Showing 4571 - 4580 of 5831 Items
Damped-driven granular chains: An ideal playground for dark breathers and multibreathers
Date: 2014-03-31
Creator: C. Chong, F. Li, J. Yang, M. O. Williams, I. G., Kevrekidis, P. G. Kevrekidis
Access: Open access
- By applying an out-of-phase actuation at the boundaries of a uniform chain of granular particles, we demonstrate experimentally that time-periodic and spatially localized structures with a nonzero background (so-called dark breathers) emerge for a wide range of parameter values and initial conditions. We demonstrate a remarkable control over the number of breathers within the multibreather pattern that can be "dialed in" by varying the frequency or amplitude of the actuation. The values of the frequency (or amplitude) where the transition between different multibreather states occurs are predicted accurately by the proposed theoretical model, which is numerically shown to support exact dark breather and multibreather solutions. Moreover, we visualize detailed temporal and spatial profiles of breathers and, especially, of multibreathers using a full-field probing technology and enable a systematic favorable comparison among theory, computation, and experiments. A detailed bifurcation analysis reveals that the dark and multibreather families are connected in a "snaking" pattern, providing a roadmap for the identification of such fundamental states and their bistability in the laboratory. © 2014 American Physical Society.
The effects of nitric oxide on the modulation of the cardiac system of the American lobster, Homarus americanus, via a peptide (GYSDRNLRFamide)
Date: 2014-08-01
Creator: Sophie Janes
Access: Open access
- The central pattern generator (CPG) is a neural network that controls the rhythmic patterned outputs, which generate locomotion as needed. The lobster provides a good model to study CPGs because it has a relatively simple CPG. The lobster CPG, or cardiac ganglion accommodates for a range of activities and changes in the environment (Cooke, 2002).The small lobster CG is made up of nine neurons that control the neurogenic heart. The lobster CG is located on the inner dorsal wall of the heart and forms long neurites that branch onto the heart muscle. The CG, through an intrinsic mechanism, generates patterned and rhythmic bursts to the heart (Cooke 2002).The H. americanus CG sends information to the heart muscle to regulate the heart beat. The patterned bursts from the CG need to be adjusted in response to changing demands, for example, activity level or blood volume. Two general mechanisms, intrinsic feedback and extrinsic neuromodulation, have been identified to facilitate this adjustment. Through an intrinsic feedback mechanism, the muscle sends information back to the CG via a positive pathway and a negative pathway. In the positive pathway, stretch-sensitive dendrites of cardiac neurons increase the frequency of heart contractions when stretched (Cooke 2002). In the negative pathway, nitric oxide (NO), produced by the cardiac muscle, slows the frequency (Mahadevan et al. 2004). The interplay between the negative and positive feedback pathways regulates the output of the CG. An extrinsic mechanism has also been identified to regulate the CG output. Chemical neuromodulators that are released either locally or as hormones signal to the heart or CG to modulate ganglion activity. The intrinsic and extrinsic mechanisms affect the contraction amplitude and frequency of the heart.Within this simple invertebrate organism, a complex layering of control exists. Studies of the effects of various extrinsic modulators suggest that these modulators may alter how the feedback pathways operate. I examined what effect the neuromodulator GYSDRNLRFamide (GYS), a peptide found in the lobster nervous system, has on the balance between the positive and negative pathways (Ma et al., 2008). Recent experiments have demonstrated that when GYS was applied at high concentrations in the whole heart, the frequency decreased. This suggests that GYS may play a role in the intrinsic feedback pathways, and likely enhances the negative pathway.I looked at if nitric oxide altered the modulation of the heartbeat frequency when enhanced by the extrinsic modulator, GYS. Based on previous experiments, I hypothesized that GYS allows the NO, or negative pathway to predominate. In order to test my hypothesis, I examined the effects of GYS when I removed nitric oxide, which allows the negative pathway to exist. I compared the characteristics of the heartbeat when saline was run through the heart to when GYS was run through the heart. I also compared the characteristics of the heartbeat when the NO inhibitor, L-NA, was applied to when GYS was applied in the presence of L-NA. I finally compared the changes in frequency between these two comparisons. I found a significant difference between the change in frequency of the heart perfused with GYS in saline as opposed to perfused with GYS in L-NA. GYS had a greater negative effect without L-NA. These results demonstrate that NO is likely the cause of the observed decrease in frequency. Final Report of research funded by the Doherty Coastal Studies Research Fellowship.
Solving which trilemma? The many interpretations of equality, Pareto, and freedom of occupational choice
Date: 2017-08-01
Creator: Kristi A. Olson
Access: Open access
- According to the trilemma claim, we cannot have all three of equality, Pareto, and freedom of occupational choice. In response to the trilemma, John Rawls famously sacrificed equality by introducing incentives. In contrast, GA Cohen and others argued that we can, in fact, have all three provided that individuals are properly motivated by an egalitarian ethos. The incentives debate, then, concerns the plausibility of the ethos solution versus the plausibility of the incentives solution. Considerable ink has been spilled on both sides of the debate. Yet, in this essay, I argue that we cannot have this debate until we clarify the terms. Once we clarify the terms, however, we might discover that there is no debate to be had. This is because, depending on how equality, Pareto, and freedom of occupational choice are interpreted, there might not be a trilemma in the first place. Specifically, I use a small but crucial distinction in how equality, the egalitarian ethos, and Pareto are assessed – what I call the internal/external distinction – to disentangle the various paths each solution – the ethos or incentives – could take. I conclude that both solutions have gained illicit plausibility by virtue of not keeping the distinction straight.
