23 research outputs found
Hyperbolic odorant mixtures as a basis for more efficient signaling between flowering plants and bees
Animals use odors in many natural contexts, for example, for finding mates or food, or signaling danger. Most analyses of natural odors search for either the most meaningful components of a natural odor mixture, or they use linear metrics to analyze the mixture compositions. However, we have recently shown that the physical space for complex mixtures is âhyperbolicâ, meaning that there are certain combinations of variables that have a disproportionately large impact on perception and that these variables have specific interpretations in terms of metabolic processes taking place inside the flower and fruit that produce the odors. Here we show that the statistics of odorants and odorant mixtures produced by inflorescences (Brassica rapa) are also better described with a hyperbolic rather than a linear metric, and that combinations of odorants in the hyperbolic space are better predictors of the nectar and pollen resources sought by bee pollinators than the standard Euclidian combinations. We also show that honey bee and bumble bee antennae can detect most components of the B. rapa odor space that we tested, and the strength of responses correlates with positions of odorants in the hyperbolic space. In sum, a hyperbolic representation can be used to guide investigation of how information is represented at different levels of processing in the CNS
Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion Emission Densities for IC 418
Recent analyses of nebular spectra have resulted in discrepant abundances
from CNO forbidden and recombination lines. We consider independent methods of
determining ion abundances for emission nebulae, comparing ion emission
measures with column densities derived from resonance absorption lines viewed
against the central star continuum. Separate analyses of the nebular emission
lines and the stellar UV absorption lines yield independent abundances for
ions, and their ratio can be expressed in terms of a parameter n_e_{em}, the
``emission density'' for each ion. Adequate data for this technique are still
scarce, but separate analyses of spectra of the planetary nebula and central
star of IC 418 do show discrepant abundances for several ions, especially Fe
II. The discrepancies are probably due to the presence of absorbing gas which
does not emit and/or to uncertain atomic data and excitation processes, and
they demonstrate the importance of applying the technique of combining
emission- and absorption-line data in deriving abundances for nebulae.Comment: 25 pages, 3 figures, accepted for publication in PAS
s-Process Abundances in Planetary Nebulae
The s-process should occur in all but the lower mass progenitor stars of
planetary nebulae, and this should be reflected in the chemical composition of
the gas which is expelled to create the current planetary nebula shell. Weak
forbidden emission lines are expected from several s-process elements in these
shells, and have been searched for and in some cases detected in previous
investigations. Here we extend these studies by combining very high
signal-to-noise echelle spectra of a sample of PNe with a critical analysis of
the identification of the emission lines of Z>30 ions. Emission lines of Br,
Kr, Xe, Rb, Ba, and Pb are detected with a reasonable degree of certainty in at
least some of the objects studied here, and we also tentatively identify lines
from Te and I, each in one object. The strengths of these lines indicate
enhancement of s-process elements in the central star progenitors, and we
determine the abundances of Br, Kr, and Xe, elements for which atomic data
relevant for abundance determination have recently become available. As
representative elements of the ``light'' and ``heavy'' s-process peaks Kr and
Xe exhibit similar enhancements over solar values, suggesting that PNe
progenitors experience substantial neutron exposure.Comment: 56 Pages, 6 figures, accepted for publication in ApJ This version
corrects missing captions in Figure 1-3 and minor typo
Independent Emission and Absorption Abundances for Planetary Nebulae
Emission-line abundances have been uncertain for more than a decade due to
unexplained discrepancies in the relative intensities of the forbidden lines
and weak permitted recombination lines in planetary nebulae (PNe) and H II
regions. The observed intensities of forbidden and recombination lines
originating from the same parent ion differ from their theoretical values by
factors of more than an order of magnitude in some of these nebulae. In this
study we observe UV resonance line absorption in the central stars of PNe
produced by the nebular gas, and from the same ions that emit optical forbidden
lines. We then compare the derived absorption column densities with the
emission measures determined from ground-based observations of the nebular
forbidden lines. We find for our sample of PNe that the collisionally excited
forbidden lines yield column densities that are in basic agreement with the
column densities derived for the same ions from the UV absorption lines. A
similar comparison involving recombination line column densities produces
poorer agreement, although near the limits of the formal uncertainties of the
analyses. An additional sample of objects with larger abundance discrepancy
factors will need to be studied before a stronger statement can be made that
recombination line abundances are not correct.Comment: 19 pages, 13 figures, accepted by ApJ. Preprint utilizes
emulateapj.cls v. 12/01/06 (included
25th annual computational neuroscience meeting: CNS-2016
The same neuron may play different functional roles in the neural circuits to which it belongs. For example, neurons in the Tritonia pedal ganglia may participate in variable phases of the swim motor rhythms [1]. While such neuronal functional variability is likely to play a major role the delivery of the functionality of neural systems, it is difficult to study it in most nervous systems. We work on the pyloric rhythm network of the crustacean stomatogastric ganglion (STG) [2]. Typically network models of the STG treat neurons of the same functional type as a single model neuron (e.g. PD neurons), assuming the same conductance parameters for these neurons and implying their synchronous firing [3, 4]. However, simultaneous recording of PD neurons shows differences between the timings of spikes of these neurons. This may indicate functional variability of these neurons. Here we modelled separately the two PD neurons of the STG in a multi-neuron model of the pyloric network. Our neuron models comply with known correlations between conductance parameters of ionic currents. Our results reproduce the experimental finding of increasing spike time distance between spikes originating from the two model PD neurons during their synchronised burst phase. The PD neuron with the larger calcium conductance generates its spikes before the other PD neuron. Larger potassium conductance values in the follower neuron imply longer delays between spikes, see Fig. 17.Neuromodulators change the conductance parameters of neurons and maintain the ratios of these parameters [5]. Our results show that such changes may shift the individual contribution of two PD neurons to the PD-phase of the pyloric rhythm altering their functionality within this rhythm. Our work paves the way towards an accessible experimental and computational framework for the analysis of the mechanisms and impact of functional variability of neurons within the neural circuits to which they belong
Chemical Origins of the Mars Ultraviolet Dayglow
Airglow optical emissions from planetary atmospheres provide remotely observable signatures of atmospheric composition, energy deposition processes, and the resulting chemical reactions. We may one day be able to detect airglow emissions from extrasolar planets. Reliable interpretation requires quantitative understanding of the energy sources and chemical mechanisms that produce them. The ultraviolet dayglow observations by the Mariner 6 and 7 (1969) and Mariner 9 (1971â72) motivated numerous modeling studies and laboratory experiments. The most obvious source reaction is photodissociation and photoionization of ambient CO2, which is known in the laboratory to produce the four strong dayglow emitting states: hÎœ + CO2 â O(1S), CO(a3Î ), CO+2(A2Î u & B2ÎŁ+u) If this simplest of models were sufficient, then the high altitude dayglow emissions would all share the same scale height, which would be that of CO2. The few Mariner dayglow observations provide weak statistics. Addition of 4 months of Mars Express dayglow data, and including radio occultation and mass spectrometry data from other missions, have made the analyses and conclusions more robust. The CO(a3Î ) and CO+2(B2ÎŁ+u) dayglow altitude profiles are consistent with the first source reaction. In contrast, the O(1S) dayglow scale heights are much larger and are consistent with a second source reaction: O+2 + eâ â O(1S) Both sets of scale heights change with respect to solar activity roughly as suggested by modeling studies