This dissertation thesis is dedicated to the heart rate variability and methods of its evaluation. It mainly focuses on nonlinear methods and especially on the Poincaré plot. First it deals with the principle and nature of the heart rate variability, then the ways of its representation, linear and also nonlinear methods of its analysis and physiological and pathophysiological influence on heart rate variability changes. In particular, there is emphasis on the metabolic syndrome. In the next section of the thesis there are compared and evaluated different ways of representation of the heart rate variability and further are tested selected methods of heart rate variability analysis on unique data from patients with the metabolic syndrome and healthy subjects provided by the Institute of Scientific Instruments, Academy of Sciences of Czech Republic. In particular, they are used the Poincaré plot and its parameters SD1 and SD2, commonly used time domain and frequency domain parameters, parameters evaluating signal entropy and the Lyapunov exponent. SD1 and SD2 combining the advantages of time and frequency domain methods of heart rate variability analysis distinguish successfully between patients with the metabolic syndrome and healthy subjects.

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Heart rate variability as a measure of cardiovascular health and autonomic activity correlates with psychological resiliency but is not consistently related to trait extraversion, a strong predictor of emotional well-being. This study intends to clarify research findings about trait extraversion and heart rate variability by identifying the context and nature of the relationship between extraversion and physiological responses. As a secondary analysis of data from a study comparing biofeedback and compassionate breathing, extraversion scores were compared with heart rate variability data at three different points including prior to a stressor, during exposure to a stressor, and recovery to a stressor to determine the influence of extraversion on stress reactivity and stress recovery. In our sample population of 80 participants who were mostly young and in good health determined by self-report, the average extraversion score 79.14. Linear regression was used to compare differences at each time point and data was analyzed for significance at p=.05; a post-hoc power analysis revealed β = .81, 1 – β. There were no significant findings between extraversion and heart rate variability at any time point. The results of this study support no relationship between extraversion, health, and stress-resiliency.

People with aphasia are known to have poor word retrieval abilities in communicative tasks. It has also been reported that they have lower, non-optimal levels of physiological arousal, which may cause lower attention levels therefore contributing to poor performance on linguistic tasks. The purpose of this study was to investigate the relationship between physiological arousal and word retrieval in adults with aphasia and neurotypical adults when presented with emotional stimuli within a confrontational naming task. Participants included 6 people with aphasia and 15 neurotypical controls. All participants completed a confrontational naming task within 3 emotional conditions (neutral, positive, negative) and physiological measures (Heart Rate Variability, Skin Conductance) were taken simultaneously. No statistically significant results were found; however, numerical trends were identified in the data that may provide direction when designing future studies.

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Major depressive disorder (MDD) is one of the most common psychiatric illnesses and causes significant disturbances in daily functioning. Research on heart-rate variability (HRV) biofeedback training suggests that HRV is an efficacious adjunct to psychotherapy in reducing depressive symptoms. The purpose of this study was to examine neurological changes in depressed individuals who were randomized to either a psychotherapy plus HRV biofeedback training or to a treatment as usual group. A control group with no history of depression was also studied. We collected psychological, physiological, and imaging data from 30 participants (10 in an experimental group, 10 in a treatment as usual group, and the other 10 in a healthy control group) at baseline and follow-up. Regions of interest (ROIs) included anterior cingulate cortex, hippocampus, and amygdala. Participants from the experimental group went through 5 weekly HRV trainings in conjunction with traditional psychotherapy approaches. The treatment as usual group only received psychotherapy. The healthy controls did not receive any HRV training or therapy services. Overall, we found significant improvements in the experimental group's depression score, overall distress level, and HRV measurements relative to the TAU and control groups. However, we did not find significant HRV and resting-state connectivity group differences among experimental group relative to healthy controls. Together, results suggest that HRV training helps to reduce depressed participants' overall distress level and depressive symptoms. However, findings do not show any changes in participants' imaging data. These findings serve as pilot data on literature related to HRV biofeedback training in a depressed population.

