Showing papers by "University of York published in 1984"

Scalar timing in memory.

Abstract: A recent report of ours’ proposed an information-processing account of temporal generalization. The account posited a clock process, which was the basic time measurement device, and working and reference memory for storing the output of the clock either temporarily or relatively permanently. Records of time intervals in working and reference memory were then compared using a binary decision process, which dictated responding or not responding. The analysis concentrated on a relativistic Weber’s law property of the data from temporal generalization, and the constraints this property imposed on sources of variance in the information-processing stages. Our purpose here is to summarize that work and generalize the model in two ways: First we consider several sources of variance operating simultaneously. The original analysis demonstrated that if only one source of variance is present, it must be a scalar source, that is, it must result in a variable memory for which variance increases with the square of the mean.’ In the generalized account proposed here, we will develop the conclusion that scalar sources dominate in some time ranges, while other sources may dominate in others. These ideas are then applied to two additional timing tasks with different characteristics.

Enemy free space and the structure of ecological communities

Abstract: We define ‘enemy free space’ as ways of living that reduce or eliminate a species' vulnerability to one or more species of natural enemies. Many aspects of species' niches, in ecological and evolutionary time have apparently been moulded by interactions with natural enemies for enemy free space. We review a large number of examples. Yet many ecologists continue to think and write as though classical resource based competition for food or space is the primary determinant of species' niches. Often it is not. The recognition that the struggle for enemy free space is an important component of many species' ecologies may have important consequences for studies of community convergence, limits to species packing, and the ratio of predator species to prey species in natural communities.

Effects of differences in mineralization on the mechanical properties of bone

Abstract: There is a considerable variation in the mineralization of bone; normal, non-pathological compact bone has ash masses ranging from 45 to 85% by mass. This range of mineralization results in an even greater range of mechanical properties. The Young modulus of elasticity can range from 4 to 32 GPa, bending strength from 50 to 300 MPa, and the work of fracture from 200 to 7000 Jm-2. It is not possible for any one type of bone to have high values for all three properties. Very high values of mineralization produce high values of Young modulus but low values of work of fracture (which is a measure of fracture toughness). Rather low values of mineralization are associated with high values of work of fracture but low values of Young modulus and intermediate values of bending strength. The reason for the high value for the Young modulus associated with high mineralization is intuitively obvious, but has not yet been rigorously modelled. The low fracture toughness associated with high mineralization may be caused by the failure of various crack-stopping mechanisms that can act when the mineral crystals in bone have not coalesced, but which become ineffective when the volume fraction of mineral becomes too high. The adoption of different degrees of mineralization by different bones, leading to different sets of mechanical properties, is shown to be adaptive in most cases studied, but some puzzles still remain.

3-Å resolution structure of a protein with histone-like properties in prokaryotes

Abstract: The 3-A structure of DNA-binding protein II, which exhibits histone-like properties in bacteria, has been determined. The molecule is dimeric and appears to bind to the phosphate backbone of DNA through two symmetry-related arms. A mechanism by which the protein induces DNA supercoiling is proposed.

The Existence of a Time-Dependent Equilibrium Distribution of Arrivals at a Single Bottleneck

Abstract: The paper gives conditions which guarantee the existence of an equilibrium arrival pattern at a single bottleneck. In the model, the times at which a driver wishes to leave the bottleneck depend on the driver. The equilibrium queue length in this model always has a continuous time derivative everywhere. However, the slope of the equilibrium cumulative arrival distribution is discontinuous at the beginning and end of congestion.

The estimation of neighbourhood parameters for plant populations

Abstract: Sewall Wright's neighbourhood model indicates that the area containing a panmictic unit within a continuous and uniform array of organisms can be estimated by 4πσ2 where σ2 is the parent-offspring dispersal variance measured around a zero mean and relative to a single reference axis passing through the population. The method has proved popular in studies on higher plants but the literature is confused as to how the two components of dispersal variance, for pollen and seeds, should be combined. It is argued that σ2=½σp2+σs2 where σ2p and σ2s are pollen and seed dispersal variances already corrected to give axial variances. The consequences of using different methods are compared and some puzzling aspects of the previous literature are discussed.

Division algebras, (pseudo)orthogonal groups and spinors

Abstract: The groups SO( nu -1), SO( nu ), SO( nu +1), SO( nu +1, 1) and SO( nu +2, 2) ( nu =1, 2, 4, 8) and their spin representations are described in terms of the division algebras R, C, H and O

Electrodermal Nonresponding, Antisocial Behavior, and Schizoid Tendencies in Adolescents

Abstract: Most studies which have tested the hypothesis of autonomic hyporeactivity in antisocial groups using skin conductance orienting response (SCOR) amplitudes have failed to confirm this prediction. However, frequency measures may represent more stable and complete indices of responsivity than amplitude measures and, therefore, provide a better test of an antisociality-hyporeactive relationship. This possibility was tested by relating electrodermal responding-nonresponding to a series of 65dB tones in 101 15-yr-old males to measures of antisocial behaviour and undersocialization. While SCOR amplitudes were unrelated to antisociality, frequency of response and incidence of nonre-sponding were significantly related in the predicted direction. A second analysis demonstrated antisocial nonresponders to be characterised by schizoid tendencies, but not by sensation-seeking. These data indicate that equating nonresponding with antisociality and interpreting such a relationship in motivational/arousal terms may be over-simplistic, and that it may be specifically a schizoid subsection of antisocials who are characterised by nonresponding. It is hypothesised that the frequently cited link between antisociality and psychosis-proneness may be underpinned by a disturbance of sustained attention as indexed by nonresponding.

