Target Selection and Enhancement During Attentional Tracking

At any waking moment, we are bombarded with more sensory information than we can fully process. Attention is necessary to deal with the dynamic world we live in. One fundamental function of vision and attention is to keep track of moving objects, but what are the targets of attention during tracking? One of the first theories of attentional tracking predicted that targets would be selected at early processing stages. By employing the double-drift illusion, which dissociates physical and perceived positions of moving objects, we investigated which of these positions is selected for tracking. Contrary to earlier theories and in line with newer findings, targets were selected rather late in visual processing, at least after the construction of illusory percepts, for both covert (Chapter One) and overt tracking (Chapter Two). Furthermore, capacity and speed limits to attentional tracking are hemifield specific. Brain activity in many areas is known to covary with the number of tracked targets, but it was previously unknown whether this effect would also show hemifield bias. Only targets presented in the contralateral hemifield influenced activity in earlier visual areas, while both contralateral and ipsilateral targets affected activity in parietal and frontal areas associated with attention (Chapter Three). Due to the hemifield specific nature of the capacity limit, we conjecture that it should emerge where load dependent activity is strongly contralateral. Overall, the studies presented in this dissertation illuminate two different aspects of attentional tracking. While selection happens late in the visual hierarchy, capacity and speed limits appear to emerge early in visual processing

Three-dimensional mapping reveals scale-dependent dynamics in biogenic reef habitat structure

Habitat structure influences a broad range of ecological interactions and ecosystem functions across biomes. To understand and effectively manage dynamic ecosystems, we need detailed information about habitat properties and how they vary across spatial and temporal scales. Measuring and monitoring variation in three-dimensional (3D) habitat structure has traditionally been challenging, despite recognition of its importance to ecological processes. Modern 3D mapping technologies present opportunities to characterize spatial and temporal variation in habitat structure at a range of ecologically relevant scales. Biogenic reefs are structurally complex and dynamic habitats, in which structure has a pivotal influence on ecosystem biodiversity, function and resilience. For the first time, we characterized spatial and temporal dynamics in the 3D structure of intertidal Sabellaria alveolata biogenic reef across scales. We used drone-derived structure-from-motion photogrammetry and terrestrial laser scanning to characterize reef structural variation at mm-to-cm resolutions at a habitat scale (~35 000 m2) over 1 year, and at a plot scale (2500 m2) over 5 years (2014–2019, 6-month intervals). We found that most of the variation in reef emergence above the substrate, accretion rate and erosion rate was explained by a combination of systematic trends with shore height and positive spatial autocorrelation up to the scale of colonies (1.5 m) or small patches (up to 4 m). We identified previously undocumented temporal patterns in intertidal S. alveolata reef accretion and erosion, specifically groups of rapidly accreting, short-lived colonies and slow-accreting, long-lived colonies. We showed that these highly dynamic colony-scale structural changes compensate for each other, resulting in seemingly stable reef habitat structure over larger spatial and temporal scales. These patterns could only be detected with the use of modern 3D mapping technologies, demonstrating their potential to enhance our understanding of ecosystem dynamics across scales

Insulin secretion profiles are modified by overexpression of glutamate dehydrogenase in pancreatic islets

Aims/hypothesis: Glutamate dehydrogenase (GDH) is a mitochondrial enzyme playing a key role in the control of insulin secretion. However, it is not known whether GDH expression levels in beta cells are rate-limiting for the secretory response to glucose. GDH also controls glutamine and glutamate oxidative metabolism, which is only weak in islets if GDH is not allosterically activated by L-leucine or (+/−)-2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH). Methods: We constructed an adenovirus encoding for GDH to overexpress the enzyme in the beta-cell line INS-1E, as well as in isolated rat and mouse pancreatic islets. The secretory responses to glucose and glutamine were studied in static and perifusion experiments. Amino acid concentrations and metabolic parameters were measured in parallel. Results: GDH overexpression in rat islets did not change insulin release at basal or intermediate glucose (2.8 and 8.3mmol/l respectively), but potentiated the secretory response at high glucose concentrations (16.7mmol/l) compared to controls (+35%). Control islets exposed to 5mmol/l glutamine at basal glucose did not increase insulin release, unless BCH was added with a resulting 2.5-fold response. In islets overexpressing GDH glutamine alone stimulated insulin secretion (2.7-fold), which was potentiated 2.2-fold by adding BCH. The secretory responses evoked by glutamine under these conditions correlated with enhanced cellular metabolism. Conclusions/interpretation: GDH could be rate-limiting in glucose-induced insulin secretion, as GDH overexpression enhanced secretory responses. Moreover, GDH overexpression made islets responsive to glutamine, indicating that under physiological conditions this enzyme acts as a gatekeeper to prevent amino acids from being inappropriate efficient secretagogue

Spin density wave induced disordering of the vortex lattice in superconducting La2−x_{2-x}Srx_xCuO4_4

We use small angle neutron scattering to study the superconducting vortex lattice in La$_{2-x}$Sr$_x$CuO$_4$ as a function of doping and magnetic field. We show that near optimally doping the vortex lattice coordination and the superconducting coherence length $\xi$ are controlled by a van-Hove singularity crossing the Fermi level near the Brillouin zone boundary. The vortex lattice properties change dramatically as a spin-density-wave instability is approached upon underdoping. The Bragg glass paradigm provides a good description of this regime and suggests that SDW order acts as a novel source of disorder on the vortex lattice.Comment: Accepted in Phys. Rev.

