On the time required for identification of visual objects

The starting point for this thesis is a review of Bundesen’s theory of visual attention. This theory has been widely accepted as an appropriate model for describing data from an important class of psychological experiments known as whole and partial report. Analysing data from this class of experiments with the help of the theory of visual attention – have proven to be an effective approach to examine cognitive parameters that are essential for a broad range of different patient groups. The theory of visual attention relies on a psychometric function that describes the ability to identify a stimulus as a function of exposure duration. An important contribution of the thesis is that it investigates whether other psychometric functions than the one originally used with the theory of visual attention could be more appropriate at describing data. The thesis points to two psychometric functions that seem more appropriate. Further the thesis shows that it is possible to incorporate any desired psychometric into the theory of visual attention. Common to the two psychometric functions suggested is that they both have a hazard function that is non-monotonic; a neural argument for this is also presented in the thesis. For the psychometric function it is further investigated how this depends on stimulus contrast. In this respect, we find that the type of psychometric function is independent of contrast, but that the parameters for the psychometric function vary systematically as a function of contrast. An analysis of the psychometric function for the individual letters of the alphabet shows that there are significant differences in the parameters of the psychometric function depending on letter identity. Here we should note that in many cases (also for Bundesen’s theory of visual attention) it has been customary to average performance over the entire set of stimuli, consisting for instance of the 26 alphabetic letters. The fact that each letter is perceived in a different way possibly reflects that each letter is represented differently in our brain. This might have to do with a difference in the set of features representing the individual letters. It is possible that some features are processed faster than others and that overlapping features representing more than one letter in the alphabet play a certain role for the tendency to confuse letters. Hopefully it should be possible, with the dataset that we collected, to directly analyse how confusability develops as a certain letter is exposed for increasingly longer time. An important scientific question is what shapes the psychometric function. It is conceivable that the function reflects both limitations and structure of the physical mechanism underlying perception. For this reason we argue that the alternative psychometric functions that we have suggested are also relevant for models trying to simulate the mechanism leading to perception. The thesis reviews a selection of stochastic models that are well-known candidates when it comes to modelling mechanisms of perception. These candidates include the Ornstein-Uhlenbeck model and the leaky competing accumulator model. A further contribution of the thesis is a demonstration that the leaky competing accumulator model (see Usher & Cohen, 1999) is able to explain a perceptual limit that characterises how many objects can in parallel be perceived. Finally, the thesis suggests five concrete topics for future work. These include as diverse themes as: determination of the visual features representing the individual letters in our brain, neurodynamical modelling of visual perception, investigation of the duration of visual short-term memory as well as psychometric functions and assumptions along with application areas in cognitive diagnostics

Robust stability conditions for feedback interconnections of distributed-parameter negative imaginary systems

Sufficient and necessary conditions for the stability of positive feedback interconnections of negative imaginary systems are derived via an integral quadratic constraint (IQC) approach. The IQC framework accommodates distributed-parameter systems with irrational transfer function representations, while generalising existing results in the literature and allowing exploitation of flexibility at zero and infinite frequencies to reduce conservatism in the analysis. The main results manifest the important property that the negative imaginariness of systems gives rise to a certain form of IQCs on positive frequencies that are bounded away from zero and infinity. Two additional sets of IQCs on the DC and instantaneous gains of the systems are shown to be sufficient and necessary for closed-loop stability along a homotopy of systems.Comment: Submitted to Automatica, A preliminary version of this paper appeared in the Proceedings of the 2015 European Control Conferenc

Constraints on New Physics from Baryogenesis and Large Hadron Collider Data

We demonstrate the power of constraining theories of new physics by insisting that they lead to electroweak baryogenesis, while agreeing with current data from the Large Hadron Collider. The general approach is illustrated with a singlet scalar extension of the Standard Model. Stringent bounds can already be obtained, which reduce the viable parameter space to a small island.Comment: 4 pages, 2 figures. References added, figures updated. Version to appear in PR

Simulating Soil Organic Matter Transformations with the New Implementation of the Daisy Model

Daisy is a well-tested deterministic, dynamic soil-plant-atmosphere model, capable of simulating water balance, nitrogen balance and losses, development in soil organic matter and crop growth and production in crop rotations under alternate management strategies. Originally it was developed as a system of single models describing each process involved, but recently it has been developed into a framework, which can be used for implementation of several different models of each of the different processes. Thus, for example a number of different models for simulating soil water dynamics can be chosen depending on the purpose of the simulation and the availability of data for parameterisation. The sub-model simulating soil organic matter is still a fixed component in the Daisy terminology. This means that there is currently only one model, which can be used to simulate soil organic matter transformations. However this sub-model can be changed considerably. Some examples are given