4,457 research outputs found
Neural signatures of cognitive flexibility and reward sensitivity following nicotinic receptor stimulation in dependent smokers : a randomized trial
IMPORTANCE Withdrawal from nicotine is an important contributor to smoking relapse. Understanding how reward-based decision making is affected by abstinence and by pharmacotherapies such as nicotine replacement therapy and varenicline tartrate may aid cessation treatment.
OBJECTIVE To independently assess the effects of nicotine dependence and stimulation of the nicotinic acetylcholine receptor on the ability to interpret valence information (reward sensitivity) and subsequently alter behavior as reward contingencies change (cognitive flexibility) in a probabilistic reversal learning task.
DESIGN, SETTING, AND PARTICIPANTS Nicotine-dependent smokers and nonsmokers completed a probabilistic reversal learning task during acquisition of functional magnetic resonance imaging (fMRI) in a 2-drug, double-blind placebo-controlled crossover design conducted from January 21, 2009, to September 29, 2011. Smokers were abstinent from cigarette smoking for 12 hours for all sessions. In a fully Latin square fashion, participants in both groups underwent MRI twice while receiving varenicline and twice while receiving a placebo pill, wearing either a nicotine or a placebo patch. Imaging analysis was performed from June 15, 2015, to August 10, 2016.
MAIN OUTCOME AND MEASURES A well-established computational model captured effects of smoking status and administration of nicotine and varenicline on probabilistic reversal learning choice behavior. Neural effects of smoking status, nicotine, and varenicline were tested for on MRI contrasts that captured reward sensitivity and cognitive flexibility.
RESULTS The study included 24 nicotine-dependent smokers (12 women and 12 men; mean [SD] age, 35.8 [9.9] years) and 20 nonsmokers (10 women and 10 men; mean [SD] age, 30.4 [7.2] years). Computational modeling indicated that abstinent smokers were biased toward response shifting and that their decisions were less sensitive to the available evidence, suggesting increased impulsivity during withdrawal. These behavioral impairments were mitigated with nicotine and varenicline. Similarly, decreased mesocorticolimbic activity associated with cognitive flexibility in abstinent smokers was restored to the level of nonsmokers following stimulation of nicotinic acetylcholine receptors (familywise error-corrected P<.05). Conversely, neural signatures of decreased reward sensitivity in smokers (vs nonsmokers; familywise error-corrected P<.05) in the dorsal striatum and anterior cingulate cortex were not mitigated by nicotine or varenicline.
CONCLUSIONS AND RELEVANCE There was a double dissociation between the effects of chronic nicotine dependence on neural representations of reward sensitivity and acute effects of stimulation of nicotinic acetylcholine receptors on behavioral and neural signatures of cognitive flexibility in smokers. These chronic and acute pharmacologic effects were observed in overlapping mesocorticolimbic regions, suggesting that available pharmacotherapies may alleviate deficits in the same circuitry for certain mental computations but not for others
Non-Fermi-liquid behavior in Ce(RuFe)Ge: cause and effect
We present inelastic neutron scattering measurements on the intermetallic
compounds Ce(RuFe)Ge (=0.65, 0.76 and 0.87). These
compounds represent samples in a magnetically ordered phase, at a quantum
critical point and in the heavy-fermion phase, respectively. We show that at
high temperatures the three compositions have the identical response of a local
moment system. However, at low temperatures the spin fluctuations in the
critical composition are given by non-Fermi-liquid dynamics, while the spin
fluctuations in the heavy fermion system show a simple exponential decay in
time. In both compositions, the lifetime of the fluctuations is determined
solely by the distance to the quantum critical point. We discuss the
implications of these observations regarding the possible origins of
non-Fermi-liquid behavior in this system.Comment: 4 figures, submitted to PR
Effective Viscosity of Dilute Bacterial Suspensions: A Two-Dimensional Model
Suspensions of self-propelled particles are studied in the framework of
two-dimensional (2D) Stokesean hydrodynamics. A formula is obtained for the
effective viscosity of such suspensions in the limit of small concentrations.
This formula includes the two terms that are found in the 2D version of
Einstein's classical result for passive suspensions. To this, the main result
of the paper is added, an additional term due to self-propulsion which depends
on the physical and geometric properties of the active suspension. This term
explains the experimental observation of a decrease in effective viscosity in
active suspensions.Comment: 15 pages, 3 figures, submitted to Physical Biolog
On the Convergence of the Born Series in Optical Tomography with Diffuse Light
We provide a simple sufficient condition for convergence of Born series in
the forward problem of optical diffusion tomography. The condition does not
depend on the shape or spatial extent of the inhomogeneity but only on its
amplitude.Comment: 23 pages, 7 figures, submitted to Inverse Problem
Spin glass behavior in URh_2Ge_2
URh_2Ge_2 occupies an extraordinary position among the heavy-electron
122-compounds, by exhibiting a previously unidentified form of magnetic
correlations at low temperatures, instead of the usual antiferromagnetism. Here
we present new results of ac and dc susceptibilities, specific heat and neutron
diffraction on single-crystalline as-grown URh_2Ge_2. These data clearly
indicate that crystallographic disorder on a local scale produces spin glass
behavior in the sample. We therefore conclude that URh_2Ge_2 is a 3D
Ising-like, random-bond, heavy-fermion spin glass.Comment: 10 pages, RevTeX, with 4 postscript figures, accepted by Physical
Review Letters Nov 15, 199
Avalanche of Bifurcations and Hysteresis in a Model of Cellular Differentiation
Cellular differentiation in a developping organism is studied via a discrete
bistable reaction-diffusion model. A system of undifferentiated cells is
allowed to receive an inductive signal emenating from its environment.
Depending on the form of the nonlinear reaction kinetics, this signal can
trigger a series of bifurcations in the system. Differentiation starts at the
surface where the signal is received, and cells change type up to a given
distance, or under other conditions, the differentiation process propagates
through the whole domain. When the signal diminishes hysteresis is observed
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