100,718 research outputs found
Geophysics applied to nuclear waste disposal investigations in Switzerland
No full textT. Spillmann, P. Blümling, E. Manukyan, S. Marelli, H. R. Maurer, S.A. Greenhalgh and A.G. Greenhttp://www.earthdoc.org/detail.php?pubid=4074
Reversal of apparent rotation in the Enigma-figure with and without motion adaptation and the effect of T-junctions
No full textAbstractWe studied the time course of apparent rotation and directional reversal in Leviant’s Enigma figure. On average, periods of clockwise rotation lasted 5.0s as opposed to 4.4s for counter-clockwise rotation, resulting in an average reversal frequency of 6.4 within 30s. At the beginning of a trial, clockwise rotation was perceived almost twice as often as counter-clockwise rotation. This bias could be shifted by previous adaptation to a black-and-white rotating sector disk, suggesting a neural interaction between real motion and illusory motion. We further studied Enigma-type motion on a chromatic bar superimposed onto a black-and-white linear grating. Illusory motion was strongest when the bar was oriented at 90deg to the grating lines and became progressively weaker with a decrease in angle. This suggests that T-junctions formed by the radial rays impinging onto the colored rings of the Enigma figure are instrumental for eliciting the rotary motion and may rule out a low-level sensory origin of the illusion
How the past gives way to the present: Evidence for Bayesian updating with repeated presentation of ambiguous motion quartets
No full textIntroduction: In a motion quartet (MQ)with one repetition, two disks that fall at the endpoints of a diameter of an invisible circle are briefly presented and then replaced by two disks that fall on a second diameter. The observer sees rotation of the first pair into the second. By varying the angle between the diameters, one can estimate a psychometric function P[L|angle]. We reported last year that recent task history exerted a strong effect on perceived direction of motion, shifting the threshold parameter of the psychometric function. In particular, the observer is strongly biased to see motion in the same direction as the motion perceived on the most recent trial. This hysteretic effect (measured as the change in threshold) is little affected by increasing the time interval between trials (to as much as 30 seconds). Purpose: We sought to determine how repetitions of the same motion quartet would affect hysteresis. It is plausible, for example, that multiple presentations of the same sensory information would eventually ‘overpower’ the influence of recent task history, reducing hysteresis to 0. But precisely how? Methods: We measured the hysteresis induced by the previous trial for two subjects who judged 3840 MQ trials each. In different blocks, the MQ was presented 1, 2, 4 or 8 times in each trial. The observer reported the direction of motion of the last presentation only. Results: For both observers we found that hysteresis decreased rapidly with number of repetitions and that a log-log plot of hysteresis versus repetition was approximately linear with slope −1. Doubling the number of repetitions halves the hysteretic effect. This outcome is consistent with a Bayesian updating model where an initial prior probability that the stimulus will go left (based on recent task history) is successively combined with independent likelihood terms, one for each repetition of the stimulus
The effect of past trials on perceived direction of motion in ambiguous motion quartets: temporal pattern detection, not priming
No full textIn a motion quartet (MQ), two disks that fall at the endpoints of a diameter of an invisible circle are briefly presented and then replaced by two disks that fall on a second diameter. With proper choice of timing, the observer sees apparent rotation of the first pair into the second. When the angle between the diameters is about 90 deg, the direction of perceived rotation is ambiguous, half the time clockwise (C) and half counterclockwis
Spontaneous patterns in the perceived direction of motion in ambiguous motion quartets: Effect of perception or judgment?
No full textIn a motion quartet (MQ), perceived direction of rotation is bistable, sometimes clockwise (C) sometimes counterclockwise (c). We reported (Martello et al, 2004 Journal of Vision 4 in press, abstract) that observers exhibited spontaneous patterns across time in their responses. Our results could be explained as completion of two kinds of patterns: (i) sequences (eg CCCC > C), and (ii) alternations (eg CcCc > C). We report an experiment intended to determine whether the effect of past perceptions is present even when the observer does not overtly judge the direction of MQs. We collected psychometric functions P(C|angle) for sixteen subjects who viewed almost 2000 MQ trials each. The trials were organised in groups of four (tetrads) and the angles in the MQs for the first three trials in each tetrad were chosen so that we could control the perceived direction of motion on these trials. The observer did not respond to these trials but simply observed them. On the fourth trial of each tetrad, the subject saw an MQ whose direction he judged. We found that the alternating sequences CcC and cCc still led to a strong bias to completion but that the effect of the cumulative sequences ccc and CCC had vanished
Letter, [Author unclear] to Paulina T. Merritt
No full textHandwritten letter to Paulina Merritt from an unknown author, October 1, 1876.