Functional significance of repressor element 1 silencing transcription factor (REST) target genes in pancreatic beta cells

Aims/hypothesis: The expression of several neuronal genes in pancreatic beta cells is due to the absence of the transcription factor repressor element 1 (RE-1) silencing transcription factor (REST). The identification of these traits and their functional significance in beta cells has only been partly elucidated. Herein, we investigated the biological consequences of a repression of REST target genes by expressing REST in beta cells. Methods: The effect of REST expression on glucose homeostasis, insulin content and release, and beta cell mass was analysed in transgenic mice selectively expressing REST in beta cells. Relevant target genes were identified in INS-1E and primary beta cells expressing REST. Results: Transgenic mice featuring a beta cell-targeted expression of REST exhibited glucose intolerance and reduced beta cell mass. In primary beta cells, REST repressed several proteins of the exocytotic machinery, including synaptosomal-associated protein (SNAP) 25, synaptotagmin (SYT) IV, SYT VII, SYT IX and complexin II; it impaired first and second phases of insulin secretion. Using RNA interference in INS-1E cells, we showed that SYT IV and SYT VII were implicated in the control of insulin release. Conclusions/interpretation: The data document the critical role of REST target genes in pancreatic beta cells. Specifically, we provide evidence that the downregulation of these genes is detrimental for the exocytosis of large dense core vesicles, thus contributing to beta cell dysfunction and impaired glucose homeostasi

Global distribution and bioclimatic characterization of alpine biomes

Although there is a general consensus on the distribution and ecological features of terrestrial biomes, the allocation of alpine ecosystems in the global biogeographic system is still unclear. Here, we delineate a global map of alpine areas above the treeline by modelling regional treeline elevation at 30 m resolution, using global forest cover data and quantile regression. We then used global datasets to 1) assess the climatic characteristics of alpine ecosystems using principal component analysis, 2) define bioclimatic groups by an optimized cluster analysis and 3) evaluate patterns of primary productivity based on the normalized difference vegetation index. As defined here, alpine biomes cover 3.56 Mkm(2) or 2.64% of land outside Antarctica. Despite temperature differences across latitude, these ecosystems converge below a sharp threshold of 5.9 degrees C and towards the colder end of the global climatic space. Below that temperature threshold, alpine ecosystems are influenced by a latitudinal gradient of mean annual temperature and they are climatically differentiated by seasonality and continentality. This gradient delineates a climatic envelope of global alpine biomes around temperate, boreal and tundra biomes as defined in Whittaker's scheme. Although alpine biomes are similarly dominated by poorly vegetated areas, world ecoregions show strong differences in the productivity of their alpine belt irrespectively of major climate zones. These results suggest that vegetation structure and function of alpine ecosystems are driven by regional and local contingencies in addition to macroclimatic factors

Smooth pursuit operates over perceived not physical positions of the double-drift stimulus

The double-drift illusion produces a large deviation in perceived direction that strongly dissociates physical position from perceived position. Surprisingly, saccades do not seem to be affected by the illusion (Lisi & Cavanagh, 2015). When targeting a double-drift stimulus, the saccade system is driven by retinal rather than perceived position. Here, using paired double-drift targets, we test whether the smooth pursuit system is driven by perceived or physical position. Participants (n = 7) smoothly pursued the inferred midpoint (Steinbach, 1976) between two horizontally aligned Gabor patches that were separated by 20° and moving on parallel, oblique paths. On the first half of each trial, the Gabors’ internal textures were static while both drifted obliquely downward. On the second half of each trial, while the envelope moved obliquely upward, the internal texture drifted orthogonally to the envelope's motion, producing a large perceived deviation from the downward path even though the upward and downward trajectories always followed the same physical path but in opposite directions. We find that smooth pursuit eye movements accurately followed the nonillusory downward path of the midpoint between the two Gabors, but then followed the illusory rather than the physical trajectory on the upward return. Thus, virtual targets for smooth pursuit are derived from perceived rather than retinal coordinates

Where have all the beetles gone? Long‐term study reveals carabid species decline in a nature reserve in Northern Germany

1. The drastic insect decline has received increasing attention in scientific as well as in public media. Long-term studies of insect diversity trends are still rare, even though such studies are highly important to assess extent, drivers and potential consequences of insect loss in ecosystems. 2. To gain insights into carabid diversity trends of ancient and sustainably managed woodlands, we analysed data of carabid beetles from a trapping study that has been run for 24 years in an old nature reserve of Northern Germany, the Luneburg Heath. We examined temporal changes in several diversity measures € (e.g. biomass, species richness, functional diversity and phylogenetic diversity) and tested diverse species traits as predictor variables for species occurrence. 3. In contrast to recently published long-term studies of insect diversity, we did not observe a decline in biomass, but in species richness and phylogenetic diversity in carabids at our study site. Additionally, hibernation stage predicted the occurrence probability of carabids: Species hibernating as imagines or both imagines and larvae and breeding in spring showed strongest declines. 4. We assume the detected trends to be the result of external effects such as climate change and the application of pesticides in the surrounding. Our results suggest that the drivers for the insect decline and the responses are multifaceted. This highlights the importance of long-term studies with identification of the catches to, at best, species level to support the understanding of mechanisms driving changes in insect diversity and abundance