Friction in inflaton equations of motion

The possibility of a friction term in the equation of motion for a scalar field is investigated in non-equilibrium field theory. The results obtained differ greatly from existing estimates based on linear response theory, and suggest that dissipation is not well represented by a term of the form $\eta\dot{\phi}$.Comment: 4 pages, 2 figures, RevTex4. An obscurity in the original version has been clarifie

Biological motion processing in autism spectrum disorders: a behavioural and fMRI investigation

There has been much controversy as to whether people with Autism Spectrum Disorders (ASDs) have a specific impairment in processing biological motion, with some studies suggesting there is an impairment (Blake, et. al. 2003; Klin et. al. 2003, Klin & Jones, 2008, Klin et. al. 2009) and others finding that people with ASDs show intact abilities to detect biological motion and categorise actions, but are impaired in emotion categorisation (Moore et. al. 1997; Hubert et. al. 2007, Parron et. al. 2008). Recent studies have found that although behavioural measures of biological motion processing show no differences, adults with ASDs show different patterns of brain activation to controls in response to intact point-light displays (PLDs), with the STS, MT+ and ITG regions showing reduced activity in this population (Herrington et. al. 2007; Parron et. al. 2009). The current thesis aimed to clarify the nature of these difficulties and to try to elucidate the brain regions used to process configural information from PLDs using novel techniques and stimuli. The first set of experiments were designed to behaviourally test people with ASDs ability to detect biological motion in noise, to categorise actions and to categorise affect from PLDs. Despite finding differences in the two groups in detection of biological motion and affect categorisation in pilot experiments, there were no significant differences between the groups in the main experiments. However, the ASD group showed slightly poorer performance at detecting biological motion and significantly more variability in the action categorisation tasks, suggesting that there may have been an underlying difference between the two groups. Furthermore, an analysis of the pattern of errors tentatively suggested that the ASD group may be using different strategies to categorise affect than controls, particularly for negative affects. We then devised a novel technique for manipulating the amount of configural information available in a PLD without the need to add different degrees of background noise and used this technique to assess the contribution of configural cues in a direction discrimination task behaviourally and neurally. The results confirmed that in typically developed individuals configural cues significantly improved the participants’ ability to correctly determine the direction of locomotion of a point light walker. Furthermore, the fMRI task found that regions of the inferotemporal, parietal and frontal regions were sensitive to the amount of configural information present in the displays that corresponded to increases in individual participants’ behavioural performance. Lastly, we used the same technique, though with a more powerful fMRI design, to assess the behavioural and neural differences between people with ASDs and controls in response to displays containing different degrees of configural information. We found that both groups were comparable in their ability to discriminate the direction of locomotion from PLDs. However, the brain regions used to process this information were found to be substantially different. In displays in which the configural information enabled participants to accurately judge the direction of locomotion, the control group utilised a similar group of regions as found in the previous experiment. The ASD group showed a pattern of activation suggesting that they predominantly used regions in the temporal and occipital cortex, and more specifically a region in the fusiform gyrus. The results of Granger Causality Mapping analysis, which allows for the mapping of directional to and from seeded regions, confirmed that whereas the control group utilised a network of regions starting from the ITG and connecting to parietal and occipital regions, the ASD group seemed to utilise two separate networks, processing form information in the fusiform gyrus and motion information separately in middle-temporal regions. The results are discussed in terms of a potential dysfunction of the ITG region in early childhood and two different models of biological motion processing that have been proposed in the recent literature. In TD individuals the model of Giese & Poggio (2003) may be more applicable, in that it proposes the integration of static form cues with motion signals in areas such as the STS. However, a dysfunctional ITG or dysfunctional connections from the ITG to more dorsal regions would disrupt the integration of form and motion processing and force the brain to place additional processing demands on form processing regions in the fusiform gyrus. This would be more in line with the model proposed by Lange and Lappe (2006) in which information can be derived from biological motion in noise without recourse to the actual motion information, through a process of temporal analysis of static postures. Both systems though, may be intact in TD individuals and may share processing requirements depending on the task. Furthermore, it is hypothesised that a dysfunctional ITG may force the brain to place additional demands on regions in the fusiform gyrus and this neural rewiring may be the cause of the developmental delay seen in processing biological motion in people with ASDs (Annaz et. al. 2009). Future studies should examine the roles of the ITG and fusiform area in more detail, both in TD people and in people with ASDs, and determine the specific nature of these neural differences and there behavioural implications for both groups

