Showing papers by "Radboud University Nijmegen published in 1989"

Oxygen 1s x-ray-absorption edges of transition-metal oxides

Abstract: The oxygen 1s x-ray-absorption edges of a series of 3d-transition-metal oxides have been measured. The structures at the edge arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The spectra can be divided into two regions: The first is a double-peaked sharp structure near threshold, which can be related to the metal 3d states; the second is a broader structure 5--10 eV above the edge and is related to the metal 4s and 4p bands. We attribute the oxygen p character up to 15 eV above threshold to mainly oxygen 2p character. The data are analyzed in terms of ligand-field and exchange splittings. It is shown that the splitting between the two sharp peaks near threshold is related closely to the ligand-field splitting, but the relative intensities of the peaks are not fully explained at the present time.

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

Abstract: We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 05 Pb 05 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV

The movement of cadmium through freshwater trout branchial epithelium and its interference with calcium transport

Abstract: Exposure of freshwater trout ( Salmo gairdneri ) to waterborne Cd 2+ results in accumulation of the metal in the branchial epithelial cells and its appearance in the blood. Cd 2+ apparently enters the cells via Ca 2+ channels in the apical membrane. Transfer of Cd 2+ through the basolateral membrane is probably by diffusion. Inhibition by Cd 2+ of transepithelial Ca 2+ influx is time- and Cd 2+ -concentration-dependent. The inhibition of transepithelial Ca 2+ influx is accompanied by blockage of apical Ca 2+ channels. In line with the assumption that cytosolic Cd 2+ inhibits Ca 2+ uptake by inhibiting the basolateral Ca 2+ pump, we hypothesize that the blockage of Ca 2+ channels is an indirect effect of Cd 2+ and results from a rise in cytosolic Ca 2+ level caused by inhibition of the basolateral membrane Ca 2+ pump.

Soft-core baryon-baryon one-boson-exchange models. II. Hyperon-nucleon potential.

Abstract: The results of the Nijmegen soft-core potential model are presented for the low-energy YN interactions. The YN version of the model is obtained by a straightforward extension of the NN model through the application of SU(3). The potentials are due to the dominant parts of the \ensuremath{\pi}, \ensuremath{\eta}, \ensuremath{\eta}', \ensuremath{\rho}, \ensuremath{\omega}, \ensuremath{\varphi}, \ensuremath{\delta}, \ensuremath{\varepsilon}, and ${S}^{\mathrm{*}}$ Regge trajectories. This gives the traditional one-boson-exchange potentials. In addition to these, the J=0 contributions from the tensor f,f',${A}_{2}$ and Pomeron trajectories are included in the potentials. The latter give potentials of the Gaussian type. Also the form factors from Regge poles are Gaussian, which guarantees that the potentials have a soft behavior near the origin. The multichannel Schr\"odinger equation is solved in configuration space for the (partially) nonlocal potentials. We work on the particle basis and include the Coulomb interaction exactly.The meson-baryon coupling constants are calculated via SU(3), using the coupling constants of the NN analysis as input. Charge symmetry breaking in the \ensuremath{\Lambda}p and \ensuremath{\Lambda}n channels is included. An excellent description is achieved of the available low-energy data per degree of freedom (${\ensuremath{\chi}}^{2}$\ensuremath{\approxeq}0.58 for 35 YN data). In particular, we were able to fit the inelastic capture ratio at rest perfectly. We have ${r}_{R}$=0.471, where experimentally the average value is ${r}_{R}$=0.468\ifmmode\pm\else\textpm\fi{}0.010. The obtained values for the adjustable mixing angles and F/(F+D) ratios agree very well with the literature. We find ${\ensuremath{\alpha}}_{\mathrm{PV}=0.355}$ and ${\ensuremath{\alpha}}_{V}^{m}$=0.275. For the scalar-meson mixing angle we obtain ${\ensuremath{\theta}}_{s}$=40.895\ifmmode^\circ\else\textdegree\fi{}, which lies between the ideal mixing angles for the scalar ${q}^{2}$q\ifmmode\bar\else\textasciimacron\fi{} $^{2}$ and qq\ifmmode\bar\else\textasciimacron\fi{} states. In the \ensuremath{\Lambda}p system we find a cusp at the ${\ensuremath{\Sigma}}^{+}$n threshold, but there is on the second Riemann sheet no pole in the vicinity causing this cusp. The predictions of the total cross sections up to the pion production threshold are given and compared to the experimental data.

