Neural processes underpinning episodic memory

Episodic memory is the memory for our personal past experiences. Although numerous functional magnetic resonance imaging (fMRI) studies investigating its neural basis have revealed a consistent and distributed network of associated brain regions, surprisingly little is known about the contributions individual brain areas make to the recollective experience. In this thesis I address this fundamental issue by employing a range of different experimental techniques including neuropsychological testing, virtual reality environments, whole brain and high spatial resolution fMRI, and multivariate pattern analysis. Episodic memory recall is widely agreed to be a reconstructive process, one that is known to be critically reliant on the hippocampus. I therefore hypothesised that the same neural machinery responsible for reconstruction might also support ‘constructive’ cognitive functions such as imagination. To test this proposal, patients with focal damage to the hippocampus bilaterally were asked to imagine new experiences and were found to be impaired relative to matched control participants. Moreover, driving this deficit was a lack of spatial coherence in their imagined experiences, pointing to a role for the hippocampus in binding together the disparate elements of a scene. A subsequent fMRI study involving healthy participants compared the recall of real memories with the construction of imaginary memories. This revealed a fronto-temporo-parietal network in common to both tasks that included the hippocampus, ventromedial prefrontal, retrosplenial and parietal cortices. Based on these results I advanced the notion that this network might support the process of ‘scene construction’, defined as the generation and maintenance of a complex and coherent spatial context. Furthermore, I argued that this scene construction network might underpin other important cognitive functions besides episodic memory and imagination, such as navigation and thinking about the future. It is has been proposed that spatial context may act as the scaffold around which episodic memories are built. Given the hippocampus appears to play a critical role in imagination by supporting the creation of a rich coherent spatial scene, I sought to explore the nature of this hippocampal spatial code in a novel way. By combining high spatial resolution fMRI with multivariate pattern analysis techniques it proved possible to accurately determine where a subject was located in a virtual reality environment based solely on the pattern of activity across hippocampal voxels. For this to have been possible, the hippocampal population code must be large and non-uniform. I then extended these techniques to the domain of episodic memory by showing that individual memories could be accurately decoded from the pattern of activity across hippocampal voxels, thus identifying individual memory traces. I consider these findings together with other recent advances in the episodic memory field, and present a new perspective on the role of the hippocampus in episodic recollection. I discuss how this new (and preliminary) framework compares with current prevailing theories of hippocampal function, and suggest how it might account for some previously contradictory data

Quantum Theory of the Classical: Einselection, Envariance, Quantum Darwinism and Extantons

Core quantum postulates including the superposition principle and the unitarity of evolutions are natural and strikingly simple. I show that -- when supplemented with a limited version of predictability (captured in the textbook accounts by the repeatability postulate) -- these core postulates can account for all the symptoms of classicality. In particular, both objective classical reality and elusive information about reality arise, via quantum Darwinism, from the quantum substrate.Comment: To appear in the ENTROPY volume "Quantum Darinism and Friends" edited by Sebastian Deffner et al. https://www.mdpi.com/journal/entropy/special_issues/quantum_darwinism. arXiv admin note: text overlap with arXiv:0707.283

Neuronal dynamics across macroscopic timescales

The brain operates in a world with rich dynamics across a wide range of timescales, including those on the order of seconds and above. Behavioral experiments on memory and timing reveal striking similarities in the behavioral patterns across a range of timescales from seconds to minutes. To subserve these behavioral patterns and adapt to natural statistics, the collective activity of the large of number of neurons in the brain should exhibit dynamics over these macroscopic timescales as well. Most established results in systems neuroscience concern the short-term responses of single neurons to static features of the world. Recently, new techniques for large-scale and chronic measurements of neural activity open up the opportunity to investigate neural dynamics across different macroscopic timescales. This dissertation presents work that reveals the temporal patterns of neural activity across a range of macroscopic timescales and explores their mechanistic basis. Chapter 1 briefly surveys the relevant empirical evidence, biophysical processes and modeling techniques. Chapter 2 presents a biophysically-realistic neural circuit model that combines a detailed simulation of a calcium-activated membrane current with the mathematical formalism of the inverse Laplace transform to produce sequential neural activity with a scale-invariant property. Chapter 3 is a theoretical analysis of the ability of linear recurrent neural networks to generate scale-invariant neural activity. It is shown that the network connectivity matrix should have a geometric series of eigenvalues and translated eigenvectors if the eigenvalues are real and distinct. Chapter 4 presents an empirical analysis of neural data motivated by the hypothesis that robust neural dynamics should simultaneously exist on multiple timescales. The analysis reveals the existence of repeatable neural dynamics on the timescale of both seconds and minutes in multiple neural recordings of rodents performing various cognitive tasks. Chapter 5 of the dissertation presents an initial effort to characterize the changes in the neural population activity during learning on the timescale of tens of minutes by analyzing neural recordings from monkeys while they learn associations between visual stimuli

Information in, on, and from the Spacetime

Information theory has become increasingly more prominent in physics research, and it is no different in gravity. In this thesis we look at several connections between information and the spacetime. Motivated in part by the black-hole information paradox, we complete a proof of an asymptotic entropy bound and consider the equivalence principle, both in attempt to answer where information may hide IN asymptotically flat space-times. We consider information bounds IN Riemann flat empy regions and ON the null conformal boundary. Our results led us to investigate the physical principles that render potential information at infinity unobservable and consider a communication protocol illustrating a connection between information and energy. We were able to show that quantum effects can account for unobservability of asymptotic charges.Holography is the idea that all the information about a spacetime is contained ON a lower dimensional surface (usually the boundary). Its most concrete realization, AdS/CFT, has already revolutionized physics. Still, that has not stopped physicists from seeking suitable generalizations, and it is in this context that holographic screens were introduced. The causal future of a spacetime region, is the subset of the spacetime where information from that region can reach. A certain characterization of the boundary of the future was needed for proving a new area theorem for holographic screens. We provide a proof of this characterization.Entanglement entropy provides a way of quantifying how information is spread in a quantum system. However, entanglement entropy alone does not provide an exhaustive picture of the distribution of information in a quantum system, and so several other entanglement measures have been studied, including, for instance, entenglement of distillation, entanglement of formation, and entenglament of purification. In many cases, holography provides an efficient way of studying entanglement measures. This allows us to learn about information properties of quantum systems FROM the spacetime. We study potential holographic duals for entanglement of purification and its multipartite generalization, and prove several of their properties

Towards Big Biology: high-performance verification of large concurrent systems

Bal, H.E. [Promotor]Fokkink, W.J. [Promotor]Kielmann, T. [Copromotor