Bilkent University
  • Ankara, Turkey
Recent publications
Spatial cues presented prior to the presentation of a static stimulus usually improve its perception. However, previous research has also shown that transient exogenous cues to direct spatial attention to the location of a forthcoming stimulus can lead to reduced performance. In the present study, we investigated the effects of transient exogenous cues on the perception of briefly presented drifting Gabor patches. The spatial and temporal frequencies of the drifting Gabors were chosen to mainly engage the magnocellular pathway. We found better performance in the motion direction discrimination task when neutral cues were presented before the drifting target compared to a valid spatial cue. The behavioral results support the hypothesis that transient attention prolongs the internal response to the attended stimulus, thus reducing the temporal segregation of visual events. These results were complemented by applying a recently developed model for perceptual decisions to rule out a speed-accuracy trade-off and to further assess cueing effects on visual performance. In a model-based assessment, we found that valid cues initially enhanced processing but overall resulted in less efficient processing compared to neutral cues, possibly caused by reduced temporal segregation of visual events.
In this study, an enhanced heat pipe performance for grooved heat pipes has been demonstrated through capillary boosting with the introduction of the bifurcation of grooves. Wider grooves regularly branch to narrower grooves such that the total cross-sectional liquid flow area remains approximately the same. Following the computational framework drawn by a recently developed heat pipe analysis toolbox (H-PAT), we develop a numerical model for the heat pipes with tree-like groove architecture. Then we utilize the model to design a flat-grooved heat pipe with one step groove bifurcation at the evaporator. To verify our numerical findings, two heat pipes with and without groove bifurcation are manufactured and experimented under the same conditions. Experimental results show that the numerical model can predict the thermal performance quite accurately. The results reveal that groove bifurcation can be a viable option for a better thermal performance than that of heat pipes with standard grooved heat pipes with straight grooves which leads to at least 25% higher maximum heat transport capacity. The effect of number of branching on the temperature flattening across the heat pipe is also demonstrated for different evaporator lengths.
Past research on how listeners weight stress cues such as pitch, duration and intensity has reported two inconsistent patternss: listeners’ weighting conforms to 1) their native language experience (e.g., language rhythmicity, lexical tone), and 2) a general “iambic-trochaic law” (ITL), favouring innate sound groupings in cue perception. This study aims to tease apart the above effects by investigating the weighting of pitch, duration and intensity cues in stress-timed (Australian English) and non-stress-timed and tonal (Taiwan Mandarin) language speaking adults using a mismatch negativity (MMN) multi-feature paradigm. Results show effects that can be explained by language-specific rhythmic influence, but only partially by the ITL. Moreover, these findings revealed cross-linguistic differences indexed by both MMN and late discriminative negativity (LDN) responses at cue and syllable position levels, and thus call for more sophisticated perspectives for existing cue-weighting models.
The Gale–Nikaido–Debreu lemma plays an important role in establishing the existence of competitive equilibrium. In this paper, we use Sperner’s lemma and basic elements of topology to prove the Gale–Nikaido–Debreu lemma.
This paper presents an alternative social/economic explanation for the Neanderthals’ demise along similar lines with the competitive exclusion principle. We represent individuals as network nodes such that each link corresponds to an acquaintanceship between the respective nodes. We rely on the findings that Sapiens were more socially developed compared to Neanderthals. Running individual-based simulations under the assumption that Sapiens initially lived in larger and more connected groups, we illustrate how the two species’ social structures might have further evolved in opposite directions and how such an evolution might have favored Sapiens in their competition against Neanderthals.
Photocatalytic conversion of CO2 into chemical fuels is a promising approach to tackle carbon emission and global warming. Herein, we promote a cobalt dicyanamide coordination compound, Co-dca, for the first time, as a selective catalyst to reduce CO2 to CO in the presence of a ruthenium photosensitizer (Ru PS) under visible light irradiation. Co-dca was prepared by a facile precipitation method and characterized by Infrared, UV-Vis, XRD, SEM, TEM, and XPS studies. A series of photocatalytic experiments under various reaction conditions were performed to reveal the role of the PS, the scavenger, and the solvent in the selectivity and the activity of the photocatalytic process. We find that Co-dca exhibits an activity of 254 μmol h-1 g-1 and a CO selectivity as high as 93%.
