Perceptual Learning – Perceptual Changes in Learning New Categories : An extended abstract (autoreferat) submitted in partial fulfillment of the requirements for the degree of Ph.D. in Psychology

Perceptual learning traditionally focuses on studying early plasticity in the sensory pathways. Categorization is a task typically attributed to relatively higher-level cognition. This dissertation explores whether visual categorization of simple objects involves perceptual learning. A simple, physiologically plausible neural network model is put forward to demonstrate how both supervised and unsupervised learning could take place in visual categorization. The model shows how early perceptual representations could be formed, adjusted and reorganized without feedback. At the same time, the re-weighting of the connections between low-level representations and mid-level perceptual structures allows the selective filtering of early-level information and the formation through supervised learning of perceptual detectors for characteristic parts of objects. The model demonstrates how perceptual learning processes could take place in seemingly higher-level tasks like categorical learning of simple objects. Several experiments, using a position transfer paradigm in categorical learning of simple objects, are presented in addition to the new model. They confirm empirically the claims of the model: that visual categorical learning involves lower-level perceptual learning processes. Evidence for such processes is found in the incomplete transfer of learning when the stimuli are presented at a new location in the visual field. This effect is observed even when the participants’ categorization strategy is very simple and explicit. These results complement a large pool of similar findings for arguably lower-level perceptual tasks like vernier discrimination or orientation judgment. This dissertation argues that visual categorical learning of simple objects involves low-level perceptual learning processes, similar to the processes in typical sensory perceptual tasks

Integrable Marginal Points in the N-Cosine Model

The integrability of the N-cosine model, a N-field generalization of the sine-Gordon model, is investigated. We establish to first order in conformal perturbation theory that, for arbitrary N, the model possesses a quantum conserved current of Lorentz spin 3 on a submanifold of the parameter space where the interaction becomes marginal. The integrability of the model on this submanifold is further studied using renormalization techniques. It is shown that for N = 2, 3, and 4 there exist special points on the marginal manifold at which the N-cosine model is equivalent to models of Gross-Neveu type and therefore is integrable. In the 2-field case we further argue that the points mentioned above exhaust all integrable cases on the marginal submanifold.Comment: 17 pages, 1 figure, LaTeX2e, AM

Regional and Ethnic Disparities of School-to-Work Transitions in Bulgaria

Bulgaria’s educational and economic landscapes are marked by substantial regional disparities that are interlaced with ethnic inequalities in school-to-work transitions. Young adults from Roma and Turkish origins particularly suffer from disadvantages with respect to education and labour market participation. We ask how ethnicity affects labour market entry in Bulgaria once educational resources of different ethnic groups are accounted for, and how regional contexts impact ethnic disparities in employment insecurities. Building on comparative school-to-work transition (STWT) concepts and on the labour queueing approach, we assume that ethnic disparities in the STWTs of youths in Bulgaria depend on the degree of urbanisation and the strength and structure of the regional economy. The study draws on data from the Bulgarian School Leaver Survey 2014 of 2103 young adults who had left education in the five years preceding the survey. Descriptive analysis and multilevel logistic regression models were applied to analyse STWT patterns with a special focus on education, regional contexts, and ethnicity. The results highlight that STWT risks differ considerably across the Bulgarian regions. The strength of the local economy thereby moderates ethnic disparities. Young people from Roma and Turkish origins are much less disadvantaged to transition towards employment compared to ethnic Bulgarians the stronger the local economy gets. Our study has several policy implications. In addition to the development of public and private employment opportunities for disadvantaged young people, special attention should also be paid to the development of quality vocational education at the national and regional leve

On the Integrability of the Bukhvostov-Lipatov Model

The integrability of the Bukhvostov-Lipatov four-fermion model is investigated. It is shown that the classical model possesses a current of Lorentz spin 3, conserved both in the bulk and on the half-line for specific types of boundary actions. It is then established that the conservation law is spoiled at the quantum level -- a fact that might indicate that the quantum Bukhvostov-Lipatov model is not integrable, contrary to what was previously believed.Comment: 11 pages, 1 figure, LaTeX2e, AMS; new references adde

Chiral stabilization of the renormalization group for flavor and color anisotropic current interactions

We propose an all-orders beta function for current-current interactions in 2d with flavor anisotropy. When the number of left-moving and right-moving flavors are unequal, the beta function has a non-trivial fixed point at finite values of the couplings. We also extend the computation to simple cases with both flavor and color anisotropy.Comment: 8 pages, v2: two references adde

On the Beta Function for Anisotropic Current Interactions in 2D

By making use of current-algebra Ward identities we study renormalization of general anisotropic current-current interactions in 2D. We obtain a set of algebraic conditions that ensure the renormalizability of the theory to all orders. In a certain minimal prescription we compute the beta function to all orders.Comment: 7 pages, 6 figures. v2: References added and typos corrected; v3: cancellation of finite parts more accurately state

Large N spin quantum Hall effect

We introduce a large N version of the spin quantum Hall transition problem. It is formulated as a problem of Dirac fermions coupled to disorder, whose Hamiltonian belong to the symmetry class C. The fermions carry spin degrees of freedom valued in the algebra sp(2N), the spin quantum Hall effect corresponding to N=1. Arguments based on renormalization group transformations as well as on a sigma model formulation, valid in the large N limit, indicate the existence of a crossover as N varies. Contrary to the N=1 case, the large N models are shown to lead to localized states at zero energy. We also present a sigma model analysis for the system of Dirac fermions coupled to only sp(2N) random gauge potentials, which reproduces known exact results.Comment: 29 pages; few references added, statement about the density of states improved; published versio

Typical 3-D localization of tumor remnants of WHO grade II hemispheric gliomas--lessons learned from the use of intraoperative high-field MRI control

Complete resection of grade II gliomas might prolong survival but is not always possible. The goal of the study was to evaluate the location of unexpected grade II gliomas remnants after assumed complete removal with intraoperative (iop) MRI and to assess the reason for their non-detection

Generalized sine-Gordon/massive Thirring models and soliton/particle correspondences

We consider a real Lagrangian off-critical submodel describing the soliton sector of the so-called conformal affine $sl(3)^{(1)}$ Toda model coupled to matter fields (CATM). The theory is treated as a constrained system in the context of Faddeev-Jackiw and the symplectic schemes. We exhibit the parent Lagrangian nature of the model from which generalizations of the sine-Gordon (GSG) or the massive Thirring (GMT) models are derivable. The dual description of the model is further emphasized by providing the relationships between bilinears of GMT spinors and relevant expressions of the GSG fields. In this way we exhibit the strong/weak coupling phases and the (generalized) soliton/particle correspondences of the model. The $sl(n)^{(1)}$ case is also outlined.Comment: 22 pages, LaTex, some comments and references added, conclusions unchanged, to appear in J. Math. Phy