Adaptive bone remodeling and biomechanical design considerations for noncemented total hip arthroplasty.

Abstract: Clinical problems with noncemented total hip arthroplasty (THA) stems, directly or indirectly related to load transfer, include mid-thigh pain due to relative (micro) motions or excessive endosteal interface stresses, subsidence and loosening due to inadequate primary stability and fit, and proximal femoral bone atrophy due to stress shielding. In this article, the load-transfer mechanisms associated with noncemented THA stems and their resulting stress patterns are discussed in relation to design features, bonding characteristics, and materials choice. Nonlinear finite-element models and computer simulation programs for strain-adaptive bone remodeling have been used for this study. Canal-filling, fully bonded metal stems have been found likely to cause proximal bone atrophy, possibly leading to long-term failure of the implant/bone composite. The use of flexible (isoelastic) materials and/or press-fit fixation reduces stress shielding, but also reduces the potential for interface stability. The stem material, the stem shape, and the coating geometry interact in relation to the load-transfer mechanism, and it is suggested that optimal combinations of these characteristics can be determined through the computer simulation methods presented.

Infectivity of cultured Plasmodium falciparum gametocytes to mosquitoes

Abstract: Various factors that may influence routine and high levels of mosquito infection with cultured Plasmodium falciparum gametocytes are considered in this paper. One of the most important is the choice of an appropriate isolate, with facilities for cryopreservation and a good technique for initiation of cultures. The use of automated culture systems with strict adherence to detail and routine has eliminated much of the variability. The quality of the serum used for the culture of gametocytes and inclusion in the feed material for mosquitoes is of the highest importance. Blood collection for culture purposes must preferably involve alcohol as an antiseptic for cleaning donor skin or suitable receptacles. Mosquito blood meals should not include plasma with citrate phosphate dextrose or sera collected in microtainer tubes or from volunteers on proguanil-chloroquine prophylaxis. Sera of individuals on chloroquine alone do not influence transmission. Haematocrits of from 5 to 10% permit the culture of equally infective gametocytes. It was impossible to predict the outcome of an infection in mosquitoes based on the number of female gametocytes or gametes. Within any experiment, the oocyst load initially increased, followed by a decline with progressively lower numbers of gametocytes accompanied by a progressive increase in the efficiency of transmission. Some of the variability of mosquito infection within an experiment was due to individual differences in the speed of blood digestion of the mosquitoes. A new membrane feeder is described with three different sizes to accommodate a variety of goals.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in the Pulsatile Properties of Circulating Growth Hormone Concentrations during Puberty in Boys

Abstract: To investigate the mechanisms subserving physiological alterations in circulating GH concentrations during puberty, we assessed the GH pulse characteristics of 60 24-h serum GH profiles obtained from healthy male volunteers of normal stature (aged 7-27 yr) whose physical development spanned the entire pubertal range. Subjects were divided into five study groups based on degree of sexual maturation. The mean 24-h concentration of GH was greater in late pubertal boys than in all other groups (P less than 0.001). This elevation primarily reflected a greater size, rather than number, of GH pulses, whether assessed as mean GH pulse area (P = 0.004 vs. all other groups), mean GH pulse amplitude (P = 0.001), or sum of the GH pulse areas (P less than 0.001). GH pulse frequency was indistinguishable among all groups (P greater than 0.05). However, circadian GH rhythms varied significantly in amplitude and mean values (but not in phase) throughout puberty. Plasma insulin-like growth factor-I levels were greatest in the late pubertal boys (1.98 +/- 0.15 U/mL) and remained elevated in the postpubertal group (1.44 +/- 0.18). The mean value for the adult men (0.74 +/- 0.06) was indistinguishable from that of prepubertal boys (0.90 +/- 0.13). In addition, all assessed characteristics of GH pulses and circadian rhythms in adults were equal to or less than corresponding values in prepubertal boys. We conclude that circulating GH concentrations transiently increase during mid- to late puberty in normal boys, primarily through augmentation of the size of GH pulses, but return to or below prepubertal levels during early adulthood.

