Most of my research interests fit more or less under the umbrella of Complex Systems.
I am currently working with Nick Jones and colleagues in the Systems and Signals group at the Dept. of Physics, University of Oxford, supported by a postdoctoral contract from the Oxford Centre for Integrative Systems Biology. My main research topics are:
Biological mechanisms for probabilistic reasoning with neurons. Given uncertain information about the world, the brain is not only able to reason logically, but seems to do so in a statistically optimal way. Using a variety of mathematical and computational techniques, as well as experimental work in vitro, we are trying to contribute to an understanding or how the brain does this.
How ensembles of cells harness noise for functional purposes. There are many instances of this kind of phenomenon, and we hope to gain insights into the mechanisms by which this comes about using ideas from complex systems science and computational methods.
I did my PhD, entitled Interplay between Network Topology and Dynamics in Neural Systems, with Joaquín Torres and Joaquín Marro at the Instituto Carlos I de Física Teórica y Computacional, University of Granada. It addresses two questions:
How the activity of individual cells might shape the topological properties of the network they form part of: Evolving networks and the development of neural systems, S. Johnson et al. J. Stat. Mech. (2010) P03003
How the structural features to emerge might in their turn influence or determine the collective behaviour of ensembles of model neurons: Functional optimization in complex excitable networks, S. Johnson et al., EPL 83, 46006 (2008); Enhancing neural-network performance via assortativity, S. de Franciscis et al., Phys. Rev. E 83, 036114 (2011); Mechanism for robust short-term memory without synaptic learning, S. Johnson et al. (2011) arXiv: 1007.3122
I have also done some work on Complex Networks: Nonlinear preferential rewiring in fixed-size networks as a diffusion process, S. Johnson et al., Phys. Rev. E 79, 050104(R) (2009); Entropic origin of disassortativity in complex networks, S. Johnson et al., Phys. Rev. Lett. 104, 108702 (2010)
I am currently involved in various other lines of research, some of which include:
Looking for possible mechanisms behind the emergence of food-webs and other ecological networks, with Miguel A. Muñoz and Virginia Domínguez, as well as several colleagues from the INIFTA, University of La Plata.
Studying the effects of degree-degree correlations in a variety of networks, as well as other related aspects, with Sebastiano de Franciscis (University of Granada) and Jorge Mejías (University of Ottawa), among others.
Together with Joaquín Torres, studying, in greater detail than in Mechanism for robust short-term memory without synaptic learning, S. Johnson et al. (2011) arXiv: 1007.3122, the conditions required for our recently proposed mechanism of Cluster Reverberation to work for short-term memory to emerge from metastable states due to a clustered network topology.
Trying to understand, with Miguel Ibáñez (Autonomous University of Madrid) and Daniele Vilone (Carlos III University of Madrid), the reason for the prevalence of sexual reproduction among higher plants and animals.
Searching for the reasons behind the different kinds of network motifs found in real-world complex networks; this with Miguel A. Muñoz and Simone Pigolotti (Niels Bohr International Academy, Copenhagen).
These are some of my friends and colleagues: