38 research outputs found
Predictive feedback control and Fitts' law
Fittsâ law is a well established empirical formula, known for encapsulating the âspeed-accuracy trade-offâ. For discrete, manual movements from a starting location to a target, Fittsâ law relates movement duration to the distance moved and target size. The widespread empirical success of the formula is suggestive of underlying principles of human movement control. There have been previous attempts to relate Fittsâ law to engineering-type control hypotheses and it has been shown that the law is exactly consistent with the closed-loop step-response of a time-delayed, first-order system. Assuming only the operation of closed-loop feedback, either continuous or intermittent, this paper asks whether such feedback should be predictive or not predictive to be consistent with Fitts law. Since Fittsâ law is equivalent to a time delay separated from a first-order system, known control theory implies that the controller must be predictive. A predictive controller moves the time-delay outside the feedback loop such that the closed-loop response can be separated into a time delay and rational function whereas a non- predictive controller retains a state delay within feedback loop which is not consistent with Fittsâ law. Using sufficient parameters, a high-order non-predictive controller could approximately reproduce Fittsâ law. However, such high-order, ânon-parametricâ controllers are essentially empirical in nature, without physical meaning, and therefore are conceptually inferior to the predictive controller. It is a new insight that using closed-loop feedback, prediction is required to physically explain Fittsâ law. The implication is that prediction is an inherent part of the âspeed-accuracy trade-offâ
Imaging the Two Gaps of the High-TC Superconductor Pb-Bi2Sr2CuO6+x
The nature of the pseudogap state, observed above the superconducting
transition temperature TC in many high temperature superconductors, is the
center of much debate. Recently, this discussion has focused on the number of
energy gaps in these materials. Some experiments indicate a single energy gap,
implying that the pseudogap is a precursor state. Others indicate two,
suggesting that it is a competing or coexisting phase. Here we report on
temperature dependent scanning tunneling spectroscopy of Pb-Bi2Sr2CuO6+x. We
have found a new, narrow, homogeneous gap that vanishes near TC, superimposed
on the typically observed, inhomogeneous, broad gap, which is only weakly
temperature dependent. These results not only support the two gap picture, but
also explain previously troubling differences between scanning tunneling
microscopy and other experimental measurements.Comment: 6 page
The pseudogap: friend or foe of high Tc?
Although nineteen years have passed since the discovery of high temperature
superconductivity, there is still no consensus on its physical origin. This is
in large part because of a lack of understanding of the state of matter out of
which the superconductivity arises. In optimally and underdoped materials, this
state exhibits a pseudogap at temperatures large compared to the
superconducting transition temperature. Although discovered only three years
after the pioneering work of Bednorz and Muller, the physical origin of this
pseudogap behavior and whether it constitutes a distinct phase of matter is
still shrouded in mystery. In the summer of 2004, a band of physicists gathered
for five weeks at the Aspen Center for Physics to discuss the pseudogap. In
this perspective, we would like to summarize some of the results presented
there and discuss its importance in the context of strongly correlated electron
systems.Comment: expanded version, 20 pages, 11 figures, to be published, Advances in
Physic
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
Composition and Diversity of Lawn Flora in Differently Managed Village Yards â A Case Study from Southwestern Hungary
Targeting neuroinflammation for therapeutic intervention in neurodegenerative pathologies: A role for the peptide analogue of thymulin (PAT)
Introduction: Inflammation has a vital task in protecting the organism, but when deregulated, it can have serious pathological consequences. The central nervous system (CNS) is capable of mounting immune and inflammatory responses, albeit different from that observed in the periphery. Neuroinflammation, however, can be a major contributor to neurodegenerative diseases and constitute a major challenge for medicine and basic research. Areas covered: Both innate and adaptive immune responses normally play an important role in homeostasis within the CNS. Microglia, astrocytes and neuronal cells express a wide array of toll-like receptors (TLR) that can be upregulated by infection, trauma, injuries and various exogenic or endogenic factors. Chronic hyper activation of brain immune cells can result in neurotoxic actions due to excessive production of several pro-inflammatory mediators. Several studies have recently described an important role for targeting receptors such as nicotinic receptors located on cells in the CNS or in other tissues for the control of inflammation. Expert opinion: Thymulin and its synthetic peptide analogue (PAT) appear to exert potent anti-inflammatory effects at the level of peripheral tissues as well as at the level of the brain. This effect involves, at least partially, the activation of cholinergic mechanisms. © 2012 Informa UK, Ltd
Functional significance of genetically different symbiotic algae Symbiodinium in a coral reef symbiosis
Modulation of the soleus H-reflex during knee rotations is not consistent with muscle fascicle length changes
The purpose of this study was to examine whether passively rotating the knee would result in parallel or differential changes to the medial gastrocnemius (MG) and soleus (SOL) H-reflex amplitudes. Since passive knee rotation alters the muscle length of the MG, but not the SOL, it was hypothesized that the MG H-reflex would reflect the lengthening or shortening actions that occur during knee rotation, whereas the SOL H-reflex would remain unaltered. MG and SOL Hoffman reflexes (H-reflexes) were evoked with the knee joint held static at 10A degrees or as the joint was passively flexed or extended past 10A degrees. Ultrasound recordings were used to confirm whether the knee rotations altered MG but not SOL muscle fascicle lengths. In contrast to our hypothesis, results indicated that the MG and SOL H-reflexes were similarly affected during knee rotations, with both MG and SOL H-max:M-max smaller during the knee extension than the knee flexion (33-43% reduction) and static (22-28% reduction) conditions. Parallel changes to the MG and SOL H-reflexes occurred despite a differential effect of knee rotation on muscle fascicle lengths. Whereas, MG muscle fascicles lengthened and shortened during knee extension and flexion, respectively, SOL fascicles length remained unchanged. Given the strong neural coupling between the MG and SOL motoneuron pools, the results highlight the difficulty in isolating specific variables (e.g., muscle length) when determining the modulatory influences on the triceps surae H-reflex amplitude