S. P. Strong

Synergy in a Neural Code

We show that the information carried by compound events in neural spike trainspatterns of spikes across time or across a population of cellscan 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 directly measure 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 flys 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 especially important patterns in the code of H1.

Interlayer tunneling and gap anisotropy in high-temperature superconductors

A quantitative analysis of a recent model of high-temperature superconductors based on an interlayer tunneling mechanism is presented. This model can account well for the observed magnitudes of the high transition temperatures in these materials and implies a gap that does not change sign, can be substantially anisotropic, and has the same symmetry as the crystal. The experimental consequences explored so far are consistent with the observations.

Field Theories for Learning Probability Distributions.

Imagine being shown

Adaptation and optimal chemotactic strategy for E. coli

N

‘Confined coherence’ in strongly correlated anisotropic metals

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 you have some prior notions about what to expect. From a Bayesian point of view one needs an a priori distribution on the space of possible probability distributions, which defines a scalar field theory. In one dimension, free field theory with a normalization constraint provides a tractable formulation of the problem, and we discuss generalizations to higher dimensions.

New types of off-diagonal long range order in quantum spin chains.

Extending the classic works of Berg and Purcell on the biophysics of bacterial chemotaxis, we find the optimal chemotactic strategy for the peritrichous bacterium {ital 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. {copyright} {ital 1998} {ital The American Physical Society}

Transition between quantum coherence and incoherence

Abstract We present a detailed discussion of both theoretical and experimental evidence in favour of the existence of states of ‘confined coherence’ in metals of sufficiently high anisotropy and with sufficiently strong correlations. 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. The most dramatic experimental manifestation of such a state is the coexistence of incoherent non-metallic transport in one or two directions (transverse to the lower dimensional subspaces) with coherent transport in at least one other direction (within the subspaces). The magnitude of the Fermi surface warping due to transverse (intersubspace) momentum plays the role of an order parameter (in a state of confined coherence, this order parameter vanishes) and the effect can occur in a pure system at zero temperature....

Single particle hopping between luttinger liquids: A spectral function approach

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.

Deconfined fermions but confined coherence

We show that a transformed Caldeira-Leggett Hamiltonian 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 nondegeneracy in the spectral function of the {open_quotes}spin-flip{close_quotes} operator. {copyright} {ital 1997} {ital The American Physical Society}

Entropy and Information in Neural Spike Trains

We present a pedagogical account of our approach to the problem of Luttinger liquids coupled by interliquid single particle hopping. It is shown that the key issue is that of coherence/incoherence of interliquid hopping, and not of relevance/irrelevance in a renormalization group sense. A clear signal of coherence, present in the case of coupled Fermi liquids, is absent for Luttinger liquids, and we argue for the existence of an incoherent regime when the interliquid hopping rate is sufficiently small. The problem is relevant to any sufficiently anisotropic, strongly correlated metal, and in particular to understanding the anomalous c-axis conductivity in the cuprate superconductors, and the physics of the quasi-1D organic conductors.