Adaptation and gain pool summation:Alternative models and masking data

Foley [J. Opt. Soc. Am. A 11 (1994) 1710] has proposed an influential psychophysical model of masking in which mask components in a contrast gain pool are raised to an exponent before summation and divisive inhibition. We tested this summation rule in experiments in which contrast detection thresholds were measured for a vertical 1 c/deg (or 2 c/deg) sine-wave component in the presence of a 3 c/deg (or 6 c/deg) mask that had either a single component oriented at -45° or a pair of components oriented at ±45°. Contrary to the predictions of Foley's model 3, we found that for masks of moderate contrast and above, threshold elevation was predicted by linear summation of the mask components in the inhibitory stage of the contrast gain pool. We built this feature into two new models, referred to as the early adaptation model and the hybrid model. In the early adaptation model, contrast adaptation controls a threshold-like nonlinearity on the output of otherwise linear pathways that provide the excitatory and inhibitory inputs to a gain control stage. The hybrid model involves nonlinear and nonadaptable routes to excitatory and inhibitory stages as well as an adaptable linear route. With only six free parameters, both models provide excellent fits to the masking and adaptation data of Foley and Chen [Vision Res. 37 (1997) 2779] but unlike Foley and Chen's model, are able to do so with only one adaptation parameter. However, only the hybrid model is able to capture the features of Foley's (1994) pedestal plus orthogonal fixed mask data. We conclude that (1) linear summation of inhibitory components is a feature of contrast masking, and (2) that the main aftereffect of spatial adaptation on contrast increment thresholds can be assigned to a single site. © 2002 Elsevier Science Ltd. All rights reserved.

Evolutionary neural networks : models and applications

The scaling problems which afflict attempts to optimise neural networks (NNs) with genetic algorithms (GAs) are disclosed. A novel GA-NN hybrid is introduced, based on the bumptree, a little-used connectionist model. As well as being computationally efficient, the bumptree is shown to be more amenable to genetic coding lthan other NN models. A hierarchical genetic coding scheme is developed for the bumptree and shown to have low redundancy, as well as being complete and closed with respect to the search space. When applied to optimising bumptree architectures for classification problems the GA discovers bumptrees which significantly out-perform those constructed using a standard algorithm. The fields of artificial life, control and robotics are identified as likely application areas for the evolutionary optimisation of NNs. An artificial life case-study is presented and discussed. Experiments are reported which show that the GA-bumptree is able to learn simulated pole balancing and car parking tasks using only limited environmental feedback. A simple modification of the fitness function allows the GA-bumptree to learn mappings which are multi-modal, such as robot arm inverse kinematics. The dynamics of the 'geographic speciation' selection model used by the GA-bumptree are investigated empirically and the convergence profile is introduced as an analytical tool. The relationships between the rate of genetic convergence and the phenomena of speciation, genetic drift and punctuated equilibrium arc discussed. The importance of genetic linkage to GA design is discussed and two new recombination operators arc introduced. The first, linkage mapped crossover (LMX) is shown to be a generalisation of existing crossover operators. LMX provides a new framework for incorporating prior knowledge into GAs.Its adaptive form, ALMX, is shown to be able to infer linkage relationships automatically during genetic search.

Pre-conception care : current practice and methods of provision

This thesis examines the present provisions for pre-conception care and the views of the providers of services. Pre-conception care is seen by some clinicians and health educators as a means of making any necessary changes in life style, corrections to imbalances in the nutritional status of the prospective mother (and father) and the assessment of any medical problems, thus maximizing the likelihood of the normal development of the baby. Pre-conception care may be described as a service to bridge the gap between the family planning clinic and the first ante-natal booking appointment. There were three separate foci for the empirical research - the Foresight organisation (a charity which has pioneered pre-conception care in Britain); the pre-conception care clinic at the West London Hospital, Hammersmith; and the West Midlands Regional Health Authority. The six main sources of data were: twenty five clinicians operating Foresight pre-conception clinics, couples attending pre-conception clinics, committee members of the Foresight organisation, staff of the West London Hospital pre-conception clinic, Hammersmith, District Health Education Officers working in the West Midlands Regional Health Authority and the members of the Ante-Natal Care Action Group, a sub-group of the Regional Health Advisory Group on Health Promotion and Preventive Medicine. A range of research methods were adopted. These were as follows: questionnaires and report forms used in co-operation with the Foresight clinicians, interviews, participant observation discussions and informal meetings and, finally, literature and official documentation. The research findings illustrated that pre-conception care services provided at the predominantly private Foresight clinics were of a rather `ad hoc' nature. The type of provision varied considerably and clearly reflected the views held by its providers. The protocol which had been developed to assist in the standardization of results was not followed by the clinicians. The pre-conception service provided at the West London Hospital shared some similarities in its approach with the Foresight provision; a major difference was that it did not advocate the use of routine hair trace metal analysis. Interviews with District Health Education Officers and with members of the Ante Natal Care Action Group revealed a tentative and cautious approach to pre-conception care generally and to the Foresight approach in particular. The thesis concludes with a consideration of the future of pre-conception care and the prospects for the establishment of a comprehensive pre-conception care service.