2D bimolecular self-assembled porphyrin-fullerene nanostructures
No full textThe formation of self-assembled monolayers of porphyrin molecules and their usefulness as surface supported templates for hosting fullerene molecules has been investigated by means of a room temperature Scanning Tunneling Microscope (STM). In particular, examples of unprecedented addressable supramolecular architectures composed of fullerenes and porphyrins were obtained. Furthermore, the first 2-dimemensional porphyrin based supramolecular host network with porous structure has been self-assembled on a solid surface. This network features a specific chemical sensitivity for different fullerene guest molecules. Various porphyrin derivatives were deposited onto diverse metal substrates. While some combinations did not result in ordered monolayers, many others revealed to form self-assembled structures. Two varieties of porphyrin molecules were examined. On the one hand, single porphyrin cores featuring different functional side-groups were investigated. On the other hand, unique triply-fused diporphyrin cores, also featuring relevant functional groups, have been researched. In addition to several close-packed monolayers, a nanoporous assembly of porphyrin molecules was discovered. This porous network features cavities with a pore-size approximately identical to the size of C60 fullerenes and a pore-pore distance of 3.3 nm. Fullerene molecules were adsorbed onto preformed porphyrin assemblies. Several of these monolayers exhibit interesting fullerene hosting capabilities. The fullerenes have been found to form lines, pairs or adsorb into the pores depending on the underlying porphyrin structure. In particular, the adsorption and dynamics of C60 and C70 fullerenes hosted in the self-assembled nanoporous network on the Ag(111) surface have been studied. Time-resolved STM studies of these supramolecular systems have revealed host-guest interactions resulting in a distinctly dissimilar mobility of the two fullerenes within the porous porphyrin network. Long-range coverage-dependent interactions have been discovered to influence the hopping rates of the adsorbed fullerene guests. These are likely mediated by a complex mechanism involving both the Ag substrate and the flexible porphyrin host network. At increased fullerene coverage this unprecedented interplay results in the formation of large fullerene chains and islands. By applying a lattice gas model with nearest-neighbor interactions and by evaluating the fullerene pair distribution functions the respective coveragedependent guest-guest interaction energies have been estimated
Handwritten biographical information on Paulina T. McClung Merritt
No full textA handwritten biography of Paulina T. McClung Merritt by an unknown author, 1892.
Heterogeneous and tissue-specific regulation of effector T cell responses by IFN-gamma during Plasmodium berghei ANKA infection.
No full textIFN-γ and T cells are both required for the development of experimental cerebral malaria during Plasmodium berghei ANKA infection. Surprisingly, however, the role of IFN-γ in shaping the effector CD4(+) and CD8(+) T cell response during this infection has not been examined in detail. To address this, we have compared the effector T cell responses in wild-type and IFN-γ(-/-) mice during P. berghei ANKA infection. The expansion of splenic CD4(+) and CD8(+) T cells during P. berghei ANKA infection was unaffected by the absence of IFN-γ, but the contraction phase of the T cell response was significantly attenuated. Splenic T cell activation and effector function were essentially normal in IFN-γ(-/-) mice; however, the migration to, and accumulation of, effector CD4(+) and CD8(+) T cells in the lung, liver, and brain was altered in IFN-γ(-/-) mice. Interestingly, activation and accumulation of T cells in various nonlymphoid organs was differently affected by lack of IFN-γ, suggesting that IFN-γ influences T cell effector function to varying levels in different anatomical locations. Importantly, control of splenic T cell numbers during P. berghei ANKA infection depended on active IFN-γ-dependent environmental signals--leading to T cell apoptosis--rather than upon intrinsic alterations in T cell programming. To our knowledge, this is the first study to fully investigate the role of IFN-γ in modulating T cell function during P. berghei ANKA infection and reveals that IFN-γ is required for efficient contraction of the pool of activated T cells
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