Strong Purifying Selection at Synonymous Sites in D. melanogaster

Synonymous sites are generally assumed to be subject to weak selective constraint. For this reason, they are often neglected as a possible source of important functional variation. We use site frequency spectra from deep population sequencing data to show that, contrary to this expectation, 22% of four-fold synonymous (4D) sites in D. melanogaster evolve under very strong selective constraint while few, if any, appear to be under weak constraint. Linking polymorphism with divergence data, we further find that the fraction of synonymous sites exposed to strong purifying selection is higher for those positions that show slower evolution on the Drosophila phylogeny. The function underlying the inferred strong constraint appears to be separate from splicing enhancers, nucleosome positioning, and the translational optimization generating canonical codon bias. The fraction of synonymous sites under strong constraint within a gene correlates well with gene expression, particularly in the mid-late embryo, pupae, and adult developmental stages. Genes enriched in strongly constrained synonymous sites tend to be particularly functionally important and are often involved in key developmental pathways. Given that the observed widespread constraint acting on synonymous sites is likely not limited to Drosophila, the role of synonymous sites in genetic disease and adaptation should be reevaluated

Nonequilibrium perturbation theory for spin-1/2 fields

A partial resummation of perturbation theory is described for field theories containing spin-1/2 particles in states that may be far from thermal equilibrium. This allows the nonequilibrium state to be characterized in terms of quasiparticles that approximate its true elementary excitations. In particular, the quasiparticles have dispersion relations that differ from those of free particles, finite thermal widths and occupation numbers which, in contrast to those of standard perturbation theory evolve with the changing nonequilibrium environment. A description of this kind is essential for estimating the evolution of the system over extended periods of time. In contrast to the corresponding description of scalar particles, the structure of nonequilibrium fermion propagators exhibits features which have no counterpart in the equilibrium theory.Comment: 16 pages; no figures; submitted to Phys. Rev.

Scales of mussel bed complexity: structure, associated biota and recruitment

Hierarchically scaled surveys were carried out on beds of the brown mussel Perna perna (Linnaeus) on the South coast of South Africa. The object was to assess spatial and temporal variations in the complexity of mussel beds and to investigate relationships between mussel bed complexity and mussel recruitment. Complexity was divided into three components: physical complexity; demographic complexity; associated biota. A series of variables within each component were recorded at two different scales (10 and 50 cm) within nested quadrats on three separate occasions. The nested ANOVA design explicitly incorporated spatial scale as levels of the ANOVA. These scales were: shores (areas 1 km in length separated by 25 km); transects (areas 20 m in length separated by 100s of meters); 50×50-cm quadrats separated by meters and 10×10-cm quadrats separated by cm) This approach was intended to generate hypotheses concerning direct associations between recruitment and complexity versus co-variation due external processes. Three main questions were addressed: (1) At what scale does each variable of complexity exhibit greatest significant variation? (2) At these scales is there similar ranking of variables of complexity and recruitment? (3) Within this/these scales, is there any significant relationship between the variables measured and mussel recruitment? On two occasions (Nov. 97 and Mar. 98) the majority of variables showed greatest significant variation at the transect-scale. On a third occasion (Oct. 97) most variables showed greatest significant variation at the quadrat-scale and the site-scale. On all occasions a markedly high percentage of the variation encountered also occurred at the smallest scale of the study, i.e., the residual scale of the ANOVA analyses. Some similarity in the ranking of variables occurred at the transect scale. Within the transect-scale, there was little indication of any relationship between variables of complexity and recruitment. Relationships were inconsistent either among transects or among sampling occasions. Overall, the results suggest that a high degree of variation in mussel bed complexity consistently occurs at very small scales. High components of variance generally also occur at one or more larger scales; however, these scales vary with season. Mussel recruitment does not appear to be directly affected by complexity of mussel beds. Instead it appears external factors may influence both complexity and recruitment independently. In addition recruitment may influence complexity rather than vice versa

