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Open Access Publications from the University of California

Scholarly Works (945 results)

  • Article
  • Peer Reviewed
Proceedings of the Annual Meeting of the Cognitive Science Society, Volume 43 (2021)
Representations held in working memory (WM) are crucial in guiding human attention in a goal-directed fashion. Currently, it is debated whether only a single or several of these representations can be active and bias behaviour at any given moment. In our present study, 25 university students performed a behavioural dense-sampling experiment to produce an estimate of the temporal activation patterns of two simultaneously held visual templates. We report two key novel results. First, the performance related to both representations was not continuous, but fluctuated rhythmically at 6 Hz. This corresponds to neural oscillations in the theta-band, whose functional importance in WM is well established. Second, our findings suggest that two concurrently held representations may be prioritized in alternation, not simultaneously. Our data extend recent research on rhythmic sampling of external information by demonstrating an analogous mechanism in the cyclic activation of internal WM representations.
    Cover page: Multiple items in working memory are cyclically activated at a theta-rhythmCreative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    Proceedings of the Annual Meeting of the Cognitive Science Society, Volume 43 (2021)
    Recently, several behavioral studies have demonstrated 4-10 Hz rhythmic fluctuations in attention. So far, this attentional sampling has only been demonstrated with regards to external stimuli. Attention, however, is often directed towards internal working memory representations. We conducted a human behavioral dense-sampling experiment on whether simultaneously held representations of two distinct feature dimensions (color and orientation) also exhibit a rhythmic temporal profile. We found an oscillatory component at 9.4 Hz in the joint time-courses of both representations, presumably reflecting a common early perceptual sampling process in the alpha-frequency range. Further, we observed an oscillatory component at 3.5 Hz with a significant phase-difference between feature dimensions. This likely corresponds to a later attentional sampling process, indicating that internal representations of distinct features are activated in alteration. In summary, we demonstrate the cyclic reactivation of internal representations, as well as the co-occurrence of perceptual- and attentional rhythmic fluctuations at distinct frequencies.
      Cover page: Cyclic reactivation of internal working memory representations of distinct feature dimensionsCreative Commons 'BY' version 4.0 license
      • Article
      Contemporary Music Score Collection (2020)
        Cover page: Oberfläche
        • Thesis
        • Peer Reviewed
        UC San Diego Electronic Theses and Dissertations (2013)
        Well-controlled laboratory experiments in simplified reactors, such as counterflow burners, most easily and accurately test the ability of chemical-kinetic mechanisms and transport descriptions to predict combustion processes. Diffusion flame investigations at atmospheric and elevated pressures have been carried out to provide insights into the structure and combustion chemistry of hydrogen, low molecular weight hydrocarbons and oxygenated fuels. A newly designed high-pressure combustion facility was used to study the structures and extinction conditions of counterflow diffusion flames in air for nitrogen- diluted hydrogen, methane, ethane, and ethylene, from 0.1MPa to 2.0 MPa. In all cases, the strain rate at extinction was found to increase with pressure in the moderate pressure range until a peak value is attained, above which a decreasing trend begins. In addition, a methane air diffusion flame was doped with hydrogen at atmospheric pressure. Irrespective of whether the hydrogen was added on the fuel or oxidizer side, the ratio of the extinction strain rate with hydrogen addition to that without was the same when the stoichiometric mixture fraction, the adiabatic flame temperature, and the proportion of oxygen that consumes the added fuel are fixed. Furthermore, an experimental investigation of the structure of large alcohols and critical conditions of methyl-esters provide new insights for the development and validation of chemical-kinetic mechanisms
          Cover page: Experimental and Computational Studies of the Combustion of Classical and Alternative Fuels
          • Article
          LBL Publications (2001)
          In-situ transmission electron microscopy is an established experimental technique that permits direct observation of the dynamics and mechanisms of dislocation motion and deformation behavior. In this paper, we detail the development of a novel specimen goniometer that allows real time observations of the mechanical response of materials to indentation loads. The technology of the scanning tunneling microscope is adopted to allow nanometer scale positioning of a sharp, conductive diamond tip onto the edge of an electron transparent sample. This allows application of loads to nanometer-scale material volumes couple with simultaneous imaging of the material response. The emphasis in this paper is experimental and descriptive, with particular attention given to sample geometry and other technical requirements. Examples of the deformation of aluminum and titanium carbide as well as the fracture of silicon will be presented.
            Cover page: Development of a nanoindenter for in-situ transmission electron microscopy
            • Article
            Journal of Transnational American Studies, Volume 8, Issue 1 (2017)
            Ulrich Adelt, "Outside Looking In: Donna Summer's Sound of Munich and David Bowie's Berlin Trilogy," in Krautrock: German Music in the Seventies (Ann Arbor: University of Michigan Press, 2016), 129-60, 209-13.
              Cover page: Outside Looking In: Donna Summer's Sound of Munich and David Bowie's Berlin TrilogyCreative Commons 'BY' version 4.0 license
              • Article
              Working Papers (1994)
                Cover page: Germany 1994 : the year of elections
                • Article
                Contemporary Music Score Collection (2020)
                  Cover page: Plastic/Fantastic/Lovers
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2016)
                  Heterogeneous and multiphase reactions of ozone are important pathways for chemical ageing of atmospheric organic aerosols. To demonstrate and quantify how moisture-induced phase changes can affect the gas uptake and chemical transformation of organic matter, we apply a kinetic multi-layer model to a comprehensive experimental data set of ozone uptake by shikimic acid. The bulk diffusion coefficients were determined to be 10(-12) cm(2) s(-1) for ozone and 10(-20) cm(2) s(-1) for shikimic acid under dry conditions, increasing by several orders of magnitude with increasing relative humidity (RH) due to phase changes from amorphous solid over semisolid to liquid. Consequently, the reactive uptake of ozone progresses through different kinetic regimes characterised by specific limiting processes and parameters. At high RH, ozone uptake is driven by reaction throughout the particle bulk; at low RH it is restricted to reaction near the particle surface and kinetically limited by slow diffusion and replenishment of unreacted organic molecules. Our results suggest that the chemical reaction mechanism involves long-lived reactive oxygen intermediates, likely primary ozonides or O atoms, which may provide a pathway for self-reaction and catalytic destruction of ozone at the surface. Slow diffusion and ozone destruction can effectively shield reactive organic molecules in the particle bulk from degradation. We discuss the potential non-orthogonality of kinetic parameters, and show how this problem can be solved by using comprehensive experimental data sets to constrain the kinetic model, providing mechanistic insights into the coupling of transport, phase changes, and chemical reactions of multiple species in complex systems.
                    Cover page: Ozone uptake on glassy, semi-solid and liquid organic matter and the role of reactive oxygen intermediates in atmospheric aerosol chemistry
                    • Article
                    • Peer Reviewed
                    Research Theses and Dissertations (2011)