Magnetic particle imaging (MPI) is a rapidly developing medical imaging modality that exploits the non-linear response of magnetic nanoparticles (MNPs). Color MPI widens the functionality of MPI, empowering it with the capability to distinguish different MNPs and/or MNP environments. The system function approach for color MPI relies on extensive calibrations that capture the differences in the harmonic responses of the MNPs. An alternative calibration-free x-space-based method called TAURUS estimates a map of the relaxation time constant, τ, by recovering the underlying mirror symmetry in the MPI signal. However, TAURUS requires a back and forth scanning of a given region, restricting its usage to slow trajectories with constant or piecewise constant focus fields (FFs). In this work, we propose a novel technique to increase the performance of TAURUS and enable τ map estimation for rapid and multi-dimensional trajectories. The proposed technique is based on correcting the distortions on mirror symmetry induced by time-varying FFs. We demonstrate via simulations and experiments in our in-house MPI scanner that the proposed method successfully estimates high-fidelity τ maps for rapid trajectories that provide orders of magnitude reduction in scanning time (over 300 fold for simulations and over 8 fold for experiments) while preserving the calibration-free property of TAURUS.
Purpose: An array-based z-gradient coil with a set of programmable power amplifiers can outperform a conventional z-gradient coil and make it highly customizable with a broader range of tunable features. Methods: A dynamically adjustable imaging volume can be achieved using a pair of independent arrays and a modified optimization procedure based on analytic equations. Two modes of operation are provided: (a) standard mode that resembles a conventional coil; (b) advanced mode, where all performance parameters can be adjusted employing a controllable feeding mechanism. Commercial software is used to demonstrate the validity and feasibility of the proposed coil. Results: Primary and shield array diameters are 24 and 30 cm, both of which comprise 12 bundles of 10 turns copper wires. Maximum feeding voltage/current is 250 V/100 A for all array elements. Four distinct magnetic profiles are provided: (a) conventional profile with 140 mm diameter spherical region of interest, 120 mT/m gradient, and up to 4500 T/m/s slew rate; (b) profile of 200 mT/m, 70 mm region of interest, and up to 6900 T/m/s slew rate; (c) 60 mm axially shifted 70 mm region of interest with 120 mT/m strength and 3600 T/m/s slew rate; and (d) profile of 370 mT/m, 120 mm region of interest, and 3700 T/m/s slew rate when the active shield is reverse fed. Conclusion: By using an active-shielded gradient array coil, the magnetic field profile of the imaging volume can be adjusted dynamically, and it can provide new features and a wide range of field profiles for diverse applications in MRI.
Seniors strive to achieve aging well by engaging in entrepreneurial activities subsequent to ceasing their organizational employment. While this is a common practice in many societies, scant research exists on what motivates seniors to engage in entrepreneurial activities once they end their formal employment. We adopt the self-determination theory (SDT) to investigate the effects of goal contents and motives on the well-being among seniors who launch their entrepreneurship journeys. Based on in-depth interviews with senior entrepreneurs in China, India, and Turkey, we contribute to extant knowledge by linking separate paradigms. These are as follows: goal contents and intrinsic motivation-driven entrepreneurship, management of inner and outer challenges, and achievement of the eventual outcome of aging well. We also investigate the culture-specific drivers of senior entrepreneurship in a comparative framework.
