Showing papers by "University of Sussex published in 1987"

A computer algorithm for reconstructing a scene from two projections

Abstract: A simple algorithm for computing the three-dimensional structure of a scene from a correlated pair of perspective projections is described here, when the spatial relationship between the two projections is unknown. This problem is relevant not only to photographic surveying1 but also to binocular vision2, where the non-visual information available to the observer about the orientation and focal length of each eye is much less accurate than the optical information supplied by the retinal images themselves. The problem also arises in monocular perception of motion3, where the two projections represent views which are separated in time as well as space. As Marr and Poggio4 have noted, the fusing of two images to produce a three-dimensional percept involves two distinct processes: the establishment of a 1:1 correspondence between image points in the two views—the ‘correspondence problem’—and the use of the associated disparities for determining the distances of visible elements in the scene. I shall assume that the correspondence problem has been solved; the problem of reconstructing the scene then reduces to that of finding the relative orientation of the two viewpoints.

Three-year-olds' difficulty with false belief: The case for a conceptual deficit

Abstract: The hypothesis, that a conceptual limitation underlies 3-year-olds' difficulty with false-belief attribution (Wimmer & Perner, 1983), was tested against three competing hypotheses. These were: (1) failure to retain essential facts, (2) failure to understand the normal expectations which give rise to false belief and (3) pragmatic misinterpretation of the test question. Results showed that false-belief attribution remained difficult for younger 3-year-olds despite their retention of essential facts and despite attempts to make expectations more explicit and prevent pragmatic misinterpretation. These findings strengthen the original hypothesis, specified here as the inability to assign conflicting truth values to propositions. This hypothesis can explain why 3-year-olds find pretend play, the distinction between expected and achieved outcomes, the real-imaginary distinction and level 1 perspective taking easier to understand than false belief, the reality-appearance distinction and level 2 perspective taking.

The stability of the fullerenes C n , with n = 24, 28, 32, 36, 50, 60 and 70

Abstract: It has been proposed1 that the geodesic and chemical properties inherent in a closed, hollow, spheroidal, carbon cage structure with the symmetry of a European football can readily explain the remarkable stability observed for the C60 molecule. Here I present a set of simple, empirical chemical and geodesic rules which relate the stability of carbon cages mainly to the disposition of pentagonal rings, or various directly fused pentagonal ring configurations. The rules yield cluster magic numbers consistent with observation and in particular predict that the fullerenes, Cn for which n = 24, 28, 32, 36, 50, 60 and 70 should have enhanced stability relative to near neighbours. These results provide further evidence for the proposal that closed hollow cages form when carbon nucleates in the vapour phase, and in particular that C60 buckminsterfullerene is indeed a truncated icosahedron as originally proposed1.

Mental models in cognitive science

Abstract: If cognitive science does not exist then it is necessary to invent it. That slogan accommodates any reasonable attitude about the subject. One attitude-an optimistic one-is that cognitive science already exists and is alive and flourishing in academe: we have all in our different ways been doing it for years. The gentleman in Moliere’s play rejoiced to discover that he had been speaking prose for forty years without realizing it: perhaps we are merely celebrating a similar discovery. And, if we just keep going on in the same way, then we are bound to unravel the workings of the mind. Another attitude-my own-is more pessimistic: experimental psychology is not going to succeed unaided in elucidating human mentality; artificial intelligence is not going to succeed unaided in modelling the mind; nor is any other discipline-linguistics, anthropology, neuroscience, philosophy-going to have any greater success. If we are ever to understand cognition, then we need a new science dedicated to that aim and based only in part on its contributing disciplines. Yet pessimism should not be confused with cynicism. We should reject the view that cognititie science is merely a clever ruse dreamed up to gain research funds-that it is nothing more than six disciplines in search of a grant-giving agency. Cognitive science does not quite exist: its precursors do, but it lacks a clear identity. Perhaps the major function of this conference should be to concentrate our minds on what that identity might be. At present, there appear to be two distinct ideas wrapped up in it: one topic-oriented, and the other methodological. The topic-oriented idea is that workers from several disciplines have converged upon a number of central problems and explanatory concepts. George Miller and 1 became aware of this convergence when we were caught in the toils