Protection against streptococcal cell wall-induced arthritis by pretreatment with the 65-kD mycobacterial heat shock protein.

Abstract: We report that streptococcal cell wall (SCW)-induced arthritis in rats, a T cell-dependent chronic, erosive polyarthritis, can be prevented by pretreatment of the rats with the mycobacterial 65-kD heat shock protein. This 65-kD protein shows extensive amino acid homology with prokaryotic and eukaryotic 65-kD heat shock proteins and is a ubiquitous bacterial common antigen. Both the clinical and histopathologic manifestations of the arthritis were prevented completely when rats were pretreated with 50 micrograms of 65-kD protein intraperitoneally at 35, 25, 15, or 5 d before administration of SCW. In such protected rats, SCW-specific T cell responses were suppressed, as compared with responses in arthritic rats. Pretreatment with 65-kD protein had no effect on the production of antibodies against SCW, on a nonspecific inflammatory reaction (zymosan-induced arthritis), or on general cellular immunity in vivo (delayed type hypersensitivity reaction to a nonrelated protein antigen). Furthermore, the protection against SCW arthritis was transferable by splenic T cells to naive recipients. Our data show that pretreatment with the 65-kD mycobacterial heat shock protein protects rats against a subsequent bacterium-induced arthritis. This protection is immunologically specific and resides in the lymphoid cell population.

Studies of copper valence states with Cu L 3 x-ray-absorption spectroscopy

Abstract: We have used x-ray-absorption spectroscopy to study a series of compounds in which Cu assumes a formal valence between 0 and 3. We find that the shape, the threshold energy, and the intensity of the Cu ${L}_{3}$ absorption edge is strongly influenced by the chemical state of the Cu atoms. We use the Cu 2p x-ray-absorption spectra of a large number of Cu compounds, including sulfides, oxides, La-Sr-Cu-O compounds, a phthalocyanine complex, and various minerals to show that the presence of a strong 2p-3d excitonic transition is a fingerprint of the Cu(${d}^{9}$) contribution to the ground state. A simple ionic picture is generally inadequate to describe these compounds.

A nonlinear model for collicular spatial interactions underlying the metrical properties of electrically elicited saccades

Abstract: An earlier model for the collicular role in the generation of saccades (Van Gisbergen et al. 1987), based on ensemble coding and linear vector addition of movement contributions from independent movement cells, yields normometric saccades in all directions over a considerable range of amplitudes. The model, however, cannot account for two nonlinear phenomena which are known from collicular electrical stimulation experiments: 1) saccade amplitude has a roughly sigmoid dependence upon current strength and 2) two electrical stimuli applied simultaneously at different sites yield a response that resembles a weighted average of the individual responses. In the present paper we propose an intracollicular mechanism which, based on lateral spatial interactions in the deeper layers of the colliculus, results in nearby excitation and remote inhibition when current is applied. Both nonlinear phenomena can thus be explained. The possibility of excitatory and inhibitory collicular interactions is supported by recent evidence in the literature. The nonlinearity in the model, essential to explain the electrical stimulation findings, resides in the input-output characteristic of the deeper layer movement cells. The results, obtained by quantitative simulations with the model, are discussed together with possible alternative explanations.

