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PubMedDate not supplied

Hemispheric asymmetries in memory processes as measured in a false recognition paradigm.

Although memory differs in important ways between the left and right cerebral hemispheres, the nature of these differences remains controversial. We examined this issue in two experiments using a false memory paradigm that allowed novel tests of two theories that have not been assessed in a common paradigm previously. Lists of semantically related words (e.g., bed, rest, wake...), all highly associated to one "critical" word (e.g., sleep), were presented auditorily during a study phase. Memory for both the related words and the critical words was measured in a subsequent old/new recognition test using divided-visual-field word presentations. The most important results were that the ability to correctly reject previously unpresented words was greater when test items were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH) and that participants were more confident in correctly rejecting unpresented words when test items were presented to the RVF/LH than to the LVF/RH. Results were in line with the theory that associative activation of semantic information is restricted in the left hemisphere but diffuse in the right; however, these results contrasted with the theory that memory traces are interpretive in the left hemisphere but veridical in the right. A potential resolution to the seemingly contradictory theories of asymmetries in memory processing is briefly discussed.

Acoustic Stimulation↗
PubMed2006

Relationship between macroscopic and microscopic models of surfactant adsorption dynamics at fluid interfaces.

In a companion preceding paper, we presented an experimental investigation into the adsorption dynamics of a diblock copolymer surfactant to a polymer/polymer interface and found them to be well-described by a microscopic model of diffusion in a potential generated using self-consistent field theory. We compare the predictions of the microscopic approach with a macroscopic (adsorption-diffusion) model and demonstrate the equivalence of the two models when the free-energy well underlying surfactant adsorption is flanked by barriers that are significantly larger than thermal energy (kT). However, when the energy barriers are nonexistent, as is the case for the experimental system of interest, a finite interfacial width must be introduced into the classical model to obtain physically meaningful results (i.e., nonnegative desorption rates). Surprisingly, we find that the predictions of the macroscopic finite interfacial width model with no adjustable parameters are in excellent agreement with experimental data presented in the companion paper even though the latter was obtained with molecular resolution. This agreement provides insight into aspects of the free-energy landscape that determine surfactant transport.

Adsorption↗
PubMed1998

Interpreting the equatorial diffraction pattern of collagenous tissues in the light of molecular motion.

The equatorial diffraction pattern associated with collagenous tissues, particularly type I collagen, is diffuse and clearly unlike that from crystals. Hukins and Woodhead-Galloway proposed a statistical model that they termed a "liquid crystal" for collagen fibers in tendons. Fratzl et al. applied this model to both unmineralized and mineralized turkey leg tendon, a model that ignores the organization imposed by the well-known cross-linking. The justification for adopting this model is that the curve fits the data. It is shown that the data can be equally well matched by fitting a least-squares curve consisting of a second-order polynomial plus a Gaussian. The peak of the Gaussian is taken as the equatorial spacing of the collagen. A physical explanation for this model is given, as is a reason for the changes in the spacing with changes in water content of the tissue. The diffusion is attributed to thermally driven agitation of the molecules, in accordance with the Debye-Waller theory including the Gaussian distribution. The remainder of the diffusion is attributed to other scattering sources like the mineral crystallites.

Animals↗
PubMed2001

An integrated sorption-diffusion model for the calculation of consistent distribution and diffusion coefficients in compacted bentonite.

A thermodynamic sorption model and a diffusion model based on electric double layer (EDL) theory are integrated to yield a surface chemical model that treats porewater chemistry, surface reactions, and the influence of charged pore walls on diffusing ions in a consistent fashion. The relative contribution of Stern and diffuse layer to the compensation of the permanent surface charge represents a key parameter; it is optimized for the diffusion of Cs in Kunipia-F bentonite, at a dry density of 400 kg/m3. The model is then directly used to predict apparent diffusivities (Da) of Cs, Sr, Cl-, I- and TcO4- and corresponding distribution coefficients (Kd) of Cs and Sr in different bentonites as a function of dry density, without any further adjustment of surface chemical and EDL parameters. Effective diffusivities (De) for Cs, HTO, and TcO4- are also calculated. All calculated values (Da, De, Kd) are fully consistent with each other. A comparison with published, measured data shows that the present model allows a good prediction and consistent explanation of (i) apparent and effective diffusivities for cations, anions, and neutral species in compacted bentonite, and of (ii) Kd values in batch and compacted systems.

Adsorption↗
PubMed2005

The effect of stimulus strength on the speed and accuracy of a perceptual decision.

