Many behaviors are learned through trial and error by matching performance to internal goals. Yet neural mechanisms of performance evaluation remain poorly understood. We recorded basal ganglia projecting dopamine neurons in singing zebra finches as we controlled perceived song quality with distorted auditory feedback. Dopamine activity was phasically suppressed after distorted syllables, consistent with a worse-than-predicted outcome, and was phasically activated at the precise moment of the song when a predicted distortion did not occur, consistent with a better-than-predicted outcome. Error response magnitude depended on distortion probability. Thus dopaminergic error signals can evaluate behaviors that are not learned for reward and are instead learned by matching performance outcomes to internal goals.
Although solid helium-4 (4He) may be a supersolid, it also exhibits many phenomena unexpected in that context. We studied relaxation dynamics in the resonance frequency f(T) and dissipation D(T) of a torsional oscillator containing solid 4He. With the appearance of the "supersolid" state, the relaxation times within f(T) and D(T) began to increase rapidly together. More importantly, the relaxation processes in both D(T) and a component of f(T) exhibited a complex synchronized ultraslow evolution toward equilibrium. Analysis using a generalized rotational susceptibility revealed that, while exhibiting these apparently glassy dynamics, the phenomena were quantitatively inconsistent with a simple excitation freeze-out transition because the variation in f was far too large. One possibility is that amorphous solid 4He represents a new form of supersolid in which dynamical excitations within the solid control the superfluid phase stiffness.
We use classical molecular dynamics simulations to study the collapse of single (SWNT) and double-walled (DWNT) carbon nanotube bundles under hydrostatic pressure. The collapse pressure (p c ) varies as 1/R 3 , where R is the SWNT radius or the DWNT effective radius. The bundles show ∼ 30 % hysteresis and the hexagonally close packed lattice is completely restored on decompression.The p c of DWNT is found to be close to the sum of its values for the inner and the outer tubes considered separately as SWNT, demonstrating that the inner tube supports the outer tube and that the effective bending stiffness of DWNT, D DW N T ∼2D SW N T . We use an elastica formulation to derive the scaling and the collapse behavior of DWNT and multi-walled carbon nanotubes.
Using a high-sensitivity torsional oscillator (TO) technique, we mapped the rotational and relaxational dynamics of solid helium-4 ((4)He) throughout the parameter range of the proposed supersolidity. We found evidence that the same microscopic excitations controlling the torsional oscillator motions are generated independently by thermal and mechanical stimulation. Moreover, a measure for the relaxation times of these excitations diverges smoothly without any indication for a critical temperature or critical velocity of a supersolid transition. Finally, we demonstrated that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the solid's shear modulus. This implies that the rotational responses of solid (4)He attributed to supersolidity are associated with generation of the same microscopic excitations as those produced by direct shear strain.
Motivated by the experimental demonstration of the actuator action of single-walled carbon nanotubes and in situ resonance Raman experiments, we have performed ab initio restricted Hatree-Fock calculations of the electronic structure of a (5,5) singlewalled nanotube in the presence of various ions. We show that the presence of ions near the nanotube causes combined axial and radial deformations of the nanotube. The presence of ions causes a small charge transfer from the ions to the nanotubes as well as changes the p-p overlap energy (c). We find that the strains developed are primarily due to the electrostatic interactions with only a small contribution from the charge transfer.
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