5,007 research outputs found
Which way do I go? Neural activation in response to feedback and spatial processing in a virtual T-maze
In 2 human event-related brain potential (ERP) experiments, we examined the feedback error-related negativity (fERN), an ERP component associated with reward processing by the midbrain dopamine system, and the N170, an ERP component thought to be generated by the medial temporal lobe (MTL), to investigate the contributions of these neural systems toward learning to find rewards in a "virtual T-maze" environment. We found that feedback indicating the absence versus presence of a reward differentially modulated fERN amplitude, but only when the outcome was not predicted by an earlier stimulus. By contrast, when a cue predicted the reward outcome, then the predictive cue (and not the feedback) differentially modulated fERN amplitude. We further found that the spatial location of the feedback stimuli elicited a large N170 at electrode sites sensitive to right MTL activation and that the latency of this component was sensitive to the spatial location of the reward, occurring slightly earlier for rewards following a right versus left turn in the maze. Taken together, these results confirm a fundamental prediction of a dopamine theory of the fERN and suggest that the dopamine and MTL systems may interact in navigational learning tasks
Intermediate phase of the one dimensional half-filled Hubbard-Holstein model
We present a detailed numerical study of the Hubbard-Holstein model in one
dimension at half filling, including full finite-frequency quantum phonons. At
half filling, the effects of the electron-phonon and electron-electron
interactions compete, with the Holstein phonon coupling acting as an effective
negative Hubbard onsite interaction U that promotes on-site electron pairs and
a Peierls charge-density wave state. Most previous work on this model has
assumed that only Peierls or U>0 Mott insulator phases are possible at half
filling. However, there has been speculation that a third metallic phase exists
between the Peierls and Mott phases. We present results confirming the
intermediate metallic phase, and show that the Luttinger liquid correlation
exponent K_rho>1 in this region, indicating dominant superconducting pair
correlations. We explore the full phase diagram as a function of onsite Hubbard
U, phonon coupling constant, and phonon frequency.Comment: 4 pages, 4 EPS figures. v2: typos corrected. To appear in Phys. Rev.
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Theory of triangular lattice quasi-one-dimensional charge-transfer solids
Recent investigations of the magnetic properties and the discovery of
superconductivity in quasi-one-dimensional triangular lattice organic
charge-transfer solids have indicated the severe limitations of the effective
1/2-filled band Hubbard model for these and related systems. Our computational
studies of these materials within a 1/4-filled band Hubbard model in which the
organic monomer molecules, and not their dimers, constitute the sites of the
Hamiltonian are able to reproduce the experimental results. We ascribe the spin
gap transition in kappa-(BEDT-TTF)_2B(CN)_4 to the formation of a
two-dimensional paired-electron crystal and make the testable prediction that
the spin gap will be accompanied by charge-ordering and period doubling in two
directions. We find enhancement of the long-range component of superconducting
pairing correlations by the Hubbard repulsive interaction for band parameters
corresponding to kappa-(BEDT-TTF)_2CF_3SO_3. The overall results strongly
support a valence bond theory of superconductivity we have proposed recently.Comment: 8 pages, 7 figure
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