Models and performances of wireless MIMO and cooperative communication systems

Multiple-antenna systems offer significant performance enhancement and will be applied to the next generation broadband wireless communications. This thesis presents the investigations of multiple-antenna systems – multiple-input multiple-output (MIMO) and cooperative communication (CC) – and their performances in more realistic propagation environments than those reported previously. For MIMO systems, the investigations are conducted via theoretical modelling and simulations in a double-scattering environment. The results show that the variations of system performances depend on how scatterer density varies in flat fading channels, and that in frequency-selective fading channels system performances are affected by the length of the coding block as well as scatterer density. In realistic propagation environments, the fading correlation also has an impact on CC systems where the antennas can be further apart than those in MIMO systems. A general stochastic model is applied to studying the effects of fading correlation on the performances of CC systems. This model reflects the asymmetry fact of the wireless channels in a CC system. The results demonstrate the varied effects of fading correlation under different protocols and channel conditions. Performances of CC systems are further studied at the packet level, using both simulations and an experimental testbed. The results obtained have verified various performance trade-offs of the cooperative relaying network (CRN) investigated in different propagation environments. The results suggest that a proper selection of the relaying algorithms and other techniques can meet the requirements of quality of service for different applications.

Forecasting the UK/US exchange rate with divisia monetary models and neural networks

This paper compares the UK/US exchange rate forecasting performance of linear and nonlinear models based on monetary fundamentals, to a random walk (RW) model. Structural breaks are identified and taken into account. The exchange rate forecasting framework is also used for assessing the relative merits of the official Simple Sum and the weighted Divisia measures of money. Overall, there are four main findings. First, the majority of the models with fundamentals are able to beat the RW model in forecasting the UK/US exchange rate. Second, the most accurate forecasts of the UK/US exchange rate are obtained with a nonlinear model. Third, taking into account structural breaks reveals that the Divisia aggregate performs better than its Simple Sum counterpart. Finally, Divisia-based models provide more accurate forecasts than Simple Sum-based models provided they are constructed within a nonlinear framework.

Changing career models and capacity for innovation in professional services

A number of professional sectors have recently moved away from their longstanding career model of up-or-out promotion and embraced innovative alternatives. Professional labor is a critical resource in professional service firms. Therefore, changes to these internal labor markets are likely to trigger other innovations, for example in knowledge management, incentive schemes and team composition. In this chapter we look at how new career models affect the core organizing model of professional firms and, in turn, their capacity for and processes of innovation. We consider how professional firms link the development of human capital and the division of professional labor to distinctive demands for innovation and how novel career systems help them respond to these demands.

Outsourcing and offshoring engineering projects:understanding the value, sourcing models, and coordination practices

In this paper, we review recent developments in the field of outsourcing and offshoring and the implications for engineering management. We examine three aspects involved in outsourcing and offshoring, namely, sourcing models, coordination, and value extracted from outsourcing projects. We conclude that additional research is needed on recent trends in outsourcing and the impact of such change process on the practice of engineering management. © 2011 IEEE.

Conceptual Foundations of Construction of the Models and Procedures for Prediction of the Avalanche-dangerous Situations Initiation

The conceptual foundations of the models and procedures for prediction of the avalanche-dangerous situations initiation are considered. The interpretation model for analysis of the avalanche-dangerous situations initiation based on the definition of probabilities of correspondence of studied parameters to the probabilistic distributions of avalanche-dangerous or avalanche non-dangerous situations is offered. The possibility to apply such a model to the real data is considered. The main approaches to the use of multiple representations for the avalanche dangerous situations initiation analysis are generalized.

Triangular Models and Asymptotics of Continuous Curves with Bounded and Unbounded Semigroup Generators

2000 Mathematics Subject Classification: Primary 47A48, Secondary 60G12.

On Generalized Models and Singular Products of Distributions in Colombeau Algebra G(R)

MSC 2010: 46F30, 46F10