Nationwide survey on nutritional habits in elite athletes. Part I. Energy, carbohydrate, protein, and fat intake.

Abstract: Information about habitual food intake was systematically obtained from elite endurance, strength, and team sport athletes. The athletes (n = 419) trained at least 1-2 h daily and competed on an international level. A 4- or 7-day food diary was kept. For analysis of the data, a computerized food table was used. Mean energy intake varied from 12.1-24.7 MJ per day for male and 6.8-12.9 MJ per day for female athletes. Protein intake was in agreement with or higher than the Dutch recommendations. Contribution of carbohydrate intake to total energy intake varied from 40%-63%. Fat intake tended to meet the criteria for a prudent diet (less than 35%). Snacks contributed about 35% to the total energy intake and the bread/cereals and dairy food groups were the most important energy sources. It is concluded that in general there are two major concerns. In those sports in which body composition plays an important role, energy and thus nutrient intake is often marginal. In all groups of athletes intake of carbohydrate is insufficient.

Interspecific variation in the visual pigments of deep-sea fishes.

Abstract: Visual pigments in the rods of 38 species of deep-sea fish were examined by microspectrophotometry. 33 species were found to have a single rhodopsin with a wavelength of maximum absorbance (lambda max) in the range 470-495 nm. Such visual pigments have absorbance maxima close to the wavelengths of maximum spectral transmission of oceanic water. 5 species, however, did not conform to this pattern and visual pigments were found with lambda max values ranging from 451 nm to 539 nm. In 4 of these species two visual pigments were found located in two types of rod. Some 2-pigment species which have unusual red sensitivity, also have red-emitting photophores. These species have both rhodopsin and porphyropsin pigments in their retinae, which was confirmed by HPLC, and the two pigments are apparently located in separate rods in the same retinal area. In deep-sea fishes the occurrence of 'unusual' visual pigments seems to be correlated with aspects of the species' depth ranges. In addition to ecological influences we present evidence, in the form of lambda max spectral clustering, that indicates the degree of molecular constraint imposed on the evolution of visual pigments in the deep-sea.

Mapping of B cell epitopes on small nuclear ribonucleoproteins that react with human autoantibodies as well as with experimentally-induced mouse monoclonal antibodies.

Abstract: Small nuclear ribonucleoprotein (snRNP) particles are a class of RNA-containing particles in the nucleus of eukaryotic cells. They consist of uridylate-rich small nuclear RNA complexed with several proteins. snRNP particles U1, U2, U4/U6, and U5 all contain a common protein core consisting of proteins B'/B, D, D', E, F, and G. In addition to this core, U1 snRNP particles contain proteins 70K, A, and C, whereas U2 snRNP particles contain proteins A' and B". Almost any of the small nuclear RNA-associated polypeptides is targeted by autoantibodies in the sera from patients with SLE or related connective tissue diseases. We immunized a genetically non-autoimmune mouse with recombinant human B" protein and obtained three mAb reactive with native U2 snRNP particles. Two of these mAb particles cross-reacted with U1 snRNP, 9A9 and 11A1, via epitopes present on the U2 snRNP B" protein as well as on the U1 snRNP-specific A protein. A third mAb 4g3, reacted exclusively with U2 snRNP via a unique epitope on protein B". Two epitopes mapped at the carboxy-terminal region of the B" protein, whereas binding of the third mAb involved both amino- and carboxy-terminal amino acids of the B" protein. Epitope mapping, employing a DNAse I fragment library of the B" cDNA, revealed that the three mAb-reactive sites were discontinuous. Autoantibodies in sera from patients with SLE and other connective tissue diseases competed for binding with the mAb, implying that the mAb define a major autoantibody-reactive region on protein B".

