Jatropha Assessment. Agronomy, socio-economic issues, and ecology. Facts from literature.

Jatropha (Jatropha curcas L.) has been promoted as a potential renewable energy source for many of its advantageous properties in comparison to other biomass feedstock. This report summarises the agronomy, socio-economic issues, and ecology facts from literature on Jatropha. Such an overview is essential to formulate recommendations and policy guidelines to stimulate best project practices and also help to avoid the promotion of unviable or unsustainable practices

Radioactive Needlework, Reconstruction of needle-positions in radiation treatment

Nucletron presented a medical problem to the SWI 2006: how to find needles used for cancer treatment in a prostate? More concretely: how to find the positions of these needles from distorted images from an ultrasound probe? Section 1 explains the background of this problem. In Section 2 we deal with physical explanations for the distortions. In Section 3 we give a brief overview of medical imaging and explain which techniques we used to clean up the images

How people achieve their amazing temporal precision in interception

People can hit rapidly moving balls with amazing precision. To determine how they manage to do so, we explored how various factors that we could manipulate influenced people's precision when intercepting virtual targets. We found that temporal precision was highest for fast targets that subjects were free to intercept wherever they wished. Temporal precision was much poorer when the point of interception was specified in advance. Examining responses to abrupt perturbations of the target's motion revealed that people adjusted where rather than when they would hit the target if given the choice. A model that combines judging how long it will take to reach the target's path with estimating the target's position at that time from its visually perceived position and velocity could account for the observed precision with reasonable values for all the parameters. The model considers all relevant sources of errors, together with the delays with which the various aspects can be adjusted. Our analysis provides a biologically plausible explanation for how light falling on the eye can guide the hand to intercept a moving ball with such high precision

Quickly 'learning' to move optimally.

People take account of the variability in their movements in a near-optimal manner in various visuo-motor tasks. Is knowledge of one’s variability needed for such near-optimal performance, or could it arise from responding to one’s success in previous attempts in some simple manner? We asked subjects to move a pen back and forth across a tablet to make a cursor move as quickly as possible between two targets. The cursor had to stop within the targets. Task difficulty was varied between blocks. Part of the variation in difficulty was explicit (three target sizes) whereas the rest had to be discovered during the movements (two mappings between the movements of pen and cursor). In all cases, subjects sped up after stopping within a target and slowed down after failing to do so. We interpret this as evidence that explicit knowledge of one’s variability is not necessary for performing close to optimally

Judging an unfamiliar object’s distance from its retinal image size

How do we know how far an object is? If an object's size is known, its retinal image size can be used to judge its distance. To some extent, the retinal image size of an unfamiliar object can also be used to judge its distance, because some object sizes are more likely than others. To examine whether assumptions about object size are used to judge distance, we had subjects indicate the distance of virtual cubes in complete darkness. In separate sessions, the simulated cube size either varied slightly or considerably across presentations. Most subjects indicated a further distance when the simulated cube was smaller, showing that they used retinal image size to judge distance. The cube size that was considered to be most likely depended on the simulated cubes on previous trials. Moreover, subjects relied twice as strongly on retinal image size when the range of simulated cube sizes was small. We conclude that the variability in the perceived cube sizes on previous trials influences the range of sizes that are considered to be likely. © ARVO

Proprioception is Robust under External Forces

Information from cutaneous, muscle and joint receptors is combined with efferent information to create a reliable percept of the configuration of our body (proprioception). We exposed the hand to several horizontal force fields to examine whether external forces influence this percept. In an end-point task subjects reached visually presented positions with their unseen hand. In a vector reproduction task, subjects had to judge a distance and direction visually and reproduce the corresponding vector by moving the unseen hand. We found systematic individual errors in the reproduction of the end-points and vectors, but these errors did not vary systematically with the force fields. This suggests that human proprioception accounts for external forces applied to the hand when sensing the position of the hand in the horizontal plane

The effect of variability in other objects’ sizes on the extent to which people rely on retinal image size as a cue for judging distance

Retinal image size can be used to judge objects’ distances because for any object one can assume that some sizes are more likely than others. It has been shown that an increased variability in the size of otherwise identical target objects over trials reduces the weight given to retinal image size as a distance cue. Here, we examined whether an increased variability in the size of objects of a different color, orientation, or shape reduces the weight given to retinal image size when judging distance. Subjects had to indicate the 3D position of a simulated target object. Retinal image size was given significantly less weight as a cue for judging the target cube’s distance when differently colored and differently oriented target objects appeared in many simulated sizes but not when differently shaped objects had many simulated sizes. We also examined whether increasing the variability in the size of cubes in the surroundings reduces the weight given to retinal image size when judging distance. It does not. We conclude that variability in surrounding or dissimilar objects’ sizes has a negligible influence on the extent to which people rely on retinal image size as a cue for judging distance