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Showing papers by "Dalhousie University published in 1983"


Journal ArticleDOI
TL;DR: This work characterize the graphs on which the cop has a winning strategy, and connects the problem with the structure theory of graphs based on products and retracts.

608 citations


Journal ArticleDOI
TL;DR: The day-to-day variations over a 70-day period in the parameters describing the lightsaturation curve of natural assemblages of coastal marine phytoplankton were examined and related to changing environmental conditions as mentioned in this paper.
Abstract: The day-to-day variations over a 70-day period in the parameters describing the lightsaturation curve of natural assemblages of coastal marine phytoplankton were examined and related to changing environmental conditions. The initial slope, ~8, and the light-saturated rate, P mB, both showed a threefold variation. Correlation and regression analysis attributed most of the variation in aS and P, B to factors relating to community structure. Changes in community structure resulting from the normal progression of spring-summer conditions were interrupted periodically by transient physical phenomena (passage of storms, periods of upwelling, etc.). The importance of physical transients to the short term dynamics of the phytoplankton production system places severe constraints on the development of predictive models of phytoplankton production.

257 citations



Journal ArticleDOI
TL;DR: These findings suggest that in multi-joint limb movements, the many degrees of freedom are organised to function temporarily as a single coherent unit that is uniquely specific to the task demands placed on it.
Abstract: In three experiments we show, using behavioural measures of movement outcome, as well as movement trajectory information and resultant kinematic profiles, that there is a strong tendency for the li...

244 citations



Journal ArticleDOI
TL;DR: In this article, the authors examined the short-range behavior of the spherically averaged Hartree-Fock exchange charge density by performing a simple Taylor expansion and constructed a theoretical model that generates gradient correction terms to the local density approximation for the exchange energy of an inhomogeneous electron gas.
Abstract: We examine the short-range behavior of the spherically averaged Hartree–Fock exchange charge density by performing a simple Taylor expansion. On the basis of this expansion, a theoretical model is constructed that generates gradient correction terms to the local density approximation for the exchange energy of an inhomogeneous electron gas. In particular, we derive the Xαβ exchange energy functional and a theoretical value for the parameter β. Our value for β agrees well with previous empirical estimates, and with empirical calculations in the present work.

225 citations


Journal ArticleDOI
TL;DR: The findings suggest that even one low-contrast letter chart could provide a valuable supplement to the standard Snellen chart, compared with other available devices, which have the advantages of cheapness, simplicity and of providing the ophthalmologist with an immediate check on patients' accuracy.

218 citations


Journal ArticleDOI
01 Jul 1983-Geology
TL;DR: In this article, fresh volcanic glass is preserved throughout the extrusive section of the Troodos ophiolite, which indicates that the lavas have not been pervasively metamorphosed.
Abstract: Fresh volcanic glass is preserved throughout the extrusive section of the Troodos ophiolite, which indicates that the lavas have not been pervasively metamorphosed. Glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals. The basal 400–500 m of the sequence consists of an andesite-dacite-rhyolite assemblage containing abundant hyaloclastites. The remainder of the section comprises a basalt–basaltic andesite assemblage with high MgO and low TiO 2 and total iron. The lower sequence is interpreted as an evolved arc-tholeiite suite; the upper has some similarities to boninitic lavas. The close association in time and space of these two suites is similar to that observed in the Mariana and Bonin arcs and suggests that all of the Troodos lavas were erupted in a subduction-zone environment, most probably in an incipient arc or fore-arc.

188 citations


Journal ArticleDOI
TL;DR: This method was simple, very effective, resulting in essentially complete removal of several endotoxins from heavily contaminated solutions (1-10 micrograms/ml by Limulus amoebocyte lysate assay) and employed mild physiological conditions.

188 citations


Journal ArticleDOI
TL;DR: The binding process is shown to be highly cooperative, and it is concluded that the hydrophobic part of the bound surfactant is not completely immersed in the Hydrophobic DNA core, but also interacts with other surfactants molecules.

162 citations


Journal ArticleDOI
TL;DR: It was found that the average relative contributions of the ankle and hip muscles were approximately 23 and 28% respectively, with the remaining 49% of the work being done by the muscles acting at the knee joint.
Abstract: A work-energy approach was used to determine the contributions of the muscles crossing the hip, knee and ankle joints to the total positive work done during maximal vertical jumps. It was found that the average relative contributions of the ankle and hip muscles were approximately 23 and 28% respectively, with the remaining 49% of the work being done by the muscles acting at the knee joint. The efficiency of jumping, i.e. the ratio of potential energy gained to the net mechanical work done by the muscles acting at the three lower limb joints was nearly 1.0. These results stress the importance of all three major leg extensor muscle groups to the performance of an explosive activity such as vertical jumping. It is suggested that the work-energy approach supplies useful information concerning joint contributions without the problems associated with other techniques.

