Mathematical models for sleep-wake dynamics: comparison of the two-process model and a mutual inhibition neuronal model

Sleep is essential for the maintenance of the brain and the body, yet many features of sleep are poorly understood and mathematical models are an important tool for probing proposed biological mechanisms. The most well-known mathematical model of sleep regulation, the two-process model, models the sleep-wake cycle by two oscillators: a circadian oscillator and a homeostatic oscillator. An alternative, more recent, model considers the mutual inhibition of sleep promoting neurons and the ascending arousal system regulated by homeostatic and circadian processes. Here we show there are fundamental similarities between these two models. The implications are illustrated with two important sleep-wake phenomena. Firstly, we show that in the two-process model, transitions between different numbers of daily sleep episodes occur at grazing bifurcations.This provides the theoretical underpinning for numerical results showing that the sleep patterns of many mammals can be explained by the mutual inhibition model. Secondly, we show that when sleep deprivation disrupts the sleep-wake cycle, ostensibly different measures of sleepiness in the two models are closely related. The demonstration of the mathematical similarities of the two models is valuable because not only does it allow some features of the two-process model to be interpreted physiologically but it also means that knowledge gained from study of the two-process model can be used to inform understanding of the mutual inhibition model. This is important because the mutual inhibition model and its extensions are increasingly being used as a tool to understand a diverse range of sleep-wake phenomena such as the design of optimal shift-patterns, yet the values it uses for parameters associated with the circadian and homeostatic processes are very different from those that have been experimentally measured in the context of the two-process model

Sleep Timing in Late Autumn and Late Spring Associates With Light Exposure Rather Than Sun Time in College Students

Timing of the human sleep-wake cycle is determined by social constraints, biological processes (sleep homeostasis and circadian rhythmicity) and environmental factors, particularly natural and electrical light exposure. To what extent seasonal changes in the light-dark cycle affect sleep timing and how this varies between weekdays and weekends has not been firmly established. We examined sleep and activity patterns during weekdays and weekends in late autumn (standard time, ST) and late spring (daylight saving time, DST), and expressed their timing in relation to three environmental reference points: clock-time, solar noon (SN) which occurs one clock hour later during DST than ST, and the midpoint of accumulated light exposure (50% LE). Observed sleep timing data were compared to simulated data from a mathematical model for the effects of light on the circadian and homeostatic regulation of sleep. A total of 715 days of sleep timing and light exposure were recorded in 19 undergraduates in a repeated-measures observational study. During each three-week assessment, light and activity were monitored, and self-reported bed and wake times were collected. Light exposure was higher in spring than in autumn. 50% LE did not vary across season, but occurred later on weekends compared to weekdays. Relative to clock-time, bedtime, wake-time, mid-sleep, and midpoint of activity were later on weekends but did not differ across seasons. Relative to SN, sleep and activity measures were earlier in spring than in autumn. Relative to 50% LE, only wake-time and mid-sleep were later on weekends, with no seasonal differences. Individual differences in mid-sleep did not correlate with SN but correlated with 50% LE. Individuals with different habitual bedtimes responded similarly to seasonal changes. Model simulations showed that light exposure patterns are sufficient to explain sleep timing in spring but less so in autumn. The findings indicate that during autumn and spring, the timing of sleep associates with actual light exposure rather than sun time as indexed by SN

A Class of Adaptive EM-based Importance Sampling Algorithms for Efficient and Robust Posterior and Predictive Simulation

A class of adaptive sampling methods is introduced for efficient posterior and predictive simulation. The proposed methods are robust in the sense that they can handle target distributions that exhibit non-elliptical shapes such as multimodality and skewness. The basic method makes use of sequences of importance weighted Expectation Maximization steps in order to efficiently construct a mixture of Student-t densities that approximates accurately the target distribution -typically a posterior distribution, of which we only require a kernel - in the sense that the Kullback-Leibler divergence between target and mixture is minimized. We label this approach Mixture of t by Importance Sampling and Expectation Maximization (MitISEM). We also introduce three extensions of the basic MitISEM approach. First, we propose a method for applying MitISEM in a sequential manner, so that the candidate distribution for posterior simulation is cleverly updated when new data become available. Our results show that the computational effort reduces enormously. This sequential approach can be combined with a tempering approach, which facilitates the simulation from densities with multiple modes that are far apart. Second, we introduce a permutation-augmented MitISEM approach, for importance sampling from posterior distributions in mixture models without the requirement of imposing identification restrictions on the model's mixture regimes' parameters. Third, we propose a partial MitISEM approach, which aims at approximating the marginal and conditional posterior distributions of subsets of model parameters, rather than the joint. This division can substantially reduce the dimension of the approximation problem

An educational audiology service delivery model : needs of teachers of children with hearing loss

In South Africa, the current movement towards the inclusion of children with disabilities, including children with hearing loss, is likely to have far-reaching consequences for both teachers and learners. Undoubtedly, needs will arise from teachers during the transition, especially in the areas pertaining to the audiological and educational management of children with hearing loss. A hearing loss often negatively impacts on the development of the child’s auditory, language, speech, communication, literacy, academic, and psychosocial skills. The educational audiologist is uniquely skilled in managing the effects of hearing loss on the child’s educational development, and is a crucial member on the educational team. The educational audiologist as specialist in the management of children with hearing loss, is able to offer a wide range of support and assistance to teachers as well as children with hearing loss in the inclusive educational system. When teachers receive appropriate educational audiology services, they are enabled to provide quality education that strives to reach the full potential of every child with hearing loss. Therefore, an urgent need existed to determine the needs of teachers of children with hearing loss regarding an educational audiology service delivery model for use within the inclusive educational system. In order to comply with this need, a descriptive research design was developed comprising of a questionnaire survey followed by focus group interviews. The questionnaire survey explored the needs of 664 teachers of children with hearing loss. Focus group interviews were conducted with 19 teachers of children with hearing loss and these results were used to substantiate findings from the questionnaire survey. The results of the study indicated that the needs of teachers differ according to the sub-groups found among teachers, namely those teachers who mainly promote the use of spoken language and those who mainly promote Sign Language. Findings revealed that, although participants realised the importance of various aspects of development of the child with hearing loss, they generally did not realise the importance of receiving support from an educational audiologist. With respect to specifics in term of support, participants strongly recommended that teachers receive support in the acquisition of knowledge re the trouble-shooting of hearing aids, advocacy for the implementation of FM systems in inclusive classrooms and the development of speech production skills of the child with hearing loss in the inclusive environment. In addition, various suggestions were made regarding the structure of services rendered within the educational context. These findings were utilised in order to propose an educational audiology service delivery model for South Africa in the current timeframe.Thesis (DPhil (Communication Pathology))--University of Pretoria, 2005.Speech-Language Pathology and Audiologyunrestricte

Improving Density Forecasts and Value-at- Risk Estimates by Combining Densities

__Abstract__ We investigate the added value of combining density forecasts for asset return prediction in a specific region of support. We develop a new technique that takes into account model uncertainty by assigning weights to individual predictive densities using a scoring rule based on the censored likelihood. We apply this approach in the context of recently developed univariate volatility models (including HEAVY and Realized GARCH models), using daily returns from the S&P 500, DJIA, FTSE and Nikkei stock market indexes from 2000 until 2013. The results show that combined density forecasts based on the censored likelihood scoring rule significantly outperform pooling based on the log scoring rule and individual density forecasts. The same result, albeit less strong, holds when compared to combined density forecasts based on equal weights. In addition, VaR estimates improve a t the short horizon, in particular when compared to estimates based on equal weights or to the VaR estimates of the individual models

Predicting Covariance Matrices with Financial Conditions Indexes

We model the impact of financial conditions on asset market volatility and correlation. We propose extensions of (factor-)GARCH models for volatility and DCC models for correlation that allow for including indexes that measure financial conditions. In our empirical application we consider daily stock returns of US deposit banks during the period 1994-2011, and proxy financial conditions by the Bloomberg Financial Conditions Index (FCI) which comprises the money, bond, and equity markets. We find that worse financial conditions are associated with both higher volatility and higher average correlations between stock returns. Especially during crises the additional impact of the FCI indicator is considerable, with an increase in correlations by 0.15. Moreover, including the FCI in volatility and correlation modeling improves Value-at-Risk forecasts, particularly at short horizons

Emotion dynamics in children and adolescents:A meta-analytic and descriptive review