The management of common property resources: collective action as an alternative to privatisation or state regulation

Abstract: Can locally based collective action be a viable way to manage common property resources? Many writers on collective action and common property are sweepingly pessimistic about the ability of the people who face problems of common property to organize sustainable patterns of use for themselves. This paper shows, with reference to Prisoners Dilemma, Garett Hardin's 'tragedy of the commons', and Mancur Olson's 'logic of collective action', that the analytical basis for this pessimism is weak in many situations of village-based common property resource use. There can thus be no general presumption that the collective action route to common property resource management will fail, any more than there can be a general presumption that it will work. The paper suggests a number of general factors to do with the characteristics of the resources, the user group, and group state relations, on which chances of success depend.

Trade and trade policy with differentiated products: a chamberlinian-ricardian model*

Abstract: This paper develops a model of international trade between economies which each contain a monopolistically-competitive industry producing differentiated products under increasing returns to scale. Firms have larger shares in their domestic market than in their export market and the implications of these uneq ual market shares for the pattern of trade and welfare are examined. Tariff policy and industrial policy are studied, and it is establishe d that a country can increase its welfare both by import tariffs and by subsidies to domestic firms. Copyright 1987 by Royal Economic Society.

Antifungal alkyl pyrones of Trichoderma harzianum

Abstract: Two strains of Trichoderma harzianum were both found to produce the volatile metabolites 6-n-pentyl-2H-pyran-2-one and 6-n-pentenyl-2H-pyran-2-one. The pentyl analogue, the major product, demonstrated potent inhibitory properties to a wide range of fungi and considerably reduced the rate of damping off in lettuce seedlings by Rhizoctonia solani during in vitro testing.

Some effects of jet-lag and their alleviation by melatonin

Abstract: Seventeen healthy volunteers (10 women and 7 men, aged 29-68) were flown from London to San Francisco between 20 November 1985 and 25 January 1986 and remained there for 14 days prior to flight home. Subjects took melatonin (N = 8, 5 women, 3 men) or placebo in a double-blind design, at 18.00h local time for three days before the return flight and at bedtime (22.00-24.00h) in Great Britain for four days. For three days before departure and on days 1-7,14,15, 21 and 22 after their return subjects collected 6-hourly sequential urine samples and kept a daily sleep log. They recorded mood and oral temperature 2 hourly and performed logical reasoning and letter cancellation tests 4 hourly from 08.00h (or wake up time) to 24.00h (or bedtime) whichever was the earlier. Urine was also collected for 48 h prior to departure from the U.S.A‥ On day 7 after their return subjects rated ‘jet lag’ (10 cm visual analogue scale—VAS) from 0 (insignificant) to 100 (very bad). Melatonin significantly improved ‘jet la...

Sexual Selection: The Handicap Principle Does Work -- Sometimes

Abstract: Zahavi's \`handicap principle' proposes that females prefer males with handicaps (mating characters that reduce survival chances) because handicaps are indicators of heritable viability. It is shown here that there are conditions under which the \`handicap principle' causes the runaway exaggeration of male handicaps and female mating preferences. The conditions required are (a) that the fitness effects of the handicap and \`viability' genes combine non-multiplicatively (Zahavi's handicap), and/or (b) that the handicap should directly reveal the presence or absence of genes for high viability (the revealing handicap). The \`handicap principle' by itself cannot initiate increases in female preference when the handicap is rare. It only works when a threshold value of female preference is exceeded, and Fisher's feedback process operates. When Fisher's feedback process occurs alone, a line of equilibria exists, where for each intensity of female preference there is a corresponding equilibrium development of the male mating character. When the `handicap principle' also operates, the internal line of equilibria is eliminated, and only boundary equilibria persist (i.e. fixation or loss of the handicap). All populations at what were previously internal equilibria, or in which the intensity of female preference is above threshold, increase in a runaway to fixation of the handicap; therefore, handicapping male mating characters are more likely to be exaggerated when they are also indicators of viability.