Both the speed and the accuracy of a perceptual judgment depend on the strength of the sensory stimulation. When stimulus strength is high, accuracy is high and response time is fast; when stimulus strength is low, accuracy is low and response time is slow. Although the psychometric function is well established as a tool for analyzing the relationship between accuracy and stimulus strength, the corresponding chronometric function for the relationship between response time and stimulus strength has not received as much consideration. In this article, we describe a theory of perceptual decision making based on a diffusion model. In it, a decision is based on the additive accumulation of sensory evidence over time to a bound. Combined with simple scaling assumptions, the proportional-rate and power-rate diffusion models predict simple analytic expressions for both the chronometric and psychometric functions. In a series of psychophysical experiments, we show that this theory accounts for response time and accuracy as a function of both stimulus strength and speed-accuracy instructions. In particular, the results demonstrate a close coupling between response time and accuracy. The theory is also shown to subsume the predictions of Piéron's Law, a power function dependence of response time on stimulus strength. The theory's analytic chronometric function allows one to extend theories of accuracy to response time.

Decision Making↗
PubMed2002

Diffusion of the diboron pair in silicon.

We propose a novel mechanism for the diffusion of a diboron pair in Si, based on first principles density functional theory. We find a reaction pathway along which the boron pair diffuses from one lowest energy configuration of [B-B](s)-< 001> to an equivalent structure at an adjacent equivalent site through three local minimum states denoted as [B-B](s)-< 111>, B(s)-B (i), and B(s)-B (s)-Si (i). The activation energy for the diffusion is estimated to be 1.81 eV in the generalized gradient approximation. A kinetic model suggests that the diboron diffusion plays an important role in determining diffusion profiles during ultrashallow junction processing (which requires high boron-dopant concentration as well as high annealing temperature).

Journal Article↗
PubMed1999

Kinetics of 4-fluoroquinolones permeation into saliva.

Following a single oral administration of ciprofloxacin, norfloxacin, pefloxacin and ofloxacin preparations to healthy volunteers simultaneously collected, saliva and plasma 4-fluoroquinolone concentrations were assayed by HPLC. Pharmacokinetic properties were determined by ordinary least squares fitting of the two compartment pharmacokinetic model to the experimental data. A good correlation between plasma and saliva data has been demonstrated. The saliva to venous plasma drug concentration ratio S/P appeared to be time-dependent in the case of norfloxacin and pefloxacin. It was demonstrated that S/P is a function of the quotient of the rate of absorption and venous plasma drug concentration. The calculated S/P ratios with the influence of absorption eliminated, (S/P)(corr) are: ciprofloxacin 0.53+/-0.02, norfloxacin 0.34+/-0.04, ofloxacin 0. 43+/-0.02 and pefloxacin 0.39+/-0.02 (mean+/-S.E.). These values are apparently independent of log D thus making it impossible to predict S/P on the basis of partition principles. The corresponding (S/P)(dif) ratios were calculated on the basis of the assumption that an equilibrium is established across the blood-saliva barrier, which is permeable only for nonionized and nonprotein bound drug fraction. Comparing (S/P)(corr) with the calculated (S/P)(dif) ratios it is evident that 4-fluoroquinolone permeation in saliva cannot be described by passive diffusion based on pH-partition theory.

Adult↗
PubMed1980

Hydrophobic chromatography of beta-galactosidase.

The hydrophobic interaction of beta-galactosidase with Sepharose 4B substituted with 3,3'-diaminodipropylamine was studied in both batch and column experiments. The equilibrium and the binding rate constants were determined for different phosphate buffer concentrations. The equilibrium constants exhibit a hysteresis effect, i.e., desorption constants are less than adsorption constants, and the higher the ionic strength to start the desorption, the larger the effect. The rate data are not satisfactorily described by a simple reversible first-order model. The column chromatographic data are semiquantitatively described by a local equilibrium theory without axial dispersion or intraparticle diffusion.

Adsorption↗
PubMed1997

Adsorption Kinetics of Alkylphosphine Oxides at Water/Hexane Interface

A theoretical model is presented to describe the adsorption of surfactant molecules at the water/oil interface for the case when transport across the interface must be considered due to the solubility of the surfactant in both liquid phases (aqueous solution drop in an oil phase). The theory is based on a diffusion-controlled adsorption model and a numerical difference scheme is used to solve the problem for closed finite systems. By simulating adsorption-desorption processes, the influence of the geometrical and thermodynamic parameters on the dynamic adsorption behavior is studied. In particular, the effects of the volume ratio, surfactant concentration, diffusion coefficients, and distribution coefficient on the process are analyzed. The model allows quantitative interpretation of the experimental data obtained for an alkyldimethylphosphine oxide at the water/hexane interface. The interfacial tension signals, presenting a minimum under some conditions, are fitted by the theoretical curves.

Journal Article↗
PubMed1979

Hysteresis, oscillations, and pattern formation in realistic immobilized enzyme systems.

