California Institute of Technology

About: California Institute of Technology is a education organization based out in Pasadena, California, United States. It is known for research contribution in the topics: Galaxy & Population. The organization has 57649 authors who have published 146691 publications receiving 8620287 citations. The organization is also known as: Caltech & Cal Tech.

On the Boundary Condition at the Surface of a Porous Medium

Abstract: A theoretical justification is given for an empirical boundary condition proposed by Beavers and Joseph [1]. The method consists of first using a statistical approach to extend Darcy's law to non homogeneous porous medium. The limiting case of a step function distribution of permeability and porosity is then examined by boundary layer techniques, and shown to give the required boundary condition. In an Appendix, the statistical approach is checked by using it to derive Einstein's law for the viscosity of dilute suspensions.

Beyond Nyquist: Efficient Sampling of Sparse Bandlimited Signals

Abstract: Wideband analog signals push contemporary analog-to-digital conversion (ADC) systems to their performance limits. In many applications, however, sampling at the Nyquist rate is inefficient because the signals of interest contain only a small number of significant frequencies relative to the band limit, although the locations of the frequencies may not be known a priori. For this type of sparse signal, other sampling strategies are possible. This paper describes a new type of data acquisition system, called a random demodulator, that is constructed from robust, readily available components. Let K denote the total number of frequencies in the signal, and let W denote its band limit in hertz. Simulations suggest that the random demodulator requires just O(K log(W/K)) samples per second to stably reconstruct the signal. This sampling rate is exponentially lower than the Nyquist rate of W hertz. In contrast to Nyquist sampling, one must use nonlinear methods, such as convex programming, to recover the signal from the samples taken by the random demodulator. This paper provides a detailed theoretical analysis of the system's performance that supports the empirical observations.

mRNA vaccine-elicited antibodies to SARS-CoV-2 and circulating variants.

Abstract: Here we report on the antibody and memory B cell responses of a cohort of 20 volunteers who received the Moderna (mRNA-1273) or Pfizer–BioNTech (BNT162b2) vaccine against SARS-CoV-21–4 Eight weeks after the second injection of vaccine, volunteers showed high levels of IgM and IgG anti-SARS-CoV-2 spike protein (S) and receptor-binding-domain (RBD) binding titre Moreover, the plasma neutralizing activity and relative numbers of RBD-specific memory B cells of vaccinated volunteers were equivalent to those of individuals who had recovered from natural infection5,6 However, activity against SARS-CoV-2 variants that encode E484K-, N501Y- or K417N/E484K/N501-mutant S was reduced by a small—but significant—margin The monoclonal antibodies elicited by the vaccines potently neutralize SARS-CoV-2, and target a number of different RBD epitopes in common with monoclonal antibodies isolated from infected donors5–8 However, neutralization by 14 of the 17 most-potent monoclonal antibodies that we tested was reduced or abolished by the K417N, E484K or N501Y mutation Notably, these mutations were selected when we cultured recombinant vesicular stomatitis virus expressing SARS-CoV-2 S in the presence of the monoclonal antibodies elicited by the vaccines Together, these results suggest that the monoclonal antibodies in clinical use should be tested against newly arising variants, and that mRNA vaccines may need to be updated periodically to avoid a potential loss of clinical efficacy The Moderna (mRNA-1273) and Pfizer–BioNTech (BNT162b2) vaccines elicit anti-RBD antibodies similar to those elicited through natural infection with SARS-CoV-2, but their potent neutralizing activity was reduced or abolished by new viral variants of concern

Coding of intention in the posterior parietal cortex

Abstract: To look at or reach for what we see, spatial information from the visual system must be transformed into a motor plan. The posterior parietal cortex (PPC) is well placed to perform this function, because it lies between visual areas, which encode spatial information, and motor cortical areas. The PPC contains several subdivisions, which are generally conceived as high-order sensory areas. Neurons in area 7a and the lateral intraparietal area fire before and during visually guided saccades. Other neurons in areas 7a and 5 are active before and during visually guided arm movements. These areas are also active during memory tasks in which the animal remembers the location of a target for hundreds of milliseconds before making an eye or arm movement. Such activity could reflect either visual attention or the intention to make movements. This question is difficult to resolve, because even if the animal maintains fixation while directing attention to a peripheral location, the observed neuronal activity could reflect movements that are planned but not executed. To address this, we recorded from the PPC while monkeys planned either reaches or saccades to a single remembered location. We now report that, for most neurons, activity before the movement depended on the type of movement being planned. We conclude that PPC contains signals related to what the animal intends to do.