                    Tidal wetlands can be important sources of methylmercury (MeHg) in aquatic ecosystems, such as the San Francisco Bay-Delta estuary. As a result of the tendency of bacteria in wetland sediments to methylate mercury, the restoration of wetland habitat may cause an increase of MeHg concentrations. To balance the need for tidal wetland habitat with concerns over increased MeHg exposure, landscape-scale techniques for minimizing the production and export of MeHg from wetland sediments are needed. One potential approach is to use an iron sediment amendment to reduce net MeHg production. The addition of Fe[II] decreases MeHg production by lowering the concentration of the inorganic Hg[II] species that are methylated by bacteria. In this research, the potential for reducing MeHg production and export via an iron amendment was evaluated in laboratory microcosm experiments and a field study in a tidal salt marsh in the San Francisco Bay estuary. Additionally, sediment incubation experiments were conducted in anaerobic containers and in in situ cores to evaluate the effect of iron and sulfur redox cycling on MeHg production.

                    Two laboratory microcosm experiments (Chapter 2) were conducted to test the iron amendment hypothesis under simulated tidal wetland conditions: one with devegetated sediments and one with live wetland vegetation. The microcosms consisted of intact sediment cores collected from Gambinini Marsh, a tidal salt marsh in the San Francisco Bay estuary dominated by pickleweed (Sarcocornia pacifica). The microcosms were maintained under simulated tidal conditions and amended at four iron doses (0, 180, 360, and 720 g-Fe/m2). Following iron addition to the devegetated sediments, porewater S[-II] concentrations decreased for each dose relative to the control. The average weekly export of MeHg in the surface water decreased by 82% and 89% for the two highest iron doses, respectively. Despite substantial variability within treatment groups, similar trends were observed in the vegetated microcosms. The results suggest that iron addition has the potential to provide a landscape-scale control on MeHg export from restored tidal wetlands under certain conditions.

                    The cycling of iron, sulfur, and mercury in tidal wetlands is a complex process, with the combination of daily tides, changes in the growth state of wetland plants, and a highly productive microbial community resulting in temporal and spatial variations in MeHg production and export. Sediment incubation experiments (Chapter 3) were used to evaluate the effect of these processes on MeHg concentrations in the sediments of Gambinini Marsh. Sediments were incubated for 7-days in sealed jars under the following conditions: untreated sediments, addition of sodium molybdate to suppress sulfate reduction, and the addition of formaldehyde as an abiotic control. Similar rates of Fe[II] production were observed in both the untreated and Mo-amended incubations, suggesting that both iron-reducing and sulfate-reducing bacteria co-existed in the same sediment layers. Additionally, MeHg production was not observed when sulfate reduction was suppressed, suggesting that mercury methylation was mediated by sulfate- reducing bacteria. The in situ incubations, which were conducted with open and closed cores, demonstrated that during the summer months when plants were active, separation of sediments from live plant roots and gas exchange with the atmosphere resulted in more reduced sediment conditions. Additionally, sediments at the surficial layers (0-1 cm and 3-4 cm depths) exhibited more reducing conditions during the winter than in the summer, suggesting that the oxidation of reduced iron species occurs more rapidly during the summer.

                    To better understand the effect of iron amendments on in situ tidal marsh biogeochemistry, a 17-month field study was conducted in the Gambinini Marsh (Chapter 4). Before and after amending the sediments with 77 g-Fe/m2, porewater from pickleweed-dominated sediments in the high marsh plain were analyzed for iron, sulfur, organic carbon, and methylmercury. Sulfide was not detected in the sediment porewater, and the iron amendment had no observable effect on net MeHg production. However, porewater iron concentrations were elevated for at least 6 weeks following the amendment. Porewater concentrations of MeHg and dissolved organic carbon were lower throughout 2010 than during the summer of 2009 when the experiment was initiated. However, these concentrations increased during the period of pickleweed flowering in 2010, further demonstrating the strong effect that wetland vegetation can have on sediment biogeochemical processes.

                    This research demonstrated that an iron sediment amendment has the potential to be an effective control of MeHg production and export in tidal wetland sediments under certain conditions. While the in situ amendment showed no effect in the high marsh plain of Gambinini Marsh, the microcosm experiments demonstrated that a strong effect may be possible in sulfide- rich sediments. Additional research is necessary to evaluate the efficacy of the iron amendment in sulfide-rich field sediments, such as those found in low marsh environments.

                      Cover page: Methylmercury Production in Tidal Salt Marsh Sediments and Potential Control Using Iron Amendments
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