Formal program construction by transformations-computer-aided, intuition-guided programming

Abstract: Formal program construction by transformations is a method of software development in which a program is derived from a formal problem specification by manageable, controlled transformation steps which guarantee that the final product meets the initial specification. This methodology has been investigated in the Munich project CIP (computer-aided intuition-guided programming). The research includes the design of a wide-spectrum language specifically tailored to the needs of transformational programming, the construction of a transformation system to support the methodology, and the study of transformation rules and other methodological issues. Particular emphasis has been laid on developing a sound theoretical basis for the overall approach. >

Nationwide survey on nutritional habits in elite athletes. Part II. Mineral and vitamin intake.

Abstract: The nutritional habits of elite athletes competing at a national and international top level were determined. Groups of endurance strength, and team sport athletes participated. All athletes trained at least 1-2 h daily. The purpose of the study was to quantify the mineral and vitamin intake and to identify the magnitude of the nutrient supplementation use. Information on food intake was obtained by a 4- or 7-day food diary. It was found that calcium and iron intake was positively related to energy intake. In low energy intakes (less than 10 MJ) iron intake might be insufficient. In general, vitamin intake with food was in agreement with the Dutch recommendations. However, if energy intake is high (greater than 20 MJ) the amount of refined carbohydrate is increased. Consequently, the nutrient density for vitamin B1 drops. Therefore, under these conditions, supplementation for vitamin B1 must be considered. The low vitamin intake found in lower energy intakes can be improved by proper nutritional advice. In body building and in professional cycling, high dosages of vitamins are used. The other groups of athletes used only moderate quantities of vitamin supplements. It is concluded that vitamin and mineral intake is sufficient, when energy intake ranges between 10 and 20 MJ/day.

Degeneration and death, by apoptosis and necrosis, of the pavement and chloride cells in the gills of the teleost Oreochromis mossambicus

Abstract: Degeneration and death of branchial epithelial cells were studied in an African cichlid fish. In both freshwater and seawater fish the superficially located pavement cells are sloughed off at the end of their lifecycle. This process is preceded by degeneration via a process of cytoplasmic shrinkage and condensation related to apoptotic (physiologically controlled) cell death. The chloride cells are pleomorphic, i.e., accessory, mature, and degenerating cells. Degeneration of chloride cells mainly occurs by apoptosis. Degenerating cells show shrinkage and densification of cytoplasm and nuclei, and swelling of the tubular system; these cells are then separated from the ambient water by pavement cells. They are finally phagocytosed and digested by macrophages. Apoptosis of chloride cells, but not of pavement cells, is greatly stimulated when the fish are in seawater; this reflects an increase in cellular turnover of the chloride cells. Accidental cell death (necrosis) of pavement cells or chloride cells is rarely observed in fully adapted freshwater and seawater fish. Its incidence increases in the first few days following transfer of fish from fresh water to seawater.

Cortisol stimulates whole body calcium uptake and the branchial calcium pump in freshwater rainbow trout.

Abstract: Exposure of rainbow trout to a reduced ambient calcium level (from 490 to 25 mumol Ca2+/l) caused hypocalcaemia and induced a rapid increase (within 1 h) in systemic cortisol levels. Under conditions of low environmental calcium concentrations, cortisol levels remained increased for at least 8 days. After this time the in-vitro Ca2+-transport capacity of branchial basolateral membrane vesicles was increased due to stimulation of Ca2+-ATPase activity, presumably as a result of chloride cell proliferation. Pituitary prolactin cells were unaffected by low ambient calcium levels. Fish kept in water containing 490 mumol Ca2+/l and treated with cortisol for 7 days displayed an increase in whole body calcium uptake and an enhancement of the branchial calcium transport capacity; concomitantly, hypercalcaemia was observed. We conclude that, in the rainbow trout, cortisol exerts hypercalcaemic effects by stimulating Ca2+ uptake from the water and that this effect forms an intrinsic part of the established mineralocorticoid action of cortisol in fish.

