Separate representations of target and timing cue locations in the supplementary eye fields

When different stimuli indicate where and when to make an eye movement, the brain areas involved in oculomotor control must selectively plan an eye movement to the stimulus that encodes the target position and also encode the information available from the timing cue. This could pose a challenge to the oculomotor system since the representation of the timing stimulus location in one brain area might be interpreted by downstream neurons as a competing motor plan. Evidence from diverse sources has suggested that the supplementary eye fields (SEF) play an important role in behavioral timing, so we recorded single-unit activity from SEF to characterize how target and timing cues are encoded in this region. Two monkeys performed a variant of the memory-guided saccade task, in which a timing stimulus was presented at a randomly chosen eccentric location. Many spatially tuned SEF neurons encoded only the location of the target and not the timing stimulus, whereas several other SEF neurons encoded the location of the timing stimulus and not the target. The SEF population therefore encoded the location of each stimulus with largely distinct neuronal subpopulations. For comparison, we recorded a small population of lateral intraparietal (LIP) neurons in the same task. We found that most LIP neurons that encoded the location of the target also encoded the location of the timing stimulus after its presentation, but selectively encoded the intended eye movement plan in advance of saccade initiation. These results suggest that SEF, by conditionally encoding the location of instructional stimuli depending on their meaning, can help identify which movement plan represented in other oculomotor structures, such as LIP, should be selected for the next eye movement

An athletic approach to studying perception-action integration: Does sport-specific training, and the impact of injury, influence how individuals visually guide navigation?

The objective of this thesis was to investigate perception-action integration capabilities of individuals during a choice navigation task. This task assessed navigation strategies in open space while individuals avoided colliding with two vertical obstacles that created a body-scaled, horizontal gap, at three varying obstacle distances from the starting location (3m, 5m, 7m). The two studies completed in this thesis employed the same paradigm to assess the hypothesized group differences. Gaze behaviours and kinematics of navigation strategies were compared between: 1) athletes specifically trained in navigating in open space versus non-athletes; and 2) athletes with post-concussion syndrome (PCS) versus non-concussed, specifically trained athletes. Specifically trained athletes have been identified as demonstrating more successful perception-action integration in discrete motor tasks related to their sport (Mann et al., 2007; Vickers, 2007). However, whether these abilities translate to the continuous motor task of obstacle avoidance in open space was unknown. The purpose of Study 1 was to identify the influence of sport-specific training on navigating in open space (i.e. navigational strategies of large field sport athletes) compared to age-matched, non-athletes. It was hypothesized that specifically-trained athletes would demonstrate fewer, longer fixations, suggesting a more successful perception-action integration strategy (as defined by Mann et al., 2007), and would employ more sport-specific navigation strategies than non-athletes by maintaining their straight trajectory toward the goal (Fajen & Warren, 2003). Athletes were found to make fewer, longer fixations than non-athletes. However, no differences were observed between navigation strategies of the two groups, nor were any kinematic measures found to differ between groups. It can be concluded that athletes and non-athletes differentially obtain visual information to perform the same actions, suggesting that athletes and non-athletes differentially perform perception-action integration when navigating in open space. Future studies are required to identify sport-specific nuances of navigation (moving obstacles, running) to better identify athletic-related navigation strategies. Although athletic training can enhance perception-action integration strategies, sport-related injuries can hinder this process. Following a concussion, individuals experience deficits of perception-action integration that persist well beyond 30 days of recovery, post-concussion (Baker and Cinelli, 2014; Slobounov et al., 2006). These perception-action integration deficits may also exist in individual with postconcussion syndrome (PCS). The purpose of the Study 2 was to identify whether perception-action integration deficits persist with the persistent physical symptoms of concussion characteristic of PCS. The current study revealed that athletes with PCS did not differ from non-concussed athletes on any measure of visual fixation strategy, nor were they found to differ on any kinematic measure assessed. These findings suggest that in the context of the current paradigm, athletes with PCS have no perception-action integration deficit. In that, athletes with PCS may have adapted perception-action integration strategies to navigate with equal efficiency as a specifically-trained group of athletes or that the paradigm was not sensitive enough to identify these differences. Such findings suggest that more research is required to assess what, if any, perception-action integration deficits persist with persisting physical symptoms of PCS to better benefit rehabilitative procedures and outcomes for these individuals. Together, these studies add to what was previously known about perception-action integration, as it relates to navigation. Both studies assessed perception-action integration in unique populations that add to understanding of behavioural dynamics in the sport setting. Study 1 builds on a line of research assessing affordance theory and behavioural dynamics in sport (Fajen, Riley, & Turvey, 2008). The findings of this study suggest that although navigation strategies did not differ between specifically trained athletes and non-athletes, visual search strategies employed in task did. Such findings add to the understanding that sport-specific training influences perception-action integration, through our understanding of how athletes obtain visual information to perform actions. This thesis did not identify perception-action integration deficits in athletes with PCS. These findings suggest that the individuals in the present study likely adapted to their injury as they demonstrated equal ability in gaze and navigation strategies to specifically-trained athletes. As such, further research is required to assess the cognitive, motor, and sensory-motor deficits that may persist with the persisting physical symptoms of PCS. As individuals with PCS do not demonstrate similar visuomotor integration deficits as individuals with acute concussions (Baker & Cinelli, 2014), such individuals must be assessed and researched as a separate population

