250 research outputs found
A neural model for the visual tuning properties of action-selective neurons
SUMMARY: The recognition of actions of conspecifics is crucial for survival and social interaction. Most current models on the recognition of transitive (goal-directed) actions rely on the hypothesized role of internal motor simulations for action recognition. However, these models do not specify how visual information can be processed by cortical mechanisms in order to be compared with such motor representations. This raises the question how such visual processing might be accomplished, and in how far motor processing is critical in order to account for the visual properties of action-selective neurons.
We present a neural model for the visual processing of transient actions that is consistent with physiological data and that accomplishes recognition of grasping actions from real video stimuli. Shape recognition is accomplished by a view-dependent hierarchical neural architecture that retains some coarse position information on the highest level that can be exploited by subsequent stages. Additionally, simple recurrent neural circuits integrate effector information over time and realize selectivity for temporal sequences. A novel mechanism combines information about the shape and position of object and effector in an object-centered frame of reference. Action-selective model neurons defined in such a relative reference frame are tuned to learned associations between object and effector shapes, as well as their relative position and motion. 
We demonstrate that this model reproduces a variety of electrophysiological findings on the visual properties of action-selective neurons in the superior temporal sulcus, and of mirror neurons in area F5. Specifically, the model accounts for the fact that a majority of mirror neurons in area F5 show view dependence. The model predicts a number of electrophysiological results, which partially could be confirmed in recent experiments.
We conclude that the tuning of action-selective neurons given visual stimuli can be accounted for by well-established, predominantly visual neural processes rather than internal motor simulations.

METHODS: The shape recognition relies on a hierarchy of feature detectors of increasing complexity and invariance [1]. The mid-level features are learned from sequences of gray-level images depicting segmented views of hand and object shapes. The highest hierarchy level consists of detector populations for complete shapes with a coarse spatial resolution of approximately 3.7°. Additionally, effector shapes are integrated over time by asymmetric lateral connections between shape detectors using a neural field approach [2]. These model neurons thus encode actions such as hand opening or closing for particular grip types. 
We exploit gain field mechanism in order to implement the central coordinate transformation of the shape representations to an object-centered reference frame [3]. Typical effector-object-interactions correspond to activity regions in such a relative reference frame and are learned from training examples. Similarly, simple motion-energy detectors are applied in the object-centered reference frame and encode relative motion. The properties of transitive action neurons are modeled as a multiplicative combination of relative shape and motion detectors.

RESULTS: The model performance was tested on a set of 160 unsegmented sequences of hand grasping or placing actions performed on objects of different sizes, using different grip types and views. Hand actions and objects could be reliably recognized despite their mutual occlusions. Detectors on the highest level showed correct action tuning in more than 95% of the examples and generalized to untrained views. 
Furthermore, the model replicates a number of electrophysiological as well as imaging experiments on action-selective neurons, such as their particular selectivity for transitive actions compared to mimicked actions, the invariance to stimulus position, and their view-dependence. In particular, using the same stimulus set the model nicely fits neural data from a recent electrophysiological experiment that confirmed sequence selectivity in mirror neurons in area F5, as was predicted before by the model.

References
[1] Serre, T. et al. (2007): IEEE Pattern Anal. Mach. Int. 29, 411-426.
[2] Giese, A.M. and Poggio, T. (2003): Nat. Rev. Neurosci. 4, 179-192.
[3] Deneve, S. and Pouget, A. (2003). Neuron 37: 347-359.

