102,849 research outputs found
Parallel search for conjunctions with stimuli in apparent motion
No full textA series of experiments was conducted to determine whether apparent motion tends to follow the similarity rule (ie is attribute-specific) and to investigate the underlying mechanism. Stimulus duration thresholds were measured during a two-alternative forced-choice task in which observers detected either the location or the motion direction of target groups defined by the conjunction of size and orientation. Target element positions were randomly chosen within a nominally defined rectangular subregion of the display (target region). The target region was presented either statically (followed by a 250 ms duration mask) or dynamically, displaced by a small distance (18 min of are) from frame to frame. In the motion display, the position of both target and background elements was changed randomly from frame to frame within the respective areas to abolish spatial correspondence over time. Stimulus duration thresholds were lower in the motion than in the static task, indicating that target detection in the dynamic condition does not rely on the explicit identification of target elements in each static frame. Increasing the distractor-to-target ratio was found to reduce detectability in the static, but not in the motion task. This indicates that the perceptual segregation of the target is effortless and parallel with motion but not with static displays. The pattern of results holds regardless of the task or search paradigm employed, The detectability in the motion condition can be improved by increasing the number of frames and/or by reducing the width of the target area. Furthermore, parallel search in the dynamic condition can be conducted with both short-range and long-range motion stimuli. Finally, apparent motion of conjunctions is insufficient on its own to support location decision and is disrupted by random visual noise. Overall, these findings show that (i) the mechanism underlying apparent motion is attribute-specific; (ii) the motion system mediates temporal integration of feature conjunctions before they are identified by the static system; and (iii) target detectability in these stimuli relies upon a nonattentive, cooperative, directionally selective motion mechanism that responds to high-level attributes (conjunction of size and orientation)
Perceived ridigity and nonridigity in the depth effect.
No full textStroboscopic simulations of three-dimensional rotating rigid structures can be perceived as highly nonrigid. To investigate this nonrigidity effect a sequence of either three (experiment 2 and 3) or thirty six frames (experiment 4) was used, each consisting of a set of dots with location on the horizontal axis corresponding to the parallel projection of a nominally defined helix. Observers were asked to judge the angle of rotation of eighty helices defined by the factorial combination of eight phase (phi) values (ie difference between the sinusoidal path of one dot and its neighbours) and ten different angular displacement values (alpha). When in each static frame the dots can be organized into curved dotted line (small values of phi), the perceived 3-D helices are highly nonrigid. But when shape information is not available in each static frame (high values of phi), the helices are perceived as rigid and rotation judgement is possible providing that alpha < 15 degrees. It appears that at small values of phi observers fail to recover the rigid structure of the helices since the input to the structure from the motion process may be distorted
Effects of water and nitrogen management on fibrous root distribution and turnover in sugar beet
No full textTwo field trials were carried Out in two years in heavy soils of NE Italy, with the aim of studying the effects of water and nitrogen management on fibrous root distribution and dynamics in sugar beet (cv. Dorotea). In conditions of moderate water deficit (year 2002, Conselice, Ravenna, clay soil), two water regimes (irrigation to 100% of potential evapotranspiration, and rainfed) were factorially combined with three rates of nitrogen application (180, 90, 0 kg ha(-1)). Irrigation increased volumetric root length density (RLD(v)) without N application and at the medium N rate - a common amount in beet cultivation - but reduced it at the maximum N dose. The medium N rate increased RLDv and shifted root distribution towards shallow layers, regardless of water regime.
In the conditions of marked drought of 2003 (Legnaro, Padova, silty-loam soil), at a single rate of N supply (90 kg ha-1) irrigation increased total production (length) of fibrous roots throughout the soil profile (1.8 m), except in the 0.5-1 m interval, and improved the length of standing living roots during the season. Although the maximum root depth at the end of the season was similar in the two water regimes (about 1.9 m), irrigated roots reached the saturated soil layers 10 days earlier than in rainfed plants. The main result was reduced root turnover in deep soil layers (>1 m) and an increase at the surface in the rainfed treatments in conditions of drought, a probable mechanism of adaptation to a more marked gradient of soil Moisture compared with irrigation
THE CRYSTAL-STRUCTURE OF PT(II)-TRANS-[(BIS-TRICYCLOHEXYLPHOSPHINE)-(ETA-1-HYDRO-FORMYL-TER-BUTYL AMIDO)(HYDRIDE)]
No full textThe title compound, C43H79O2NPt, crystallizes in the monoclinic system, space group P2(1)/c, with a = 20.444(8) angstrom, b = 12.220(4) angstrom, c = 18.303(7) angstrom, beta = 101.6(4), Z = 4. The coordination of Pt is slightly distorted square-planar, the backbone of the hydro-aldoamidic ligand is almost planar; its mean plane is orthogonal to the coordination plane of Pt. The crystal structure has been refined to R = 0.054 for 3518 observed reflections
CRYSTAL AND MOLECULAR-STRUCTURE OF AN ETA-1-KETENYL METAL-COMPLEX - TRANS-([BIS(TRICYCLOHEXYL)PHOSPHINE](ETA-1-FORMYLKETENYL) (HYDRIDE))PLATINUM(II) - PT(H)(PCY3)2[ETA-1-C(CHO)CO] .16.
No full textREACTIONS OF C3O2 WITH STABILIZED TRIPHENYLPHOSPHORANES PH(3)P=CHX (X=CN, COME, COPH)
No full textLinear neutral compound like 1 or zwitterionic heterocycles like 2 form exclusively from the reaction of C3O2 with the phosphorus ylides Ph3P =CHX (X = CN for 1, and COMe or COPh for 2) bearing an H atom on the ylidic C atom. In contrast to earlier investigations, neither Wittig reactions nor the formation of cyclobutanediones were observed
- …
