Integrable impurity spin ladder systems

Two different types of integrable impurities in a spin ladder system are proposed. The impurities are introduced in such a way that the integrability of the models is not violated. The models are solved exactly with the Bethe ansatz equations as well as the energy eigenvalues obtained. We show for both models that a phase transition between gapped and gapless spin excitations occurs at a critical value of the rung coupling J. In addition, the dependence of the impurities on this phase transition is determined explicitly. In one of the models the spin gap decreases by increasing the impurity strength A. Moreover, for a fixed A, a reduction in the spin gap by increasing the impurity concentration is also observed.

Integrable generalized spin ladder models based on the SU(1|3) and SU(3|1) algebras

We present two integrable spin ladder models which possess a general free parameter besides the rung coupling J. The models are exactly solvable by means of the Bethe ansatz method and we present the Bethe ansatz equations. We analyze the elementary excitations of the models which reveal the existence of a gap for both models that depends on the free parameter. (C) 2003 American Institute of Physics.

Graduate entry to medical studies: thoughts from 'down under' (editorial)

This invited editorial, reflecting on expectations of changing to graduate entry, eg enhanced maturity in the student cohort with greater self-sufficiency and taking of responsibility for learning in the context of adoption of a problem-based learning model, examines experiences of early post-change years and raises questions for contemplation by medical schools considering graduate entry.

Alterations of neocortical pyramidal cell phenotype in the Ts65Dn mouse model of Down syndrome: Effects of environmental enrichment

Mental retardation in individuals with Down syndrome (DS) is thought to result from anomalous development and function of the brain; however, the underlying neuropathological processes have yet to be determined. Early implementation of special care programs result in limited, and temporary, cognitive improvements in DS individuals. In the present study, we investigated the possible neural correlates of these limited improvements. More specifically, we studied cortical pyramidal cells in the frontal cortex of Ts65Dn mice, a partial trisomy of murine chromosome 16 (MMU16) model characterized by cognitive deficits, hyperactivity, behavioral disruption and reduced attention levels similar to those observed in DS, and their control littermates. Animals were raised either in a standard or in an enriched environment. Environmental enrichment had a marked effect on pyramidal cell structure in control animals. Pyramidal cells in environmentally enriched control animals were significantly more branched and more spinous than non-enriched controls. However, environmental enrichment had little effect on pyramidal cell structure in Ts65Dn mice. As each dendritic spine receives at least one excitatory input, differences in the number of spines found in the dendritic arbors of pyramidal cells in the two groups reflect differences in the number of excitatory inputs they receive and, consequently, complexity in cortical circuitry. The present results suggest that behavioral deficits demonstrated in the Ts65Dn model could be attributed to abnormal circuit development.

Tympanometry and TEOAE testing of children with Down Syndrome in Special Schools

Despite widespread awareness that children with Down syndrome are particularly susceptible to hearing pathologies, the audiological status of students with Down syndrome in special schools is all too often unknown. Unfortunately, hearing screening for this population is unable to rely on standard, behavioural test batteries. To facilitate future improvements in screening protocols, this study investigated the results of tympanometry and transient evoked otoacoustic emission (TEOAE) testing for a group of children with Down syndrome. Assessments were not conducted in the artificial context of a clinic or laboratory, but within the school environment. Outcomes are reported for 27 subjects with a mean age of 10 years 5 months (SD = 4;11). Tympanometry testing was failed in at least one ear by 41.7% of subjects, while a failure rate of 81.5% of subjects was observed for TEOAE testing. Therefore, it is concluded that immediate review of hearing screening programs for students with Down syndrome is highly advisable.