bluuebunny/crossref_metadata_2025 huggingface dataset:
Finite quotients of the multiplicative group of a finite dimensional division algebra are solvable
Authors: Rapinchuk Andrei , Segev Yoav , Seitz Gary
Published: October 2002

We prove that finite quotients of the multiplicative group of a finite dimensional division algebra are solvable. Let

D
D

be a finite dimensional division algebra having center

K
K

, and let

N
⊆

D

×

N\subseteq D^{\times }

be a normal subgroup of finite index. Suppose

D

×

/

N

D^{\times }/N

is not solvable. Then we may assume that

H
:=

D

×

/

N

H:=D^{\times }/N

is a minimal nonsolvable group (MNS group for short), i.e. a nonsolvable group all of whose proper quotients are solvable. Our proof now has two main ingredients. One ingredient is to show that the commuting graph of a finite MNS group satisfies a certain property which we denote Property

(
3

1
2

)

(3\frac {1}{2})

. This property includes the requirement that the diameter of the commuting graph should be

≥
3

\ge 3

, but is, in fact, stronger. Another ingredient is to show that if the commuting graph of

D

×

/

N

D^{\times }/N

has Property

(
3

1
2

)

(3\frac {1}{2})

, then

N
N

is open with respect to a nontrivial height one valuation of

D
D

(assuming without loss of generality, as we may, that

K
K

is finitely generated). After establishing the openness of

N
N

(when

D

×

/

N

D^{\times }/N

is an MNS group) we apply the Nonexistence Theorem whose proof uses induction on the transcendence degree of

K
K

over its prime subfield to eliminate

H
H

as a possible quotient of

D

×

D^{\times }

, thereby obtaining a contradiction and proving our main result.
Weighted Paley-Wiener spaces
Authors: Lyubarskii Yurii , Seip Kristian
Published: October 2002

We study problems of sampling and interpolation in a wide class of weighted spaces of entire functions. These weights are characterized by the property that their natural regularization as the envelop of the unit ball of the corresponding space is equivalent to the original weight. We give an independent description of such weights and also show that, in a sense, this is the widest class of weights and associated spaces for which results on sets of uniqueness, sampling, and interpolation related to the classical Paley-Wiener spaces can be extended in a direct and natural way, keeping the basic features of the theory intact. One of the basic tools for our study is the De Brange theory of spaces of entire functions.
The cranial anatomy ofDiscosauricsusKuhn, a seymouriamorph tetrapod from the Lower Permian of the Boskovice Furrow (Czech Republic)
Authors: Klembara Jozef
Published: October 2002

A detailed description is given of the cranial anatomy of the Lower Permian tetrapodDiscosauriscus. This material is three–dimensionally preserved and represents one of the best know seymouriamorph tetrapods. Two species,D. austriacusandD. pulcherrimus, are distinguished in the material from the Boskovice Furrow.D. austriacusis abundant,D. pulcherrimusis rare. The two species differ in the following features: (1) the shape of the postorbital and the jugal and the nature of their junction; (2) the shape of the prefrontal and the postfrontal and the position of their contact; and (3) the morphology of the ventral surfaces fo the palatal bones and the arrangement of the denticles on them. The genusDiscosauriscusrepresents larval, metamorphic and early juvenile individuals. Comparison of the cranial structures ofDiscosauriscuswith those ofSeymouria baylorensis,Seymouria sanjuanensis, andSeymouriacf.S. sanjuanensis(which is of similar size to the largest individuals ofDiscosauriscus) shows that (i)DiscosauriscusandSeymouriaare structurally similar but different genera, (ii)Discosauriscusis a member of the taxon Seymouriamorpha, (iii)DiscosauriscusandAriekanerpetoncomprise the family Discosauriscidae and (iv)Seymouriais not a reptile (an amniote animal).Ariekanerpeton,Utegenia,Urumquia,kotlassiids,leptorophidsandEnosuchusrequire revision.
The three–dimensional leading–edge vortex of a ‘hovering’ model hawkmoth
Authors: van den Berg Coen , Ellington Charles P.
Published: October 2002

Recent flow visualisation experiments with the hawkmoth,
Manduca sexta
, revealed small but clear leading–edge vortex and a pronounced three–dimensional flow. Details of this flow pattern were studied with a scaled–up, robotic insect (‘the flapper’) that accurately mimicked the wing movements of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing confirmed the existence of a small, strong and stable leading–edge vortex, increasing in size from wingbase to wingtip. Between 25 and 75 % of the wing length, its diameter increased approximately from 10 to 50 % of the wing chord. The leading–edge vortex had a strong axial flow veolocity, which stabilized it and reduced its diamater. The vortex separated from the wing at approximately 75 % of the wing length and thus fed vorticity into a large, tangled tip vortex. If the circulation of the leading–edge vortex were fully used for lift generation, it could support up to two–thirds of the hawkmoth's weight during the downstroke. The growth of this circulation with time and spanwise position clearly identify dynamic stall as the unsteady aerodynamic mechanism responsible for high lift production by hovering hawkmoths and possibly also by many other insect species.
Of mice and wrens: the relation between abundance and geographic range size in British mammals and birds
Authors: Blackburn T. M. , Gaston K. J. , Quinn R. M. , Arnold H. , Gregory R. D.
Published: October 2002

