Showing papers by "University of Crete published in 1991"

A Corneal Flap Technique for Laser In Situ Keratomileusis: Human Studies

Abstract: • Using a modified microkeratome, nasally based central corneal flaps were created on six human blind eyes at a depth of 300 μm. The flaps were allowed to heal with a bandage soft contact lens, using neither sutures nor bioadhesives. The optical quality of the corneas was evaluated with biomicroscopic examination, computer-assisted topography, and optical pachometry during a 3-month follow-up period. The maintenance of transparency and lack of distortion of the corneal surface 3 months after surgery support the concept that the flap technique may be useful in laser in situ keratomileusis.

On fully discrete Galerkin methods of second-order temporal accuracy for the nonlinear Schrödinger equation

Abstract: We approximate the solutions of an initial- and boundary-value problem for nonlinear Schrodinger equations (with emphasis on the `cubic' nonlinearity) by two fully discrete finite element schemes based on the standard Galerkin method in space and two implicit. Crank-Nicolson-type second-order accurate temporal discretizations. For both schemes we study the existence and uniqueness of their solutions and proveL 2 error bounds of optimal order of accuracy. For one of the schemes we also analyze one step of Newton's method for solving the nonlinear systems that arise at every time step. We then implement this scheme using an iterative modification of Newton's method that, at each time stept n , requires solving a number of sparse complex linear systems with a matrix that does not change withn. The effect of this `inner' iteration is studied theoretically and numerically.

Minimum and Maximum Delay Problems in Real-Time Systems

Abstract: We consider a finite-state system with a finite number of clocks, where the transitions may depend on the values of the clocks, and may reset some of the clocks. We address the complexity and provide algorithms for the following problems. Suppose that the system starts from a given current state with a given assignment of values to the clocks. Can a given target state ever appear in the history of the system? What is the earliest time it can appear? What is the latest time it can appear?

Minos, a new transposable element from Drosophila hydei, is a member of the Tc1-like family of transposons.

Abstract: We have cloned and sequenced a new repetitive element from Drosophila hydei. The element was isolated in a screen of a genomic library from D. hydei strain bb (1), performed to recover clones containing non-ribosomal DNA adjacent to ribosomal DNA sequences. A sequence 1775 nucleotides long was found inserted within the external transcribed spacer the rDNA locus, between bases 4257 and 4258 of the published sequence (2) (Figure 1). Southern blots of restricted DNA from two D. hydei strains showed distinct banding patterns (Figure 1), suggesting that the element is, or has until recently been mobile. The element was named Minos, after the legendary king who inhabited the palace located near our laboratories. The sequence of Minos has the following salient features: 1. Perfect inverted repeats 255 nt long (nucleotides 1 to 255 and 1521 to 1775) are found at the ends of the element. 2. Two non-overlapping open reading frames exist on the same strand. The first, designated ORF1, is located between nucleotides 334 and 792, begins with an ATG, and can encode a 153 amino acid long peptide. The second, designated ORF2, is located between nucleotides 898 and 1476 and does not begin with an ATG. The peptide sequence encoded by ORF2 has significant similarity with the putative transposase encoded by the long open reading frame of the transposable element Tel of Caenorhabditis elegans (3). Of the 201 amino acid residues in the two aligned sequences, 64 (~ 32%) are identical and 95 (—47%) are related (Figure 2). Similar sequence homology was detected between ORF2 and regions of the other members of the Tel-like transposon family (4, 5, 6, 7). Based on the genomic distribution of the element, the presence of perfect terminal repeats, and the sequence similarity with Tel, we conclude that Minos is a newly discovered member of the widely dispersed class of Tel-like transposons. The sequenced element, because of the stop codon within the putative transposase gene, presumably cannot encode active transposase.

Model-checking for probabilistic real-time systems

Abstract: Model-checking is a method of verifying concurrent systems in which a state-graph model of the system behavior is compared with a temporal logic formula. This paper extends model-checking to stochastic real-time systems, whose behavior depends on probabilistic choice and quantitative time. The specification language is TCTL, a branching-time temporal logic for expressing real-time properties. We interpret the formulas of the logic over generalized semi-Markov processes. Our model can express constraints like “the delay between the request and the response is distributed uniformly between 2 to 4 seconds”.