Hysteresis, oscillations, and pattern formation in realistic biochemical systems governed by P.D.E.s are considered from both numerical and mathematical points of view. Analysis of multiple steady states in the case of hysteresis, and bifurcation theory in the cases of oscillations and pattern formation, account for the observed numerical results. The possibility to realize these systems experimentally is their main interest, thus bringing further arguments in favor of theories explaining basic biological phenomena by diffusion and reaction.

Biochemical Phenomena↗
PubMed1986

Somatosensory evoked potentials after experimental head injury in the awake rat.

Experimental acceleration concussion was induced in 30 male rats who were immobilized with curare, artificially ventilated but not anaesthetised. Serial recordings of cortical somatosensory evoked potentials (SEPs) were made from the onset of concussion and for the following 30 min. Immediately after the head blow the initial three components of the SEP (P1, N1 and P2) were all either absent or markedly increased in latency. The late negative component (N2) was always abolished. All components reappeared and returned to approximately pre-concussion latencies within 5-6 min. The most persisting abnormality was in N2 whose post-concussion amplitude stabilized and remained depressed at only 50% of baseline value. The results suggest that experimental head injury produces a significant abnormality in every component of the evoked potential. The findings are therefore inconsistent with the theory that concussion differentially affects the diffuse nonspecific pathways whose activity is mediated by the reticular formation while leaving the lemniscal system comparatively intact. It is also shown that the morphology of the post-concussion SEP waveform can be quite accurately simulated by recording SEPs from normal animals at high rates of stimulation (50/s). This implies that the pathophysiology of concussion involves a temporary dysfunction in synaptic transmission although the level at which this is occurring has yet to be determined.

Afferent Pathways↗
PubMed1984

Line-of-descent and genealogical processes, and their applications in population genetics models.

A variety of results for genealogical and line-of-descent processes that arise in connection with the theory of some classical selectively neutral population genetics models are reviewed. While some new results and derivations are included, the principle aim is to demonstrate the central importance and simplicity of genealogical Markov chains in this theory. Considerable attention is given to "diffusion time scale" approximations of such genealogical processes. A wide variety of results pertinent to (diffusion approximations of) the classical multiallele single-locus Wright-Fisher model and its relatives are simplified and unified by this approach. Other examples where such genealogical processes play an explicit role, such as the infinite sites and infinite alleles models, are discussed.

Alleles↗
PubMed1989

Properties of gramicidin A channels in erythrocyte membranes.

Studies for the cation permeability properties of the gramicidin A channel in erythrocyte membranes are presented. It is shown that gramicidin A interacts with the membrane in a cooperative manner, creating aggregates of the antibiotic molecules in the lipid lattice of the membrane. Cationic channels exist in these aggregates with the following order of selectivity: Rb+ greater than Cs+ greater K+ greater than Na+. The cation permeability of the channels depends on the media surrounding the membrane. This finding has been explained on the basis of Hodgkin-Keynes theory for single-file ion diffusion through extra-narrow pores.

Cations↗
PubMed2005

Kinetics of ultrasonic extraction of extractive substances from garden (Salvia officinalis L.) and glutinous (Salvia glutinosa L.) sage.

The kinetics of ultrasonic extraction of extractive substances (ES) from dry herbs of garden (Salvia officinalis L.) and glutinous (Salvia glutinosa L.) sage using petroleum ether, 70% ethanol or water at 40 degrees C, as well as the composition of dry extracts, were studied. The mechanism of ultrasonic extraction is confirmed to occur in two steps: first, dissolution of the ES near the particle surface (washing) and, second, diffusion from the solid particles to the bulk of the liquid extract (slow extraction). The process is described mathematically using three concepts of the unsteady diffusion through plant material, the film theory and the empirical equation of Ponomaryov. The yield of ES increases with increasing solvent polarity, and nearly the maximum concentration of ES in liquid extracts is achieved for about 20 min. The composition of extracts depends on both the extraction conditions applied and the plant material.

Kinetics↗
PubMed2002

Adsorption of single and mixed ionic surfactants at fluid interfaces.