We examine the relation between population size and geographic range size for British breeding birds and mammals. As for most other assemblages studied, a strong positive interspecific correlation is found in both taxa. The relation is also recovered once the phylogenetic relatedness of species has been controlled for using an evolutionary comparative method. The slope of the relation is steeper for birds than for mammals, but this is due in large part to two species of mammals that have much higher population sizes than expected from their small geographic ranges. These outlying mammal species are the only ones in Britain to be found only on small offshore islands, and so may be exhibiting density compensation effects. With them excluded, the slope of the abundance–range size relation for mammals is not significantly different to that for birds. However, the elevation of the relation is higher for mammals than for birds, indicating that mammals are approximately 30 times more abundant than birds of equivalent geographic range size. An earlier study of these assemblages showed that, for a given body mass, bats had abundances more similar to birds than to non–volant mammals, suggesting that the difference in abundance between mammals and birds might be due to constraints of flight. Our analyses show that the abundance–range size relation for bats is not different for that from other mammals, and that the anomalously low abundance of bats for their body mass may result because they have smaller than expected geographic extents for their size. Other reasons why birds and mammals might have different elevations for the relation between population size and geographic range size are discussed, together with possible reasons for why the slopes of these relations might be similar.
A comparison of fertility control and lethal control of bovine tuberculosis in badgers: the impact of perturbation induced transmission
Authors: Swinton J. , Tuyttens F. , Macdonald D. , Nokes D. J. , Cheeseman C. L. , Clifton-hadley R.
Published: October 2002

In this paper we use mathematical modelling to consider the broad advantages and disadvantages of fertility control over lethal control for bovine tuberculosis in badger populations.

We use a deliberately simple model, attempting to capture only the key transmission processes. The model is parametrized with reference to the long–term Woodchester Park study. Estimates of mortality rate from this study suggest no significant extra mortality risk for animals with evidence of infection as indicated by the presence of anti-
Mycobacterium bovis
antibodies or
M. bovis
isolation.

We find that large reductions in prevalence are sometimes the consequence of only moderate reductions in population numbers. If we assume that the act of control does not in itself affect transmission rates, then as far as eradication is concerned, both fertility control and mortality control operate through the same epidemiological mechanism, the removal of susceptibles: if one is in principle capable of keeping a population low enough to be infection free then so is the other. It is necessary to continue either form of control at regular intervals to maintain a constant level of infection in the long term. If control were to be stopped, return to precontrol levels of badger population and infection prevalence would be expected within a few years. Fertility control is less effective in reducing population density than lethal control since it can only act, at maximum, to remove one age cohort per year. It is also less effective in reducing transmission as it can only ever remove susceptibles, while lethal control also removes infectious badgers.

However, if the social disturbance caused by lethal control does in fact increase contact rates for the remaining infectious badgers, the relative efficacies of the two strategies become a great deal less clear. While we have no quantitative data on the extent to which social perturbation does act to promote transmission, model simulations show that it is possible to develop plausible scenarios in which the lethal control may actually act to increase the absolute numbers of animals infected, while reducing the number of uninfected animals to very low numbers.
Introduction to a Discussion on Near–infrared spectroscopy and imaging of living systems.
Authors: Cooper C. , Delpy D. T.
Published: October 2002

Introduction to a Discussion on Near–infrared spectroscopy and imaging of living systems. A Discussion held at the Royal Society on 23rd and 24th October 1996. Organized and edited by B. Chance, C. E. Cooper, D. T. Delpy and E. O. R. Reynolds.
Use of near–infrared spectroscopy in the adult
Authors: Kirkpatrick Peter J.
Published: October 2002

Adult near–infrared spectroscopy is a potential method for observing changes in cerebral oxygenation non–invasively. Access of light to the adult brain requires penetration through extracranial tissues; hence the detection of changes in cerebral chromophore concentration can only be achieved by using near–infrared spectroscopy in the reflectance–mode thereby adding variables which are difficult to control. These include the effects of variable anatomy, different intra–optode distances and the presence of an extra– to intracranial collateral blood supply. Although movements of oxygenated haemoglobin concentration following specific cerebral stimuli can be demonstrated, the challenge of separating changes which occur within the extracranial compartment from those occurring in the intracranial compartments remains. Our experience with near–infrared spectroscopy in the three adult clinical scenarios of carotid endarterectomy, head injury and carbon dioxide stress testing will be presented. The influence of extracranial contamination is demonstrated, as are the methods we have developed to help control for extracranial contamination. Provisional experience with spatially resolved spectroscopy technology will also be presented.
Predicting variation in sperm precedence
Authors: Cook Penny A. , Harvey Ian F. , Parker Geoffrey A.
Published: October 2002

