680 research outputs found
Multitasking versus multiplexing: Toward a normative account of limitations in the simultaneous execution of control-demanding behaviors
Why is it that behaviors that rely on control, so striking in their diversity and flexibility, are also subject to such striking limitations? Typically, people cannot engage in more than a fewâand usually only a singleâcontrol-demanding task at a time. This limitation was a defining element in the earliest conceptualizations of controlled processing; it remains one of the most widely accepted axioms of cognitive psychology, and is even the basis for some laws (e.g., against the use of mobile devices while driving). Remarkably, however, the source of this limitation is still not understood. Here, we examine one potential source of this limitation, in terms of a trade-off between the flexibility and efficiency of representation (âmultiplexingâ) and the simultaneous engagement of different processing pathways (âmultitaskingâ). We show that even a modest amount of multiplexing rapidly introduces cross-talk among processing pathways, thereby constraining the number that can be productively engaged at once. We propose that, given the large number of advantages of efficient coding, the human brain has favored this over the capacity for multitasking of control-demanding processes.National Science Foundation (U.S.). Graduate Research Fellowship Progra
Active current sheets and hot flow anomalies in Mercury's bow shock
Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities
interacting with collisionless bow shocks. They are typically formed when the
normal component of motional (convective) electric field points toward the
embedded current sheet on at least one of its sides. The core region of an HFA
contains hot and highly deflected ion flows and rather low and turbulent
magnetic field. In this paper, we report first observations of HFA-like events
at Mercury identified over a course of two planetary years. Using data from the
orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and
Ranging (MESSENGER) mission, we identify a representative ensemble of active
current sheets magnetically connected to Mercury's bow shock. We show that some
of these events exhibit unambiguous magnetic and particle signatures of HFAs
similar to those observed earlier at other planets, and present their key
physical characteristics. Our analysis suggests that Mercury's bow shock does
not only mediate the flow of supersonic solar wind plasma but also provides
conditions for local particle acceleration and heating as predicted by previous
numerical simulations. Together with earlier observations of HFA activity at
Earth, Venus and Saturn, our results confirm that hot flow anomalies are a
common property of planetary bow shocks, and show that the characteristic size
of these events is of the order of one planetary radius.Comment: 39 pages, 15 figures, 2 table
Large Numbers of Matings Give Female Field Crickets a Direct Benefit but not a Genetic Benefit
Female crickets can potentially gain both direct and indirect benefits from mating multiple times with different males. Most studies have only examined the effects of small numbers of matings, although female crickets are capable of mating many times. The goal of this paper is to examine the direct and indirect benefits of mating large numbers of times for female reproductive success. In a previous experiment, female Gryllus vocalis were found to gain diminishing direct benefits from mating large numbers of times. In this study I attempt to determine whether mating large numbers of times yields similar diminishing returns on female indirect benefits. Virgin female Gryllus vocalis crickets were assigned to mate five, ten or 15 times with either the same or different males. Females that mated more times gained direct benefits in terms of laying more eggs and more fertilized eggs. Females that mated with different males rather than mating repeatedly with the same male did not have higher offspring hatching success, a result that is contrary to other published results comparing female reproductive success with repeated versus different partners. These results suggest that females that mate large numbers of times fail to gain additional genetic benefits from doing so
Upstream UltraâLow Frequency Waves Observed by MESSENGERâs Magnetometer: Implications for Particle Acceleration at Mercuryâs Bow Shock
We perform the first statistical analysis of the main properties of waves observed in the 0.05â0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the â30 sâ waves observed at the Earthâs foreshock, the normalized wave amplitude (ÎŽB/|B0|âŒ0.2) and occurrence rate (âŒ0.5%) are much smaller. This could be associated with relatively lower backstreaming proton fluxes, the smaller foreshock size and/or less stable solar wind (SW) conditions around Mercury. Furthermore, we estimate that the speed of resonant backstreaming protons in the SW reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the SW speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.Key PointsWe perform the first statistical analysis (4,536 events) of the main properties of the lowestâfrequency waves in the Hermean foreshockSmall normalized wave amplitude (0.2) and occurrence (0.5%) are likely due to low backstreaming proton flux and variable external conditionsThe normalized backstreaming protons speed (âŒ0.95â2.6) suggests similar acceleration processes occur at several planetary shocksPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155492/1/grl60476.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155492/2/grl60476_am.pd