A search for context-stimulus associations in latent inhibition

Abstract: Six experiments investigated the effects of pre-exposure to a tone on the subsequent acquisition of conditioned suppression by rats. In Experiments 1–3 the response suppressed was drinking; in Experiments 4–6 it was food-rewarded lever pressing. Repeated exposure to the tone resulted in latent inhibition, i.e., a retardation in the acquisition of suppression. The size of the latent inhibition effect was reduced when a different context was used for conditioning from that used for pre-exposure (Experiment 2). When the context remained the same throughout, a phase of exposure to the context alone, interposed between pre-exposure and conditioning, had no influence on the size of the latent inhibition effect ultimately observed. This last result casts doubt upon Wagner's (1976, 1979) theory of the role of contextual factors in latent inhibition, and alternative accounts are considered.

Ultrastructure of Paired Helical Filaments of Alzheimer's Neurofibrillary Tangle

Abstract: Isolated paired helical filaments (PHF) of Alzheimer's disease and senile dementia of the Alzheimer type (AD/SDAT) were studied by negative stain techniques and electron microscopy (EM). The helical structure of the PHF was confirmed by tilt analysis of negatively stained isolated PHF. A substructure of four protofilaments, each 3-5 nm in diameter, was also observed. A comparison of PHF substructure, neurofilaments and microtubules, revealed no similarity among the three structures.

Bonding of molecular oxygen to T state human haemoglobin

Abstract: Haemoglobin (Hb) is the tetrameric protein molecule that in vertebrate blood transports oxygen from the lungs to the tissues. This function depends on four subunits in the molecule binding cooperatively so that their affinity for oxygen increases as the level of oxygenation increases. X-ray analysis has shown that deoxyhaemoglobin, which has a low oxygen affinity, and oxyhaemoglobin, which has a high oxygen affinity, differ principally in their subunit or quaternary structures, referred to as the T and R states, respectively. As it switches from the T state to the R state during oxygenation, Hb increases its oxygen affinity. However, the structural pathway between deoxy- and oxy-haemoglobin is not known, principally because there has been no accurate structural knowledge of the intermediate states. We report here the crystal structure of T state human Hb in which the alpha chains are oxygenated and the beta subunits are oxygen-free. In this crystal the Hb appears to be in an intermediate state between the unliganded T state and the liganded R state. There is also evidence that the Hb molecule operates by loading and unloading the beta haems and thus the alpha-oxy, beta-deoxy Hb crystal may represent a physiologically important state.

Reduction of cerebral edema with GM1 ganglioside

Abstract: Administration of exogenous gangliosides has been reported to accelerate neurite outgrowth in vitro, and to enhance peripheral nerve regeneration and central nervous system recovery subsequent to damage. After injury, facilitation of CNS recovery with GM1 ganglioside treatment has been postulated to be due to enhanced neuronal regeneration. Since maximal recovery is achieved when experimental animals are treated before injury with GM1 ganglioside, an alternative or parallel mechanism is that gangliosides are "protecting" the CNS by limiting the extent of damage (ie, cell loss, process degeneration, membrane disruption). This may be due to a reduction in the edema subsequent to injury. In this study, rats were treated for 2 days with 20 mg/kg/day of GM1 ganglioside. On the third day they were subjected to a unilateral lesion (mechanical) of one cerebral hemisphere and given another 20 mg/kg of GM1. On the fourth day brains were removed for analysis of edema resulting from the injury. In treated animals there was a significant reduction in edema as measured either in the entire injured hemisphere (23%) or in the area of injury (33%). No effect was seen outside the damaged area. Since exogenous gangliosides can spontaneously "insert" into membranes, it is postulated that the effect of the GM1 may be due to alterations of membrane processes (eg, lipid hydrolysis, phospholipase activation, levels and membrane action of arachidonic acid, ionic permeation) that are characteristic of edema.

Cognitive brain potential components in adolescents.

Abstract: Event-related potentials (ERPs) were recorded in a cross-sectional study from 70 adolescents ranging in age from 11 to 18. A paradigm was used in which background, standard auditory events (66 percent occurrence) were randomly replaced by either a change in pitch, or a missing stimulus, each occurring 17 percent of the time. Subjects were instructed to respond to one of the infrequent events, with each designated as relevant (i.e., a target) on alternate blocks of trials. Factor score analyses showed that P300 and frontal negative slow wave were larger to relevant than irrelevant events, whereas parietal positive slow wave was not. Both N200 and frontal negative slow wave showed modest age effects. Significant effects of sex, mainly for the exogenous components, were also found. These data highlight the importance of the negative task-related potentials as correlates of development. They also point to the functional independence of P300 and slow wave and add to the evidence for the functional distinction between the frontal negative and parietal positive aspects of slow wave.