Effect of waterborne and dietary cadmium on plasma ions of the teleost Oreochromis mossambicus in relation to water calcium levels

Abstract: The effects of cadmium administered via ambient water or food on plasma ions of the African freshwater cichlidOreochromis mossambicus were studied for 2, 4, 14, and 35 days, in low calcium (0.2 mM) and high calcium (0.8 mM) water. In low calcium water, an environmentally relevant concentration of 10 μg/L water-borne cadmium induced a significant and dramatic hypocalcemia on days 2 and 4. Recovery of plasma calcium was observed on days 14 and 35. Hypermagnesemia was observed on day 2, but normal levels were already found on day 4. In high calcium water adapted fish, the extent of hypocalcemia and hypermagnesemia was less pronounced than in fish from low calcium water. Water-borne cadmium caused no significant changes in plasma phosphate, sodium, potassium, or osmolality. On days 2 and 4, dietary cadmium (averaging 10 μg Cd/fish/day) caused hypermagnesemia and hypocalcemia in low calcium wateradapted fish. Recovery was observed on days 4 and 14, respectively. In fish from high calcium water, dietary cadmium caused a significant reduction in plasma calcium on day 4 only; plasma magnesium was unaffected. Hyperphosphatemia was apparent on day 14, irrespective of the water calcium concentration. No changes in plasma sodium, potassium, or osmolality were found. The results show that sublethal concentrations of cadmium, administered via the water as well as via the food, affect calcium and magnesium metabolism in tilapia. High water calcium ameliorates the effects of both water and dietary cadmium on plasma calcium and magnesium levels. Among the various heavy metal pollutants, cadmium is frequently present in natural water bodies as a result of discharges from industrial processes or other anthropogenic contamination. The harmful effects of cadmium on mammals and other terrestrial animals have been widely studied and reviewed (Flicket al. 1971; Vallee and Ulmer 1972; Webb 1979; Korte 1983; Foulkes 1986). Aquatic vertebrates such as fish, live in very intimate contact with the environment through their gills. This makes them very susceptible to aquatic pollutants. Since it is well established that freshwater fish take up most of the ions necessary for homeostasis from the water via the gills (Eddy 1982), cadmiuminduced plasma ionic disturbances are apparently caused by impaired uptake and diffusional losses of ions via these organs (Larssonet al. 1981; Giles 1984). Ionic disturbances have also been reported after exposure of fish to sublethal concentrations of heavy metals. For example, changes in the plasma ionic composition have been observed in fish exposed to copper and zinc (Lewis and Lewis 1971; Spry and Wood 1985), mercury (Locket al. 1981), and chromium (Van der Putteet al. 1983). With respect to cadmium, exposure of rainbow trout to sublethal levels induced hypocalcemia, with reduced plasma sodium, potassium, chloride and increased plasma magnesium (Giles 1984). In European flounder, cadmium-induced hypocalcemia and elevated levels of plasma phosphate, magnesium and potassium were observed (Larssonet al. 1981). In addition to water, food could also be a source of cadmium for fish, since it accumulates in aquatic organisms through trophic transfers (Anonymous 1971; Williams and Giesy 1978; Coombs 1979). Indeed, Bryan (1976) concluded that food as a source of Zn, Mn, Co, and Fe for molluscs, crustaceans and fish was more important than water. From various studies on both water-borne and food-containing metals, reviewed by Dallingeret al. (1987), there is evidence that uptake of heavy metals such as Cd, Cu, Co, Pb, Hg, and Zn from food is also the predominant pathway in freshwater fish. Koyama and Itazawa (1977) reported significant hypocalcemia and elevated plasma phosphate levels in cadmium-fed carps. Similarly, plaice and thornback ray both accumulated more cadmium from food than from seawater (Pentreath 1977). In general, cadmium concentrations in natural waters are extremely low and a more important route of cadmium uptake by fish may be represented via the gut. Experiments with dietary cadmium may therefore yield more representative information for field situations. In this investigation, we have compared the effects of a sublethal concentration of cadmium administered via the water or via the food in the African cichlid fishOreochromis mossambicus (tilapia). Plasma ions and osmolality were determined. Cadmium was administered at sublethal concentrations, in the order of magnitude that may occur in natural waters (⩽10 μg Cd/L). In many studies aimed at evaluating the effects of cadmium on fishes, high concentrations (>1 mg Cd/L) of cadmium have been used. Hence severe physiological, behavioral and detrimental effects have been reported. Such high concentrations are rarely found in nature, except in cases of spillage or heavily polluted waters. The Working Group on Cadmium Toxicity (EIFAC 1977) has suggested that chronic exposure to low cadmium concentrations is more relevant to understanding the mechanisms involved in the intoxication process in teleost fish. We further studied the influence of relatively low and high calcium concentration of the water on the toxic effects of cadmium. The effects of water hardness (mainly Ca2+ and Mg2+ ions) on heavy metal toxicity have been demonstrated in various species of teleosts (Partet al. 1985). Increased toxicity of cadmium to fish in soft water as compared to hard water has been demonstrated in catfish and guppies (Kinkade and Erdman 1975), goldfish (McCartyet al. 1978), striped bass (Palawskiet al. 1985), brook trout (Carrollet al. 1979) and rainbow trout (Calamariet al. 1980; Pascoet al. 1986). Similar observations on teleosts exposed to zinc, copper and lead (Sinleyet al. 1974; Zitko and Carson 1976; Judy and Davies 1979; Lauren and McDonald 1986) indicate a protective role of calcium against the toxic effects of heavy metals. It was also investigated whether the protective effect of the water-calcium concentration is limited to water-borne cadmium only, or also applies to dietary cadmium.

