When energy is added to a system, for example by stirring, the ensuing equilibration dynamics usually leads to more disorder. However, in a bounded two-dimensional fluid containing quantized vortices, Onsager found a surprising result: continuing to add energy leads to highly-ordered, persistent vortex clusters. Here, for the first time, we realize these high-energy vortex clusters in a planar superfluid and demonstrate that they persist for long times, despite being far from equilibrium with the surrounding fluid. Our experiments explore a new regime of vortex matter at negative absolute temperature, diametrically opposed to the Kosterlitz-Thouless transition at positive temperature. Such vortex cluster states are directly relevant to studies of two-dimensional turbulence, and are potentially realizable in helium films, exciton-polariton superfluids, nonlinear optical materials, and fermion superfluids. arXiv:1801.06951v2 [cond-mat.quant-gas]
The present study compared the contribution of visual information of hand and target position to the online control of goal-directed arm movements. Their respective contributions were assessed by examining how human subjects reacted to a change of the position of either their seen hand or the visual target near the onset of the reaching movement. Subjects, seated head-fixed in a dark room, were instructed to look at and reach with a pointer towards visual targets located in the fronto-parallel plane at different distances to the right of the starting position. LEDs mounted on the tip of the pointer were used to provide true or erroneous visual feedback about hand position. In some trials, either the target or the pointer LED that signalled the actual hand position was shifted 4.5 cm to the left or to the right during the ocular saccade towards the target. Because of saccadic suppression, subjects did not perceive these displacements, which occurred near arm movement onset. The results showed that modifications of arm movement amplitude appeared, on average, 150 ms earlier and reached a greater extent (mean difference=2.7 cm) when there was a change of target position than when a change of the seen hand position occurred. These findings highlight the weight of target position information to the online control of arm movements. Visual information relative to hand position may be less contributive because proprioception also provides information about limb position.
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