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Travel Characteristics and Yellow Fever Vaccine Usage Among US Global TravEpiNet Travelers Visiting Countries with Risk of Yellow Fever Virus Transmission, 2009â2011
Yellow fever (YF) vaccine-associated serious adverse events and changing YF epidemiology have challenged healthcare providers to vaccinate only travelers whose risk of YF during travel is greater than their risk of adverse events. We describe the travel characteristics and YF vaccine use among US travelers visiting Global TravEpiNet clinics from January of 2009 to March of 2011. Of 16,660 travelers, 5,588 (34%) had itineraries to areas with risk of YF virus transmission. Of those travelers visiting one country with YF risk (N = 4,517), 71% were vaccinated at the visit, and 20% were presumed to be immune from prior vaccination. However, travelers visiting friends and relatives (odds ratio [OR] = 2.57, 95% confidence interval [95% CI] = 1.27â5.22) or going to Nigeria (OR = 3.01, 95% CI = 1.37â6.62) were significantly more likely to decline vaccination. To optimize YF vaccine use, clinicians should discuss an individual's riskâbenefit assessment of vaccination and close knowledge gaps regarding vaccine use among at-risk populations
Interpreting ~1âHz magnetic compressional waves in Mercury's inner magnetosphere in terms of propagating ionâBernstein waves
We show that ~1âHz magnetic compressional waves observed in Mercury's inner magnetosphere could be interpreted as ionâBernstein waves in a moderate proton beta ~0.1 plasma. An observation of a proton distribution with a large planetary loss cone is presented, and we show that this type of distribution is highly unstable to the generation of ionâBernstein waves with low magnetic compression. Ray tracing shows that as these waves propagate back and forth about the magnetic equator; they cycle between a state of low and high magnetic compression. The group velocity decreases during the highâcompression state leading to a pileup of compressional wave energy, which could explain the observed dominance of the highly compressional waves. This bimodal nature is due to the complexity of the index of refraction surface in a warm plasma whose upper branch has high growth rate with low compression, and its lower branch has low growth/damping rate with strong compression. Two different cycles are found: one where the compression maximum occurs at the magnetic equator and one where the compression maximum straddles the magnetic equator. The later cycle could explain observations where the maximum in compression straddles the equator. Ray tracing shows that this mode is confined within ±12° magnetic latitude which can account for the bulk of the observations. We show that the Doppler shift can account for the difference between the observed and model wave frequency, if the wave vector direction is in opposition to the plasma flow direction. We note that the WentzelâKramersâBrillouin approximation breaks down during the pileup of compressional energy and that a study involving full wave solutions is required.Key PointsThe ionâBernstein (IB) mode is highly unstable to proton loss cones at MercuryThe IB mode can become highly compressional as it propagatesRay tracing of the IB mode predicts compression peaking the off equatorPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112180/1/jgra51808.pd
Ion-scale kinetic Alfvén turbulence: MMS measurements of the Alfvén ratio in the magnetosheath
Turbulence in the Earth's magnetosheath at ion kinetic scales is investigated with the magnetospheric multiscale spacecraft. Several possibilities in the wave paradigm have been invoked to explain plasma turbulence at ion kinetic scales such as kinetic AlfvĂ©n, slow, or magnetosonic waves. To differentiate between these different plasma waves is a challenging task, especially since some waves, in particular, kinetic slow waves and kinetic AlfvĂ©n waves, share some properties making the possibility to distinguishing between them very difficult. Using the excellent time resolution data set provided from both the fluxgate magnetometer and the Fast Plasma Instrument, the ratio of trace velocity fluctuations to the magnetic fluctuations (in AlfvĂ©n units), which is termed the AlfvĂ©n ratio, can be calculated down to ion kinetic scales. Comparison of the measured AlfvĂ©n ratio is performed with respect to the expectation from twoâfluid magnetohydrodynamic theory for the kinetic slow wave and kinetic AlfvĂ©n wave. Moreover, the plasma data also allow normalized fluctuation amplitudes of density and magnetic field to be compared differentiating between magnetosonicâlike and kinetic AlfvĂ©nâlike turbulence. Using these two different ratios, we can rule out that the fluctuations at ion scales are dominated by magnetosonicâlike fluctuations or kinetic slowâlike fluctuations and show that they are consistent with kinetic AlfvĂ©nâlike fluctuations. This suggests that in the wave paradigm, heating in the direction of the parallel magnetic field is predominantly by the Landau damping of the kinetic AlfvĂ©n wave
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