Sperm competition theory predicts that males are adapted for success in sperm competition by the production of large numbers of sperm. This is supported by both inter– and intraspecific studies showing that males mating under high sperm competition risk increase investment in sperm production. Such an increase in sperm production is an advantage if sperm mix randomly or if sperm displacement occurs. When two males mate with the same female, the measurement of the proportion of eggs fertilized by the second male to mate (termed
P
2
) has been used to help elucidate sperm competition mechanisms.
P
2
is usually quoted as a mean value, with little attention being paid to its variance, although
P
2
estimates are notoriously variable. By predicting an expected variance for
P
2
, additional information on sperm competition mechanisms may be obtained. Here we present a technique for analysing the variance in
P
2
when a given mechanism of
P
2
is assumed. We apply this technique to
P
2
data collected from
Plodia interpunctella
(Lepidoptera, Pyralidae), assuming a ‘fair raffle’ mechanism of sperm competition. We compare observed distributions of
P
2
with theoretical distributions generated assuming random mixing of two ejaculates drawn randomly from a population of known mean and variance in sperm numbers. Ejaculates of known size were obtained by counting the number of sperm ejaculated by males mating for the first (large ejaculate) or second (small ejaculate) time. Females either received two small or one small and one large ejaculate, and the distribution of
P
2
(estimated using the sterile male technique) was compared with our theoretical predictions. The observed variance in
P
2
was greater than our model prediction, thus we conclude that sperm from
P. interpunctella
do not mix randomly before fertilization.
The economic determinants of land degradation in developing countries
Authors: Barbier Edward B.
Published: October 2002

The following paper investigates the economic determinants of land degradation in developing countries. The main trends examined are rural household's decisions to degrade as opposed to conserve land resources, and the expansion of frontier agricultural activity that contributes to forest and marginal land conversion. These two phenomena appear often to be linked. In many developing areas, a poor rural household's decision whether to undertake long–term investment in improving existing agricultural land must be weighed against the decision to abandon this land and migrate to environmentally fragile areas. Economic factors play a critical role in determining these relationships. Poverty, imperfect capital markets and insecure land tenure may reinforce the tendency towards short–term time horizons in production decisions, and may bias land use decisions against long–term land management strategies. In periods of commodity booms and land speculation, wealthier households generally take advantage of their superior political and market power to ensure initial access to better quality resources, in order to capture a larger share of the resource rents. Poorer households are confined either to marginal environmental areas where resource rents are limited, or only have access to resources once they are degraded and rents dissipated.

Overall trends in land degradation and deforestation are examined, followed by an overview of rural household's resource management decisions with respect to land management, frontier agricultural expansion, and migration from existing agricultural land to frontiers. Finally, the discussion focuses on the scope for policy improvements to reduce economic constraints to effective land management.
Nutrient resources for crop production in the tropics
Authors: Vlek Paul L. G. , Kühne Ronald F. , Denich Manfred
Published: October 2002

For the foreseeable future a majority of the population, and almost all the mal– and under–nourished, will continue to be found in the tropics and subtropics. Food security in these parts of the world will have to be met largely from local resources. The productivity of the land is to a large extent determined by the fertlity of the soil, which in turn is mostly determined by its organic matter content and stored nutrients. Soil organic matter is readily lost when organic matter inputs are reduced upon cultivation and more so upon intensification. The concomitant loss of topsoil and possible exposure of subsoil acidity may cause further soil degradation.

Plant nutrients to replenish what is yearly taken from the soil to meet the demands for food and fibre amount to 230 million tonnes (Mt). Current fertilizer consumption stands at about 130 Mt of N, P
2
O
5
,and K
2
O, supplemented by an estimated 90 Mt of N from biological nitrogen fixation worldwide. Although 80 per cent of the population lives in the developing world, only half the world's fertilizer is consumed there. Yet, as much as 50% of the increase in agricultural productivity in the developing world is due to the adoption of fertilizers. World population growth will cause a doubling in these nutrients requirements for the developing world by 2020, which, in the likely case of inadequate production, will need to be met from soil reserves. Because expansion of the cultivable land area is reaching its limits, the reliance on nutrient inputs and their efficient use is bound to grow.