Landmark maps for honeybees

Abstract: Experiments by Fabre (1915), Thorpe (1950), Chmurzynski (1964), and most recently Gould (1986) suggest that insects have "maps" of their terrain which enable them to find their way directly to a goal when they are displaced several hundred metres from it. This paper discusses what might constitute an insect's map in terms of a two-part computational model. The first part describes how an insect reaches a goal when the insect is sufficiently close that it can see some of the landmarks which are visible from the goal. The second part considers the problem of navigating when there is no similarity between the view from the release-site and the view from the goal. We start from a model designed to explain how a bee might return to a goal using a two-dimensional "snapshot" of the landscape seen from the goal (Collett and Cartwright 1983). To guide its return, the model bee continuously compares its snapshot with its current retinal image and moves so as to reduce the discrepancy between the two. Bees can only be guided in the right direction by the difference between current retinal image and snapshot when there is some resemblance between the two. In a realistically cluttered world, snapshot and retinal image become very dis-similar only a short distance from the goal. To increase the distance from which a model bee can return, the bee takes two snapshots at the goal. The first snapshot excludes landmarks near to the goal and the second snapshot includes them. With close landmarks filtered from both snapshot and retinal image, the match between the two deteriorates gradually as the bee moves away from the goal. A model bee using a filtered snapshot and image finds its way back to the neighbourhood of the goal from a relatively long distance (Fig. 2). The bee then switches to the second snapshot and is guided to the precise spot by its memory of the close landmarks. For longer range guidance, the model bee is equipped with an album of snapshots, each taken at a different location within the terrain. Linked to each snapshot is a vector encoding the distance and direction from the place where the snapshot was taken to the hive. When the bee is displaced to a new position, it selects the snapshot which best matches its current image and follows the associated home-vector back to the hive (Fig. 3). Such a hive-centred map can also be used to devise novel routes to places other than the hive. For instance, a bee can reach a foraging site from anywhere in its terrain by adding the home-vector recalled at the starting position to a vector specifying the distance and direction of the foraging site from the hive. The sum of these two vectors defines a direct trajectory to the foraging site.

On magnetic braking of late-type stars

Abstract: This report deals with the rotational history of late-type stars subsequent to their reaching the main sequence. The question of relevance to this Symposium is whether detailed comparison of magnetic braking theory with observations of young stellar clusters will yield any constraints on dynamo activity, in particular on the relation between the surface field strength B s and the angular velocity Ω.

Histochemical staining of clonal mammalian cell lines expressing E. coli beta galactosidase indicates heterogeneous expression of the bacterial gene.

Abstract: An evaluation has been made of the E. coli beta-galactosidase (beta-gal) gene for use as a reporter gene in mammalian cells in culture. We have adopted a histochemical procedure which enables identification of those cells within a population that express the introduced bacterial gene. Data is presented concerning the sensitivity of the histochemical method relative to an immunological method of detection. It has been found that several clonal cell lines generated after transfection of human 293 cells with a Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter-beta-gal construction are mosaic for expression of the introduced mini-gene. Furthermore, after treatment of these clonal cell lines with the nucleoside analog 5-aza-cytidine (5-aza-C), an increase in production of beta-gal under control of this promoter element was observed.

The response of hair cells in the basal turn of the guinea‐pig cochlea to tones.