Social categorization and person memory: The pervasiveness of race as an organizing principle

Abstract: Two studies investigated the pervasiveness of race as a social categorization and whether the organization of information around racial categories is sensitive to contextual factors. Both studies measured accentuation effects (more intra- than inter-race errors) and own-group bias (fewer confusions between own- than other-group members) in person memory, using the paradigm developed by Taylor, Fiske, Etcoff and Ruderman (1978). Experiment 1 studies the generalization of these effects across ethnic group membership (black/white) and topic (categorization-relevant/irrelevant) in a 2 × 2 [× 4] between-subject design, with type of error as a repeated measure. There was a highly significant accentuation effect, which was not affected by either topic or group membership. Experiment 2, using white subjects only, manipulated anticipated future interaction/no interaction, which affected overall accuracy/error rate, but not the strong accentuation effect. Neither study found any support for an own-group bias. Results are discussed in terms of the automaticity of race as a basis for social categorization.

Appearance of lipid-absorption capacities in larvae of the sea bass Dicentrarchus labrax during transition to the exotrophic phase

Abstract: The development of intestinal lipid absorption capacities was studied in larvae of the sea bassDicentrarchus labrax from the time of mouth opening to 25 d after hatching, in individuals fed with brine shrimp,Artemia sp. (Brazil commercial strain) or artificial diets. At the time of mouth opening, before the first feed, enterocytes synthesized lipoprotein particles from endogenous lipids. Starting with the first feed, enterocytes absorbed food lipids regardless of the diet, but since these cells have a low capacity for lipoprotein synthesis, they accumulated lipids in the form of “free” lipid droplets. In larvae fed withArtemia sp., the rapid development of enterocytes during growth was combined with increasingly effective lipoprotein synthesis (becoming even greater than that observed in the adult), starting on Days 18 to 19. Although lipoprotein synthesis and transport were observed in larvae given artificial feed, they showed abnormalities in their intestinal mucosa. Lipid droplets formed in association with the membrane structures of specimens given egg yolk during the first days of feeding, and enterocytes degenerated. In 18 to 19 d-old larvae given artificial feed, intestinal folds disappeared and the constituent cells showed limited differentiation. In the last-mentioned individuals, the abnormalities appeared to be related to malnutrition resulting from inadequacy of the food offered, which led to total mortality of larvae, beginning on Day 20.

Plant NAD(H)-Glutamate Dehydrogenase Consists of Two Subunit Polypeptides and Their Participation in the Seven Isoenzymes Occurs in an Ordered Ratio

Abstract: The structure and function of NAD(H)-glutamate dehydrogenase in plants was studied by using grapevine (Vitis vinifera L. cv Sultanina) callus grown under different nitrogen sources. The enzyme consists of two subunit-polypeptides, α and β, with similar antigenic properties but with different molecular mass and charge. The two polypeptides have molecular masses of 43.0 and 42.5 kilodaltons, respectively. The holoenzyme is hexameric and is resolved into seven isoenzymes by native gel electrophoresis. Two-dimensional native/SDS-PAGE revealed that the 1 and 7 isoenzymes are homohexamers and the isoenzymes 2 through 6 are hybrids of the two polypeptides following an ordered ratio. The total quantity of α- and β-polypeptides and the isoenzymic pattern was altered by the exogenous nitrogen source. The sample derived from callus grown on nitrate or glutamic acid contained a slightly greater amount of β-polypeptide and of the more cathodal isoenzymes, whereas α-polypeptide and the more anodal isoenzymes predominated in callus grown in the presence of either ammonium or glutamine. The anabolic reaction was correlated with the α- and the catabolic reaction with the β-polypeptide; this could suggest that each isoenzyme exhibits anabolic and catabolic function of different magnitude. The isoenzymic patterns did not obey the expected binomial distribution proportions.

