The rapid explosion in the use of wireless devices, combined with the simultaneous infrastructure expansion required to suport a massive number of users, has made ubiquitous access to data a reality. When a large number of users simultaneously accesses global data in a mobile computing environment, an efficient means to manage a large amount of concurrent transactions is required. Current multi‐database concurrency control schemes do not address the limited bandwidth and frequent disconnections associated with wireless networks. This article describes a new hierarchical concurrency control algorithm, v‐lock, that addresses the shortcomings of existing multi‐database concurrency control schemes. The algorithm uses global locking tables created with semantic informantion contained within a hierarchy to serialize global transactions, and remove global deadlocks. Additionally, wireless transmission of data is subject to weak or frequent disconnection. With the increased amount of local memory available at the mobile unit, data replication can be used to provide local data availability, limiting the restrictions imposed by a wireless mobile environment. In a mobile, multi‐database environment, local autonomy restrictions prevent the use of a page‐or‐file‐based data replication scheme. This article describes a new data replication scheme to address to limited bandwidth and local autonomy restrictions. Consistency is maintained by using a parity‐based invalidation scheme of data cached at the mobile unit. Additionally, a simple prefetching scheme is used to further improve the effectiveness of the proposed scheme. Finally, simulated results of the concurrency control and replication algorithms are presented and discussed.
HIGHLIGHTS This study tested the effects of adolescent-stress on adult learning and memory.Adolescent-stressed rats had enhanced reversal learning compared to unstressed rats.Adolescent-stress exposure made working memory more vulnerable to disturbance.Adolescent-stress did not affect adult associative learning or reference memory.Exposure to acute stress can cause a myriad of cognitive impairments, but whether negative experiences continue to hinder individual as they age is not as well understood. We determined how chronic unpredictable stress during adolescence affects multiple learning and memory processes in adulthood. Using male Sprague Dawley rats, we measured learning (both associative and reversal) and memory (both reference and working) starting 110 days after completion of an adolescent-stress treatment. We found that adolescent-stress affected adult cognitive abilities in a context-dependent way. Compared to rats reared without stress, adolescent-stressed rats exhibited enhanced reversal learning, an indicator of behavioral flexibility, but showed no change in associative learning and reference memory abilities. Working memory, which in humans is thought to underpin reasoning, mathematical skills, and reading comprehension, may be enhanced by exposure to adolescent-stress. However, when adolescent-stressed animals were tested after a novel disturbance, they exhibited a 5-fold decrease in working memory performance while unstressed rats continued to exhibit a linear learning curve. These results emphasize the capacity for stress during adolescence to transform the cognitive abilities of adult animals, even after stress exposure has ceased and animals have resided in safe environments for the majority of their lifespans.
Spatial abilities allow animals to retain and cognitively manipulate information about their spatial environment and are dependent upon neural structures that mature during adolescence. Exposure to stress in adolescence is thought to disrupt neural maturation, possibly compromising cognitive processes later in life. We examined whether exposure to chronic unpredictable stress in adolescence affects spatial ability in late adulthood. We evaluated spatial learning, reference and working memory, as well as long-term retention of visuospatial cues using a radial arm water maze. We found that stress in adolescence decreased the rate of improvement in spatial learning in adulthood. However, we found no overall performance impairments in adult reference memory, working memory, or retention caused by adolescent-stress. Together, these findings suggest that adolescent-stress may alter the strategy used to solve spatial challenges, resulting in performance that is more consistent but is not refined by incorporating available spatial information. Interestingly, we also found that adolescent-stressed rats showed a shorter latency to begin the water maze task when re-exposed to the maze after an overnight delay compared with control rats. This suggests that adolescent exposure to reoccurring stressors may prepare animals for subsequent reoccurring challenges. Overall, our results show that stress in adolescence does not affect all cognitive processes, but may affect cognition in a context-dependent manner.
A mobile computing environment involves accessing information through a wireless network connection. The mobile unit may be stationary, in motion, and/or intermittently connected to a fixed (wired) network. As technology advances are made in software and hardware, the feasibility of accessing information "anytime, anywhere" is becoming a reality.There are similarities involved in accessing information in a multidatabase environment and mobile computing environment. We propose to superimpose a wireless-mobile computing environment on a multidatabase system to realize a system capable of effectively accessing a large amount of data over a wireless medium (MDAS -Mobile Data Access System). We show how one can easily map solutions from one environment to another.As a potentially large number of users may simultaneously access the available data, there are several issues involved in the ability to concurrently manage transactions. Current concurrency control schemes do not efficiently manage these accesses because they do not address the limited bandwidth and frequent disconnections associated with wireless networks. This paper proposes a new hierarchical concurrency control algorithm. The proposed concurrency control algorithm uses global locking tables created with semantic information contained within the hierarchy. The performance of the new algorithm is simulated and the results are presented.
Exposure to chronic stress during adolescence can shape behaviour, cognition and physiology in adulthood, but the consequences of these longterm changes remain unclear. Prior studies reporting altered performance following exposure to stress in adolescence have generally interpreted lasting changes as impairments. However, we have recently shown that exposure to chronic unpredictable stress during adolescence (from postnatal days 30-70) can enhance performance in a context-dependent manner during a foraging task. Increases in foraging performance, (previously measured by the number of rewards obtained), are often associated with trade-offs in other behaviours, such as vigilance. Here, we examined the effect of stress exposure in adolescence on adult foraging in male Sprague Dawley rats to determine (1) whether the increase in foraging performance exhibited by animals exposed to stress in adolescence is balanced by a decrease in vigilance, and (2) whether stress in adolescence alters time allocation between foraging and vigilance behaviours in low-and high-threat conditions. We found no evidence of a trade-off between foraging and vigilance; under low-threat conditions, rats exposed to stress in adolescence spent more time being vigilant compared with unstressed rats, suggesting that exposure to stress in adolescence enhances anticipation of threat in adulthood. Under high-threat conditions, adolescentstressed and unstressed rats did not differ in foraging and vigilance behaviours. Given that we have previously found that rats exposed to stress in adolescence nearly double food intake under high-threat, and we now show that high-performing rats do not spend more time foraging, it appears that stress exposure in adolescence may enhance foraging efficiency (food consumed/time) under high-threat conditions rather than time allocation between foraging and competing behaviours. We also examined the relationship, at the level of the individual, between foraging performance and foraging and vigilance behaviours. We found that changes in individual foraging performance between low-and high-threat conditions were independent of behavioural changes (i.e. both highly and poorly performing rats were equally active and contacted a similar number of patches). This suggests that the ability to obtain many rewards under high-threat conditions may be related to efficiency, rather than the frequency of foraging and effort-related behaviours.
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