Purpose To investigate the effect of botulinum neurotoxin-A (BTX-A) treatment on dry eye symptoms, tear meniscus, corneal topography and corneal aberrometry in patients with benign essential blepharospasm (BEB) and hemifacial spasm (HFS). Materials and methods This prospective study comprised of 6 patients with BEB and 20 patients with HFS. Tear meniscus height (TMH) and depth (TMD), tear break-up time (TBUT), corneal fluorescein staining score (CFSS), Schirmer I test, ocular surface disease index (OSDI) score, corneal topography [corneal power of flat axis (K1), corneal power of steep axis (K2), mean corneal power (Km), astigmatism and thinnest pachymetry] and anterior corneal aberrometry [spherical aberration (SA), vertical coma (vcoma), horizontal coma (hcoma), higher order root mean square (hRMS) and total RMS] were evaluated before BTX-A treatment, 3 weeks after BTX-A treatment and 2 months after BTX-A treatment. Results Six patients with BEB and 20 patients with HFS treated with BTX-A were evaluated in this study. Twenty contralateral spasm free eyes of 20 HFS patients were taken as control group. TMH and TMD were found to be significantly higher in eyes with spasm at both 3 weeks and 2 months after injection (TMH: 279.0 ± 123.2 at pretreatment, 380.5 ± 174.7 at third week and 317.0 ± 125.5 at second month p < 0.001 and p = 0.02, respectively), (TMD: 183.7 ± 59.7 at pretreatment, 235.7 ± 91.1 at third week and 209.8 ± 77.1 at second month p < 0.01 and p = 0.015, respectively). TBUT, CFSS, Schirmer I test values were similar (p > 0.05). OSDI scores decreased significantly from 29.6 ± 25.3 to 19.8 ± 20. p = 0.03 at third week and increased again by second month. K2 (43.9 ± 1.7 vs. 43.7 ± 1.6, p = 0.03) and astigmatism (0.8 ± 0.5 vs. 0.6 ± 0.4, p = 0.04) values were significantly lower at third week and increased again by second month. Pachymetry and aberrometric values did not change significantly. In the control group only Schirmer I test value decreased significantly at second month (10.5 ± 6.5 vs. 7.2 ± 5.6, p = 0.008), other parameters did not change. Conclusion BTX-A injection increases tear meniscus and decrease symptoms related to dry eye disease in BEB and HFS patients. It decrease astigmatism and keratometry values, it does not cause a significant change in corneal aberrations. However the positive effects of BTX-A injection on ocular surface is temporary.
Optoelectronic biointerfaces offer a wireless and nongenetic neurostimulation pathway with high spatiotemporal resolution. Fabrication of low‐cost and flexible optoelectronic biointerfaces that have high photogenerated charge injection densities and clinically usable cell stimulation mechanism is critical for rendering this technology useful for ubiquitous biomedical applications. Here, supercapacitor technology is combined with flexible organic optoelectronics by integrating RuO2 into a donor–acceptor photovoltaic device architecture that facilitates efficient and safe photostimulation of neurons. Remarkably, high interfacial capacitance of RuO2 resulting from reversible redox reactions leads to more than an order‐of‐magnitude increase in the safe stimulation mechanism of capacitive charge transfer. The RuO2‐enhanced photoelectrical response activates voltage‐gated sodium channels of hippocampal neurons and elicits repetitive, low‐light intensity, and high‐success rate firing of action potentials. Double‐layer capacitance together with RuO2‐induced reversible faradaic reactions provide a safe stimulation pathway, which is verified via intracellular oxidative stress measurements. All‐solution‐processed RuO2‐based biointerfaces are flexible, biocompatible, and robust under harsh aging conditions, showing great promise for building safe and highly light‐sensitive next‐generation neural interfaces.
We propose an algorithm to generate inner and outer polyhedral approximations to the upper image of a bounded convex vector optimization problem. It is an outer approximation algorithm and is based on solving norm-minimizing scalarizations. Unlike Pascoletti–Serafini scalarization used in the literature for similar purposes, it does not involve a direction parameter. Therefore, the algorithm is free of direction-biasedness. We also propose a modification of the algorithm by introducing a suitable compact subset of the upper image, which helps in proving for the first time the finiteness of an algorithm for convex vector optimization. The computational performance of the algorithms is illustrated using some of the benchmark test problems, which shows promising results in comparison to a similar algorithm that is based on Pascoletti–Serafini scalarization.
