No description available.

No description available.

This thesis presents four largely independent essays on industrial organisation. The first three essays examine how search and switching costs distort competitive markets, whilst the fourth essay studies how firms themselves might eliminate competition by agreeing to fix the market price. The key insight from the first two essays is that by choosing to pay a search cost, a consumer reveals to a retailer some private information about their product valuations. In this context the first essay re-examines the well-known theoretical Diamond Paradox in which markets completely break down if firms sell only a single product. I demonstrate that multiproduct retailers offer an elegant and realistic way of overcoming this Paradox, and then apply the model to supermarkets. In particular, this essay provides new insights into why convenience stores charge high prices and why grocery stores use selected loss-leaders. The second essay looks at internet search and therefore focuses on the special case in which search frictions become very small. It seeks to explain why retailers pay so much for online advertising, when consumers can easily click on whichever links they like. I show that if consumers have prior information about products, their search behaviour is limited but very informative about their preferences. This places prominent firms in a privileged position, and makes them substantially more profitable. The third essay provides a simple model in which small switching costs are pro-competitive and beneficial to consumers. This challenges the conventional wisdom, but also argues that switching costs should be less of a policy priority than search costs. The final essay examines a game in which two firms bargain over a collusive price. It is shown that entry into such a market may make collusion easier, and may increase price.

This thesis is about what happens in the brain when people switch between tasks. Each task requires a particular assembly of cognitive processes, an orientation of attention and set of rules relating action to input — a "task-set". The research reported used a task-cueing paradigm to study preparatory control of task-set. On each trial a stimulus (a coloured shape) was preceded by a verbal task-cue specifying which task to do (judge the shape or the colour of the stimulus). Reaction time and error rate increase on trials when the task changes relative to trials on which it does not. When the cue stimulus interval (CSI) is increased, this "switch cost" is reduced, indexing a process of task-set reconfiguration in which top-down control is employed to reconfigure the task-set parameters. Effective reconfiguration may also be indicated by a reduction in the "response congruence effect" — poorer performance on stimuli mapped to different responses for the two tasks than for stimuli mapped to the same response. I present six experiments using transcranial magnetic stimulation (TMS), a technique for interfering briefly and harmlessly with neuronal activity in a small region of cortex, to address the question of which brain regions contribute to anticipatory control of task-set as indexed by these behavioural measures. To help guide the selection of candidate brain regions, I first present a review and meta-analysis of neuroimaging studies of task-switching in the literature. Many fMRI studies, comparing brain activation on task-switch and -repeat trials have been published. Some have also tried to isolate activations related specifically to pro-active control of task-set. The activations reported are quite inconsistent over studies. I used a quantitative meta-analysis technique to identify which brain regions are most consistently found by studies reporting switch minus repeat contrasts and which may be specifically important for preparation on switch trials. The experiments examined the effect of stimulating several regions during the long cue-stimulus interval of a task-cueing paradigm, relative to control conditions. A first pair of experiments suggests an important role in proactive task-set control for two regions in dorsal medial frontal cortex, the supplementary motor area (SMA) and an area known as pre-SMA, though the former region appeared to contribute to reducing the switch cost while the latter appeared to reduce the effects of response congruence. In a further three experiments, I examined the role of the right intra-parietal sulcus (rIPS); this appears to play a crucial role in preparation for a task-switch but not post-stimulus task-set reconfiguration. In a final experiment, I used TMS guided by fMRI activations in the same participants to study the effects of stimulation over the left inferior frontal junction (IFJ). The results indicate that a region just anterior to the left IFJ is specifically important for preparing for a switch trial. I discuss the roles that may be played by these three regions in task-set control.

