A threshold hypothesis of institutional change: collective action in the Italian Alps during the 13th - 19th centuries.

This dissertation is about collective action issues in common property resources. Its focus is the “threshold hypothesis,” which posits the existence of a threshold in group size that drives the process of institutional change. This hypothesis is tested using a six-century dataset concerning the management of the commons by hundreds of communities in the Italian Alps. The analysis seeks to determine the group size threshold and the institutional changes that occur when groups cross this threshold. There are five main findings. First, the number of individuals in villages remained stable for six centuries, despite the population in the region tripling in the same period. Second, the longitudinal analysis of face-to-face assemblies and community size led to the empirical identification of a threshold size that triggered the transition from informal to more formal regimes to manage common property resources. Third, when groups increased in size, gradual organizational changes took place: large groups split into independent subgroups or structured interactions into multiple layers while maintaining a single formal organization. Fourth, resource heterogeneity seemed to have had no significant impact on various institutional characteristics. Fifth, social heterogeneity showed statistically significant impacts, especially on institutional complexity, consensus, and the relative importance of governance rules versus resource management rules. Overall, the empirical evidence from this research supports the “threshold hypothesis.” These findings shed light on the rationale of institutional change in common property regimes, and clarify the mechanisms of collective action in traditional societies. Further research may generalize these conclusions to other domains of collective action and to present-day applications.

Femtosecond laser-based investigations of magnetic circular dichroism in near-threshold photoemission

During the last decades magnetic circular dichroism (MCD) has attracted much interest and evolved into various experimental methods for the investigation of magnetic thin films. For example, synchrotron-based X-ray magnetic circular dichroism (XMCD) displays the absolute values of spin and orbital magnetic moments. It thereby benefits from large asymmetry values of more than 30% due to the excitation of atomic core-levels. Similarly large values are also expected for threshold photoemission magnetic circular dichroism (TPMCD). Using lasers with photon energies in the range of the sample work function this method gives access to the occupied electronic structure close to the Fermi level. However, except for the case of Ni(001) there exist only few studies on TPMCD moreover revealing much smaller asymmetries than XMCD-measurements. Also the basic physical mechanisms of TPMCD are not satisfactorily understood. In this work we therefore investigate TPMCD in one- and two-photon photoemission (1PPE and 2PPE) for ferromagnetic Heusler alloys and ultrathin Co films using ultrashort pulsed laser light. The observed dichroism is explained by a non-conventional photoemission model using spin-resolved band-structure calculations and linear response theory. For the two Heusler alloys Ni2MnGa and Co2FeSi we give first evidence of TPMCD in the regime of two-photon photoemission. Systematic investigations concerning general properties of TPMCD in 1PPE and 2PPE are carried out at ultrathin Co films grown on Pt(111). Here, photon-energy dependent measurements reveal asymmetries of 1.9% in 1PPE and 11.7% in 2PPE. TPMCD measurements at decreased work function even yield larger asymmetries of 6.2% (1PPE) and 17% (2PPE), respectively. This demonstrates that enlarged asymmetries are also attainable for the TPMCD effect on Co(111). Furthermore, we find that the TPMCD asymmetry is bulk-sensitive for 1PPE and 2PPE. This means that the basic mechanism leading to the observed dichroism must be connected to Co bulk properties; surface effects do not play a crucial role. Finally, the enhanced TPMCD asymmetries in 2PPE compared to the 1PPE case are traced back to the dominant influence of the first excitation step and the existence of a real intermediate state. The observed TPMCD asymmetries cannot be interpreted by conventional photoemission theory which only considers direct interband transitions in the direction of observation (Γ-L). For Co(111), these transitions lead to evanescent final states. The excitation to such states, however, is incompatible with the measured bulk-sensitivity of the asymmetry. Therefore, we generalize this model by proposing the TPMCD signal to arise mostly from direct interband transitions in crystallographic directions other than (Γ-L). The necessary additional momentum transfer to the excited electrons is most probably provided by electron-phonon or -magnon scattering processes. Corresponding calculations on the basis of this model are in reasonable agreement with the experimental results so that this approach represents a promising tool for a quantitative description of the TPMCD effect. The present findings encourage an implementation of our experimental technique to time- and spatially-resolved photoemission electron microscopy, thereby enabling a real time imaging of magnetization dynamics of single excited states in a ferromagnetic material on a femtosecond timescale.

Blood glucose and prognosis in children with presumed severe malaria: is there a threshold for 'hypoglycaemia'?

OBJECTIVES: Hypoglycaemia (glucose <2.2 mmol/l) is a defining feature of severe malaria, but the significance of other levels of blood glucose has not previously been studied in children with severe malaria. METHODS: A prospective study of 437 consecutive children with presumed severe malaria was conducted in Mali. We defined hypoglycaemia as <2.2 mmol/l, low glycaemia as 2.2-4.4 mmol/l and hyperglycaemia as >8.3 mmol/l. Associations between glycaemia and case fatality were analysed for 418 children using logistic regression models and a receiver operator curve (ROC). RESULTS: There was a significant difference between blood glucose levels in children who died (median 4.6 mmol/l) and survivors (median 7.6 mmol/l, P < 0.001). Case fatality declined from 61.5% of the hypoglycaemic children to 46.2% of those with low glycaemia, 13.4% of those with normal glycaemia and 7.6% of those with hyperglycaemia (P < 0.001). Logistic regression showed an adjusted odds ratio (AOR) of 0.75 (0.64-0.88) for case fatality per 1 mmol/l increase in baseline blood glucose. Compared to a normal blood glucose, hypoglycaemia and low glycaemia both significantly increased the odds of death (AOR 11.87, 2.10-67.00; and 5.21, 1.86-14.63, respectively), whereas hyperglycaemia reduced the odds of death (AOR 0.34, 0.13-0.91). The ROC [area under the curve at 0.753 (95% CI 0.684-0.820)] indicated that glycaemia had a moderate predictive value for death and identified an optimal threshold at glycaemia <6.1 mmol/l, (sensitivity 64.5% and specificity 75.1%). CONCLUSIONS: If there is a threshold of blood glucose which defines a worse prognosis, it is at a higher level than the current definition of 2.2 mmol/l.

