Theta activity is one of the most prominent rhythms in the brain and appears to be conserved among mammals.  These 4-12 Hz oscillations have been predominantly studied in the dorsal hippocampus where they are correlated with a broad range of voluntary and exploratory behaviors. Theta activity has been also implicated in a number of mnemonic processes, long-term potentiation (LTP) induction and even acting as a global synchronizing mechanism. Moving along the dorso-ventral axis theta activity is reduced in power and desynchronized from the dorsal part. However, theta activity can also be generated in the ventral hippocampus itself during anxiety- and fear-related behaviors. Until now it was unknown which hippocampal cell population was capable to generate theta activity and it was controversial if its origin was local, in the hippocampus, or driven by other brain regions. In this thesis I present compelling in vitro and in vivo  evidence that   a subpopulation of OLM interneurons (defined by the Chrna2-cre line)  distinctively enriched  in the CA1 region of  the ventral hippocampus is implicated in LTP function (paper I,II), information control (paper V) and the induction of theta activity that is under cholinergic  control (paper IV). Importantly, a concomitant effect of the optogenetically induced theta activity is reduction in anxiety (Paper IV). Another innovation of this work was the development of a methodological approach to avoid artefactual signals when combining electrophysiology with light activation during optogenetic experiments (Paper III). In summary, the work presented in this thesis elucidates the role of a morphologically and electrophysiologially identified cell population, OLMα2 interneurons, first on the cellular, then on the circuit and ultimately on the behavioral level.

No description available.

The topic of this dissertation, modeling and identification of nonlinear oscillation, represents an area of mathematical systems theory that has received little attention in the past. Primarily, the types of oscillation of interest are those found in biological systems where theoretical foundations for mathematical models are insufficient. These oscillations are also observed in other systems including electrical, mechanical, and chemical. The contributions of this dissertation are a generalized class of autonomous differential equations that are found to exhibit stable limit cycles, and an investigation of a method of system identification that can be used to estimate the model parameters. Here the observed signal is modeled as the response of a nonlinear system that can be described by differential equations. Modeling the signal in this way shifts the emphasis from signal characteristics, such as spectral content, to system characteristics, such as parameter values and system structure. This shift in emphasis may provide a better method for monitoring complex systems that exhibit periodic behavior such as patients under anesthesia. A class of autonomous differential equations, called the generalized oscillator models, are presented as one nᵗʰ-order differential equations with nonlinear coefficients. The coefficients are chosen to change sign depending on the magnitude of the phase variables. The coefficients are negative near the origin and positive away from the origin. Motivated by the generalized Routh-Hurwitz criterion, this coefficient sign changing produces the desired oscillation. Properties of the generalized oscillator model are investigated using the describing function method of analysis and numerical simulation. Several descriptive examples are presented. Based on the generalized oscillator model as a set of candidate models, the system identification problem is formed as a mathematical programming problem. The method of quasilinearization is investigated as method of solving the identification problem. Two examples are presented that demonstrate the method. It is shown that in general, the method of quasilinearization as a solution to the system identification problem will not converge regardless of the initial starting point. This result indicates that although the quasilinearization method is useful for solving two-point boundary value problems, it is not useful (in its present form) for solving the system identification problem.

This work analyzes data taken in 1979 using a modification of the solar detector at SCLERA (Santa Catalina Laboratory for Experimental Relativity) designed to enhance spatial properties of the previously observed solar oscillations. Unlike previous solar observations taken at SCLERA, where the data consisted of single solar diameter measurements, the 1979 data consisted of six recorded limb profiles. This has important ramifications for the amount of signal present in the data which was generated by the terrestrial atmosphere, for the origin of the observed solar oscillations in fluctuations of the solar limb darkening function, and, most importantly, for the spatial symmetry properties of the observed solar eigenfunctions. The data consisted of 18 days of observations averaging ten hours per day and covering a total of 41 days. A linked Fourier transform of all 18 days was done for signal generated from each limb profile, and combinations of these six Fourier transforms made to increase sensitivity to symmetric or antisymmetric properties of the observed solar eigenmodes. The following results were found: 1. The observed oscillations are manifestations of fluctuations in the solar limb darkening function. 2. Terrestrial atmospheric contributions to the observed signal are negligible; thus, the sun constitutes the only possible source of the signal. 3. Given a resolution element of 1/(41 days) or 0.28 μHz, the solar oscillations observed represent individual solar eigenstates. 4. The spatial properties of the eigenstates are consistent with their interpretation in terms of spherical harmonics defined with respect to the observed solar rotational axis. 5. The eigenstates are temporally coherent for > 2 days and, in selected samples, for > 41 days. 6. The observed spacing of groups of eigenmodes is shown to be indicative of solar rotational effects; this spacing implies that the core of the sun is rotating approximately six times faster than the observed surface rotational velocity.