Military training and operations can place significant demands on cognitive and physical resources of service members, resulting in heightened stress and fatigue, elevated risk of accidents and injuries, and diminished cognitive and occupational performance. Transcutaneous electrical stimulation (TES) is a novel, non-invasive neuromodulatory technique being investigated as a means to improve alertness and preserve performance under stress with few-to-no side effects. Despite the recent increase in research using TES, few studies have explored the effects of stimulation of the trigeminal nerve on cognition and the human stress response. Therefore, the aims of this study were to elucidate the effects of TES on biochemical and physiological responses to stress, cognition, and marksmanship performance under cognitive load. Participants in this repeated measures, crossover-design study included 23 healthy male (n = 18) and female (n = 5) civilians and members of the military ranging in age from 19 to 37 (mean 24.00 ± 5.65) years. Study procedures occurred in the afternoon on five consecutive days, including two testing days involving administration of active or sham TES to the right supraorbital region of the face using a commercially-available device (Thync One, Cerevast Therapeutics). To evaluate the effects of TES on the stress response, participants were required to complete a prolonged, cognitively challenging target discrimination task using a simulated firing range, which has been previously demonstrated to induce a reliable stress response in human research volunteers. Computer-assisted cognitive tasks were administered before and after rifle marksmanship in order to provide complementary assessment of functional domains challenged during the marksmanship task. Salivary markers of cortisol and α-amylase were collected at several time points during the testing day, and electrocardiography (ECG) and photoplethysmography (PPG), both markers of heart rate variability and stress responding, were monitored continuously. Linear mixed models with random slopes were used to analyze the effect of stimulation condition (active versus sham TES) on marksmanship and cognitive, physiological, and salivary outcomes across the testing period and at each measurement time point. No significant effects of stimulation condition or the interactions between stimulation condition and measurement time point were found for salivary stress biomarkers (punadj range 0.12 – 0.98) or for cognitive (punadj range 0.25 – 0.88) and physical workload (punadj range 0.31 – 0.79). There were no significant effects of stimulation condition on time-series indicators of heart rate variability (punadj range 0.10 – 0.96) except for pNN50 when measured with PPG (β = -4.97, punadj = 0.04, padj = n.s., d < 0.01). There were, however, significant stimulation condition by time interaction effects on mean heart rate, mean R-R interval, SDNN, RMSSD, and pNN50 (punadj range 0.12 – 0.98, d range < 0.01 – 0.02), indicating that trigeminal TES using the Thync One device increased activity of both the sympathetic and parasympathetic nervous systems during marksmanship and cognitive testing. Similar effects were noted on frequency-series indicators of heart rate variability using both ECG and PPG, in which stimulation condition effects were noted on ECG high frequency absolute (β = 8.50, punadj < 0.01, padj = 0.01, d < 0.01) and relative powers (β = -8.54, punadj < 0.01, padj = 0.01, d < 0.01), as well as PPG very low frequency power (β = -367.98, punadj < 0.01, padj = n.s., d = 0.12). Effects of the interaction between stimulation condition and measurement time point were noted on very low, low, and high frequency powers (punadj range < 0.01 – 0.048, d range < 0.01 – 0.21), as well as the ratio of low- to high-frequency powers in ECG (punadj range < 0.01 – 0.048, d < 0.01 for all). These results also suggest that trigeminal TES increased activity of both the sympathetic and parasympathetic nervous systems during marksmanship and cognitive testing. Furthermore, significant effects of stimulation condition were noted on marksmanship shot accuracy (β = 0.14, punadj = 0.01, padj = n.s., d = 0.60) and distance of shots from the targets’ center of mass (β = -0.08, punadj = 0.02, padj = n.s., d = 0.56), indicating that trigeminal TES impaired shot accuracy. There were also significant condition-by-time interaction effects on target detection latency (β = 220.46, punadj = 0.04, padj = n.s., d = 0.49); significant impairments in shot latency observed during the first marksmanship session in the active TES condition only resolved by the second marksmanship session. There were no significant effects of TES on accuracy or response times for neuropsychological tasks assessing response inhibition, sustained attention, and working memory (punadj range 0.09 – 0.98). Active trigeminal TES did, however, significantly reduce the standard deviation of response times on a measure of sustained attention and response inhibition (β = -16.29, punadj = 0.045, padj = n.s., d = 0.43). Although the literature suggests that TES may benefit stress and performance, these results do not support that conclusion. Overall, these analyses found that TES using a commercially available device did not influence chemical biomarkers of stress, but did influence markers of physiological stress, as well as cognitive and marksmanship performance under high cognitive load. TES was associated with impairments in marksmanship performance as well as increases in both sympathetic and parasympathetic nervous system activity. Further studies using different stimulation parameters, including multiple sessions of stimulation, will be necessary to more fully characterize possible influences of trigeminal nerve stimulation on stress responding and marksmanship performance or other military relevant tasks. In addition, this project underscores the need for more investigation into the mechanisms of effect of the Thync One device and other devices applying TES of the trigeminal nerve.

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