Journal ArticleDOI
08 Apr 1983-Science
TL;DR: These models are discussed with respect to the kinds of evidence needed for their corroboration and to their significance for questions related to macroevolutionary punctuated equilibria and genetic revolutions.
Abstract: Growing recognition that much of the evolutionary history of eukaryotic genomes reflects the operation of turnover processes involving repetitive DNA sequences has led to the recent formulation of models describing speciation as a consequence of such turnover. These models are of three general kinds: those attributing hybrid infertility to the process of transposition, those attributing hybrid infertility to mispairing between chromosomes of divergent repetitive DNA composition, and those assuming that change in repetitive DNA's can reset coordinated gene regulation. These models are discussed with respect to the kinds of evidence needed for their corroboration and to their significance for questions related to macroevolutionary punctuated equilibria and genetic revolutions.


Journal ArticleDOI
TL;DR: It is concluded that the coupling of hypothalamic opiate receptors to the LH regulatory mechanisms is dependent upon gonadal steroids, the most important of these appear to be estradiol and testosterone, since careful priming of long-term gonadectomized rats with these steroids is able to largely restore the LH responses to naloxone and FK 33-824.
Abstract: We have studied the stimulatory effects of naloxone and the inhibitory influence of the opiate peptide, FK 33–824,on LH secretion in the gonadectomized rat. Our results indicate that endogenous and exogenous opiate involvement in LH release disappear coincident with the removal of gonadal steroid feedback. At 7 days post-surgery naloxone is no longer able to stimulate LH secretion in male or female rats. Similarly, by 7 days in the male, and 21 days in the female, FK 33–824 is unable to inhibit LH secretion. We conclude that the coupling of hypothalamic opiate receptors to the LH regulatory mechanisms is dependent upon gonadal steroids. The most important of these appear to be estradiol and testosterone, since careful priming of long-term gonadectomized rats with these steroids is able to largely restore the LH responses to naloxone and FK 33–824.

Journal ArticleDOI
TL;DR: Some general theorems on the existence of minimal points of compact sets in linear spaces with respect to closed convex orderings are proved and compared to the axiom of choice.
Abstract: We prove some general theorems on the existence of minimal points of compact sets in linear spaces with respect to closed convex orderings. These results are compared to the axiom of choice.

Journal ArticleDOI
TL;DR: The results imply that the initial main effect on fascia dentata granule cells of activity in the contralateral hilus is a γ‐aminobutyric‐acid‐mediated inhibition, which most probably involves commissural excitation of local inhibitory interneurons.
Abstract: Stimulation of the contralateral hippocampus in the hilar region had a marked effect on granule cell excitability in the fascia dentata. The primary effect was to block the population spike that otherwise occurred in response to perforant path stimulation. In contrast, the size of the excitatory post-synaptic potential component of the perforant path-evoked field potential was only slightly reduced. The population spike diminution began at short latency (3.5 msec), beginning at about 1.0 msec after the onset of the slow component of the potential evoked by the contralateral stimulus. The completeness and duration of this population spike diminution depended on the contralateral stimulus intensity. The maximum duration was less than 40 msec in unanesthetized rats and more than 100 msec in rats under pento-barbital anaesthesia. Bicuculline did not diminish the field potential evoked bythe contralateral stimulus but it did prevent the stimulus from blocking the perforant path population spike and, instead, permitted a weak facilitation of the population spike. The normal reduction of the population spike was not mediated by recurrent inhibition, secondary to granule cell activation, since it occurred whether or not the granule cells were inhibited at the time of contralateral stimulation. These results imply that the initial main effect on fascia dentata granule cells of activity in the contralateral hilus is a γ-aminobutyric-acid-mediated inhibition. This effect most probably involves commissural excitation of local inhibitory interneurons. The direct excitatory action of commissural fibers on granule cells, by comparison, is very weak.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the volume Vt of the tracer domain grows much more rapidly than volume Vp of the particle domain if the merging of streaks is ignored and streaks must merge and Vp can be used to provide a rough estimate of peak concentration, or concentration variance.