With current urban expansion, nutrients in harvested products are increasingly lost from the rural environment as a whole. Estimates of soil nutrient depletion rates for sub–Saharan Africa (SSA) are alarmingly high. The situation may be more favourable in Latin America and Asia where fertilizer inputs are tenfold those of SSA. Closing the nutrient cycle at a community level in rural areas may be tedious; on an inter–regional level it is associated with considerable costs of collection, detoxification and transportation to the farms. Yet, at the rate at which some of the non–renewable resources such as phosphorus and potassium are being exploited, recycling of these nutrients will soon be required.
Genetics of common disease: implications for therapy, screening and redefinition of disease
Authors: Bell John
Published: October 2002

Susceptibility to most common human diseases is, at least in part, determined by genetic factors. Rapid progress is being made in defining these genetic determinants for a range of diseases including breast cancer, colon cancer, diabetes, arthritis and dementia. The ability to define susceptibility in genetic terms has already led to a reclassification of some of these diseases on genetic and mechanistic grounds. This information is likely to have a profound effect on our approach to human diseases as it will allow a better definition of these disorders, permitting more effective therapeutic intervention, and will lead to both a more precise understanding of the natural history of these diseases and the possibility of identifying populations at risk. An understanding of the mechanisms underlying disease susceptibilty will also improve our ability to develop rational therapeutic interventions for many of these diseases. The role of genetic screening in these common diseases will be discussed, particularly in regard to the application of health care in populations.
How will improved forecasts of individual lifetimes affect underwriting?
Authors: Macdonald Angus S.
Published: October 2002

The combined effects of underwriting and adverse selection among heterogeneous populations are considered, using a simple Markov model. We illustrate the possible extent of the costs of adverse selection; in all cases, above–average sums assured is the most significant factor.
Should there be property rights in genes?
Authors: Dworkin Gerald
Published: October 2002

This paper deals with the following questions. Are there property rights in the human body or its parts? What legal control is, or should be, available in respect of genetic material? Can, or should, patents be granted for genes or for products incorporating human genetic material? How extensive are patent rights over genetic material? Should ethical matters be a critical part of the patent granting process?
Conclusions – scientific research and its implications
Authors: Anderson R. M.
Published: October 2002

The organizers of this meeting felt that it would be appropriate to have two short papers at the end of this volume to summarize the key points that are of relevance to scientists and the insurance and actuarial professions. This was felt to be of particular importance given the very heterogeneous backgrounds of members of the audience at the meeting, namely, scientific, medical, legal, social and financial. The objectives are to summarize the key issues in layman's terms—trying to avoid jargon, exaggeration and a partisan approach. In some ways my task is somewhat easier than Chris Daykin's job, since there are only four scientific and medical presentations compared with seven from the actuarial, social and legal side.
Introduction to Knowledge–based vision in man and machine. A Discussion held at the Royal Society
Authors: Anderson James , Barlow Horace , Gregory Richard
Published: October 2002

Introduction to Knowledge–based vision in man and machine. A Discussion held at the Royal Society organised and edited by J. Anderson, H. B. Barlow and R. L. Gregory on the 12 and 13 February 1997
Host—parasite interaction and morbidity in malaria endemic areas
Authors: Marsh Kevin , Snow Robert W.
Published: October 2002

Severe morbidity due to
Plasmodium falciparum
is a major health problem in African children. The patterns of morbidity in endemic areas are modified by the immune response, and vary markedly with transmission intensity. Severe disease falls into three overlapping syndromes: coma, respiratory distress, and severe anaemia. Recently, it has become clear that metabolic acidosis plays a major role in the pathogenesis of severe disease and is particularly important in the overlap between the different clinical syndromes. We propose that the different manifestations of severe malarial morbidity arise from the interaction of a limited number of pathogenic processes: red cell destruction, toxin–mediated activation of cytokine cascades, and infected cell sequestration in tissue microvascular beds. The pattern of severe morbidity varies with age within any one endemic area, with severe anaemia predominating in the youngest children and coma having its highest incidence in older children. Between endemic areas there is a marked variation in mean age of children with severe malaria, and therefore in the importance of different clinical syndromes. The shift in mean age is due to a combination of increased challenge and more rapid development of immunity at higher levels of transmission. Recent comparative studies indicate that at higher levels of transmission the net effect of these shifts may be a paradoxical reduction in total severe malarial morbidity.
Spatial frames of reference and somatosensory processing: a neuropsychological perspective
Authors: Vallar Giuseppe
Published: October 2002

In patients with lesions in the right hemisphere, frequently involving the posterior parietal regions, left–sided somatosensory (and visual and motor) deficits not only reflect a disorder of primary sensory processes, but also have a higher–order component related to a defective spatial representation of the body. This additional factor, related to right brain damage, is clinically relevant: contralesional hemianaesthesia (and hemianopia and hemiplegia) is more frequent in right brain–damaged patients than in patients with damage to the left side of the brain. Three main lines of investigation suggest the existence of this higher–order pathological factor. (i) Right brain–damaged patients with left hemineglect may show physiological evidence of preserved processing of somatosensory stimuli, of which they are not aware. Similar results have been obtained in the visual domain. (ii) Direction–specific vestibular, visual optokinetic and somatosensory or proprioceptive stimulations may displace spatial frames of reference in right brain–damaged patients with left hemineglect, reducing or increasing the extent of the patients’ ipsilesional rightward directional error, and bring about similar directional effects in normal subjects. These stimulations, which may improve or worsen a number of manifestations of the neglect syndrome (such as extrapersonal and personal hemineglect), have similar effects on the severity of left somatosensory deficits (defective detection of tactile stimuli, position sense disorders). However, visuospatial hemineglect and the somatosensory deficits improved by these stimulations are independent, albeit related, disorders. (iii) The severity of left somatosensory deficits is affected by the spatial position of body segments, with reference to the midsagittal plane of the trunk. A general implication of these observations is that spatial (non–somatotopic) levels of representation contribute to corporeal awareness. The neural basis of these spatial frames includes the posterior parietal and the premotor frontal regions. These spatial representations could provide perceptual–premotor interfaces for the organization of movements (e.g. pointing, locomotion) directed towards targets in personal and extrapersonal space. In line with this view, there is evidence that the sensory stimulations that modulate left somatosensory deficits affect left motor disorders in a similar, direction–specific, fashion.
From visual affordances in monkey parietal cortex to hippocampo–parietal interactions underlying rat navigation
Authors: Arbib Michael A.
Published: October 2002

