Visuomotor coordination requires both the accurate alignment of spatial information from different sensory streams and the ability to convert these sensory signals into accurate motor commands. Both ...of these processes are highly plastic, as illustrated by the rapid adaptation of goal-directed movements following exposure to shifted visual feedback. Although visual-shift adaptation is a widely used model of sensorimotor learning, the multifaceted adaptive response is typically poorly quantified. We present an approach to quantitatively characterizing both sensory and task-dependent components of adaptation. Sensory aftereffects are quantified with "alignment tests" that provide a localized, two-dimensional measure of sensory recalibration. These sensory effects obey a precise form of "additivity," in which the shift in sensory alignment between vision and the right hand is equal to the vector sum of the shifts between vision and the left hand and between the right and left hands. This additivity holds at the exposure location and at a second generalization location. These results support a component transformation model of sensory coordination, in which eye-hand and hand-hand alignment relies on a sequence of shared sensory transformations. We also ask how these sensory effects compare with the aftereffects measured in target reaching and tracking tasks. We find that the aftereffect depends on both the task performed during feedback-shift exposure and on the testing task. The results suggest the presence of both a general sensory recalibration and task-dependent sensorimotor effect. The task-dependent effect is observed in highly stereotyped reaching movements, but not in the more variable tracking task.
The HERMES polarized atomic beam source Nass, A.; Baumgarten, C.; Braun, B. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
06/2003, Letnik:
505, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The HERMES atomic beam source is used to provide nuclear polarized hydrogen or deuterium atoms for the HERMES target. The setup including the dissociator, the sextupole magnet system and the ...high-frequency transitions is described. Measurements of the hydrogen intensity of up to
6.4×10
16
atoms/
s
in two hyperfine substates and of the deuterium intensity of up to
5.8×10
16
atoms/
s
in three hyperfine substates are presented. A degree of dissociation of 92.8% for H (94.5% for D) at the entrance of the storage cell and a nuclear polarization of around 0.97 (H) and 0.92 (D) have been found constant within a couple of percent over the whole running period of the HERMES experiment.
An atomic beam polarimeter for measurement of the hyperfine population numbers and the absolute polarization of thermal atomic hydrogen (H) and deuterium (D) beams is described. The principle of ...measurement and the method of calibration are given. The polarimeter measures the 4 (6) relative hyperfine population numbers of a thermal H (D) beam in the region of
10
13
atoms
s
−1
to an absolute error of less than 0.01. The polarimeter has been in continuous operation with the internal polarized hydrogen and deuterium gas target used in the HERMES experiment at DESY since 1996.
The flavor asymmetry of the polarized light sea, Δū−Δd̄, discriminates between different model calculations of helicity densities. We show that the chiral chromodielectric model, differently from ...models based on a 1/Nc expansion, predicts a small value for this asymmetry, what seems in agreement with preliminary HERMES data.
A measurement of the proton spin structure function g1p(x,Q{sup 2}) in deep-inelastic scattering is presented. The data were taken with the 27.6 GeV longitudinally polarised positron beam at HERA ...incident on a longitudinally polarised pure hydrogen gas target internal to the storage ring. The kinematic range is 0.021<x<0.85 and 0.8 GeV{sup 2}<Q{sup 2}<20 GeV{sup 2}.
Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel pK0S→pπ+π−. A peak is observed in the pK0S invariant mass spectrum at ...1528±2.6(stat)±2.1(syst) MeV. Depending on the background model, the naive statistical significance of the peak is 4–6 standard deviations and its width may be somewhat larger than the experimental resolution of σ=4.3–6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Θ+(uudds̄) pentaquark baryon. No signal for an hypothetical Θ++ baryon was observed in the pK+ invariant mass distribution. The absence of such a signal indicates that an isotensor Θ is excluded and an isovector Θ is unlikely.
A gas analyzer for the internal polarized target of the HERMES experiment Baumgarten, C; Braun, B; Contalbrigo, M ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2003, Letnik:
508, Številka:
3
Journal Article
Recenzirano
Odprti dostop
A gas analyzer has been developed for the internal polarized target of the HERMES experiment at DESY in order to determine the relative amount of atomic and molecular hydrogen or deuterium in a gas ...sample. The precise quantitative knowledge of this ratio is crucial because the nucleons in atoms and molecules contribute differently to the average nuclear polarization of the target gas. A new calibration technique used to derive the relative sensitivity to atoms and molecules is presented. As an example, it is shown how the gas analyzer is used within the HERMES environment to divide the molecules in the gas sample into an unpolarized and a potentially polarized fraction.
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