Bimanual interference associated with the selection of target locations.

Four experiments were conducted to identify the locus of interference observed during the preparation of bimanual reaching movements. Target locations were specified by color, and the right-hand and left-hand targets could be either the same or a different color. Movements of different amplitudes (Experiment 1) or different directions (Experiment 2) to targets of the same color were initiated more quickly than symmetric movements to targets of different colors. These results indicate that costs observed during bimanual movements arise during target selection rather than during motor programming. Experiments 3 and 4 further examined the interference associated with target selection. Reaction time costs were found with unimanual movements when the target was presented among distractors associated with responses for the other hand. Interference observed during bimanual reaching appears to reflect difficulty in segregating the response rules assigned to each hand. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The right view from the wrong location: depth perception in stereoscopic multi-user virtual environments

Stereoscopic depth cues improve depth perception and increase immersion within virtual environments (VEs). However, improper display of these cues can distort perceived distances and directions. Consider a multi-user VE, where all users view identical stereoscopic images regardless of physical location. In this scenario, cues are typically customized for one "leader" equipped with a head-tracking device. This user stands at the center of projection (CoP) and all other users ("followers") view the scene from other locations and receive improper depth cues. This paper examines perceived depth distortion when viewing stereoscopic VEs from follower perspectives and the impact of these distortions on collaborative spatial judgments. Pairs of participants made collaborative depth judgments of virtual shapes viewed from the CoP or after displacement forward or backward. Forward and backward displacement caused perceived depth compression and expansion, respectively, with greater compression than expansion. Furthermore, distortion was less than predicted by a ray-intersection model of stereo geometry. Collaboration times were significantly longer when participants stood at different locations compared to the same location, and increased with greater perceived depth discrepancy between the two viewing locations. These findings advance our understanding of spatial distortions in multi-user VEs, and suggest a strategy for reducing distortion.     

Understanding the behavior of compiler optimizations

Compiler optimizations are difficult to implement and add complexity to a compiler. For this reason, compiler writers are selective about implementing them: they implement only the ones that they believe will be beneficial. To support compiler writers in this, we describe a method for measuring the cost and benefits of compiler optimizations, both individually and in synergy with other optimizations. We demonstrate our method by presenting results for the optimizations implemented in the Jikes Research Virtual Machine on the PowerPC and IA32 platforms. Copyright © 2006 John Wiley & Sons, Ltd.     

ATP/GTP hydrolysis is required for oxazole and thiazole biosynthesis in the peptide antibiotic microcin B17

In the maturation of the Escherichia coli antibiotic Microcin B17, the product of the mcbA gene is modified posttranslationally by the multimeric Microcin synthetase complex (composed of McbB, C, and D) to cyclize four Cys and four Ser residues to four thiazoles and four oxazoles, respectively. The purified synthetase shows an absolute requirement for ATP or GTP in peptide substrate heterocyclization, with GTP one-third as effective as ATP in initial rate studies. The ATPase/GTPase activity of the synthetase complex is conditional in that ADP or GDP formation requires the presence of substrate; noncyclizable versions of McbA bind to synthetase, but do not induce the NTPase activity. The stoichiometry of ATP hydrolysis and heterocycle formation is 5:1 for a substrate that contains two potential sites of modification. However, at high substrate concentrations (>50Km) heterocycle formation is inhibited, while ATPase activity occurs undiminished, consistent with uncoupling of NTP hydrolysis and heterocycle formation at high substrate concentrations. Sequence homology reveals that the McbD subunit has motifs reminiscent of the Walker B box in ATP utilizing enzymes and of motifs found in small G protein GTPases. Mutagenesis of three aspartates to alanine in these motifs (D132, D147, and D199) reduced Microcin B17 production in vivo and heterocycle formation in vitro, suggesting that the 45 kDa McbD has a regulated ATPase/GTPase domain in its N-terminal region necessary for peptide heterocyclization.     

Bridging temporal gaps between CS and US in autoshaping: Insertion of other stimuli before, during, and after CS.

In 4 experiments, female White Carneaux pigeons were exposed to key-light illuminations separated from food delivery by 12–60 sec. Approach to the key light did not develop on conventional trace-conditioning arrangements but occurred consistently whenever some auditory or visual stimulus filled the CS–UCS gap (serial conditioning) or was always present except during the gap. The CS approach was strong only when the stimulus present during the intertrial interval remained on until the termination of CS; if the stimulus ended at CS onset, conditioning did not occur. Although discriminability of CS–UCS gaps from intertrial periods seemed necessary for conditioning to occur in the absence of close CS–UCS contiguity, the outcome of the final experiment indicates that such discriminability was not sufficient for conditioning. Results are discussed in terms of possible 2nd-order conditioning effects and the changes in the associative strength of the "local context" existing when the CS appears, which may lead to superconditioning of CS. (35 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pavlovian appetitive contingencies and approach versus withdrawal to conditioned stimuli in pigeons.

Investigated the directed skeletal movements of 21 female White Carneaux pigeons toward signals of food or no food in 3 experiments. Ss approached and pecked an illuminated key that was positively correlated with food delivery, and positioned themselves relatively far from an illuminated key that was negatively correlated with food delivery. Key illuminations alone, random presentations of key illuminations and food, and backward pairings of key illuminations and food did not produce keypecking or consistent approach-withdrawal. Therefore, directed skeletal behavior-often believed to be conditioned and maintained primarily or exclusively by operant procedures-also emerges on Pavlovian procedures. Several kinds of alternative explanations (e.g., conditioned reinforcement effects, and stimulus substitution) for these phenomena are considered, and some potential implications for operant discrimination learning are briefly explored. (17 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)