This paper explores the hypothesis that various subregions (but by no means all) of the posterior parietal cortex are specialized to process visual information to extract a variety of affordances for behaviour. Two biologically based models of regions of the posterior parietal cortex of the monkey are introduced. The model of the lateral intraparietal area (LIP) emphasizes its roles in dynamic remapping of the representation of targets during a double saccade task, and in combining stored, updated input with current visual input. The model of the anterior intraparietal area (AIP) addresses parietal–premotor interactions involved in grasping, and analyses the interaction between the AIP and premotor area F5. The model represents the role of other intraparietal areas working in concert with the inferotemporal cortex as well as with corollary discharge from F5 to provide and augment the affordance information in the AIP, and suggests how various constraints may resolve the action opportunities provided by multiple affordances. Finally, a systems–level model of hippocampo–parietal interactions underlying rat navigation is developed, motivated by the monkey data used in developing the above two models as well as by data on neurons in the posterior parietal cortex of the monkey that are sensitive to visual motion. The formal similarity between dynamic remapping (primate saccades) and path integration (rat navigation) is noted, and certain available data on rat posterior parietal cortex in terms of affordances for locomotion are explained. The utility of further modelling, linking the World Graph model of cognitive maps for motivated behaviour with hippocampal–parietal interactions involved in navigation, is also suggested. These models demonstrate that posterior parietal cortex is not only itself a network of interacting subsystems, but functions through cooperative computation with many other brain regions.
A new view of hemineglect based on the response properties of parietal neurones
Authors: Pouget A. , Sejnowski T. J.
Published: October 2002

Lesion studies of the parietal cortex have led to a wide range of conclusions regarding the coordinate reference frame in which hemineglect is expressed. A model of spatial representation in the parietal cortex has recently been developed in which the position of an object is not encoded in a particular frame of reference, but instead involves neurons computing basis functions of sensory inputs. In this type of representation, a nonlinear sensorimotor transformation of an object is represented in a population of units having the response properties of neurons that are observed in the parietal cortex. A simulated lesion in a basis–function representation was found to replicate three of the most important aspects of hemineglect: (i) the model behaved like parietal patients in line–cancellation and line–bisection experiments; (ii) the deficit affected multiple frames of reference; and (iii) the deficit could be object–centred. These results support the basis–function hypothesis for spatial representations and provide a testable computational theory of hemineglect at the level of single cells.
Right medial temporal–lobe contribution to object–location memory
Authors: Milner Brenda , Johnsrude Ingrid , Crane Joelle
Published: October 2002

An important aspect of normal human memory, and one humans share with many other species, is the ability to remember the location of objects in their environment. There is by now strong evidence from the study of epileptic patients undergoing brain surgery that right temporal–lobe lesions that encroach extensively upon the hippocampal and parahippocampal gyrus impair the delayed, but not the immediate, recall of the location of objects within a random array. These findings have now been extended to a multiple–trial, spatial–array learning task; by including not only patients tested after unilateral anterior temporal lobectomy but also those with a selective left or right amygdalohippocampectomy, it has been shown that the deficits associated with right hippocampal lesions are not dependent upon conjoint damage to the lateral temporal neocortex. Furthermore, the fact that on the learning task no group differences were seen on Trial 1, at zero delay, strengthened the view that the impairment was in the maintenance and subsequent retrieval of information rather than in its initial encoding. These results left unresolved the question of whether the deficit was in the mediation of object–place associations or whether it could be reduced to a more general impairment in memory for location as such. Also left unanswered was the neuroanatomical question as to the relative contributions of the hippocampus and the parahippocampal gyrus to the performance of the experimental tasks. These questions were addressed in two blood–flow activation studies that made use of positron emission tomography (PET) and magnetic resonance imaging (MRI) and incorporated computerized versions of object–location and simple–location memory tasks. Taken together, the results point to a special contribution from the anterior part of the right parahippocampal gyrus, probably corresponding to the entorhinal cortex, to the retrieval of object–place associations, a result consonant with neurophysiological findings in non–human primates.
Hippocampal synaptic plasticity: role in spatial learning or the automatic recording of attended experience?
Authors: Morris R. G. M. , Frey U.
Published: October 2002