Airlines tend to design their flights schedules with the primary concern of the minimization of operational costs. However, the recently emerging idea of resilient scheduling defined as staying operational in case of unexpected disruptions and adaptability should be of great importance for airlines as well due to the high opportunity costs caused by the flight cancellations and passenger inconvenience caused by delays in the schedule. In this study, we integrate resilient airline schedule design, aircraft routing and fleet assignment problems with uncertain non-cruise times and controllable cruise times. We follow a data-driven method to estimate flight delay probabilities to calculate the airport congestion coefficients required for the probability distributions of non-cruise time random variables. We formulate the problem as a bi-criteria nonlinear mixed integer mathematical model with chance constraints. The nonlinearity caused by the fuel consumption and CO2 emission function associated with the controllable cruise times in our first objective is handled by second order conic inequalities. We minimize the total absolute deviation of the aircraft path variability’s from the average in our second objective to generate balanced schedules in terms of resilience. We compare the recovery performances of our proposed schedules to the minimum cost schedules by a scenario-based posterior analysis.
Recessive or dominant inborn errors of type I interferon (IFN) immunity can underlie critical COVID-19 pneumonia in unvaccinated adults. The risk of COVID-19 pneumonia in unvaccinated children, which is much lower than in unvaccinated adults, remains unexplained. In an international cohort of 112 children (<16 yr old) hospitalized for COVID-19 pneumonia, we report 12 children (10.7%) aged 1.5-13 yr with critical (7 children), severe (3), and moderate (2) pneumonia and 4 of the 15 known clinically recessive and biochemically complete inborn errors of type I IFN immunity: X-linked recessive TLR7 deficiency (7 children) and autosomal recessive IFNAR1 (1), STAT2 (1), or TYK2 (3) deficiencies. Fibroblasts deficient for IFNAR1, STAT2, or TYK2 are highly vulnerable to SARS-CoV-2. These 15 deficiencies were not found in 1,224 children and adults with benign SARS-CoV-2 infection without pneumonia (P = 1.2 × 10-11) and with overlapping age, sex, consanguinity, and ethnicity characteristics. Recessive complete deficiencies of type I IFN immunity may underlie ∼10% of hospitalizations for COVID-19 pneumonia in children.
Photocatalytic wastewater treatment is expected to become a sustainable way of eliminating toxic chemicals. Due to the surface-driven mechanism of the photocatalysis, surface area of the catalyst material plays a crucial role in the efficiency of the process, which is usually achieved by nanoparticles. However, using powder materials introduces a new problem: removing the catalyst materials out of clean water. As an alternative, atomic layer deposition (ALD) can form conformal thin films on high surface area substrates providing an immobilization route with high photocatalytic activity. Textile materials are inexpensive and accessible therefore good candidates for the substrate materials. Here, we deposit thin films on TiO2 on fiberglass fabrics and investigate the photocatalytic activity. Since the as-deposited ALD TiO2 films are amorphous, they have very limited photocatalytic activity. Upon thermal treatment of the films after deposition, photocatalytic activity is achieved. After four hours of exposure to the solar simulator and UV lamp, TiO2-coated fibers demonstrated much higher photocatalytic activity than films on planar substrates previously described in the literature. The photocatalytic activity and structure of the coated fibers were investigated using XRD, XPS, UV–Vis, and PL analyses.
Molybdenum disulfides and molybdenum trioxides are structures that possess the potential to work as efficient charge transport layers in optoelectronic devices. In the present study, as opposed to the existing Mo-based nanostructures in flake, sheet, or spherical forms, an extremely simple and low-cost hydrothermal method is used to prepare nanowires (NWs) of MoS2@MoO3 (MSO) composites. The synthesis method includes several advantages including easy handling and processing of inexpensive precursors to reach stable MSO NWs without the need for an oxygen-free medium, which would facilitate the possibility of mass production of these nanostructures. The structural analysis confirmed the formation of MSO nanocomposites with different Mo valence states, as well as NWs of average length and diameter of 70 nm and 5 nm, respectively. In order to demonstrate their potential for optoelectronic applications, MSO NWs were blended into hole injection layers (HILs) in quantum dot-based light-emitting diodes (QLEDs). Electroluminescence measurements show a substantial enhancement in both luminance (from 44,330 to 68,630 cd.m–2) and external quantum efficiency (from 1.6 to 2.3%), based on the increase in the ratio of MSO NWs from 3 to 10%. Interestingly, the addition of 10% volume of MSO NWs resulted in a remarkably smoother HIL with improved current efficiency and stability in green-emitting QLEDs. The simplicity and cost-effective features of the synthesis method along with outstanding favorable morphology demonstrated their ability to enhance the QLED performance and mark them as promising agents for optoelectronics.