Flexible and goal-driven behaviour requires a process by which the appropriate task-set is selected and maintained in a privileged state of activation. This process can be conceptualised as loading a task-set into a procedural working memory (PWM) buffer. Task switching experiments, which exercise this process, reveal “switch costs”: increased reaction times and error rates when the task changes, compared to when it repeats. The process of loading a task-set into PWM may be one source of these costs. The switch cost is reduced with preparation, suggesting that at least some of the processes involved in a successful change of task can be achieved in advance of the stimulus. The aim of this thesis was to investigate the properties of PWM, and its contribution to task-set control. One account of PWM distinguishes between the level at which recently exercised (but currently irrelevant) task-sets are represented, and the level at which only the currently relevant task-set is maintained in a most active state. To distinguish between these levels of representation, and to assess the extent to which the process of getting a task-set into a most-active state (loading it into the PWM buffer) is subject to a capacity limit at each level, the experiments varied the number of tasks participants switched among (Experiments 1 and 2), and the complexity of individual task-sets (Experiments 3-6) in a task-cueing paradigm. In Experiments 1 and 2, participants switched among three or five tasks, in separate sessions. There was no effect of the number of tasks on the switch cost, or its reduction with preparation, provided that recency and frequency of task usage were matched. When recency and frequency were not matched, there appeared to be a larger switch cost with five tasks at a short preparation interval, suggesting that the time consumed by getting a task-set into a most active state is influenced by its recency and frequency of usage, not the number of alternatives per se. However, Experiment 3 showed that the time required to select an S-R mapping within a task-set does increase as a function of the number of alternatives (even when stimulus frequency and recency are matched), suggesting that representation of the most active task-set in a PWM buffer is subject to a strict capacity limit. Experiments 4-6 further investigated the capacity limit of this PWM buffer, and found that task-set preparation was more effective for task-sets that are less complex (i.e. specified by fewer S-R rules). These findings suggest that only very few S-R rules can be maintained in a most active state in the PWM buffer. Finally, Experiments 7-9 investigated whether S-R rules are represented phonologically for task-set maintenance and preparation, by manipulating the phonological properties of the stimulus terms. But task-cueing performance was not affected by the name length (Experiment 7) or phonological similarity (Experiments 8 and 9) of the stimulus terms. These results suggest that phonological representations of S-R rules do not make a functional contribution to task-set control, possibly because the rules are compiled into a non-linguistic PWM. The results of these experiments are discussed in terms of a procedural working memory which is separate from declarative working memory, and distinguishes between two levels of task-set control: the level of task-sets, which are maintained in a capacity unlimited state of representation, and the level at which the currently relevant task-set is maintained in a most-active but highly capacity limited state of representation.

No description available.

The U-curves of five different test objects, three of which contain a rod floating object at different positions within the gap, are characterised. During the testing, a high speed camera was used to photograph the discharges. The results are compared to Rizk’s theoretical model for determining the flashover voltage of gaps with floating objects are presented. It is concluded that the position of the floating object within the gap affects which discharge mechanism exists in each of the gaps. The effect that each discharge mechanism has on the flashover voltage and time to crest of the gap is shown. Time interval photographs are presented showing the formation of a discharge channel due to the streamer mechanism. In evaluating the high speed photographs it is seen that the extent of the branching of the discharge channel is a function of the time to crest of the applied impulse, more branching is evident for shorter times to crest.

University of Macau / Faculty of Arts and Humanities. / Department of English

University of Macau / Faculty of Arts and Humanities. / Department of English

The <i>spin</i> of an electron has for long excited researchers both with respect to its fundamental physics and technological applications. Consequently, the traditional field driven switching of ferromagnets gave way for more scalable current driven switching based on the well-known spin transfer torque phenomenon. However, in the quest for better energy-efficiency, the manipulation of electron spin through pure voltage driven or voltage-assisted mechanisms are being intensely explored. In this research, we demonstrate that the very physics and the characteristics of such voltage driven devices enable interesting possibilities with respect to memory, neuromorphic and logic applications. We rely on the recent experimental demonstrations of two novel voltage effects on nano-magnets - the voltage controlled magnetic anisotropy (VCMA) and the pure voltage driven magneto-electric (ME) effect. Specifically, we propose in-situ, in-memory, vector logic operations by exploiting the voltage asymmetry and precessional switching dynamics of the VCMA effect to construct 'stateful' logic gates. Stateful logic are those in which the same device acts as a storage element and compute engine, simultaneously. In addition, we show that the pure voltage driven mono-domain switching and domain-wall motion of nano-magnets through the ME effect can be leveraged to construct neuro-mimetic devices exhibiting leaky-integrate-fire dynamics of biological neurons and as well as non-volatile synaptic elements. Further, we propose a voltage driven logic-device using the ME switching and demonstrate that the proposed logic-device can be used to construct a complete cascadable logic family including XNOR, IMP (implication), NAND and NOR gates. Additionally, we present an energy and area efficient content addressable memory using a logic compatible ME-XNOR device. The presented research shows that voltage driven switching can augment the very functionality and widen the application scope of spin based devices and circuits.