Continuous attractors for dynamic memories

Episodic memory has a dynamic nature: when we recall past episodes, we retrieve not only their content, but also their temporal structure. The phenomenon of replay, in the hippocampus of mammals, offers a remarkable example of this temporal dynamics. However, most quantitative models of memory treat memories as static configurations, neglecting the temporal unfolding of the retrieval process. Here, we introduce a continuous attractor network model with a memory-dependent asymmetric component in the synaptic connectivity, which spontaneously breaks the equilibrium of the memory configurations and produces dynamic retrieval. The detailed analysis of the model with analytical calculations and numerical simulations shows that it can robustly retrieve multiple dynamical memories, and that this feature is largely independent of the details of its implementation. By calculating the storage capacity, we show that the dynamic component does not impair memory capacity, and can even enhance it in certain regimes

Gravitational capture opportunites for asteroid retrieval missions

Asteroids and comets are of strategic importance for science in an effort to uncover the formation, evolution and composition of the Solar System. Near-Earth objects (NEOs) are of particular interest because of its accessibility from Earth, but also because of their speculated wealth of resources. The exploitation of these resources has long been discussed as a means to lower the cost of future space endeavours. In this paper, we analyze the possibility of retrieving entire objects from accessible heliocentric orbits and moving them into the Earth’s neighbourhood. The asteroid retrieval transfers are sought from the continuum of low energy transfers enabled by the dynamics of invariant manifolds; specifically, the retrieval transfers target planar, vertical Lyapunov and halo orbit families associated with the collinear equilibrium points of the Sun-Earth Circular Restricted Three Body problem. The judicious use of these dynamical features provides the best opportunity to find extremely low energy Earth transfers for asteroidal material. With the objective to minimise transfer costs, a global search of impulsive transfers connecting the unperturbed asteroid’s orbit with the stable manifold phase of the transfer is performed. A catalogue of asteroid retrieval opportunities of currently known NEOs is presented here. Despite the highly incomplete census of very small asteroids, the catalogue can already be populated with 12 different objects retrievable with less than 500 m/s of Δv. All, but one, of these objects have an expected size and transfer requirements that can be met by current propulsion technologies. Moreover, the methodology proposed represents a robust search for future retrieval candidates that can be automatically applied to a growing survey of NEOs

An interactive analysis of harmonic and diffusion equations on discrete 3D shapes

AbstractRecent results in geometry processing have shown that shape segmentation, comparison, and analysis can be successfully addressed through the spectral properties of the Laplace–Beltrami operator, which is involved in the harmonic equation, the Laplacian eigenproblem, the heat diffusion equation, and the definition of spectral distances, such as the bi-harmonic, commute time, and diffusion distances. In this paper, we study the discretization and the main properties of the solutions to these equations on 3D surfaces and their applications to shape analysis. Among the main factors that influence their computation, as well as the corresponding distances, we focus our attention on the choice of different Laplacian matrices, initial boundary conditions, and input shapes. These degrees of freedom motivate our choice to address this study through the executable paper, which allows the user to perform a large set of experiments and select his/her own parameters. Finally, we represent these distances in a unified way and provide a simple procedure to generate new distances on 3D shapes

Space, time and memory in the medial temporal lobe

This thesis focuses on memory and the representation of space in the medial temporal lobe, their interaction and their temporal structure. Chapter 1 briefly introduces the topic, with emphasis on the open questions that the subsequent chapters aim to address. Chapter 2 is dedicated to the issue of spatial memory in the medial entorhinal cortex. It investigates the possibility to store multiple independent maps in a recurrent network of grid cells, from a theoretical perspective. This work was conducted in collaboration with Remi Monasson, Alexis Dubreuil and Sophie Rosay and is published in (Spalla et al. 2019). Chapter 3 focuses on the problem of the dynamical update of the representation of space during navigation. It presents the results of the analysis of electrophysiological data, previously collected by Charlotte Boccara (Boccara et al., 2010), investigating the encoding of self-movement signals (speed and angular velocity of the head) in the parahippocampal region of rats. Chapter 4 addresses the problem of the temporal dynamics of memory retrieval, again from a computational point of view. A continuous attractor network model is presented, endowed with a mechanism that makes it able to retrieve continuous temporal sequences. The dynamical behaviour of the system is investigated with analytical calculations and numerical simulations, and the storage capacity for dynamical memories is computed. Finally, chapter 4 discusses the meaning and the scope of the results presented, and highlights possible future directions

Spectroscopic Study of the Absorption Properties of Ozone and Methane for use in the Remote Sensing Applications

Monitoring of the atmospheric composition of the Earth is essential for studying the processes occurring in different layers of the atmosphere and, consequently, for air quality control and the climate change prediction. The most important conclusions are based on the data from the long-term global observations. Nowadays this data is coming from both space- or airborne instruments and ground based monitoring networks. The ozone (O3) and the methane (CH4) are among the most important trace gases. They play crucial role in the physical and chemical processes in the atmosphere, like formation of the protective stratospheric ozone layer or contribution to the greenhouse effect by absorption of the solar radiation (by both ozone and methane) or radiation re-emitted by the Earthâ s surface. Another important factor is that both ozone and methane concentration and distribution changes serve as the important markers of the anthropogenic influence on the environment. The atmospheric composition is derived after processing the spectra acquired by the satellite-borne instruments. Most methods utilize the fitting of the observed and synthetic spectra. Modelled spectra are produced based on the preliminary assumptions of approximate concentrations, temperatures and altitude distribution of the trace gases. These methods require a precise knowledge of the electromagnetic radiation absorption characteristics of the different atmospheric gases in a wide spectral range, as well as the temperature and pressure dependences of these parameters. The uncertainties in these tabulated spectral data lead to the uncertainties in the resulting retrieved concentrations and distribution profiles of atmospheric gases. Most of the modern instruments operate in the ultraviolet, visible and infrared parts of the spectrum between 250 nm and 1000 nm for ozone observations. For methane detection, infrared regions of the spectrum around 1,6 or 2,4 micron are mostly used. Despite the fact that different research groups have been analysing the absorption spectra of both ozone and methane with a lot of scrutiny for decades, there still is a room for improvement of the quality of the data. Consequently, the new detailed spectroscopic data would allow to further increase the quality of the atmospheric observations. Demand for an updated and improved (in terms of uncertainties and parameterization capabilities) spectroscopic data for ozone and methane from the remote sensing community was a major motivating factor for this study. This work is dedicated to the acquisition of the new high-quality broadband absorption spectra of ozone and methane, corresponding data analysis and the methods of achieving these goals