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S. P. Strong

Publications and source records attributed to S. P. Strong.

11 recordsLinked to original sources
arXiv1999

Symbols and synergy in a neural code

Understanding a neural code requires knowledge both of the elementary symbols that transmit information and of the algorithm for translating these symbols into sensory signals or motor actions. We show that these questions can be separated: the information carried by any candidate symbol in the code--- a pattern of spikes across time or across a population of cells---can be measured, independent of assumptions about what these patterns might represent. By comparing the information carried by a compound pattern with the information carried independently by its parts, we measure directly the synergy among these parts. We illustrate the use of these methods by applying them to experiments on the motion sensitive neuron H1 of the fly's visual system, where we confirm that two spikes close together in time carry far more than twice the information carried by a single spike. We analyze the sources of this synergy, and provide evidence that pairs of spikes close together in time may be special symbols in the code of H1.

physics.bio-ph↗
arXiv1997

An Order Parameter for the Mott-Hubbard Transition in One Dimension

We propose an order parameter for the one dimensional Mott-Hubbard transition and provide numerical evidence and general theoretical arguments for the correctness of our proposal. In addition, we discuss some of the implications of this picture of the one dimensional Mott-Hubbard transition and speculate about possible higher dimensional analogs.

cond-mat.str-el↗
arXiv1997

Entropy and Information in Neural Spike Trains

The nervous system represents time-dependent signals in sequences of discrete action potentials or spikes, all spikes are identical so that information is carried only in the spike arrival times. We show how to quantify this information, in bits, free from any assumptions about which features of the spike train or input waveform are most important. We apply this approach to the analysis of experiments on a motion-sensitive neuron in the fly visual system.

cond-mat↗
arXiv1997

Adaptation and Optimal Chemotactic Strategy for E. Coli

Extending the classic works of Berg and Purcell on the biophysics of bacterial chemotaxis, we find the optimal chemotactic strategy for the peritrichous bacterium E. Coli in the high and low signal to noise ratio limits. The optimal strategy depends on properties of the environment and properties of the individual bacterium and is therefore highly adaptive. We review experiments relevant to testing both the form of the proposed strategy and its adaptability and propose extensions of them which could test the limits of the adaptability in this simplest sensory processing system.

adap-org↗
arXiv1997

A Creation Operator for Spinons in One Dimension

We propose a definition for a creation operator for the spinon, the fractional statistics elementary excitation of the Haldane-Shastry model, and give numerical and analytical evidence that our operator creates a single spinon with nearly unit amplitude in the ISE model. We then discuss how the operator is useful in more general contexts such as studying the underlying spinons of other spin chain models, like the XXX- and XY-model, and of the one dimensional Hubbard model.

cond-mat.str-el↗
arXiv1997

The Transition Between Quantum Coherence and Incoherence

We show that a transformed Caldeira-Leggett Hamltonian has two distinct families of fixed points, rather than a single unique fixed point as often conjectured based on its connection to the anisotropic Kondo model. The two families are distinguished by a sharp qualitative difference in their quantum coherence properties and we argue that this distinction is best understood as the result of a transition in the model between degeneracy and non-degeneracy in the spectral function of the ``spin-flip'' operator.

cond-mat.str-el↗
arXiv1996

Deconfined Fermions but Confined Coherence?

The cuprate superconductors and certain organic conductors exhibit transport which is qualitatively anisotropic, yet at the same time other properties of these materials strongly suggest the existence of a Fermi surface and low energy excitations with substantial free electron character. The former of these features is very difficult to account for if the material possesses three dimensional coherence, while the latter is inconsistent with a description based on a two dimensional fixed point. We therefore present a new proposal for these materials in which they are categorized by a fixed point at which transport in one direction is not renormalization group irrelevant, but is intrinsically incoherent, i.e. the incoherence is present in a pure system, at zero temperature. The defining property of such a state is that single electron coherence is confined to lower dimensional subspaces (planes or chains) so that it is impossible to observe interference effects between histories which involve electrons moving between these subspaces.

cond-mat↗
arXiv1996

Field Theories for Learning Probability Distributions

Imagine being shown $N$ samples of random variables drawn independently from the same distribution. What can you say about the distribution? In general, of course, the answer is nothing, unless we have some prior notions about what to expect. From a Bayesian point of view we need an {\it a priori} distribution on the space of possible probability distributions, which defines a scalar field theory. In one dimension, free field theory with a constraint provides a tractable formulation of the problem, and we also discus generalizations to higher dimensions.

cond-mat↗
arXiv1994

New Types of Off-Diagonal Long Range Order in Spin-Chains

We discuss new possibilities for Off-Diagonal Long Range Order (ODLRO) in spin chains involving operators which add or delete sites from the chain. For the Heisenberg and Inverse Square Exchange models we give strong numerical evidence for the hidden ODLRO conjectured by Anderson \cite{pwa_conj}. We find a similar ODLRO for the XY model (or equivalently for free fermions in one spatial dimension) which we can demonstrate rigorously, as well as numerically. A connection to the singlet pair correlations in one dimensional models of interacting electrons is made and briefly discussed.

cond-mat↗
arXiv1994

Spectral Functions of Local Operators In The Tomonaga-Luttinger Model

I give a simple general prescription for computing the spectral functions of local operators in the Tomonaga-Luttinger model from the space-time correlation functions. The method is significantly simpler than directly transforming the space-time Greens function and allows a physical interpretation of the singularities encountered in the spectral function.

cond-mat↗