Theories on children and adolescents emotion dynamics were reviewed using data from 102 ecological momentary assessment studies with 19.928 participants and 689 estimates. We examined age-graded differences in emotional intensity, variability, instability, inertia, differentiation, and augmentation/blunting. Outcomes included positive versus negative affect scales, discrete emotions (anger, sadness, anxiety, and happiness), and we compared samples of youth with or without mental or physiological problems. Multi-level models showed more variable positive affect and sadness in adolescents compared to children, and more intense negative affect. Our additional descriptive review suggests a decrease in instability of positive and negative emotions from early to late adolescence. Mental health problems were associated with more variable and less intense positive affect, and more intense anxiety and heightened sadness variability. These results suggest systematic changes in emotion dynamics throughout childhood and adolescence, but the supporting literature proved to be limited, fragmented, and based on heterogeneous concepts and methodology

Development of a music perception test for adult hearing-aid users

The purpose of this research was twofold: firstly, to develop a music perception test (MPT) for hearing-aid users, and secondly, to evaluate the influence of non-linear frequency compression (NFC) on music perception with the use of the self-compiled test. This article focuses on the description of the development and validation of the MPT. To date, the main direction in frequency-lowering hearing-aid studies has been in relation to speech perception abilities. As hearing-aid technology has improved, interest has grown in musical perception as a dimension that could improve hearing-aid users’ quality of life. The MPT was designed to evaluate different aspects of rhythm, timbre, pitch and melody. The development of the MPT could be described as design-based. Phase 1 of the study included test development and recording, while phase 2 entailed presentation of stimuli to normal hearing listeners (n=15) and hearing-aid users (n=4). Based on the findings of phase 2, item analysis was performed to eliminate or change stimuli that resulted in high error rates. During phase 3 the adapted version of the test was performed on a smaller group of normal hearing listeners (n=4) and 20 hearing-aid users. Results proved that adults with normal hearing as well as adults using hearing aids were able to complete all the sub-tests of the MPT, although hearing-aid users scored lower on the various sub-tests than normal hearing listeners. For the rhythm section of the MPT normal hearing listeners scored on average 93.8% versus 75.5% of hearing-aid users; for the timbre section the scores were 83% versus 62.3% respectively. Normal hearing listeners obtained an average score of 86.3% for the pitch section and 88.2% for the melody section, compared with the 70.8% and 61.9% respectively obtained by hearing-aid users. This implies that the MPT can be used successfully for assessment of music perception in hearing-aid users within the South African context and may therefore result in more effective hearing-aid fittings taking place. The test can be used as a counselling tool to assist audiologists and patients in understanding the problems they experience regarding music perception, and might be used for future musical training in areas where participants experience problems in customising individual fittings.http://www.sajcd.org.za/index.php/SAJCDnf201

Antidepressants in primary care:patients' experiences, perceptions, self-efficacy beliefs, and nonadherence

PURPOSE: Patient adherence to antidepressants is poor. However, this is rather unsurprising, given the equivocal efficacy, side effects, and practical problems of antidepressants. The aim of this study was to examine a wide array of patient experiences and perceptions regarding the efficacy, side effects, and practical problems of antidepressants, as well as their associations with nonadherence, and whether patients' perceived self-efficacy moderated these associations.PATIENTS AND METHODS: Experiences and perceptions of 225 patients, recruited through community pharmacies, were efficiently assessed with the Tailored Medicine Inventory. Nonadherence was assessed through self-report and pharmacy refill data.RESULTS: Many patients were not convinced of the efficacy, thought the efficacy to be limited or did not believe antidepressants to prevent relapse, were worried about or had experienced one or more side effects, and/or had experienced one or more practical problems regarding information, intake, and packaging. Being convinced of efficacy was associated with lower intentional nonadherence (odds ratio [OR] 0.9, 95% confidence interval [CI] 0.8-0.96). A higher number of practical problems experienced was associated with increased unintentional nonadherence (OR 1.3, 95% CI 1.1-1.7). Higher perceived self-efficacy regarding taking antidepressants was associated with lower unintentional nonadherence (OR 0.7, 95% CI 0.5-0.9). Perceived self-efficacy did not moderate associations of patient experiences and perceptions with nonadherence.CONCLUSION: Assessing a wide array of patients' experiences and perceptions regarding the efficacy, side effects, and practical problems of antidepressants contributes to better understanding of nonadherence to antidepressants. Guiding physician-patient conversations by patients' experiences and perceptions may reduce both unintentional and intentional nonadherence. Also, it may give rise to considerations of prudent discontinuation, eg, when patients are not convinced of the efficacy.</p