We study high-resolution crime prediction and introduce a new generative model applicable to any spatiotemporal data with graph convolutional gated recurrent units (Graph-ConvGRU) and multivariate Gaussian distributions. We introduce a subdivision algorithm and create a graph representation to tackle the sparsity and complexity problem in high-resolution spatiotemporal data. By leveraging the flexible structure of graph representation, we model the spatial, temporal, and categorical relations of crime events and produce state vectors for each region. We create a multivariate probability distribution from the state vectors and train the distributions by minimizing the KL divergence between the generated and the actual distribution of the crime events. After creating the distributions, crime can be predicted in any resolution as the first time in the literature. In our experiments on real-life and synthetic datasets, our model obtains the best score with respect to the state-of-the-art models with statistically significant improvements. Hence, our model is not only generative but also precise. We also provide the source code of our algorithm for reproducibility.
Lithium metal is the ultimate anode choice for high energy rechargeable lithium batteries owing to its ultra-high theoretical capacity, however, Li dendrites and low Coulombic efficiency (CE) caused by disordered Li plating restrict its practical application. Herein, we develop an ultrathin Sn-decorated Cu substrate ([email protected]) fabricated by an electroless plating method to induce ordered Li nucleation and growth behavior. The lithiophilic Sn interfacial layer is found to play a critical role to lower the Li nucleation over-potential and promote fast Li-migration kinetics, and the underlying mechanism is revealed using the first principle calculations. Accordingly, a dense dendrite-free and Li deposition with large granular morphology is obtained, which significantly improved the CE and cycling performance of Li||[email protected] half cells symmetric cells. Symmetric cells using the [email protected] electrode display a much-prolonged life span (>1200 h) with low overpotential (∼18 mV) at a high current density of 1 mA cm⁻². Moreover, full cells paired with commercial LiFePO4 cathode (1.8 mAh cm⁻²) deliver enhanced cycling stability (0.5 C, 300 cycles) and excellent rate performance. This work provides a simple and effective way to bring about high efficiency and long lifespan substrates for practical applications.
Real Time Mission Critical Communication (RTMCC) in emergency situations can include real‐time video and audio calls between peers and first responders all occurring simultaneously. RTMCC also requires secure end‐to‐end (E2E) group communication (GC) sessions against potential security threats during such incidents. In this paper, we explore all aspects of the possible methods that are suitable for a software implementation of for session key change during GC in E2E encryption of RTMCC. Later, we introduce our Entropy Service concept, which can be very effective in secure E2E RTMCC sessions. The proposed method ensures E2E security in real‐time communication systems while allowing very fast session key change for clients involved in an RTMCC session with a computational complexity of 𝒪(1). Our experimental results show that the proposed Entropy Service can reduce total time by 99.6% and 99.2%, the idle time by 99.4% and 98.99%, and the number of messages by 51.4% and 35.33% compared to the key refreshing and hash methods, respectively, when the number of users in the system increases to 45. These results show that both communication and computation complexity are significantly reduced with the proposed RTMCC session key change.
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5,978 members
Hodjat Hajian
  • Nanotechnology Research Center (NANOTAM)
Hamit Kalaycioglu
  • Department of Physics
Alipaşa Ayas
  • Graduate School of Education
Aydan Yeltik
  • Department of Physics
Ihsan Gursel
  • Mol Biol Genet
Bilkent, TR-06800, Ankara, Turkey
Head of institution
Abdullah Atalar