Journal ArticleDOI
TL;DR: The demonstration of a maturation-related reduction in opiate inhibition of LH suggests a critical role for endogenous (hypothalamic?) opiate peptides and opiate receptors in the onset of sexual maturation.
Abstract: We have examined (1) the effects of naloxone and an opiate peptide, FK 33-824, on LH and FSH secretion in immature male and female rats and (2) the influence of sexual maturation on the ability of this peptide to inhibit LH secretion. FK 33-824 potently inhibits LH secretion in the 48-h-gonadectomized rat. This effect could be blocked by coinjection of naloxone, suggesting that the peptide exerts its influence through opiate receptors. An endogenous opiate component in the control of LH secretion was demonstrated by examining the effect of naloxone injection alone. Naloxone rapidly stimulated LH secretion in the intact or the acutely gonadectomized rat. Naloxone or FK 33-824 did not modify LHRH-stimulated release of gonadotropins from organ cultures of anterior pituitary obtained from immature rats. This suggests, in agreement with other reports, that naloxone or FK 33-824 controls LH secretion via a central, possibly hypothalamic site. We further observed a clear age-related reduction in LH sensitivity t...

Journal ArticleDOI
TL;DR: It is concluded that management procedures can have strong selective effects and that genetic changes may be expected to occur rapidly if the obvious genetic conditions are met.

Journal ArticleDOI
TL;DR: The results show that monocular deprivation is more effective in the experimental animals than in normal kittens of the same age, but less effective than innormal 2-month-old animals.

Journal ArticleDOI
TL;DR: A physiologically based discrimination model is proposed: Asymmetrically tuned cortical cells feed a ratio-tuned neural mechanism whose properties are formally analogous to those of ratio- Tuned neurons that have recently been found in cat visual cortex.
Abstract: We found that inspecting a sine-wave grating elevated threshold for spatial-frequency discrimination as it does for contrast detection, but discrimination threshold was maximally elevated at about twice the adapting frequency, where detection threshold was little affected; and detection threshold was maximally elevated at the adapting frequency, where discrimination threshold was not elevated at all. Orientation tuning was roughly similar for contrast and for discrimination threshold elevations; elevations fell by half at between 7 and 17 deg from the adapting orientation. We compared our findings with the predictions of three models of discrimination: (1) The data are inconsistent with the idea that the most strongly stimulated channels are the most important channels for discrimination. (2) With an additional assumption, the Hirsch-Hylton scaled-lattice model could account for our finding that discrimination threshold elevations are asymmetric. (3) With no additional assumptions, the idea that discrimination is determined by the relative activities of multiple overlapping spatial-frequency channels or size-tuned neurons can account for our finding that discrimination thresholds are asymmetric. We propose a physiologically based discrimination model: Asymmetrically tuned cortical cells feed a ratio-tuned neural mechanism whose properties are formally analogous to those of ratio-tuned neurons that have recently been found in cat visual cortex. The linear relation between firing frequency and contrast can explain why discrimination threshold is substantially independent of contrast.


Journal ArticleDOI
TL;DR: In this paper, the authors considered photodesorption due to a laser resonantly coupled into an internal vibrational mode of an adsorbed molecule and calculated desorption rates as a function of temperature and laser intensity.
Abstract: Photodesorption due to a laser resonantly coupled into an internal vibrational mode of an adsorbed molecule is considered. Based on a master equation with transition probabilities calculated quantum statistically for laser-induced vibrational transitions, phonon-mediated bound-state---bound-state and bound-state---continuum transitions, and tunneling transitions from bound states degenerate with the continuum, we calculate desorption rates as a function of temperature and laser intensity. They are highly nonlinear in both, and become saturated for high intensities. The theory is applied to the C${\mathrm{H}}_{3}$F-NaCl and CO-NaCl systems.

Journal ArticleDOI
TL;DR: Partial or complete inhibition of hemolymph phenoloxidase activity occurred in host species attacked by some parasitoid wasps.
Abstract: Partial or complete inhibition of hemolymph phenoloxidase activity occurred in host species attacked by some parasitoid wasps. In one system, inhibition of enzyme activity could be achieved by injection of a virus purified from parasitoid ovaries.