Verifying Automata Specifications of Probabilistic Real-time Systems

Abstract: We present a model-checking algorithm for a system presented as a generalized semi-Markov process and a specification given as a deterministic timed automaton. This leads to a method for automatic verification of timing properties of finite-state probabilistic real-time systems.

Statism, Social Policy and the Middle Classes in Greece

Abstract: This article discusses welfare policy and the trends of development of public expenditure and the social budget in Greece in the post war period. Particular emphasis is given to the expansion of income transfers during the 1980s under the conditions of economic recession. An understanding of the relationship between the state and civil society in Greece is crucial for explaining social policy developments. Our analysis focuses on the strong grip of politics on civil society, which constitutes a major aspect of Greek statism and contributes to an increased fragmentation of social demands and conflicts.The main argument of this article is that the growing size and political weight of the middle classes since the mid-1970s intensified social fragmentation and the contradictions of Greek statism, and made it more difficult to achieve any consensus about social policy aims. Public expenditure and the social budget increased significantly during the 1980s, as a result of presstrre from various middle-class stra...

Review of Citrobacter bacteremia in cancer patients over a sixteen-year period.

Abstract: A review was conducted of 65 episodes ofCitrobacter bacteremia in cancer patients during a 16-year period. Cases of polymicrobial bacteremia were excluded from this analysis. The infection occurred most commonly in patients with acute leukemia. Most patients acquired the infection in the hospital, and 57 % had received antibiotic therapy during the preceding ten days. Fever occurred in 98 % of cases and shock in 17 %. Thirty-eight percent of patients had concomitant pneumonia. Patients with shock, pneumonia or hemorrhage had a substantially poorer prognosis. The response rate was 72 % for patients who received appropriate antibiotics. Patients who continued to have positive blood culture results while receiving appropriate antibiotic therapy had a poor prognosis. Beta-lactam antibiotics were more effective than aminoglycosides.

Effect of the manganese complex on the binding of the extrinsic proteins (17, 23 and 33 kDa) of Photosystem II.

Abstract: Selective extraction-reconstitution experiments with the extrinsic Photosystem II polypeptides (33 kDa, 23 kDa and 17 kDa) have demonstrated that the manganese complex and the 33 kDa polypeptide are both necessary structural elements for the tight binding of the water soluble 17 and 23 kDa species. When the manganese complex is intact the 33 kDa protein interacts strongly with the rest of the photosynthetic complex. Destruction of the Mn-complex has two dramatic effects: i) The binding of the 33 kDa polypeptide is weaker, since it can be removed by exposure of the PS II system to 2 M NaCl, and ii) the 17 and 23 kDa species do not rebind to Mn-depleted Photosystem II membranes that retain the 33 kDa protein.

Masers and Star Formation

Abstract: The physics of astronomical masers and their connection with star-forming regions are reviewed. For a better understanding of the subject, a qualitative discussion is given of the basic concepts about masers. These are: Amplification, saturation, thermalization, beaming, apparent size, variability, line width and polarization. The difference between laboratory and astronomical masers is discussed and a few examples of the usefulness of astronomical masers are given. The basic requirements for the construction of a maser model are presented and the accuracy with which the various inputs are known is commented on. A qualitative discussion is given of the most common models, which are collisional and radiative. Specific pumping mechanisms for OH and H2O masers in star-forming regions are presented and criticized. The current status of the observations of these masers is reviewed and the implications on the theoretical models is discussed.

Nonlinear transport in hydrogen-bonded chains: Free solitonic excitations.

Abstract: The longitudinal dynamics of protons in hydrogen-bonded chains is described with a one-component, one-dimensional model with a two-parameter doubly periodic on-site potential. Ionic and bonding defects are described as soliton solutions of this model. Their energy, momentum, mass, width, and charge are calculated in the continuum limit. Small-amplitude oscillating solutions of breather and envelope (or dark) soliton type have also been calculated. Exact kink-antikink solutions for the discrete system are obtained numerically using a very efficient technique based on well-known minimization procedures. Producing such accurate initial conditions, the defect dynamics are explored numerically for some realistic sets of the physical parameters of the model. The microscopic behavior of the proton interbond and intrabond transfer is thus well understood for the conservative, nondriven hydrogen-bonded chain.