Two new analysis methods for solar seismology are developed. Called the continuous orthonormalization (CON) and adjoint methods, their use enables both solar eigenfrequencies and eigenfrequency sensitivities (partial derivatives with respect to solar model parameters) to be computed more accurately and efficiently than with existing methods. The CON method integrates an eighth-order nonlinear system of ordinary differential equations (ODEs) which defines the linear adiabatic nonradial oscillation modes of the Sun. (The Cowling approximation is not used.) All normal modes of oscillation are treated identically, regardless of their type (pressure, gravity or fundamental) or their predominant location inside the Sun. The adjoint method integrates a related eighth-order linear inhomogeneous system of ODEs. From the resultant solution, an eigenfrequency's partial derivatives with respect to an extensive set of solar model parameters may be computed simultaneously. Extensive numerical tests confirm the validity of the two new methods. Eigenfrequencies obtained via the CON method have seven significant digits and match within 1% the eigenfrequencies obtained via finite difference or mesh approaches. (Exact agreement is neither expected nor attainable because differently defined solar models are analyzed. The CON method analyzes models which are functionally specified on a continuum of radial points; the other methods analyze models defined on discrete sets of radial points.) Eigenfrequency sensitivities obtained via the adjoint method match within 2% the results obtained by explicitly perturbing the solar model parameters and recomputing the eigenfrequencies. The usefulness and power of the two new methods are demonstrated by applying them to the solution of an elementary solar inversion problem. A sample solar model's f-mode frequencies (obtained via the CON method) are iteratively driven into agreement with an observed set of f-mode frequencies. Adjoint sensitivity results are used to alter solar model parameters within hundreds of radial bins. The frequency movement is large, comparable to the frequency separation between adjacent degree f-modes. Model changes are also large; the density near the base of the convection zone is roughly doubled, while slightly further out it is halved.

This work investigates the response of the solar atmosphere to mechanical and thermal driving due to global solar oscillations. It was discovered that the coupling of thermal and mechanical modes was very important in reconciling theoretical predictions of the expected change in the solar limb due to solar oscillations and experimental observations of the variability in the solar limb darkening function undertaken at SCLERA (Santa Catalina Laboratory for Experimental Relativity). The coupling between the thermal and mechanical modes occur mainly due to the nonlocal nature of the radiation field. Previous theoretical calculations that used approximations for the radiative transfer that ignored the nonlocal nature of the radiation field predicted expected temperature perturbations (compared to the fluid displacement) that were much too small to be observed. Much larger ratios were found when the radiative transfer was treated properly. A particular solar oscillation can be influenced by the presence of a large number of other modes, if these modes can change the average properties of the medium. If the basic nonlinear equations are statistically averaged, the influence of the "mean field" can be investigated. This nonlinear effect can become important in the analysis for single modes in the upper photosphere.

No description available.

With recent advances in CCD technology, it has become possible to detect low-amplitude variability in stars. Thus, the number of low-amplitude variables has increased at an exceptional rate over the past decade. Many of these low-amplitude variables are pulsating stars such as Delta Scuti or Gamma Doradus stars, whose periods are on the orders of hours and days, respectively. One particular place where these variables are being found is in close binary systems. A close binary system has two components separated on the order of tens of solar radii and whose periods are on the order of days. Eclipsing binary systems occur when the orbital plane of the system is aligned such that the stars eclipse each other with respect to Earth’s line of sight. Soydugan et al. (2006) presented a paper in which a small number of eclipsing systems with a Delta Scuti-type pulsating component were analyzed. The group derived an observational relationship between the pulsation and orbital periods, thus indicating a physical phenomenon. The proposed project herein will seek to further determine whether there is a statistically significant relationship between the pulsation period and orbital parameters of close binary systems with a Delta Scuti-type pulsating component by searching for such pulsations in close binary systems using the method of high-precision CCD photometry. / Stellar dynamics -- Observations -- [Delta] Scuti stars in close binary systems. / Department of Physics and Astronomy

Space missions like Kepler have revolutionized asteroseismology, the science that infers the stellar interiors by studying oscillation frequency spectra of pulsating stars. Great advancements have been made in understanding solar-like oscillators. However, this is not the case for variable stars of intermediate masses, such asScutiand Doradus variables. By studying these stars in eclipsing binaries (EBs), model independent funda- mental parameters such as mass and radius can be inferred. On one hand, this synergy constrains the parameter space and facilitates the asteroseismic modeling, and this is shown for the Scuti type pulsating EB KIC 9851944. On the other hand, studies of binary stars must address the complexities such as mass transfer. KIC 8262223 is such an example, which consists of a mass-gaining Scuti primary and a pre-He white dwarf secondary. Some of the eccentric binary systems, the ‘heartbeat’ stars, show tidally excited oscillations. After briefly reviewing the linear theory of tidally forced stellar oscillations, we study the tidal pulsating binary KIC 3230227 and demonstrate that both amplitude and phase can be used to identify the tidally excited pulsation modes. We also discuss the variability of a Slowly Pulsating B-star KOI-81 and a Cataclysmic variable KIC 9406652. In the second part of this dissertation, we apply Bayesian statistics to some problems in binaries and asteroseismology with the help of packages BUGS and JAGS. Special attention is paid to the inverse problems (tomography) encountered in studying the double-line spectroscopic binaries.

In making this study it is important to search for ways to enhance and, if possible, make detection of MMO signals simpler in order that this technique for obtaining FS measurements may be extended to other materials. This attempt to improve measurement techniques has resulted in a significant discovery: the eddy-current techniques described in detail in a later section which should allow MMO to be observed and sensitively measured in many additional solids. The second major thrust of the study has been to use the newly discovered eddy-current technique in obtaining the first indisputable observation of MMO in zinc.