Journal ArticleDOI
TL;DR: Wangersky et al. as mentioned in this paper described the chemical characteristics of samples collected with a bubble adsorptive technique at monthly intervals during the course of a year in Halifax Harbour, and the chemical composition of the coastal samples varied with the seasons; the aerosols generally contained a higher proportion of low molecular weight compounds than those generated from oceanic water samples (10% < 500 mol wt).
Abstract: Samples of organic matter were isolated f?om seawater of the North Atlantic Ocean and Halifax Harbour by a bubble adsorptive technique. The samples thus collected, representative of the organic materials important in the transport of gcochcmical species across the airsea interface, were characterized by exclusion chromatography, elemental analysis, stable carbon isotope analysis, and assays for “carbohydrate” and “protein” levels. The concentration of organic carbon in the aerosol samples was enriched relative to the original seawater samples by factors of 47-573. The chemical composition of the coastal samples varied with the seasons; the aerosols generally contained a higher proportion of low molecular weight compounds (41% < 500 mol wt) than those generated from oceanic water samples (10% < 500 mol wt). The influence of the Northeast Atlantic Current can be discerned in the composition of aerosols generated from North Atlantic surface waters. Described elsewhere (Gcrshey 1983) is a device that concentrates and collects from seawater large quantities of organic materials which are important in physicochemical transport processes at the airsea interface. I describe here the chemical characteristics of samples collected with this device at monthly intervals during the course of a year in Halifax Harbour. Samples collected in the mid-North Atlantic Ocean were also analyzed. I thank P. J. Wangersky, R. A. Duce, R.

Journal ArticleDOI
TL;DR: Orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing and are as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies.
Abstract: This study of form vision explores the relationships between orientation and spatial frequency in suprathreshold discrimination tasks. Orientation discrimination thresholds for sine-wave gratings were 0.3–0.5 deg, much less than the roughly 10–24-deg orientational bandwidth of channels; spatial-frequency discrimination thresholds were 3–7%, much less than the roughly 1.2-octave spatial-frequency bandwidth of channels. We find that spatial-frequency discrimination between two gratings was as acute when the two gratings were orthogonal as when they were parallel. Orientation discrimination between two gratings was as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies. Thus orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing.

Journal ArticleDOI
TL;DR: It is concluded that the hypothesis of antagonistic interactions between life- history characters, generalized to take genetic variability into account, is the most satisfactory theory of life-history evolution available.
Abstract: SYNOPSIS. In order to assess current scientific understanding of life-history evolution, the alternative fundamental theories are formulated in a refutable form and compared with the available empirical evidence. The hypothesis that life-history does not evolve is rejected on the grounds that life-history can be readily modified by artifical selection. The hypothesis that life-history evolves according to mechanisms other than natural selection acting on genetic variation is shown to have no sound experimental basis. The hypothesis that life-history evolution depends primarily on group selection is undermined by the absence of the predicted group adaptations. The hypothesis that life-history is a unitary character which evolves in the same fashion as fitness is rejected because of the disparity between life-history genetics and basic theory concerning the evolution of fitness. The hypothesis that life-history is composed of a set of autonomous characters which are subject to mutation accumulation at later ages is refuted by the lack of any detectable increase in genetic variability with age and the evidence for the interdependence of lifehistory characters. It is concluded that the hypothesis of antagonistic interactions between life-history characters, generalized to take genetic variability into account, is the most satisfactory theory of life-history evolution available.

Journal ArticleDOI
01 Jul 1983-Geology
TL;DR: The extrusive section on the northern flank of the Troodos ophiolite is 1,000 to 1,500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia.
Abstract: The extrusive section on the northern flank of the Troodos ophiolite is 1,000 to 1,500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia. Some pillowed units, which average 100 to 200 m in thickness, vary systematically from lava tubes up to 6 m in diameter at the base, through common pillows less than 1 m in diameter, to small pillows and extrusive breccias at the top. These assemblages appear to be individual volcanoes, suggesting episodic volcanic accretion. The upper 100 to 200 m of the section is pervasively altered and oxidized, but fresh volcanic glass is preserved throughout most of the extrusive section. The lavas are subdivided on geochemical evidence into a lower, differentiated andesite to dacitic andesite series resembling evolved island-arc tholeiites and an upper series of picrite, mafic basalt, and basaltic andesite very low in TiO2, K2O, P2O5, Nb, Rb, and Ba. The highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs. Lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process. The occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion, located above a subduction zone.

Journal ArticleDOI
TL;DR: A combination of 1.0mg dl-norgestrel and 0.1mg ethinylestradiol (EE) was administered orally at 36 hours after the detection of the luteinizing hormone peak and again at 48 hours in 12 healthy volunteers with normal menstrual cycles.