546 research outputs found
Learning Hybrid System Models for Supervisory Decoding of Discrete State, with applications to the Parietal Reach Region
Based on Gibbs sampling, a novel method to identify mathematical models of neural activity in response to temporal changes of behavioral or cognitive state is presented. This work is motivated by the developing field of neural prosthetics, where a supervisory controller is required to classify activity of a brain region into suitable discrete modes. Here, neural activity in each discrete mode is modeled with nonstationary point processes, and transitions between modes are modeled as hidden Markov models. The effectiveness of this framework is first demonstrated on a simulated example. The identification algorithm is then applied to extracellular neural activity recorded from multi-electrode arrays in the parietal reach region of a rhesus monkey, and the results demonstrate the ability to decode discrete changes even from small data sets
Prominences in SDO/EVE spectra: contributions from large solar structures
The EVE instrument on SDO is making accurate measurements of the solar spectral irradiance in the EUV between 30 and 1069 Å, with 1 Å spectral resolution and 10 s sampling rate. These data define solar variability in the “Sun-as-a-star” mode and reveal many interesting kinds of variation. Its high sensitivity also makes it suitable for spectroscopic diagnostics of solar features such as flares. Here we present EVE's potential contribution to the diagnostics of large-scale, slowly evolving features such as prominences and active regions, and what we can learn from this
A quantum de Finetti theorem in phase space representation
The quantum versions of de Finetti's theorem derived so far express the
convergence of n-partite symmetric states, i.e., states that are invariant
under permutations of their n parties, towards probabilistic mixtures of
independent and identically distributed (i.i.d.) states. Unfortunately, these
theorems only hold in finite-dimensional Hilbert spaces, and their direct
generalization to infinite-dimensional Hilbert spaces is known to fail. Here,
we address this problem by considering invariance under orthogonal
transformations in phase space instead of permutations in state space, which
leads to a new type of quantum de Finetti's theorem that is particularly
relevant to continuous-variable systems. Specifically, an n-mode bosonic state
that is invariant with respect to this continuous symmetry in phase space is
proven to converge towards a probabilistic mixture of i.i.d. Gaussian states
(actually, n identical thermal states).Comment: 5 page
Guided Curriculum Learning for Walking Over Complex Terrain
Reliable bipedal walking over complex terrain is a challenging problem, using
a curriculum can help learning. Curriculum learning is the idea of starting
with an achievable version of a task and increasing the difficulty as a success
criteria is met. We propose a 3-stage curriculum to train Deep Reinforcement
Learning policies for bipedal walking over various challenging terrains. In the
first stage, the agent starts on an easy terrain and the terrain difficulty is
gradually increased, while forces derived from a target policy are applied to
the robot joints and the base. In the second stage, the guiding forces are
gradually reduced to zero. Finally, in the third stage, random perturbations
with increasing magnitude are applied to the robot base, so the robustness of
the policies are improved. In simulation experiments, we show that our approach
is effective in learning walking policies, separate from each other, for five
terrain types: flat, hurdles, gaps, stairs, and steps. Moreover, we demonstrate
that in the absence of human demonstrations, a simple hand designed walking
trajectory is a sufficient prior to learn to traverse complex terrain types. In
ablation studies, we show that taking out any one of the three stages of the
curriculum degrades the learning performance.Comment: Submitted to Australasian Conference on Robotics and Automation
(ACRA) 202
Stability of Bott--Samelson Classes in Algebraic Cobordism
In this paper, we construct stable Bott--Samelson classes in the projective
limit of the algebraic cobordism rings of full flag varieties, upon an initial
choice of a reduced word in a given dimension. Each stable Bott--Samelson class
is represented by a bounded formal power series modulo symmetric functions in
positive degree. We make some explicit computations for those power series in
the case of infinitesimal cohomology. We also obtain a formula of the
restriction of Bott--Samelson classes to smaller flag varieties.Comment: 23 pages. Comments are welcom
Learning When to Switch: Composing Controllers to Traverse a Sequence of Terrain Artifacts
Legged robots often use separate control policiesthat are highly engineered
for traversing difficult terrain suchas stairs, gaps, and steps, where
switching between policies isonly possible when the robot is in a region that
is commonto adjacent controllers. Deep Reinforcement Learning (DRL)is a
promising alternative to hand-crafted control design,though typically requires
the full set of test conditions to beknown before training. DRL policies can
result in complex(often unrealistic) behaviours that have few or no
overlappingregions between adjacent policies, making it difficult to
switchbehaviours. In this work we develop multiple DRL policieswith Curriculum
Learning (CL), each that can traverse asingle respective terrain condition,
while ensuring an overlapbetween policies. We then train a network for each
destinationpolicy that estimates the likelihood of successfully switchingfrom
any other policy. We evaluate our switching methodon a previously unseen
combination of terrain artifacts andshow that it performs better than heuristic
methods. Whileour method is trained on individual terrain types, it
performscomparably to a Deep Q Network trained on the full set ofterrain
conditions. This approach allows the development ofseparate policies in
constrained conditions with embedded priorknowledge about each behaviour, that
is scalable to any numberof behaviours, and prepares DRL methods for
applications inthe real worl
Leadership and Change in Asia-Pacific: Where Does Political Will Come From?
We introduce this thematic issue by exploring the role of leadership in social and political change. In current times, the importance of leadership and choice has proved as important as ever. Leadership is often the critical variable separating success or failure, legitimacy and sustainability or collapse. This thematic issue explores a range of in-depth case studies across the Asia-Pacific region that help illustrate the critical elements of leadership. Collectively they demonstrate that leadership is best understood as a collective process involving motivated agents overcoming barriers to cooperation to form coalitions that have enough power, legitimacy and influence to transform institutions. Five themes emerge from the thematic issue as a whole: leadership is political; the centrality of gender relations; the need for a more critical localism; scalar politics; and the importance of understanding informal processes of leadership and social change
Behaviour of molecular hydrogen emission in three solar flares
We have systematically investigated ultraviolet (UV) emission from molecular
hydrogen (H) using the Interface Region Imaging Spectrometer (IRIS),
during three X-ray flares of C5.1, C9.7 and X1.0 classes on Oct. 25, 2014.
Significant emission from five H spectral lines appeared in the flare
ribbons, interpreted as photo-excitation (fluorescence) due to the absorption
of UV radiation from two Si IV spectral lines. The H profiles were broad
and consisted of two non-stationary components in red and in the blue wings of
the line in addition to the stationary component. The red (blue) wing
components showed small redshifts (blue shifts) of ~5-15 km s (~5-10 km
s). The nonthermal velocities were found to be ~5-15 km s. The
interrelation between intensities of H lines and their branching ratios
confirmed that H emission formed under optically thin plasma conditions.
There is a strong spatial and temporal correlation between Si IV and H
emission, but the H emission is more extended and diffuse, further
suggesting H fluorescence, and - by analogy with flare ''back-warming''
providing a means to estimate the depth from which the H emission
originates. We find that this is 1871157 km and 1207112 km below the
source of the Si IV emission, in two different ribbon locations.Comment: 14 pages, 12 figures, accepted for publication in MNRA
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