Two different approaches have been used in the literature to describe the effects of ionisation of surfactants on the surface pressure, pi. One approach is based on a molecular model for a charged monolayer, in which the mutual repulsion of the long-chain surfactant ions results in an additional surface pressure, piel, calculated with the Gouy-Chapman theory for the formation of a diffuse electric double layer, and with counterion binding in the Stern-Helmholtz layer adjacent to the surfactant monolayer. The other approach regards the surface as a two-dimensional solution defined as a Gibbs dividing surface, which is electroneutral by definition. In this approach, the adsorption of any ion is the sum of its excesses in the monolayer and the electrical double layer; no assumptions are made about the spatial distribution of charges. It has been shown that both models can produce a reasonable description of experimental results obtained for solutions of a single ionic surfactant (RX) with or without inorganic electrolyte (XY). In many cases, measurements of pi vs. mean ionic activity at different salt concentrations (cXY) are found to coincide on a single curve, implying that at given mean ionic activity both adsorption and pi are independent of cXY, i.e. that double-layer contributions to the surface pressure are negligible. In addition, the electroneutral 2-D solution approach has resulted in a simple explanation of several typical features of mixed ionic surfactant solutions, in particular for mixtures of anionic and cationic surfactants. In mixed solutions too, double-layer effects appear to be negligible. We present arguments for such negligibility. One reason is a significant degree of binding between adsorbed surface active ions (R) and counterions (X); another is that for 1:1 electrolytes, the contribution of the diffuse double layer to the adsorption of the combination (RX) vanishes. As a result, it is possible to interpret the same experimental data in terms of both models.

Journal Article↗
PubMed1991

Surface charging by large multivalent molecules. Extending the standard Gouy-Chapman treatment.

Traditionally, Gouy-Chapman theory has been used to calculate the distribution of ions in the diffuse layer next to a charged surface. In recent years, the same theory has found application to adsorption (incorporation, partitioning) of charged peptides, hormones, or drugs at the membrane-water interface. Empirically it has been found that an effective charge, smaller than the physical charge, must often be used in the Gouy-Chapman formula. In addition, the large size of these molecules can be expected to influence their adsorption isotherms. To improve evaluation techniques for such experiments, comparatively simple extensions of the standard Gouy-Chapman formalism have been studied which are based on a discrete charge virial expansion. The model allows for the mobility of charged groups at the interface. It accounts for finite size of the adsorbed macromolecules and for discrete charge effects arising from pair interactions in the interface plane. In contrast to previous discrete charge treatments this model nearly coincides with the Gouy-Chapman formalism in the case where the adsorbing molecules are univalent. Large discrepancies are found for multivalent molecules. This could explain the reduced effective charges needed in the standard Gouy-Chapman treatment. The reduction factor can be predicted. The model is mainly limited to low surface coverage, typical for the adsorption studies in question.

Adsorption↗
PubMed2000

Social networks, ideation, and contraceptive behavior in Bangladesh: a longitudinal analysis.

Longitudinal survey data from a panel of married women in Bangladesh is used to estimate the impact of a social network approach to family planning field worker communication and to test a theoretical model of behavior change that explains why women adopt modern contraceptives. Government field workers were trained to organize group discussions with women in the homes of opinion leaders located at central points in each village's social network. A set of intervening variables, referred to collectively as 'ideation', are derived from diffusion of innovation and social network theory to explain how the social network approach affects contraceptive behavior. The rate of increase in modern contraceptive use was found to be five times greater among women in the social network approach than among women who were visited by field workers at home. The impact of the social network approach on modern contraceptive use was almost double that of conventional field worker visits after controlling for the effects of prior contraceptive use and intention, prior home visits, and selected socio-demographic characteristics. Both approaches had the same degree of impact on ideation. The results confirm the influence of ideation on fertility change and suggest that family planning programs would benefit from training field workers to use a social network approach.

Adolescent↗
PubMed2001

Cortisol suppresses prolactin release through a non-genomic mechanism involving interactions with the plasma membrane.

In the classical theory of steroid hormone action, steroids diffuse through the membrane and alter transcription of specific genes resulting in synthesis of proteins important for modulating cell function. Most often, steroids work solely through the genome to exert their physiological actions in a process that normally takes hours or days to occur. In tilapia (Oreochromis mossambicus), cortisol inhibits prolactin (PRL) release within 10-20 min in vitro. This action is accompanied by similarly rapid reductions in cellular Ca(2+) and cAMP levels, second messengers known to transduce the membrane effects of peptide hormones. We further examined whether cortisol might inhibit PRL release through a non-genomic, membrane-associated mechanism using the protein synthesis inhibitor, cycloheximide, and a membrane impermeant form of cortisol, cortisol-21 hemisuccinate BSA (HEF/BSA). Cycloheximide (2 and 10 microg/ml) was ineffective in overcoming PRL release induced by hyposmotic medium or that inhibited by cortisol over 4 h static incubations. These dosages reduced protein synthesis as measured by amino acid incorporation in pituitaries by 75 and 99%, respectively. During 4-h incubation, HEF/BSA and HEF significantly reduced PRL release in a dose-dependent fashion. These studies suggest that cortisol inhibits PRL release through a plasma membrane-associated, protein-synthesis independent (non-genomic) pathway.

Animals↗