78 research outputs found
Divergence alone cannot guarantee stable sparse activity patterns if connections are dense
No description supplie
Quantum dynamics in strong fluctuating fields
A large number of multifaceted quantum transport processes in molecular
systems and physical nanosystems can be treated in terms of quantum relaxation
processes which couple to one or several fluctuating environments. A thermal
equilibrium environment can conveniently be modelled by a thermal bath of
harmonic oscillators. An archetype situation provides a two-state dissipative
quantum dynamics, commonly known under the label of a spin-boson dynamics. An
interesting and nontrivial physical situation emerges, however, when the
quantum dynamics evolves far away from thermal equilibrium. This occurs, for
example, when a charge transferring medium possesses nonequilibrium degrees of
freedom, or when a strong time-dependent control field is applied externally.
Accordingly, certain parameters of underlying quantum subsystem acquire
stochastic character. Herein, we review the general theoretical framework which
is based on the method of projector operators, yielding the quantum master
equations for systems that are exposed to strong external fields. This allows
one to investigate on a common basis the influence of nonequilibrium
fluctuations and periodic electrical fields on quantum transport processes.
Most importantly, such strong fluctuating fields induce a whole variety of
nonlinear and nonequilibrium phenomena. A characteristic feature of such
dynamics is the absence of thermal (quantum) detailed balance.Comment: review article, Advances in Physics (2005), in pres
Understanding resonant charge transport through weakly coupled single-molecule junctions
Off-resonant charge transport through molecular junctions has been
extensively studied since the advent of single-molecule electronics and it is
now well understood within the framework of the non-interacting Landauer
approach. Conversely, gaining a qualitative and quantitative understanding of
the resonant transport regime has proven more elusive. Here, we study resonant
charge transport through graphene-based zinc-porphyrin junctions. We
experimentally demonstrate an inadequacy of the non-interacting Landauer theory
as well as the conventional single-mode Franck-Condon model. Instead, we model
the overall charge transport as a sequence of non-adiabatic electron transfers,
the rates of which depend on both outer and inner-sphere vibrational
interactions. We show that the transport properties of our molecular junctions
are determined by a combination of electron-electron and electron-vibrational
coupling, and are sensitive to the interactions with the wider local
environment. Furthermore, we assess the importance of nuclear tunnelling and
examine the suitability of semi-classical Marcus theory as a description of
charge transport in molecular devices.Comment: version accepted in Nature Communications; SI available at
https://researchportal.hw.ac.uk/en/publications/understanding-resonant-charge-transport-through-weakly-coupled-s
A Corticothalamic Circuit Model for Sound Identification in Complex Scenes
The identification of the sound sources present in the environment is essential for the survival of many animals. However, these sounds are not presented in isolation, as natural scenes consist of a superposition of sounds originating from multiple sources. The identification of a source under these circumstances is a complex computational problem that is readily solved by most animals. We present a model of the thalamocortical circuit that performs level-invariant recognition of auditory objects in complex auditory scenes. The circuit identifies the objects present from a large dictionary of possible elements and operates reliably for real sound signals with multiple concurrently active sources. The key model assumption is that the activities of some cortical neurons encode the difference between the observed signal and an internal estimate. Reanalysis of awake auditory cortex recordings revealed neurons with patterns of activity corresponding to such an error signal
Charge Transport in DNA-Based Devices
Charge migration along DNA molecules has attracted scientific interest for
over half a century. Reports on possible high rates of charge transfer between
donor and acceptor through the DNA, obtained in the last decade from solution
chemistry experiments on large numbers of molecules, triggered a series of
direct electrical transport measurements through DNA single molecules, bundles
and networks. These measurements are reviewed and presented here. From these
experiments we conclude that electrical transport is feasible in short DNA
molecules, in bundles and networks, but blocked in long single molecules that
are attached to surfaces. The experimental background is complemented by an
account of the theoretical/computational schemes that are applied to study the
electronic and transport properties of DNA-based nanowires. Examples of
selected applications are given, to show the capabilities and limits of current
theoretical approaches to accurately describe the wires, interpret the
transport measurements, and predict suitable strategies to enhance the
conductivity of DNA nanostructures.Comment: A single pdf file of 52 pages, containing the text and 23 figures.
Review about direct measurements of DNA conductivity and related theoretical
studies. For higher-resolution figures contact the authors or retrieve the
original publications cited in the caption
Sparse, decorrelated odor coding in the mushroom body enhances learned odor discrimination
Sparse coding may be a general strategy of neural systems for augmenting memory capacity. In Drosophila melanogaster, sparse odor coding by the Kenyon cells of the mushroom body is thought to generate a large number of precisely addressable locations for the storage of odor-specific memories. However, it remains untested how sparse coding relates to behavioral performance. Here we demonstrate that sparseness is controlled by a negative feedback circuit between Kenyon cells and the GABAergic anterior paired lateral (APL) neuron. Systematic activation and blockade of each leg of this feedback circuit showed that Kenyon cells activated APL and APL inhibited Kenyon cells. Disrupting the Kenyon cell–APL feedback loop decreased the sparseness of Kenyon cell odor responses, increased inter-odor correlations and prevented flies from learning to discriminate similar, but not dissimilar, odors. These results suggest that feedback inhibition suppresses Kenyon cell activity to maintain sparse, decorrelated odor coding and thus the odor specificity of memories
Multiple network properties overcome random connectivity to enable stereotypic sensory responses
Connections between neuronal populations may be genetically hardwired or random. In the insect olfactory system, projection neurons of the antennal lobe connect randomly to Kenyon cells of the mushroom body. Consequently, while the odor responses of the projection neurons are stereotyped across individuals, the responses of the Kenyon cells are variable. Surprisingly, downstream of Kenyon cells, mushroom body output neurons show stereotypy in their responses. We found that the stereotypy is enabled by the convergence of inputs from many Kenyon cells onto an output neuron, and does not require learning. The stereotypy emerges in the total response of the Kenyon cell population using multiple odor-specific features of the projection neuron responses, benefits from the nonlinearity in the transfer function, depends on the convergence:randomness ratio, and is constrained by sparseness. Together, our results reveal the fundamental mechanisms and constraints with which convergence enables stereotypy in sensory responses despite random connectivity
Understanding resonant charge transport through weakly coupled single-molecule junctions
Off-resonant charge transport through molecular junctions has been
extensively studied since the advent of single-molecule electronics and it is
now well understood within the framework of the non-interacting Landauer
approach. Conversely, gaining a qualitative and quantitative understanding of
the resonant transport regime has proven more elusive. Here, we study resonant
charge transport through graphene-based zinc-porphyrin junctions. We
experimentally demonstrate an inadequacy of the non-interacting Landauer theory
as well as the conventional single-mode Franck-Condon model. Instead, we model
the overall charge transport as a sequence of non-adiabatic electron transfers,
the rates of which depend on both outer and inner-sphere vibrational
interactions. We show that the transport properties of our molecular junctions
are determined by a combination of electron-electron and electron-vibrational
coupling, and are sensitive to the interactions with the wider local
environment. Furthermore, we assess the importance of nuclear tunnelling and
examine the suitability of semi-classical Marcus theory as a description of
charge transport in molecular devices.Comment: version accepted in Nature Communications; SI available at
https://researchportal.hw.ac.uk/en/publications/understanding-resonant-charge-transport-through-weakly-coupled-s
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