Allocentric spatial learning can sometimes occur in one trial. The incorporation of information into a spatial representation may, therefore, obey a one–trial correlational learning rule rather than a multi–trial error–correcting rule. It has been suggested that physiological implementation of such a rule could be mediated by
N
–methyl–D–aspartate (NMDA) receptor–dependent long–term potentiation (LTP) in the hippocampus, as its induction obeys a correlational type of synaptic learning rule. Support for this idea came originally from the finding that intracerebral infusion of the NMDA antagonist AP5 impairs spatial learning, but studies summarized in the first part of this paper have called it into question. First, rats previously given experience of spatial learning in a watermaze can learn a new spatial reference memory task at a normal rate despite an appreciable NMDA receptor blockade. Second, the classical phenomenon of ‘blocking’ occurs in spatial learning. The latter finding implies that spatial learning can also be sensitive to an animal's expectations about reward and so depend on more than the detection of simple spatial correlations.

In this paper a new hypothesis is proposed about the function of hippocampal LTP. This hypothesis retains the idea that LTP subserves rapid one–trial memory, but abandons the notion that it serves any specific role in the geometric aspects of spatial learning. It is suggested that LTP participates in the ‘automatic recording of attended experience’: a subsystem of episodic memory in which events are temporarily remembered in association with the contexts in which they occur. An automatic correlational form of synaptic plasticity is ideally suited to the online registration of context–event associations. In support, it is reported that the ability of rats to remember the most recent place they have visited in a familiar environment is exquisitely sensitive to AP5 in a delay–dependent manner. Moreover, new studies of the lasting persistence of NMDA–dependent LTP, known to require protein synthesis, point to intracellular mechanisms that enable transient synaptic changes to be stabilized if they occur in close temporal proximity to important events. This new property of hippocampal LTP is a desirable characteristic of an event memory system.
Variable place–cell coupling to a continuously viewed stimulus: evidence that the hippocampus is part of a perceptual system
Authors: Rotenberg Alexander , Muller Robert U.
Published: October 2002

A key feature of perception is that the interpretation of a single, continuously available stimulus can change from time to time. This aspect of perception is well illustrated by the use of ambiguous figures that can be seen in two different ways. When people view such a stimulus they almost universally describe what they are seeing as jumping between two states. If it is agreed that this perceptual phenemonon is causally linked to the activity of nerve cells, the state jumps would have to occur in conjunction with changes in neural activity somewhere in the nervous system.

The experiments described in this paper suggest that hippocampal place cells are part of a perceptual system. Variations were made of a ‘cue–card rotation’ experiment on rats in which the angular position of a prominent visual stimulus on the wall of cylinder is changed in the rat's presence. The three main results are as follows. (i) Place–cell firing fields remain stationary if the cue is rotated by 180° so that the relation between the cue and the field is altered. (ii) Firing fields rotate by 45° when the cue is rotated by 45°and the relation between the field and the card is maintained. (iii) If the cue is first rotated by 180°and then rotated in a series of 45° steps, the field finishes at a different angular position relative to the card when the card is back in its original position. Thus, place cells can fire in two different ways in reponse to a continuously viewed stimulus. It is concluded that place cells reveal that the hippocampal mapping system also has properties expected of a perceptual system.
The ageing brain: normal and abnormal memory
Authors:   The American Association for Research into Nervous and Mental Diseases, Albert Marilyn S.
Published: October 2002

With advancing age, the majority of individuals experience declines in their ability to learn and remember. An examination of brain structure and function in healthy older persons across the age range indicates that there are substantial changes in the brain that appear to be related to alterations in memory. The nature of the cognitive and neurobiological alterations associated with age-related change is substantially different from that seen in the early stages of a dementing illness, such as Alzheimer's disease. These differences have implications for potential intervention strategies.
Introduction to Ageing: science, medicine, and society
Authors: Klug Aaron
Published: October 2002

Introduction to Ageing: science, medicine, and society, the proceedings of a Discussion held at The Royal Society on 7 and 8 May 1997. Organized and edited by J. Grimley Evans, R. Holliday, T. B. L. Kirkwood, P. Laslett and L. Tyler.
Changes in the ageing brain in health and disease
Authors: Anderton Brian H.
Published: October 2002

