441 research outputs found
THE USE OF PHYSIOLOGICAL PARAMETERS IN MEASURING THE VEGETATIVES CONSEQUENCES OF MENTAL IMAGERY
INTRODUCTION The effects of the mental repetition of an activity entail not only the improvement of motor performances, but create vegetative consequences, which are connected to the degree of imagined effort (Decety, Jeannerod, Germain et Pastene, 1991 ). These beneficial effects of imagery have been explained by considering that the central structures of programming allow the anticipation of the metabolic requirement according to the established goals. The authors who do not agree on the explanatory theory of vegetative changes observed during the mental imagery phase, also have differing opinions concerning the existence of measurable muscular activity. Traditional instruments of measure such as heart rate and electromyography were used as well as force measurement. The force allowed us to know the, mechanical force produced, which should be zero during imagery phase not to have a conclusion in favor of a mechanical action. METHODS If we consider, with Jagasinski and Nicholls (1984,1987) that the central structure involved in the programming of a real activity, as in an imagined activity, defines the contents of this activity according to a goal, we can then assume that the mental operations occuring before the beginning of an action affect the level of effort and the energetic or metabolic resources used. We asked 24 male subjects who had obtained good results at the Sheehan imagery questionnaire to imagine a precise isometric contraction : bending the elbow. Through the use of instructions, one group of 8 subjects had to complete the task (called the goal of implication in a task), 8 other subjects had to do better than the others, surpass them (ego implication goal) while the 8 remaining subjects were given no instructions to achieve the goal. All the subjects went through two sessions including positive\negative feed-backs. In the real execution phase as well as in the imaginary phase, the measurements of heart rate and the electromyography of the biceps brachi and the force used are the primary indicators concerning the vegetative changes. RESULTS They show no trace of electromyography activity during the imagined movement if we make sure of the absence of movement during this phase by using a force transducer. Besides, there is a significant rise in the heart rate, during a real movement as well as an imagined one and this is even more evident when the subjects go from the conditions of implication in a task and implication of ego. At the same time, the subjects stated that they made less effort when they were given a positive feed-back and made higher levels of effort when they received a negative feed-back in an implication of ego situation. DISCUSSION In the mental imagery phase, all significant muscular activity is due to a mechanical action. In the absence of force production, even with the creation of an image, the muscle produces no activity. The potential of a subliminal muscular activity would be the real action in weak proportions but mechanically not significant for the chosen movement. The benefits of the mental repetition are here considered according to a symbolic perspective. and emphasize the cognitive components of the mental repetition, in other words, the individual imagined the activity to be accomplished. This is a matter of purely cognitive process. If a muscular activity is visible, the individual has gone from the stage of an intention of movement to his effective achievement, this change would come from the representation of the activity that the subject creates himself. We may conclude that measurements made with biomechanical devices can be very useful to check psychological hypotheses in sport. REFERENCES 1-BERGUS ; JONES, E.E. : Drug choice as self handicapping strategy in response to no contingent success. Journal of personnality and social psychology 76 : 14E158.1978. 2-CARVER, C.S ; SCHEIER. M F. : Attention and self regulation. A confrol theory approach lo human behavior. New York : Sp~nger- Verlag. 1981 3JAGASINSK1, C.D.; NICHOLLS. J.G. : Concepttons of a Ability and Related in Task lnvolment and Ego Involrnanl. Journal of Educational Psychology 76 : 909-919. 198
Magnetic impurities on superconducting Pb surfaces
It has been predicted theoretically and found experimentally that magnetic
impurities induce localized bound states within the superconducting energy gap,
called Yu-Shiba-Rusinov (YSR) states. Combining symmetry analysis with
experimental findings provides a convincing argument for the energy splitting
and distribution of the YSR peaks, but the full details of the electronic
structure remain elusive and simple models with point scatterers lack the full
orbital complexity required to meet this challenge. In this work we combine a
Greens function based first-principles method, which incorporates a
phenomenological parameterization of the superconducting state, with orbitally
complex impurity potentials to make material-specific predictions of realistic
systems. We study the effect of 3d transition elements on the superconducting
energy gap of a Pb (001) surface. Not only do we find a good agreement with
experiment, we also show that the energetic position, strength and orbital
composition of the YSR states depend strongly on the chemical makeup of the
impurity and its position with respect to the surface. Such quantitative
results cannot be derived from simplified models but require full material
specific calculations
Probing the early phase of rapid instructed rule encoding