Mitochondrial tRNA(Leu(UUR)) mutation m.3302A > G presenting as childhood-onset severe myopathy: threshold determination through segregation study

Mitochondrial tRNA(Leu(UUR)) mutation m.3302A > G is associated with respiratory chain complex I deficiency and has been described as a rare cause of mostly adult-onset slowly progressive myopathy. Five families with 11 patients have been described so far; 5 of them died young due to cardiorespiratory failure. Here, we report on a segregation study in a family with an index patient who already presented at the age of 18 months with proximal muscular hypotonia, abnormal fatigability, and lactic acidosis. This early-onset myopathy was rapidly progressive. At 8 years, the patient is wheel-chair bound, requires nocturnal assisted ventilation, and suffers from recurrent respiratory infections. Severe complex I deficiency and nearly homoplasmy for m.3302A > G were found in muscle. We collected blood, hair, buccal swabs and muscle biopsies from asymptomatic adults in this pedigree and determined heteroplasmy levels in these tissues as well as OXPHOS activities in muscle. All participating asymptomatic adults had normal OXPHOS activities. In contrast to earlier reports, we found surprisingly little variation of heteroplasmy levels in different tissues of the same individual. Up to 45% mutation load in muscle and up to 38% mutation load in other tissues were found in non-affected adults. The phenotypic spectrum of tRNA(Leu(UUR)) m.3302A > G mutation seems to be wider than previously described. A threshold of more than 45% heteroplasmy in muscle seems to be necessary to alter complex I activity leading to clinical manifestation. The presented data may be helpful for prognostic considerations and counseling in affected families.

The role of rotation thromboelastometry in early prediction of massive transfusion

Early prediction of massive transfusion (MT) is critical in the management of severely injured trauma patients. Variables available early after injury including physiologic, laboratory, and rotation thromboelastometric (ROTEM) parameters were evaluated as predictors for the need of MT.

Fatal pulmonary embolism in a premature neonate after twin-to-twin transfusion syndrome

Thrombotic events are being increasingly recognized during the neonatal period. An infant girl was born at 29 weeks' gestation after a pregnancy complicated by twin-to-twin transfusion syndrome. After an initial uncomplicated clinical course, her oxygen requirement increased, which was interpreted as an early sign of bronchopulmonary dysplasia. At 3 weeks of age, she suddenly collapsed and died of severe pulmonary hypertension. At autopsy, multiple pulmonary artery emboli and several older renal vein thromboses were found. Results of genetic analyses of the infant and her family were negative for thrombophilia. Although embolism represents a frequent emergency in adults, fatal pulmonary embolism has never, to our knowledge, been described for premature infants. This case suggests that thrombotic events are underdiagnosed and that additional studies are needed to define infants at risk and optimal treatment strategies.

Properties of low-threshold motor axons in the human median nerve

This study investigated the excitability and accommodative properties of low-threshold human motor axons to test whether these motor axons have greater expression of the persistent Na(+) conductance, I(NaP). Computer-controlled threshold tracking was used to study 22 single motor units and the data were compared with compound motor potentials of various amplitudes recorded in the same experimental session. Detailed comparisons were made between the single units and compound potentials that were 40% or 5% of maximal amplitude, the former because this is the compound potential size used in most threshold tracking studies of axonal excitability, the latter because this is the compound potential most likely to be composed entirely of motor axons with low thresholds to electrical recruitment. Measurements were made of the strength-duration relationship, threshold electrotonus, current-voltage relationship, recovery cycle and latent addition. The findings did not support a difference in I(NaP). Instead they pointed to greater activity of the hyperpolarization-activated inwardly rectifying current (I(h)) as the basis for low threshold to electrical recruitment in human motor axons. Computer modelling confirmed this finding, with a doubling of the hyperpolarization-activated conductance proving the best single parameter adjustment to fit the experimental data. We suggest that the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel(s) expressed on human motor axons may be active at rest and contribute to resting membrane potential.

Threshold-dependent effects on peripheral nerve in vivo excitability properties in the rat

Various factors, including maturity, have been shown to influence peripheral nerve excitability measures, but little is known about differences in these properties between axons with different stimulation thresholds. Multiple nerve excitability tests were performed on the caudal motor axons of immature and mature female rats, recording from tail muscles at three target compound muscle action potential (CMAP) levels: 10%, 40% ("standard" level), and 60% of the maximum CMAP amplitude. Compared to lower target levels, axons at high target levels have the following characteristics: lower strength-duration time constant, less threshold reduction during depolarizing currents and greater threshold increase to hyperpolarizing currents, most notably to long hyperpolarizing currents in mature rats. Threshold-dependent effects on peripheral nerve excitability properties depend on the maturation stage, especially inward rectification (Ih), which becomes inversely related to threshold level. Performing nerve excitability tests at different target levels is useful in understanding the variation in membrane properties between different axons within a nerve. Because of the threshold effects on nerve excitability and the possibility of increased variability between axons and altered electric recruitment order in disease conditions, excitability parameters measured only at the "standard" target level should be interpreted with caution, especially the responses to hyperpolarizing currents.