Controlling Rescattering Effects in Constraints on the CKM Angle arising from Decays
It has recently been pointed out that the observables of the decay and its charge conjugate allow us to take into account
rescattering effects in constraints on the CKM angle arising from
and modes, and that they play an
important role to obtain insights into final-state interactions. In this paper,
the formalism needed to accomplish this task is discussed in detail.
Furthermore, using a transparent model to describe the rescattering processes,
as well as electroweak penguins, we calculate the quantities parametrizing the
and decay amplitudes for specific
examples, and illustrate the constraints on arising from the
corresponding observables. Although this model is very crude, it shows nicely
the power of both to include the rescattering effects in the
bounds on and to obtain insights into final-state interactions.
Moreover this model exhibits the interesting feature that the combined
branching ratio BR may be considerably enhanced through
rescattering processes, as was recently pointed out within a general framework.Comment: 14 pages, LaTeX, 4 figure
Weak phases and from , or and decays
An improved flavor SU(3) method is presented for determining the weak angle
of the unitarity triangle using decay rates for and (or and ),
their CP-conjugate modes and the CP-averaged rate for .
Rescattering (color-suppressed) contribution in , for which
an improved bound is obtained, is subtracted away. The only significant SU(3)
breaking effects are accounted for in the factorization approximation of tree
amplitudes. The weak angle is obtained as a byproduct.Comment: 9 pages with 2 included eps figures; revised argument, taking into
account the special shape of the amplitude quadrangle. An improved bound on
rescattering effects is give
Penguin Topologies, Rescattering Effects and Penguin Hunting with and
In the recent literature, constraints on the CKM angle arising from
the branching ratios for and decays
received a lot of attention. An important theoretical limitation of the
accuracy of these bounds is due to rescattering effects, such as
. We point out that these processes are related
to penguin topologies with internal up quark exchanges and derive SU(2) isospin
relations among the and decay amplitudes by
defining ``tree'' and ``penguin'' amplitudes in a proper way, allowing the
derivation of generalized bounds on the CKM angle . We propose
strategies to obtain insights into the dynamics of penguin processes with the
help of the decays and , derive a
relation among the direct CP-violating asymmetries arising in these modes, and
emphasize that rescattering effects can be included in the generalized bounds
on completely this way. Moreover, we have a brief look at the impact
of new physics.Comment: Some mistakes in the equations have been corrected, conclusions have
been changed slightly. The complete paper, including figures, is also
available via anonymous ftp at ftp://ttpux2.physik.uni-karlsruhe.de/, or via
www at http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints
U-Spin Symmetry in Charmless B Decays
We prove a general theorem about equal CP rate differences within pairs of
U-spin related charmless and decays. Large deviations from equalities
would be evidence for new physics. Six pairs of decays into two pseudoscalar
mesons are identified where such relations hold. Ratios of corresponding rate
differences and certain ratios of rates measure U-spin breaking. These
processes provide useful information on the weak phase . Applications of U-spin symmetry to other decays are discussed.Comment: A few typos corrected, to appear in Phys. Lett.
Model independent sum rules for B-> pi K decays
We provide a set of sum rules relating CP-averaged branching ratios and
CP-asymmetries of the modes. They prove to be useful as a
mechanism to `test' experimental data given our expectations of the size of
isospin breaking. A set of observables emerges providing a simpler
interpretation of data in terms of isospin breaking. Moreover, the derivation
is done in a completely model independent way, i.e., they can accommodate also
New Physics contributions.Comment: 17 pages, 9 figure
The Role of in Determining the Weak Phase
The decay rates for , , and the
charge-conjugate processes were found to provide information on the weak phase
when the ratio of weak tree and penguin
amplitudes was taken from data on or semileptonic
decays. We show here that the rates for and can provide the necessary information on , and estimate the
statistical accuracy of forthcoming measurements at the Fermilab Tevatron.Comment: 8 pages, LaTeX, no figures, submitted to Physics Letters B,
corrections to discussion of SU(3) breaking adde
Flavor Physics and the CKM Matrix: An Overview
I review the current status of our knowledge of CP violation and flavor
physics. I discuss where one should look for future improvements, and outline
the experimental and theoretical priorities of the field.Comment: 11 pages. Presentation at the Fifth KEK Topical Conference,
"Frontiers in Flavor Physics", November 20-22, 2001. References adde
The second order QCD contribution to the semileptonic b -> u decay rate
The order alpha_s^2 contribution to the inclusive semileptonic decay width of
a b quark \Gamma(b -> X_u e \bar{\nu}_e) is calculated analytically for zero
mass u quarks.Comment: 10 pages, 1 figur
Correlated neutral B meson decays into CP eigenstates
In the two correlated meson decay experiment we propose to measure
intensities relating CP eigenstate () decays on sides,
which will be measurable in future upgrades of KEK and PEP. As a CP-forbidden
transition, we obtain . We calculate in a model independent way all the possible intensities
relating final CP and flavour eigenstate decays. Under CPT-invariance, the
asymmetries for processes related by CP vanish for
and measure linearly. We notice the impossibility to isolate
the sign of \cos (2\bet a) without an independent knowledge of the sign of
. This exhaustion of the possible Golden Plate and flavour
decays provides new observables which may throw light in our present
understanding of CKM physics.Comment: 9 pages, no figures. Minor changes to coincide with published PLB
versio
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