The Economics of Optimism and Pessimism

Abstract: SUMMARY This paper suggests a way of incorporating the important concepts of optimism and pessimism into the accepted model of decision-making under uncertainty. We exploit the primitive notion that an optimist is someone who over-estimates (underestimates) the likelihood of favourable (unfavourable) outcomes. We show that this incorporation enables us to explain several commonly observed apparent violations of Subjective Expected Utility Theory. Several illustrations and economic applications are presented, and we show that attitude to ‘fate’ (as evidenced in optimism and pessimism) is a different dimension of personality than attitude to risk. We conclude by relating our extension of SEU theory to other recent extensions.

Anatomical variation in first leaves of nine tritucum genotypes, and its relationship to photosynthetic capacity

Abstract: Summary The variation in over 20 leaf characters including morphological, anatomical and cellular features have been examined in nine Triticum genotypes at three levels of ploidy. The characters included transectional leaf area, leaf width, distance between vascular bundles, volume occupied by each cell type, cell size and number, and chloroplast size and number. Analysis of these characters showed that the major components of the variation were related to the ploidy level and the genotype of the species and cultivars examined. The measured characters most strongly influenced by ploidy level were cell size, chloroplast number and leaf thickness. Genetic variation accounted for the variation in all other measured characters. The interaction of genetic variation with the major effects attributable to ploidy level creates a characteristic anatomical ‘fingerprint’ for each genotype. The observation that photosynthetic capacity is negatively correlated with the ratio of mesophyll cell size to nuclear genome size, and with no other character, is discussed.

A dipteran from south of the Antarctic Circle: Belgica antarctica (Chironomidae) with a description of its larva

Abstract: Belgica antarctica Jacobs is recorded for the first time from two localities south of the Antarctic circle: Orford Cliff (66°55'S) on the Continental mainland and the Refuge Islands (68°21'S) The larva is described, and measurements of head capsule lengths indicate four instars The larvae south of the Antarctic circle appear to be slightly smaller than those from within the main distributional range Belgica antarctica is certainly the earth's southernmost free-living holometabolous insect, but the flea Glaciopsyllus antarcticus has been recorded 14 minutes of latitude farther south than the southernmost records of B antarctica

Daily Meal Anticipation: Interaction of Circadian and Interval Timing

Abstract: Both short-interval and circadian timing systems support anticipatory response accelerations prior to food reinforcement. In the first case, the behavior pattern is determined by a scalar timing process with an arbitrary-reset property. In contrast, under daily cycles of food-availability, behavior reflects a self-sustaining oscillation. With rats as subjects, the concurrent operation of timing of both kinds was studied by addition of premeal auditory cues on the circadian baseline, in the absence of a day-night illumination cycle. Cues within both minute and hour ranges served to lower the level of premeal anticipatory responding, although exponential accelerations were similar to the uncued case. Cues within the minutes range yielded interval-timing functions that reflected approximate superposition. Cues within the hours range suppressed respondings at their outset, in proportion to cue duration. When one of the shorter cues was suddenly lengthened, short-interval accelerations appeared at inappropriate circadian phases. When a premeal cue was extended through mealtime, anticipation rates increased markedly, suggesting that cue termination at the start of mealtime is a potent anchor for premeal anticipation regardless of cue duration. By use of meal-omission probes without external cues, peak rates were located after the onset of expected mealtime, often near its termination. The results suggest interactions between the scalar interval timer and the circadian anticipation timer, as modulated by the circadian free-run timer.

The Response of Multispecies Systems to Perturbations

Abstract: This paper considers evidence from ecological theory and observation relevant to the response of multispecies marine communities to exploitation. The problems of reversibility, multiple stable states, and the differences between complex and simple communities are reviewed.

Comparative Mechanical Properties and Histology of Bone

Abstract: Different bone tissues differ in their amounts of porosity, mineralization,reconstruction, and preferred orientation. All these have important effects on mechanical properties. Very porous, cancellous bone is always weaker and morecompliant than compact bone on a weight for weight basis, yet it occurs in placeswhere its energyabsorbing ability, or its low density, is advantageous. Bonevaries considerably in its mineralization, and such variations have quite disproportionate effects on mechanical properties. These variations can be shown to be adaptive. In particular, there must always be a compromise between stiffnessandresistance to fracture; these two properties run contrary to each other. The reason for secondary remodeling is an unresolved problem, though in a few places the role of such remodeling in changing the grain of the bone is clearly mechanically adaptive. The mechanical properties of non-mammalian bone are obscure, and as the histology of such bone is often quite different from that of mammalian bone, we are no doubt in for some surprises when the mechanical properties ofnonmammalian bone are discovered.