Online Learning in Discrete Hidden Markov Models

We present and analyse three online algorithms for learning in discrete Hidden Markov Models (HMMs) and compare them with the Baldi-Chauvin Algorithm. Using the Kullback-Leibler divergence as a measure of generalisation error we draw learning curves in simplified situations. The performance for learning drifting concepts of one of the presented algorithms is analysed and compared with the Baldi-Chauvin algorithm in the same situations. A brief discussion about learning and symmetry breaking based on our results is also presented.Comment: 8 pages, 6 figure

Evidence of a Role for Three Neuropeptides that Mediate Steroid Negative Feedback on Gonadotropin Releasing Hormone/Luteinizing Hormone Secretion in the Ewe: Kisspeptin, Neurokinin B and Orphanin FQ

Gonadotropin-releasing hormone (GnRH) is the final common output from the central nervous system for controlling luteinizing hormone (LH) secretion from the anterior pituitary. GnRH, and subsequently LH, secretion for most of the menstrual/estrous cycle occurs in an episodic pattern that is controlled by the inhibitory actions of estradiol and progesterone. However, GnRH neurons are devoid of the appropriate steroid receptors, therefore, estradiol and progesterone must mediate their effects on GnRH/LH secretion through interneurons. Three likely candidates are neurons containing kisspeptin, neurokinin B (NKB) and orphanin FQ (OFQ).;Puberty in the ewe is marked by an increase in GnRH/LH secretion which in part results from a lessening of estradiol negative feedback. Kisspeptin and NKB are coexpressed in the same subsets of neurons in the arcuate nucleus (ARC) of the hypothalamus and are proposed to be critical for normal timing of puberty. Given that greater than 95% of kisspeptin/NKB neurons colocalize with estrogen receptor-alpha (ERalpha) and progesterone receptor in the ewe, we examined changes in kisspeptin- and NKB-positive neurons in the ARC of pre- and postpubertal ewes in the presence and absence of sex steroids. We observed a greater number of kisspeptin, but not NKB, positive neurons in the ARC of postpubertal ewes compared to prepubertal ewes, which corresponded with changes in LH secretion. Also, we showed that an intravenous injection of senktide, an NKB receptor agonist, stimulates LH secretion in prepubertal ewes. These results support a role for kisspeptin and NKB in ovine puberty.;OFQ when given centrally has been shown to inhibit LH secretion in rats and ewes and OFQ localizes to the external zone of the median eminence. Given its effect on LH secretion and its location, we investigated whether OFQ acts at the pituitary to inhibit LH release and/or at the hypothalamus to inhibit GnRH secretion. OFQ had no direct influence on ovine anterior pituitary cells in cell culture. However, we did observe a decrease in GnRH secretion in hypophyseal portal blood samples following infusion of an OFQ agonist into the third cerebroventricle of ovariectomized ewes. From this we conclude that OFQ acts centrally at the hypothalamus to inhibit GnRH secretion.;Given its central inhibitory action on GnRH secretion in the ewe, we examined if OFQ neurons in the preoptic area (POA) and hypothalamus contain steroid receptors. Furthermore, we investigated a role for endogenous OFQ by infusing an OFQ receptor antagonist into steroid-treated ewes. POA OFQ neurons did not contain steroid receptors, while a majority of OFQ neurons in the ARC contained both ERalpha and progesterone receptor. Furthermore, ovariectomized (OVX) ewes with luteal phase concentrations of progesterone and estradiol, but not OVX ewes implanted with only estradiol or ovary-intact anestrous ewes, showed a significant increase in LH pulse frequency during infusion of an OFQ receptor antagonist. Therefore, we conclude that OFQ mediates, at least in part, the negative feedback action of progesterone on GnRH/LH secretion in the ewe

REACTOR PHYSICS CALCULATIONS FOR THE MSRE

A compilation is presented of results obtained to date from a number of reactor physics calculations for the molten salt reactor experiment (MSRE). Included are one-dimensional multigroup and two-dimensional twogroup calculations of critical mass, flux, and power density distributions; gamma heating in the core can, reactor vessel, and core support grid; drain tank criticality; and an estimate of the beta , gamma , and delayed neutron dose rates due to fission products in the fuel contained in the pump bowl. For a cylindrical core 54 in. in diameter and 66 in. high, graphite-mcderated with 8 vol% fuel salt, the calculated critical loading is 0.76 mole% uranium (93.3% U/sup 235/), which is equivalent to a critical mass of 16 kg. At a reactor power of 10 mw, the peak power density in the core assuming a homogeneous mixure of fuel salt and graphite is 10 watts/cm/sup 3/, the average power density is 4 watts/cm/sup 3/. The computed peak thermal flux is 7.3 x 10/sup 13/ neutrons/cm/sup 2/ sec and the average is 2.5 x l0/sup 13/ neutrons/cm/sup 2/ sec. Gamma heating prcduces a power density of 0.2 watts/cm/sup 3/ in the core wall at the midplane and 0.4 watts/cm/sup 3/ in the support grid at the bottom of the core at the reactor center line. (auth

Ferromagnetic bubble clusters in Y0.67_{0.67}Ca0.33_{0.33}MnO3_3 thin films

We studied the ferromagnetic topology in a Y$_{0.67}$Ca$_{0.33}$MnO$_3$ thin film with a combination of magnetic force microscopy and magnetization measurements. Our results show that the spin-glass like behavior, reported previously for this system, could be attributed to frustrated interfaces of the ferromagnetic clusters embedded in a non-ferromagnetic matrix. We found temperature dependent changes of the magnetic topology at low temperatures, which suggests a non-static Mn$^{3+}$/Mn$^{4+}$ ratio