Item does not contain fulltextHumans can rapidly convert instructions about a rule into functional neural structures used to apply the rule. The early stages of this encoding process are poorly understood. We designed a stimulus–response (SR) task in which participants were first shown a SR rule on a screen for 200 ms, and then had to apply it to a test stimulus T, which either matched the S in the rule (SR trial) or not (catch trial). To investigate the early stages of rule encoding, the delay between the end of rule display and the onset of the test stimulus was manipulated and chosen between values of 50 ms to 1300 ms. Participants conducted three sessions of 288 trials each, separated by a median of 9 h. Random sequences of 20 rules were used. We then analysed the reaction times and the types of errors made by participants in the different conditions. The analysis of practice effects in session 1 suggests that the neural networks that process SR and catch trials are at least partially distinct, and improve separately during the practice of respectively SR and catch trials. The rule-encoding process, however, is common to both tasks and improves with the number of trials, irrespective of the trial type. Rule encoding shows interesting dynamic properties that last for 500 ms after the end of the stimulus presentation. The encoding process increases the response time in a non-stochastic way, simply adding a reaction time cost to all responses. The rule-retrieval system is functional before the encoding has stabilized, as early as 50 ms after the end of SR rule presentation, with low response errors. It is sensitive to masking however, producing errors with brief (100 ms) test stimulus presentations. Once encoding has stabilized, the sensitivity to masking disappears. It is suggested that participants do encode rules as a parametrized function, using the same neural encoding structure for each trial, rather than reconfiguring their brain anew for each new SR rule. This structure would have been implemented from instructions received prior to the experiment, by using a library of neural functions available in the brain. The observed errors are consistent with this view.13 p
Dynamic nonlinear (cubic) susceptibility in quantum Ising spin glass
Dynamic nonlinear (cubic) susceptibility in quantum d-dimensional Ising spin
glass with short-range interactions is investigated on the basis of quantum
droplet model and quantum-mechanical nonlinear response theory. Nonlinear
response depends on the tunneling rate for a droplet which regulates the
strength of quantum fluctuations. It shows a strong dependence on the
distribution of droplet free energies and on the droplet length scale average.
Comparison with recent experiments on quantum spin glasses like disordered
dipolar quantum Ising magnet is discussed.Comment: 15 pages, 3 figure
Properties of the random field Ising model in a transverse magnetic field
We consider the effect of a random longitudinal field on the Ising model in a
transverse magnetic field. For spatial dimension , there is at low
strength of randomness and transverse field, a phase with true long range order
which is destroyed at higher values of the randomness or transverse field. The
properties of the quantum phase transition at zero temperature are controlled
by a fixed point with no quantum fluctuations. This fixed point also controls
the classical finite temperature phase transition in this model. Many critical
properties of the quantum transition are therefore identical to those of the
classical transition. In particular, we argue that the dynamical scaling is
activated, i.e, the logarithm of the diverging time scale rises as a power of
the diverging length scale
Disorder-induced rounding of certain quantum phase transitions
We study the influence of quenched disorder on quantum phase transitions in
systems with over-damped dynamics. For Ising order parameter symmetry disorder
destroys the sharp phase transition by rounding because a static order
parameter can develop on rare spatial regions. This leads to an exponential
dependence of the order parameter on the coupling constant. At finite
temperatures the static order on the rare regions is destroyed. This restores
the phase transition and leads to a double-exponential relation between
critical temperature and coupling strength. We discuss the behavior based on
Lifshitz-tail arguments and illustrate the results by simulations of a model
system.Comment: 4 pages, 3 eps figure
Numerical Study of Order in a Gauge Glass Model
The XY model with quenched random phase shifts is studied by a T=0 finite
size defect energy scaling method in 2d and 3d. The defect energy is defined by
a change in the boundary conditions from those compatible with the true ground
state configuration for a given realization of disorder. A numerical technique,
which is exact in principle, is used to evaluate this energy and to estimate
the stiffness exponent . This method gives in
2d and in 3d, which are considerably larger than
previous estimates, strongly suggesting that the lower critical dimension is
less than three. Some arguments in favor of these new estimates are given.Comment: 4 pages, 2 figures, revtex. Submitted to Phys. Rev. Let
Non-Fermi liquid behavior and Griffiths phase in {\it f}-electron compounds
We study the interplay among disorder, RKKY and Kondo interactions in {\it
f}-electron alloys. We argue that the non-Fermi liquid behavior observed in
these systems is due to the existence of a Griffiths phase close to a quantum
critical point. The existence of this phase provides a unified picture of a
large class of materials. We also propose new experiments that can test these
ideas.Comment: 4 pages, 1 Figure. NEW version of the original manuscript. A single
framework for NFL behavior in different kinds of alloys is presented. Final
version finally allowed to appear on the glorious Physical Review Letter
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