A short-latency transition in saccade dynamics during square-wave tracking and its significance for the differentiation of visually-guided and predictive saccades.

Abstract: Several recent studies indicate that saccades elicited in the absence of a visual target are slower than visually-guided movements of the same size. In addition, we have shown earlier that the slower saccades observed in two different paradigms had more asymmetrical (skewed) velocity profiles. Recently, it has been reported that predictive saccades are also slower. An interesting question, which arises if predictive and visually-guided saccades do have different velocity profiles, is whether the time when the transition occurs can be determined from their dynamic characteristics (peak velocity and skewness) and whether this transition latency can serve as a plausible criterion for distinguishing predictive and visually-guided saccades. To investigate this problem, visually-guided and predictive saccades were elicited by various experimental paradigms in six normal human subjects. Eye movements were measured using the double-magnetic induction method. We found that scatter plots of normalized peak velocity against latency showed an abrupt, small (10-20%) increase at a surprisingly short latency (about 30-70 ms). Furthermore, skewness of the saccadic velocity profile showed a significant drop at comparable latencies. There was a tight correlation between the peak velocity and skewness transition latencies of each subject. Considering the shape of the latency histograms in this and earlier studies, as well as other data, it appears unlikely that these very short transition latencies demarcate the distinction between predictive and fully visually-guided saccades. Instead, we suggest the possibility that the visual stimulus can speed up saccades at an earlier time than it can initiate and guide them. If this is the case, the very short transition latencies (mean: about 50 ms) probably represent the sum of afferent and efferent pure time delays in the system and do not include the time needed for the computation of saccade metrical properties.

Fear of Crime in Residential Environments: Testing a Social Psychological Model

Abstract: We examined fear of crime in the residential environment from a theoretical and social psychological perspective by constructing a model that relates feelings of unsafety to attributions about self, the potential criminal, and the situation in which the criminal act might occur. Using data from a questionnaire field survey of 440 residents in four urban neighborhoods, the model was compared with a socio-demographic alternative (comprising such variables as gender, age, and educational level). LISREL, a relatively new method of analysis that takes measurement error into account, indicated that the model has superior explanatory power and greater in-terpretability. Multiple regression analyses confirmed these results and point to ways in which operationalization of the theoretical model might be improved.