Practical considerations for high spatial and temporal resolution dynamic transmission electron microscopy

Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5 x 10(7) electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10(-6)s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed.     

The changing face of bullying: An empirical comparison between traditional and internet bullying and victimization

Electronic aggression, or cyberbullying, is a relatively new phenomenon. As such, consistency in how the construct is defined and operationalized has not yet been achieved, inhibiting a thorough understanding of the construct and how it relates to developmental outcomes. In a series of two studies, exploratory and confirmatory factor analyses (EFAs and CFAs respectively) were used to examine whether electronic aggression can be measured using items similar to that used for measuring traditional bullying, and whether adolescents respond to questions about electronic aggression in the same way they do for traditional bullying. For Study I (n = 17 551; 49% female), adolescents in grades 8-12 were asked to what extent they had experience with physical, verbal, social, and cyberbullying as a bully and victim. EFA and CFA results revealed that adolescents distinguished between the roles they play (bully, victim) in a bullying situation but not forms of bullying (physical, verbal, social, cyber). To examine this further, Study II (n = 733; 62% female), asked adolescents between the ages of 11 and 18 to respond to questions about their experience sending (bully), receiving (victim), and/or seeing (witness) specific online aggressive acts. EFA and CFA results revealed that adolescents did not differentiate between bullies, victims, and witnesses; rather, they made distinctions among the methods used for the aggressive act (i.e. sending mean messages or posting embarrassing pictures). In general, it appears that adolescents differentiated themselves as individuals who participated in specific mode of online aggression, rather than as individuals who played a particular role in online aggression. This distinction is discussed in terms of policy and educational implications.     

Are Cyberbullies really bullies? An investigation of reactive and proactive online aggression

Cyberbullying, or online aggression, is an issue of increasing concern, however little research has been conducted on the motivations underlying this form of aggression. Using a mixed-method approach, by means of survey and interview data, we explored whether online aggressive acts were motivated by proactive (intentionally aggressing to obtain a resource or a goal), and/or reactive (aggression that occurs in reaction to provocation) reasons. Participants for the survey portion of the study included 733 adolescents between the ages of 10 and 18, while a subset of 15 adolescents participated in semi-structured interviews. Factor analysis revealed that, in contrast to traditional forms of bullying, adolescents do not identify themselves according to the role they played in an internet aggressive situation (i.e. bully, victim, witness), but according to the method of aggression they used (i.e. sending mean messages, posting embarrassing photos, and developing hostile websites). More interestingly, regression analyses demonstrated that motivations for aggressing online also varied according to method of aggression rather than role. For example, adolescents who chose to aggress by posting mean messages or posting embarrassing photos were more likely to do so for reactive reasons, while adolescents who spent time creating hostile websites did so for proactive reasons.     

A sparse Gaussian process regression model for tourism demand forecasting in Hong Kong

In recent years, Gaussian process (GP) models have been popularly studied to solve hard machine learning problems. The models are important due to their flexible non-parametric modeling abilities using Mercer kernels and the Bayesian framework for probabilistic inference. In this paper, we propose a sparse GP regression (GPR) model for tourism demand forecasting in Hong Kong. The sparsification procedure of the GPR model not only decreases the computational complexity but also improves the generalization ability. We experiment the proposed model with monthly demand data that are relevant to Hong Kong’s tourism industry, and compare the performance of the sparse GPR model with those of various kernel-based models to show its effectiveness. The proposed sparse GPR model shows that its forecasting capability outperforms those of the ARMA model and the two state-of-the-art SVM models.     

A quantitative analysis of microtubule elongation

Methods have been developed for differentially inhibiting microtubule nucleation and elongation in vitro. By use of polyanions, assembly-competent tubulin solutions of several milligrams/milliliter can be prepared which do not exhibit appreciable spontaneous assembly during the time-course of an experiment. Microtubule elongation can be initiated by the addition of known numbers of microtubule fragments. A detailed analysis of the resulting process demonstrates that: (a) rings are not obligatory intermediates in the nucleation sequence, and neither rings nor protofilament sheets are obligatory intermediates in the elongation reaction. (b) The end of an elongating microtubule often has a short region of open protofilament sheet or "C-microtubule" similar to that observed in vivo. (c) The development of turbidity follows a simple exponential approach to an equilibrium value. (d) The final equilibrium values are independent of the number of added nucleating fragments, while the initial growth rates and half-times to reach equilibrium are dependent on the number of added nuclei. (e) The final lengths of the microtubules at equilibrium are inversely proportional to the number of added fragments. (f) The equilibrium constants are independent of microtubule length. (g) The number of assembly and disassembly sites per microtubule is not a function of microtubule length. (h) The forward rate constants, the final polymer concentrations, and growth rates of microtubules are dependent upon the concentration of polyanion present. These results are strongly supportive of the idea that microtubule assembly is a "condensation-polymerization" and provide basic information on the kinetics and length distributions of the elongation in vitro.     

Brain-computer interfaces for 1-D and 2-D cursor control: designs using volitional control of the EEG spectrum or steady-state visual evoked potentials.

We have developed and tested two electroencephalogram (EEG)-based brain-computer interfaces (BCI) for users to control a cursor on a computer display. Our system uses an adaptive algorithm, based on kernel partial least squares classification (KPLS), to associate patterns in multichannel EEG frequency spectra with cursor controls. Our first BCI, Target Practice, is a system for one-dimensional device control, in which participants use biofeedback to learn voluntary control of their EEG spectra. Target Practice uses a KPLS classifier to map power spectra of 62-electrode EEG signals to rightward or leftward position of a moving cursor on a computer display. Three subjects learned to control motion of a cursor on a video display in multiple blocks of 60 trials over periods of up to six weeks. The best subject's average skill in correct selection of the cursor direction grew from 58% to 88% after 13 training sessions. Target Practice also implements online control of two artifact sources: 1) removal of ocular artifact by linear subtraction of wavelet-smoothed vertical and horizontal electrooculograms (EOG) signals, 2) control of muscle artifact by inhibition of BCI training during periods of relatively high power in the 40-64 Hz band. The second BCI, Think Pointer, is a system for two-dimensional cursor control. Steady-state visual evoked potentials (SSVEP) are triggered by four flickering checkerboard stimuli located in narrow strips at each edge of the display. The user attends to one of the four beacons to initiate motion in the desired direction. The SSVEP signals are recorded from 12 electrodes located over the occipital region. A KPLS classifier is individually calibrated to map multichannel frequency bands of the SSVEP signals to right-left or up-down motion of a cursor on a computer display. The display stops moving when the user attends to a central fixation point. As for Target Practice, Think Pointer also implements wavelet-based online removal of ocular artifact; however, in Think Pointer muscle artifact is controlled via adaptive normalization of the SSVEP. Training of the classifier requires about 3 min. We have tested our system in real-time operation in three human subjects. Across subjects and sessions, control accuracy ranged from 80% to 100% correct with lags of 1-5 s for movement initiation and turning. We have also developed a realistic demonstration of our system for control of a moving map display (http://ti.arc.nasa.gov/).