2,558 research outputs found
Is implicit motor imagery a reliable strategy for a brain computer interface?
Explicit motor imagery (eMI) is a widely used brain computer interface (BCI) paradigm, but not everybody can accomplish this task. Here we propose a BCI based on implicit motor imagery (iMI). We compared classification accuracy between eMI and iMI of hands. Fifteen able bodied people were asked to judge the laterality of hand images presented on a computer screen in a lateral or medial orientation. This judgement task is known to require mental rotation of a person’s own hands which in turn is thought to involve iMI. The subjects were also asked to perform eMI of the hands. Their electroencephalography (EEG) was recorded. Linear classifiers were designed based on common spatial patterns. For discrimination between left and right hand the classifier achieved maximum of 81 ± 8% accuracy for eMI and 83 ± 3% for iMI. These results show that iMI can be used to achieve similar classification accuracy as eMI. Additional classification was performed between iMI in medial and lateral orientations of a single hand; the classifier achieved 81 ± 7% for the left and 78 ± 7% for the right hand which indicate distinctive spatial patterns of cortical activity for iMI of a single hand in different directions. These results suggest that a special brain computer interface based on iMI may be constructed, for people who cannot perform explicit imagination, for rehabilitation of movement or for treatment of bodily spatial neglect
A Diffie-Hellman based key management scheme for hierarchical access control
All organizations share data in a carefully managed fashion\ud
by using access control mechanisms. We focus on enforcing access control by encrypting the data and managing the encryption keys. We make the realistic assumption that the structure of any organization is a hierarchy of security classes. Data from a certain security class can only be accessed by another security class, if it is higher or at the same level in the hierarchy. Otherwise access is denied. Our solution is based on the Die-Hellman key exchange protocol. We show, that the theoretical worst case performance of our solution is slightly better than that of all other existing solutions. We also show, that our performance in practical cases is linear in the size of the hierarchy, whereas the best results from the literature are quadratic
A Tight Bound for Shortest Augmenting Paths on Trees
The shortest augmenting path technique is one of the fundamental ideas used
in maximum matching and maximum flow algorithms. Since being introduced by
Edmonds and Karp in 1972, it has been widely applied in many different
settings. Surprisingly, despite this extensive usage, it is still not well
understood even in the simplest case: online bipartite matching problem on
trees. In this problem a bipartite tree is being revealed
online, i.e., in each round one vertex from with its incident edges
arrives. It was conjectured by Chaudhuri et. al. [K. Chaudhuri, C. Daskalakis,
R. D. Kleinberg, and H. Lin. Online bipartite perfect matching with
augmentations. In INFOCOM 2009] that the total length of all shortest
augmenting paths found is . In this paper, we prove a tight upper bound for the total length of shortest augmenting paths for
trees improving over bound [B. Bosek, D. Leniowski, P.
Sankowski, and A. Zych. Shortest augmenting paths for online matchings on
trees. In WAOA 2015].Comment: 22 pages, 10 figure
Universal decoherence due to gravitational time dilation
The physics of low-energy quantum systems is usually studied without explicit
consideration of the background spacetime. Phenomena inherent to quantum theory
on curved space-time, such as Hawking radiation, are typically assumed to be
only relevant at extreme physical conditions: at high energies and in strong
gravitational fields. Here we consider low-energy quantum mechanics in the
presence of gravitational time dilation and show that the latter leads to
decoherence of quantum superpositions. Time dilation induces a universal
coupling between internal degrees of freedom and the centre-of-mass of a
composite particle. The resulting correlations cause decoherence of the
particle's position, even without any external environment. We also show that
the weak time dilation on Earth is already sufficient to decohere micron scale
objects. Gravity therefore can account for the emergence of classicality and
the effect can in principle be tested in future matter wave experiments.Comment: 6+4 pages, 3 figures. Revised manuscript in Nature Physics (2015
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