Universality of the gauge-ball spectrum of the four-dimensional pure U(1) gauge theory

We continue numerical studies of the spectrum of the pure U(1) lattice gauge theory in the confinement phase, initiated in our previous work. Using the extended Wilson action $S = -\sum_P [\beta \cos(\Theta_P) + \gamma \cos(2\Theta_P)]$ we address the question of universality of the phase transition line in the ($\beta,\gamma$) plane between the confinement and the Coulomb phases. Our present results at $\gamma= -0.5$ for the gauge-ball spectrum are fully consistent with the previous results obtained at $\gamma= -0.2$. Again, two different correlation length exponents, $\nu_{ng} = 0.35(3)$ and $\nu_{g} = 0.49(7)$, are obtained in different channels. We also confirm the stability of the values of these exponents with respect to the variation of the distance from the critical point at which they are determined. These results further demonstrate universal critical behaviour of the model at least up to correlation lengths of 4 lattice spacings when the phase transition is approached in some interval at $\gamma\leq -0.2$.Comment: 16 page

The Synthesis and Surface Chemistry of Colloidal Quantum Dots

Colloidal semiconductor nanocrystals, also known as quantum dots, are an extraordinary class of material, combining many of the most attractive properties of semiconductors with the practicality of solution chemistry. As such, they lie at a unique interface between inorganic chemistry, organic chemistry, solid-state physics, and colloidal chemistry. The rapid advance in knowledge of quantum dots over the past 30 years has largely been driven by interest in their fundamental physical properties and their broad applicability to challenges in nanoscience. However, much less attention has been paid to the chemistry underlying these features. In this dissertation, we discuss the state of nanocrystal chemistry and new insights we have unlocked by taking a bottom-up, chemistry-based approach to nanocrystal synthesis. We will cover these in a case-by-case fashion in the context of four chapters. Chapter 1 covers our CdTe nanocrystal synthesis surface chemistry studies with an eye toward CdTe photovoltaic technology, in which the role of CdTe surfaces is poorly understood. CdTe nanocrystals are traditionally a difficult material to synthesize, particularly with well-defined surface chemistry. In order to enable quantitative surface studies, we looked upstream and re-evaluated CdTe synthesis from the ground up. We identified a CdTe precursor largely overlooked since 1990, cadmium bis(phenyltellurolate) (Cd(TePh)2), and harnessed its excellent reactivity toward a synthesis of CdTe nanocrystals solely bound by cadmium carboxylate (Cd(O2CR)2) ligands. We then use this well-defined material to show that Cd(O2CR)2 ligands bind less tightly to CdTe nanocrystals than CdSe nanocrystals. This finding holds promise for the development of photovoltaics from colloidal CdTe feedstocks. Chapter 2 covers a tunable library of substituted thiourea precursors to metal sulfide nanocrystals. Controlling the size of nanocrystals produced in a given reaction is paramount to their use in opto-electronic devices, but the most widely used technique to control size is prematurely arresting crystal growth. We introduce a library of thiourea precursors whose organic substituents tune the rate of precursor conversion, which dictates the number of nanocrystals formed and the final nanocrystal size following complete precursor conversion. We use PbS as a model system to 1) demonstrate the concept of kinetically controlled nanocrystal size, 2) quantify substituent trends, and 3) optimize multigram scale syntheses. We then expand the thiourea methodology to a broad range of materials and nanocrystal morphologies. This work represents a paradigm shift that will greatly accelerate the pace of progress in nanocrystal science as it transitions from academia to a multibillion-dollar industry. Chapter 3 covers an analogously tunable library of substituted selenourea precursors, but focuses on the synthesis of PbSe nanocrystals. PbSe nanocrystal synthesis is notoriously low-yielding and poorly tunable, but the remarkable properties of PbSe nanocrystals in photovoltaics and electrical transport have driven interest in the material for decades. We develop a library of N,N,N’-trisubstituted selenourea precursors and leverage their fine conversion rate tunability to synthesize PbSe nanocrystals of many sizes in quantitative yields. Interestingly, the nanocrystals produced in this reaction are demonstrably less polydisperse than literature samples, exhibiting absorption linewidths approaching the single-particle limit. We quantify this narrowness using a transient absorption spectroscopy technique called spectral hole burning. Chapter 4 covers our efforts to dig deeper into nanocrystal nucleation and growth and use that new knowledge to develop luminescent downconverters ready for on-chip integration into LED lighting. By studying early time points in PbS and PbSe nanocrystal synthesis, we estimate solute concentrations, nucleation thresholds, and nanocrystal growth rates. In particular, we find that metal selenides and sulfides have very different nucleation and growth behavior, as well as that PbS nucleation is a surprisingly slow process. The lessons learned from these fundamental experiments have enabled us to rapidly develop red-emitting CdS/CdSe/CdS “spherical quantum well” emitters whose photoluminescence quantum yields are 90 – 95%

Facts, trends and challenges in modern software development

The IT industry is not new to change and evolution, however, we are now in an era of two fundamental waves of IT changes. First, the post-PC era, where mobile devices and tablet-like devices are giving end-users the ability to consume information when they want it and where they want it. Second, the post-server era where companies no longer need to neither buy nor provision servers in their own data centres but instead rent the compute resources as needed. This twin change has direct consequences to how end-users consume software, how that software is produced, and how it is delivered.info:eu-repo/semantics/publishedVersio