessive signal artifact, which affected the fidelity of the HRV scores. When these two cases were excluded from the group analyses, the resulting correlations were high-very high for all time and frequency domain measures (r = 0.70-0.93). The means for each response HRV time and frequency domain variable between the Polar method and Schiller method were significantly different (P < 0.05). Additional correlational analyses did not reveal any systematic associations between HRV measures and simple markers of sympathetic activity (urinary NE or E) and aerobic fitness (VO2pk) in this small sample of subjects. Conclusions: Due to this important change in reproducibility with the Polar method, the consequence of artifact-free recordings is unmistakable. Within the limitations of this small study sample it is concluded that, while HRV in apparently healthy adults may not be measured reliably with brief data collection periods, longer daytime sampling periods of 8 hours (e.g. Polar device) yields acceptable reliability for both time and frequency domain parameters of HRV. / Master of Science