Climate change and health costs of air emissions from biofuels and gasoline
Date: 2009-02-10
Creator: Jason Hill, Stephen Polasky, Erik Nelson, David Tilman, Hong, Huo, Lindsay Ludwig, James Neumann, Haochi Zheng, Diego Bonta
Access: Open access
- Environmental impacts of energy use can impose large costs on society. We quantify and monetize the life-cycle climate-change and health effects of greenhouse gas (GHG) and fine particulate matter (PM2.5) emissions from gasoline, corn ethanol, and cellulosic ethanol. For each billion ethanol-equivalent gallons of fuel produced and combusted in the US, the combined climate-change and health costs are $469 million for gasoline, $472-952 million for corn ethanol depending on biorefinery heat source (natural gas, corn stover, or coal) and technology, but only $123-208 million for cellulosic ethanol depending on feedstock (prairie biomass, Miscanthus, corn stover, or switchgrass). Moreover, a geographically explicit life-cycle analysis that tracks PM2.5 emissions and exposure relative to U.S. population shows regional shifts in health costs dependent on fuel production systems. Because cellulosic ethanol can offer health benefits from PM2.5 reduction that are of comparable importance to its climate-change benefits from GHG reduction, a shift from gasoline to cellulosic ethanol has greater advantages than previously recognized. These advantages are critically dependent on the source of land used to produce biomass for biofuels, on the magnitude of any indirect land use that may result, and on other as yet unmeasured environmental impacts of biofuels. © 2009 by The National Academy of Sciences of the USA.
Lie to Me: Linguistic Markers of Deception in Relation to Individual Differences in Executive Control Access to this record is restricted to members of the Bowdoin community. Log in here to view.
Date: 2014-05-01
Creator: Lauren Pashkowski
Access: Access restricted to the Bowdoin Community
Systematic ENSO-driven nutrient variability recorded by central equatorial Pacific corals
Date: 2013-08-16
Creator: Michèle LaVigne, Intan S. Nurhati, Kim M. Cobb, Helen V. McGregor, Daniel, Sinclair, Robert M. Sherrell
Access: Open access
- Variations in ocean productivity are driven largely by nutrient supply to the photic zone, but temporal records of nutrient variability are sparse. Here we show scleractinian coral P/Ca proxy records of variations in phosphate concentrations during El Niño Southern Oscillation (ENSO) cycles in the central equatorial Pacific. Covarying P/Ca records in Porites corals from Christmas and Fanning Islands show a regional ∼40% decrease during the upwelling relaxation of the 1997-1998 El Niño, consistent with less frequent nutrient measurements from this area. Similar ∼35-45% skeletal P/Ca decreases occur during the 1982-1983 and 1986-1987 El Niño events, which predate satellite color and regional nutrient measurements. After each El Niño event, nutrient increases lag temperature recovery by 4-12 months, likely reflecting uptake by massive phytoplankton blooms that followed resumption of upwelling. The results support the utility of coral P/Ca to probe the mechanisms linking ENSO, equatorial upwelling, and carbon cycling in the past. © 2013. American Geophysical Union. All Rights Reserved.
KLT-type relations for QCD and bicolor amplitudes from color-factor symmetry
Date: 2018-03-01
Creator: Robert W. Brown, Stephen G. Naculich
Access: Open access
- Color-factor symmetry is used to derive a KLT-type relation for tree-level QCD amplitudes containing gluons and an arbitrary number of massive or massless quark-antiquark pairs, generalizing the expression for Yang-Mills amplitudes originally postulated by Bern, De Freitas, and Wong. An explicit expression is given for all amplitudes with two or fewer quark-antiquark pairs in terms of the (modified) momentum kernel. We also introduce the bicolor scalar theory, the “zeroth copy” of QCD, containing massless biadjoint scalars and massive bifundamental scalars, generalizing the biadjoint scalar theory of Cachazo, He, and Yuan. We derive KLT-type relations for tree-level amplitudes of biadjoint and bicolor theories using the color-factor symmetry possessed by these theories.
Complaint-making as political participation in contemporary Russia
Date: 2012-09-01
Creator: Laura A. Henry
Access: Open access
- Prior to December 2011, instances of widespread collective mobilization were relatively rare in contemporary Russia. Russian citizens are more likely to engage in a different means of airing grievances: making an official complaint to the authorities. This article considers how complaint-making, as a variety of political participation, may contribute either to authoritarian resilience or to political liberalization. The political significance of complaints made to the Commissioner for Human Rights of the Russian Federation is examined. Since it is the broader political context that shapes the significance of complaints, in the absence of meaningful elections individualized appeals to the state are unlikely to promote democratic change, although they may allow for redress of individual rights violations. © 2012 The Regents of the University of California.