The brains of individuals, who are cognitively normal, show age–related changes that include an overall reduction in brain volume and weight, which are associated with gyral atrophy and widening of the sulci of the cerebral cortex, and enlargement of the brain ventricles. These changes are partly the result of nerve cell loss but accurate estimates of neuronal loss are notoriously difficult to make. Microscopically, there are increasing amounts of the age–related pigment, lipofuscin, granulovacuolar degeneration in neurons, Hirano bodies, variable amounts of diffuse deposits of β–amyloid in the parenchyma, the presence of neurofibrillary tangles mainly confined to the hippocampus and amygdala, and sparse numbers of senile plaques in these brain regions and also in other cortical areas. Of these changes, neurofibrillary tangles and senile plaques are the neuropathological hallmark of Alzheimer's disease in which they are more abundant and widespread. Alzheimer's disease has therefore been regarded as accelerated brain ageing; however, the realization that there is a strong genetic contribution to developing the disease at least implies that it may not be the inevitable, even if frequent, consequence of old age. Understanding the molecular basis of plaque and tangle formation is advancing greatly and is the main focus of research into the cellular and molecular changes observed in the ageing brain.
The economic consequences of ageing populations
Authors: Mirrlees James A.
Published: October 2002

The effect of low birth rates and lengthening lives on the economy is discussed. Two extreme cases are examined: where pensions are entirely on a pay–as–you–go basis, and where they are entirely funded. It is argued that the economy would grow faster in the latter case. The impact on the levels of consumption of each age cohort during its lifetime is assessed. The possible magnitude of changes in consumption as a result of an increase in the retired part of the population is illustrated. It is shown that, comparing later cohorts to earlier cohorts, the former are better off under a funded system. An argument is then sketched showing that a pay–as–you–go system favours earlier cohorts too much; while most probably, but not certainly, a fully funded system favours the later cohorts excessively. It is claimed that a gradual introduction of partial funding, and some increase in the length of working lives, can deal with the effects of an ageing population without an excessive burden on any cohort or age–group.
Long live the Queen's subjects
Authors: Perutz M. F.
Published: October 2002

In 1952, the Queen congratulated 255 people on their hundredth birthdays and 1135 couples on their sixtieth wedding anniversaries. By 1996, these numbers had risen to 5218 and 11 688, respectively. Semilogarithmic plots, normalized to constant numbers of births and marriages, show steady exponential rises in the number of centenarians with a doubling time of 11 years, and of diamond weddings with a doubling time of 19 years. An alternative plot of the numbers of those reaching a hundred between 1910 and 1990, based on registers of births and deaths and normalized to constant births, shows an annual rise of only 1% from 1910 to 1946, followed by a steady exponential rise with a doubling time of 12 years, closely matching that of 11 years derived from the Queen's figures.

The exponential rise in the number of those born from 1846 onwards living to a hundred precedes by many years the general rise in the expectation of life at birth and the general drop in mortality from infectious diseases, but it coincides with the beginning of a steady rise in real wages. Another important factor may be improved medical treatment at old age from 1946 onwards.
Sex ratios in bumble bees
Authors: Bourke Andrew F. G.
Published: October 2002

The median proportion of investment in females among 11 populations of seven bumble bee (Bombus) species was 0.32 (range 0.07 to 0.64). By contrast, two species of workerless social parasites in the related genusPsithyrushad female–biased sex allocation, the reasons for which remain unclear. Male–biased sex allocation inBombuscontradicts the predictions of Trivers and Hare's sex ratio model for the social Hymenoptera, which are that the population sex investment ratio should be 0.5 (1:1) under queen control and 0.75 (3:1 females:males) under worker control (assuming single, once–mated, outbred queens and non–reproductive workers). Male bias inBombusdoes not appear to be either an artefact, or purely the result of symbiotic sex ratio distorters. According to modifications of the Trivers—Hare model, the level of worker male–production inBombusis insufficient to account for observed levels of male bias. There is also no evidence that male bias arises from either local resource competition (related females compete for resources) or local mate enhancement (related males cooperate in securing mates). Bulmer presented models predicting sexual selection for protandry (males are produced before females) in annual social Hymenoptera and, as a consequence (given some parameter values), male–biased sex allocation. Bumble bees fit the assumptions of Bulmer's models and are protandrous. These models therefore represent the best current explanation for the bees' male–biased sex investment ratios. This conclusion suggests that the relative timing of the production of the sexes strongly influences sex allocation in the social Hymenoptera.
EXO1-dependent single-stranded DNA at telomeres activates subsets of DNA damage and spindle checkpoint pathways in budding yeast yku70Δ mutants
Authors: Maringele Laura , Lydall David
Published: October 2002

