1,249 research outputs found
Divisive Normalization and Neuronal Oscillations in a Single Hierarchical Framework of Selective Visual Attention
Divisive normalization models of covert attention commonly use spike rate modulations as indicators of the effect of top-down attention. In addition, an increasing number of studies have shown that top-down attention increases the synchronization of neuronal oscillations as well, particularly in gamma-band frequencies (25–100 Hz). Although modulations of spike rate and synchronous oscillations are not mutually exclusive as mechanisms of attention, there has thus far been little effort to integrate these concepts into a single framework of attention. Here, we aim to provide such a unified framework by expanding the normalization model of attention with a multi-level hierarchical structure and a time dimension; allowing the simulation of a recently reported backward progression of attentional effects along the visual cortical hierarchy. A simple cascade of normalization models simulating different cortical areas is shown to cause signal degradation and a loss of stimulus discriminability over time. To negate this degradation and ensure stable neuronal stimulus representations, we incorporate a kind of oscillatory phase entrainment into our model that has previously been proposed as the “communication-through-coherence” (CTC) hypothesis. Our analysis shows that divisive normalization and oscillation models can complement each other in a unified account of the neural mechanisms of selective visual attention. The resulting hierarchical normalization and oscillation (HNO) model reproduces several additional spatial and temporal aspects of attentional modulation and predicts a latency effect on neuronal responses as a result of cued attention
Morphological development and cytochrome c oxidase activity in Streptomyces lividans are dependent on the action of a copper bound Sco protein
Copper has an important role in the life cycle of many streptomycetes, stimulating the developmental switch between vegetative mycelium and aerial hyphae concomitant with the production of antibiotics. In streptomycetes, a gene encoding for a putative Sco-like protein has been identified and is part of an operon that contains two other genes predicted to handle cellular copper. We report on the Sco-like protein from
Streptomyces lividans
(Sco
Sl
) and present a series of experiments that firmly establish a role for Sco
Sl
as a copper metallochaperone as opposed to a role as a thiol-disulphide reductase that has been assigned to other bacterial Sco proteins. Under low copper concentrations, a Δ
sco
mutant in
S. lividans
displays two phenotypes; the development switch between vegetative mycelium and aerial hyphae stalls and cytochrome
c
oxidase (CcO) activity is significantly decreased. At elevated copper levels, the development and CcO activity in the Δ
sco
mutant are restored to wild-type levels and are thus independent of Sco
Sl
. A CcO knockout reveals that morphological development is independent of CcO activity leading us to suggest that Sco
Sl
has at least two targets in
S. lividans
. We establish that one Sco
Sl
target is the dinuclear Cu
A
domain of CcO and it is the cupric form of Sco
Sl
that is functionally active. The mechanism of cupric ion capture by Sco
Sl
has been investigated, and an important role for a conserved His residue is identified.
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On the relation between decoherence and spontaneous symmetry breaking
We have recently shown that there is a limit to quantum coherence in
many-particle spin qubits due to spontaneous symmetry breaking. These results
were derived for the Lieb-Mattis spin model. Here we will show that the
underlying mechanism of decoherence in systems with spontaneous symmetry
breaking is in fact more general. We present here a generic route to finding
the decoherence time associated with spontaneous symmetry breaking in many
particle qubits, and subsequently we apply this approach to two model systems,
indicating how the continuous symmetries in these models are spontaneously
broken and discussing the relation of this symmetry breaking to the thin
spectrum. We then present in detail the calculations that lead to the limit to
quantum coherence, which is due to energy shifts in the thin spectrum.Comment: 14 pages, 5 figure
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