Distribution of fishes on the Cretan shelf

Abstract: Information on the abundance of demersal fishes on the shelf of Crete has been collected through a 3-year bottom trawl survey carried out at fixed sampling stations. Cluster analysis identified three major station-groups delimited by the nature of the substrate. Two of the identified substrate zones were situated on the continental shelf while the beginning of the third zone coincided with the start of the continental slope. Discriminant analysis applied to the abiotic variables defined that depth was the most important variable in discriminating between the identified groups. Discriminant analysis also identified that the most important species in discriminating among station-groups were Capros aper (Linnaeus, 1758), Gadicuh argenieus argenteus Guichenot, 1850, Merluccius merluccius (Linnaeus, 1758), Mullus surmuletus Linnaeus, 1758 and Spicara flexuosa Rafinesque, 18 10. Association analysis revealed few species pairs having significant associations which in most cases seem to be related to the reproductive migration of these species. The results demonstrate that there is a pronounced change with the substrate and by extension with depth in the ichthyofaunal composition of the shelf of Crete. It has also been suggested that there may be a particular relationship between the fish communities of the two zones of the continental shelf.

An exact discrete-event model and control policies for production lines with buffers

Abstract: The authors present a hybrid simulation/analytic model for the analysis and design of unreliable production lines with buffers and discrete workparts. The model is exact and much faster than conventional piece-by-piece simulators. It can analyze, efficiently and accurately, lines of any size during either transient or steady-state periods by taking into account effectively all transients associated with the function of the line. It is demonstrated that the model is superior to both brute-force and event-driven models and is an efficient practical tool in the analysis and design of production lines. By incorporating the perturbation analysis technique into the proposed algorithm. the gradient estimation of the system's throughput can be performed during a single simulation run for various design parameters. A number of experimental results are reported for the repair allocation problem, and the optimality of various control policies is investigated. >

EEG coherence during hemispheric activation in schizophrenics.

Abstract: We studied the change of EEG alpha-band coherence between resting and motor performance ("reactivity of EEG coherence") in 18 chronic schizophrenics and 30 normal controls, examining differences between left and right hemisphere tasks. Three coherences were examined for each hemisphere. The central-parietal coherence in normals increased on the left for left hemisphere tasks, and decreased on the right for right hemisphere tasks, whereas the patients showed a decrease of both measures, more on the left side. Thus, the interhemispheric difference of central-parietal coherence reactivity was reversed for the schizophrenics (P less than 0.06) and suggested a left-sided deviation. The lateral-percentral and precentral-central coherences showed no left-right deviations of reactivity in the patients. Since coherence increase is considered to be a sign of the coupled function between the studied regions, our findings suggest a defective coordination of left central-parietal regions during a task in schizophrenics.

Geometry based mapping strategies for PDE computations

Abstract: In this paper we formulate new mapping strategies for partial differential equations (PDE) computations into MIMD architectures. These mappings are based on decompositions of the geometric data (meshes) associated with the PDE domain, and distribute the solution of large linear systems across many parallel processors in such a way that the processor workload is balanced and the synchronization / communication cost among them is a minimal. For the allocation of computations associated with each subdomain to individual processors, we use an intermediate 2-D Euclidean space to represent the decomposition graph of the computation and the topology of the architecture. Then correlation between subdomains and processors is pursued according to a number of performance criteria and iterative improvement techniques. ated with the PDE domain, the discrete algebraic equations associated with some discretization of the PDE equations, and the data flow graph of the PDE solver. In this paper we study mapping techniques formulated at the geometric data structures of the PDE problem. Mapping approaches formulated on other levels have been studied in [Fox 86], [Sada 87], [Pomm 90], [Ayka 88] and [Hous 87, 90 b]. The mapping met hodology employed in this paper consists of two phases: the partitioning and allocation. In the partitioning phase we decompose the geometric data structures (meshes) to a prespecified number (usually equal to the number of processors) of sub domains or substructures such that the following criteria are approximately satisfied: (i) the subdomains have approximately the same number of elements (finite element meshes) or grid points (grids),