Supply-side ecology of the brown mussel, Perna perna: an investigation of spatial and temporal variation in, and coupling between, gamete release and larval supply

Sampling of recruitment-associated variables of Perna perna was done approximately monthly for 14 months at intertidal locations 500 m apart, nested within sites 25 km apart. Paired with intertidal locations were nearshore locations, 600 m to sea. Sampling assessed spawning, densities of larvae in the water column and densities of late plantigrades and juveniles on the shore. Major events in each variable were synchronous over larger scales (10s of kilometres) while subsidiary events were synchronised at smaller scales, varying within sites (100s of metres) or even within locations (metres). This suggests that the processes driving major events operated over large scales while processes operating at much more local scales drove less intense, more localised events. A major spawning event occurred at all locations in May–June 1998. Weaker spawning events occurred at different times in different locations. Larvae were found on 80% of sampling occasions, densities peaking in January–March 1998 and 1999 at all locations. Plantigrades and juveniles showed less clear patterns, with considerable residual variation. There was no sign of strong coupling among variables with few significant direct or cross correlations. The major sources of variability shifted from time to space as one progressed from spawning, to plantigrade density to juvenile density. For spawning, time was the most important source (58%) of heterogeneity and space accounted for little (8%) of the total variance. For larvae and late plantigrades, time was still the most important source of variability (41% and 33%, respectively), but space was a much more substantial component. For juveniles, small-scale (residual) spatial variability dominated total variability (75%). This strongly suggests the importance of hydrography and its effects on variation in delivery of larvae to the intertidal from offshore. These findings also indicate greater spatial heterogeneity as recruits age, reflecting small-scale variations in larval delivery and the increasing importance of post-settlement mortality

Dissipation in equations of motion of scalar fields

The methods of non-equilibrium quantum field theory are used to investigate the possibility of representing dissipation in the equation of motion for the expectation value of a scalar field by a friction term, such as is commonly included in phenomenological inflaton equations of motion. A sequence of approximations is exhibited which reduces the non-equilibrium theory to a set of local evolution equations. However, the adiabatic solution to these evolution equations which is needed to obtain a local equation of motion for the expectation value is not well defined; nor, therefore, is the friction coefficient. Thus, a non-equilibrium treatment is essential, even for a system that remains close to thermal equilibrium, and the formalism developed here provides one means of achieving this numerically.Comment: 17 pages, 5 figure

Scaling in high-temperature superconductors

A Hartree approximation is used to study the interplay of two kinds of scaling which arise in high-temperature superconductors, namely critical-point scaling and that due to the confinement of electron pairs to their lowest Landau level in the presence of an applied magnetic field. In the neighbourhood of the zero-field critical point, thermodynamic functions scale with the scaling variable $(T-T_{c2}(B))/B^{1/2\nu}$, which differs from the variable $(T - T_c(0))/B^{1/2\nu}$ suggested by the gaussian approximation. Lowest-Landau-level (LLL) scaling occurs in a region of high field surrounding the upper critical field line but not in the vicinity of the zero-field transition. For YBaCuO in particular, a field of at least 10 T is needed to observe LLL scaling. These results are consistent with a range of recent experimental measurements of the magnetization, transport properties and, especially, the specific heat of high-$T_c$ materials.Comment: 22 pages + 1 figure appended as postscript fil