We have examined the role of checkpoint pathways in responding to ayku70Δ defect in budding yeast. We show thatCHK1, MEC1, and RAD9 checkpoint genes are required for efficient cell cycle arrest of yku70Δ mutants cultured at 37°C, whereas RAD17,RAD24, MEC3, DDC1, and DUN1play insignificant roles. We establish that cell cycle arrest ofyku70Δ mutants is associated with increasing levels of single-stranded DNA in subtelomeric Y‘ regions, and find that the mismatch repair-associated EXO1 gene is required for both ssDNA generation and cell cycle arrest of yku70Δ mutants. In contrast, MRE11 is not required for ssDNA generation. The behavior of yku70Δ exo1Δ double mutants strongly indicates that ssDNA is an important component of the arrest signal inyku70Δ mutants and demonstrates a link between damaged telomeres and mismatch repair-associated exonucleases. This link is confirmed by our demonstration that EXO1 also plays a role in ssDNA generation in cdc13-1 mutants. We have also found that the MAD2 but not the BUB2 spindle checkpoint gene is required for efficient arrest of yku70Δ mutants. Therefore, subsets of both DNA-damage and spindle checkpoint pathways cooperate to regulate cell division of yku70Δ mutants.
Nr-CAM is a target gene of the β-catenin/LEF-1 pathway in melanoma and colon cancer and its expression enhances motility and confers tumorigenesis
Authors: Conacci-Sorrell Maralice E. , Ben-Yedidia Tamar , Shtutman Michael , Feinstein Elena , Einat Paz , Ben-Ze'ev Avri
Published: October 2002

β-catenin and plakoglobin (γ-catenin) are homologous molecules involved in cell adhesion, linking cadherin receptors to the cytoskeleton. β-catenin is also a key component of the Wnt pathway by being a coactivator of LEF/TCF transcription factors. To identify novel target genes induced by β-catenin and/or plakoglobin, DNA microarray analysis was carried out with RNA from cells overexpressing either protein. This analysis revealed that Nr-CAM is the gene most extensively induced by both catenins. Overexpression of either β-catenin or plakoglobin induced Nr-CAM in a variety of cell types and the LEF/TCF binding sites in the Nr-CAM promoter were required for its activation by catenins. Retroviral transduction of Nr-CAM into NIH3T3 cells stimulated cell growth, enhanced motility, induced transformation, and produced rapidly growing tumors in nude mice. Nr-CAM and LEF-1 expression was elevated in human colon cancer tissue and cell lines and in human malignant melanoma cell lines but not in melanocytes or normal colon tissue. Dominant negative LEF-1 decreased Nr-CAM expression and antibodies to Nr-CAM inhibited the motility of B16 melanoma cells. The results indicate that induction of Nr-CAM transcription by β-catenin or plakoglobin plays a role in melanoma and colon cancer tumorigenesis, probably by promoting cell growth and motility.
Chromatin remodeling by RSC involves ATP-dependent DNA translocation
Authors: Saha Anjanabha , Wittmeyer Jacqueline , Cairns Bradley R.
Published: October 2002

Chromatin-remodeling complexes couple ATP hydrolysis to alterations in histone–DNA interactions and nucleosome mobility, allowing transcription factors access to chromatin. Here, we use triple-helix strand-displacement assays, DNA length-dependent ATPase assays, and DNA-minicircle ATPase assays to establish that RSC, as well as its isolated ATPase subunit Sth1, are DNA translocases. RSC/Sth1 ATPase activity is stimulated by single-stranded DNA, suggesting that Sth1 tracks along one strand of the DNA duplex. Each RSC complex appears to contain a single molecule of Sth1, and isolated Sth1 is capable of nucleosome remodeling. We propose that the remodeling enzyme remains in a fixed position on the octamer and translocates a segment of DNA (with accompanying DNA twist), which breaks histone–DNA contacts and propagates as a wave of DNA around the octamer. The demonstration of DNA translocation presented here provides a mechanistic basis for this DNA wave. To test the relative contribution of twist to remodeling, we use nucleosomes containing nicks in precise locations to uncouple twist and translocation. Nucleosomes bearing nicks are remodeled less efficiently than intact nucleosomes. These results suggest that RSC and Sth1 are DNA translocases that use both DNA translocation and twist to remodel nucleosomes efficiently.
The lytic switch protein of KSHV activates gene expression via functional interaction with RBP-Jκ (CSL), the target of the Notch signaling pathway
Authors: Liang Yuying , Chang Jean , Lynch Stephen J. , Lukac David M. , Ganem Don
Published: October 2002

The RTA protein of the Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is responsible for the switch from latency to lytic replication, a reaction essential for viral spread and KS pathogenesis. RTA is a sequence-specific transcriptional activator, but the diversity of its target sites suggests it may act via interaction with host DNA-binding proteins as well. Here we show that KSHV RTA interacts with the RBP-Jκ protein, the primary target of the Notch signaling pathway. This interaction targets RTA to RBP-Jκ recognition sites on DNA and results in the replacement of RBP-Jκ's intrinsic repressive action with activation mediated by the C-terminal domain of RTA. Mutation of such sites in target promoters strongly impairs RTA responsiveness. Similarly, such target genes are induced poorly or not at all by RTA in fibroblasts derived from RBP-Jκ−/− mice, a defect that can be reversed by expression of RBP-Jκ. In vitro, RTA binds to two adjacent regions of RBP-Jκ, one of which is identical to the central repression domain that binds the Notch effector fragment. These results indicate that KSHV has evolved a ligand-independent mechanism for constitutive activation of the Notch pathway as a part of its strategy for reactivation from latency.