Abstract: 1. Intracellular recordings were made from inner and outer hair cells in the basal turn of the guinea-pig cochlea. The resting membrane potentials of the inner hair cells are more positive than -50 mV while those of outer hair cells are usually more negative than -70 mV. 2. At low frequencies the receptor potentials of inner hair cells are predominantly depolarizing while those from outer hair cells are hyperpolarizing at low and moderate sound pressure (e.g. less than 90 dB re 2 X 10(-5) Pa at 600 Hz). The potentials then become predominantly depolarizing at high sound pressure. 3. The asymmetry of the inner and outer hair cell receptor potentials are manifested instantaneously except at high stimulus levels when the depolarizing responses of outer hair cells take several cycles to develop. 4. At the offset of intense tones outer hair cell membrane potentials remain depolarized by 1-2 mV above their resting value and return to normal over a period depending on the level and duration of the tone. 5. In response to tones above about 2 kHz and at levels below about 90 dB the wave forms of outer hair cell receptor potentials are virtually symmetrical without measurable d.c. components. In response to tones close to their best frequencies (16-21 kHz), inner hair cells in the basal turn generate large depolarizing (d.c.) receptor potentials while outer hair cells from this region of the cochlea do not generate significant voltage responses. 6. Frequency tuning curves were derived for inner and outer hair cells from the amplitude-intensity relationships of their d.c. and phasic (a.c.) receptor potentials respectively. When the latter were compensated for the low-pass characteristics of the recording system and the hair cell time constant, the frequency selectivity of inner and outer hair cells are similar. 7. The response properties of inner and outer hair cells in the basal turn of the guinea-pig cochlea are discussed in relation to their proposed roles in mechano-electric transduction.

The use of visual landmarks by honeybees: Bees weight landmarks according to their distance from the goal

Abstract: 1 Honeybees learn the location of a source of sucrose in relation to the positions of visual landmarks It is shown here that bees pay most attention to those landmarks which are close to the sucrose and which will therefore guide a bee most accurately to its goal 2 Individually marked bees were trained to collect sucrose from a reservoir whose location was specified by an array of landmarks, with individual landmarks placed at different distances from the reservoir Once trained, the bees' flight path was recorded in tests with the sucrose removed In these tests, the array of landmarks was transformed from the training configuration to provide two possible sites for the sucrose One was defined by the landmarks close to the sucrose reservoir in training, the other by more distant landmarks In all experiments (Figs1, 2 and 3), bees spent more time flying in the site defined by the close landmarks They did so even when the apparent sizes of the close and distant landmarks viewed from the reservoir were the same We conclude that bees learn the distance of landmarks from the goal, and that, during their search for the goal, they weight nearer landmarks more heavily than distant ones 3 Landmarks are also weighted according to their (apparent) size Bees were trained to find sucrose at a reservoir placed equidistantly from different-sized landmarks Tests were given with the training array stretched to provide two possible sites for the sucrose One site was specified by the large, the other by the small landmarks Bees spent more time flying in the site defined by the larger landmarks (Fig 5) 4 We discuss, with the aid of computer simulation, different ways in which the distance between landmark and goal could influence a bee's searching behaviour

The vanadium nitrogenase of Azotobacter chroococcum. Purification and properties of the VFe protein.

Abstract: 1. Nitrogenase activity of a strain of Azotobacter chroococcum lacking the structural genes for conventional nitrogenase (nifHDK) was separated into two components: an Fe-containing protein and a vanadoprotein. 2. The larger protein was purified to homogeneity by the criterion of electrophoresis of 10% (w/v) acrylamide gels in the presence of SDS. Two types of subunit, of Mr 50,000 and 55,000, were present in equal amounts. 3. The protein had an Mr of 210,000 and contained 2 V atoms, 23 Fe atoms and 20 acid-labile sulphide groups per molecule. The Mo content was less than 0.06 g-atom/mol. All the common amino acids were present, with a predominance of acidic residues. Ultracentrifugal analysis gave a maximum sedimentation coefficient of 9.7 S and a symmetrical boundary at 5 mg of protein X ml-1; dissociation occurred at lower concentrations. The specific activities (nmol of product/min per mg of protein), when assayed under optimum conditions with the complementary Fe protein from this strain, were 1348 for H2 evolution, 350 for NH3 formation and 608 for acetylene reduction. Activity was O2-labile, with a t1/2 of 40 s in air. At low temperatures the dithionite-reduced protein showed e.p.r. signals at g = 5.6, 4.35, 3.77 and 1.93, consistent with an S = 3/2 ground state with an additional S = 1/2 centre giving rise to the feature at g = 1.93. The u.v. spectra of dithionite-reduced and thionine-oxidized protein were very similar. Oxidation resulted in a general increase in absorbance in the visible region. The shoulder at 380 nm in the spectrum of reduced protein was replaced with shoulders near 330 nm and 420 nm on oxidation.

