A simple Hebbian/anti-Hebbian network learns the sparse, independent components of natural images

Slightly modified versions of an early Hebbian/anti-Hebbian neural network are shown to be capable of extracting the sparse, independent linear components of a prefiltered natural image set. An explanation for this capability in terms of a coupling between two hypothetical networks is presented. The simple networks presented here provide alternative, biologically plausible mechanisms for sparse, factorial coding in early primate vision.     

EuLisp in education

We present our experience withEuLisp as a teaching language, focussing on the level of the language which was specifically designed for this purpose (level-0).EuLisp has been used in undergraduate and postgraduate teaching since 1990, in lectures and laboratories, where in many cases it has replaced Scheme or Common Lisp. It has been used extensively in programming courses, parallelism courses, as a vehicle for advanced courses in symbolic computing and programming language design; it has also been used as a platform for final year undergraduate projects. This experience has demonstrated thatEuLisp is well suited to teaching and far reaching in its capabilities: it supports the relevant concepts in a consistent and versatile framework, so that the language serves to facilitate the educational process. The discussion is illustrated with examples, and where appropriate we draw a comparison with the Lisp dialects used previously in these courses.     

Integrating security and real-time requirements using covertchannel capacity

Database systems for real-time applications must satisfy timing constraints associated with transactions in addition to maintaining data consistency. In addition to real-time requirements, security is usually required in many applications. Multi-level security requirements introduce a new dimension to transaction processing in real-time database systems. In this paper, we argue that, due to the conflicting goals of each requirement, tradeoffs need to be made between security and timeliness. We first define mutual information, a measure of the degree to which security is being satisfied by a system. A secure two-phase locking protocol is then described and a scheme is proposed to allow partial violations of security for improved timeliness. Analytical expressions for the mutual information of the resultant covert channel are derived, and a feedback control scheme is proposed that does not allow the mutual information to exceed a specified upper bound. Results showing the efficacy of the scheme obtained through simulation experiments are also discussed     

Semiempirical and ab initio conformational analysis of 2-methylene-8,8-dimethyl-1,4,6,10-tetraoxaspiro[4.5] decane with application of GIAO-SCF methods to NMR spectrum interpretation

The GIAO-SCF method for calculating isotropic nuclear magnetic shielding values has been utilized to explain certain features in the ¹H-NMR spectrum of 2-methylene-8,8-dimethyl-1,4,6,10-tetraoxaspiro[4.5] decane. Population distributions of the low-energy conformers based on their ab initio energies were used to produce weighting factors for the individual calculated shielding values to calculate the weighted average of the shielding values for a complete set of conformers. The differences in ¹H chemical shifts between the hydrogens of the two methyl groups and between the axial and equatorial hydrogens in 2-methylene-8,8-dimethyl-1,4,6,10-tetraoxaspiro[4.5] decane were shown to be due to energy differences between the chair and boat orientations of the six-membered ring and contribution from a twist-boat conformation. Results suggest a hypothesis that intramolecular differences in chemical shift might be calculated to a greater degree of accuracy than chemical shifts calculated relative to a standard.     

Global structure of families of multivariable linear systems with an application to identification

The identification problem for linear stochastic systems may be stated roughly as follows: given observations on two stochastic processes which are the input and output of some unknown linear system, determine some estimate of the parameters of the system. A set of candidate linear systems which contains the “true” system is introduced, and probabilistic assumptions on the two stochastic processes turn the identification problem into the deterministic problem of minimizing some objective function over this candidate model set. If this set is a manifold, the existence of globally convergent identification algorithms hinges on the critical point behavior of the objective functions which it carries. By way of Morse Theory, the critical point behavior of objective functions on a manifold has implications with regard to the topology of the manifold. This paper analyzes the topology and critical point behavior of objective functions on a specific manifold of linear systems which appears frequently as the candidate model set in identification problems. This manifold is the set Σ of allm-input,p-output linear systems of fixed McMillan degree with real or complex coefficients. Over this manifold sits the principal bundle \(\tilde \Sigma\) of minimal realizations of systems in Σ It is shown that there exist three natural analytic metrics on the associated vector bundle. It is also shown that, in the real case, the first Stiefel-Whitney class of the bundle \(\tilde \Sigma\) has min(m, p)-1 nonvanishing powers; the same conclusion is drawn about the first Chern class of \(\tilde \Sigma\) in the complex case. These results, which follow from Morse Theory and some elementary homotopy and homology theory, imply that the category of the bundle \(\tilde \Sigma\) is at least min(m, p), and hence that the Lusternik-Schnirelmann category of Σ is at least min(m, p). It follows that canonical forms (i.e. sections of \(\tilde \Sigma\) ) may exist only when min(m, p) = 1 and that any objective function on Σ with compact sublevel sets has at least min(m, p) critical points. In particular, there exist on Σ no globally convergent gradient algorithms when min(m, p) > 1.     

Definition of the Linear Region of X-Ray -Induced Cable Response

Cable response to X-rays is linear with incident fluence, provided the deposited charge in cable dielectrics is directly proportional to the X-ray flux. To estimate the level at which the linear region ends, we discuss three nonlinear processes that modify the deposited charge profile in a hypothetical cable model: field-limiting in vacuum gaps, ionization effects in air-filled gaps, and radiation-induced dielectric conductivity. The exact level at which limiting of the Norton driver in an elemental length of cable begins depends on the cable geometry and the X-ray source. Estimates of the onset of nonlinearities caused by field-limiting and by dielectric conductivity are found in terms of cable and source parameters. With air-filled gaps, the Norton driver is always nonlinear. In addition to limiting of the Norton drivers, the load response of a long cable may be limited because propagating currents are attenuated by the induced conductivity of the bulk of the dielectric.     

Proof of termination within a weak logic of programs

Summary A weak logic of programs is a formal system in which statements that mean the program halts cannot be expressed. In order to prove termination, we would usually have to use a stronger logical system. In this paper we show how we can prove termination of both iterative and recursive programs within a weak logic by augmenting the programs with counters and adding bound assertions on the counters as loop invariants and entry conditions. Thus, most of the existing verifiers which are based on a weak logic of programs can be used to prove termination of programs without any modification. We give examples of proofs of termination and of upper bounds on computation time that were obtained using a Pascal program verifier. The use of the method to locate causes of non-termination is also illustrated.This research was supported inpart by the Advanced Research Agency of the Office of the Secretary of Defence under contract DAHC 15-73-C-0435     

A model for pattern perception with musical applications part II: The information content of pitch structures

This is the second paper of a series which begins by treating the perception of pitch relations in musical contexts and the perception of timbre and speech. The first paper discusses in some detail those properties of musical scales required in order for them to function as reference frames which provide for the measurement of intervals such that ([1], p. 270),Every melodic phrase, every chord, which can be executed at any pitch, can be also executed at any other pitch in such a way that we immediately perceive the characteristic marks of their similarity. Here we continue this discussion by developing quantitative measures of the degree to which different scales possess the above properties. Then that property of musical scales which permits a listener to code the pitches of which it is constituted into degrees is examined and a corresponding quantitative measure developed. Musical scales are shown to be optimal choices with respect to both the former and latter measures, and a theory limiting those scales which are musically useful to a small fraction of possible sets of pitches is proposed. Existing scales which have been examined fall within the theory, which links the techniques of composition which may be used (i.e., those which produce perceptible relations between musical segments) to the above properties of the scale structures. This paper is not self-contained—reading of the previous paper in this series is required.This research was supported in part by grants and contracts AF-AFOSR 881-65, AF 49(638)-1738 and AF-AFOSR 68-1596.