153 research outputs found
On the relative homology of cleft extensions of rings and abelian categories
AbstractWe study the relative homological behaviour of the omnipresent class of cleft extensions of abelian categories. This class of extensions is a natural generalization of the trivial extensions studied in detail by Fossum, Griffith and Reiten and by Palmer and Roos. We apply our results to the relative homology of cleft extensions of rings
C*-Algebras over Topological Spaces: Filtrated K-Theory
We define the filtrated K-theory of a C*-algebra over a finite topological
space X and explain how to construct a spectral sequence that computes the
bivariant Kasparov theory over X in terms of filtrated K-theory. For finite
spaces with totally ordered lattice of open subsets, this spectral sequence
becomes an exact sequence as in the Universal Coefficient Theorem, with the
same consequences for classification. We also exhibit an example where
filtrated K-theory is not yet a complete invariant. We describe a space with
four points and two C*-algebras over this space in the bootstrap class that
have isomorphic filtrated K-theory but are not KK(X)-equivalent. For this
particular space, we enrich filtrated K-theory by another K-theory functor, so
that there is again a Universal Coefficient Theorem. Thus the enriched
filtrated K-theory is a complete invariant for purely infinite, stable
C*-algebras with this particular spectrum and belonging to the appropriate
bootstrap class.Comment: Changes to theorem and equation numbering
Both low and high spatial frequencies drive the early posterior negativity in response to snake stimuli
Previous event-related potential (ERP) studies have shown that snake pictures elicit greater early posterior negativity (EPN) compared to other animal pictures. The EPN reflects early selective visual processing of emotionally significant stimuli. Evidence for the role that high and low spatial frequencies play in the early detection of snakes is still inconsistent. The current study aims to clarify this by studying the effect of high and low spatial frequencies on the elevated EPN for snakes separately. Using a rapid serial visual presentation paradigm, participants viewed images of snakes, spiders and birds in three different conditions of filtered spatial frequencies: high spatial frequency, low spatial frequency, and full spatial frequency (the original image). P1 and mean EPN activity in a time window of 225–300 ms after stimulus onset were measured at the occipital cluster (O1, O2, Oz). The results show smaller P1 amplitudes and shorter P1 latencies in response to full-spectrum snake pictures compared to full-spectrum spider and bird pictures, and an increased EPN in response to snake pictures compared to spider and bird pictures in all three filtering conditions. The EPN in response to full-spectrum snake pictures was larger than the EPN in response to filtered snake images. No difference in EPN was found between the snake pictures in the high and low spatial frequency conditions. The results suggest that the roles of high and low spatial frequencies in early automatic attention to snakes are equally important.</p
On exact categories and applications to triangulated adjoints and model structures
We show that Quillen's small object argument works for exact categories under
very mild conditions. This has immediate applications to cotorsion pairs and
their relation to the existence of certain triangulated adjoint functors and
model structures. In particular, the interplay of different exact structures on
the category of complexes of quasi-coherent sheaves leads to a streamlined and
generalized version of recent results obtained by Estrada, Gillespie, Guil
Asensio, Hovey, J{\o}rgensen, Neeman, Murfet, Prest, Trlifaj and possibly
others.Comment: 38 pages; version 2: major revision, more explanation added at
several places, reference list updated and extended, misprints correcte
High spatial frequencies drive the early posterior negativity in response to snake pictures
__Introduction__
As snakes were probably the first predators of
mammals, they may have been important agents of
evolutionary changes in the primate visual system
allowing fast visual detection of fearful stimuli (Isbell,
2006). Many EEG studies have established larger
early posterior negativity (EPN) in response to
snake stimuli when compared to other animal stimuli
(e.g., Van Strien et al., 2014). The EPN is an eventrelated
potential that reflects early selective visual
processing of emotionally significant information. A
recent study (Van Strien & Isbell, 2017) has
emphasized the importance of the typical scales and
scale patterns of the snake skin for the enhanced
EPN in response to snake pictures. In the present
research, we examined whether the EPN snake
effect still exists when these scales are made less
visible by blurring snake pictures, that is, we
examined the influence of spatial frequency on the
EPN snake effect
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