Magnetic fields and non-uniform rotation in stellar radiative zones

Abstract: Effets des instabilites dynamiques et resistives sur la redistribution magnetique du moment angulaire a l'interieur d'une etoile

Evolution of N-body open clusters

Abstract: On discute de simulations realistes a r corps de l'evolution dynamique d'amas ouverts et on les compare aux observations. La plupart comportent 1000 corps dont les masses initiales suivent une loi de puissance de pente α=−2,75 et dont la valeur moyenne est 0,5 MO

An analysis of the costs and benefits of physiological integration between ramets in the clonal perennial herb Glechoma hederacea

Abstract: The costs and benefits, measured in terms of dry weight, of physiological integration between clonal ramets, were analysed in two experiments conducted on the clonal herb Glechoma hederacea. Firstly, integration between consecutively-produced ramets was examined in an experiment in which stolons grew from one set of growing conditions (either unshaded or shaded and either nutrient-rich or nutrient-poor) into conditions in which light or nutrient level was altered. Comparisons were made between the dry weight of the parts of the clones produced before and after growing conditions were changed, and the dry weights of the corresponding part of control clones subjected to constant growing conditions. In a second experiment, integration between two distinct parts of G. hederacea clones was investigated. In this experiment clones were grown from two connected parent ramets and the parts of the clone produced by each parent ramet were subjected independently to either nutrient-rich or nutrient-poor conditions. Ramets in resource-rich conditions provided considerable physiological support to those in resource-poor conditions. This was measured as a dry weight gain compared with the weight of the corresponding part of the control clones growing in resource-poor conditions. However, when stolons grew from resource-poor conditions into resource-rich conditions, there was no similar evidence of the resourcepoor ramtes receiving support from resource-rich ramets. Physiological integration did not result in dry weight gains when this would have necessitated basipetal translocation of resources.Because of the predominantly acropedal direction of movement of translocates in G. hederacea, the structure of the clone was important in determining the effectiveness of integration between ramets. Where physiological integration was effective, the cost to the supporting ramets in terms of dry weight was insignificant. Physiological integration allows clones to maintain a presence in less favourable sites with insignificant cost to ramets in favourable sites, thereby reducing the probability of invasion by other plants, and providing the potential for rapid clonal growth if conditions improve. Integrated support of ramets in unfavourable conditions also enables the clone to grow through unfavourable sites, thus increasing the probability of encountering more favourable conditions by wider foraging.

Quntitative ion distribution within root cells of salt-sensitive and salt-tolerant maize varieties.

Abstract: Significant varietal differences were apparent in the survival of seedlings of maize in saline conditions but only at relatively high external concentrations (200 mol m−3 NaCl), where there was a range from 0 to 66% survival, 25 d after salinization. For the varieties examined there was a strong negative correlation between Na concentrations in the third leaf and survival. Two resistant varieties (Across 8024 and Protador) and one salt-sensitive variety (LG11) were identified. The characteristics of ion accumulation were clearly different in salt-tolerant and salt-sensitive types, the difference becoming more pronounced with plant age. The distribution of ions, particular those of Na, K and Cl, was determined within subcellular compartments of roots cells using X-ray microanalysis of freeze-substituted tissue. Salinity induced a greater increase (about 1.7 times) in cytoplasmic Na concentration in the salt-sensitive variety (LG11) than in resistant varieties (Across 8024 or Protador). The mean K:Na ratio in the cytoplasm of the root cortical cells in the salt-resistant varieties grown for 15 d in saline conditions (100 mol m−3 NaCl) was twice that found for LG11. Sodium and Cl concentrations in the vacuoles decreased radially inwards from the epidermal cells in salt-treated roots of LG11.