Showing papers by "University of California, Santa Barbara published in 1993"

A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy

Abstract: The spring constant of microfabricated cantilevers used in scanning force microscopy (SFM) can be determined by measuring their resonant frequencies before and after adding small end masses These masses adhere naturally and can be easily removed before using the cantilever for SFM, making the method nondestructive The observed variability in spring constant—almost an order of magnitude for a single type of cantilever—necessitates calibration of individual cantilevers in work where precise knowledge of forces is required Measurements also revealed that the spring constant scales with the cube of the unloaded resonant frequency, providing a simple way to estimate the spring constant for less precise work

Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures

Abstract: A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q(230) proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.

Mechanism of mitotic block and inhibition of cell proliferation by taxol at low concentrations.

Abstract: Taxol inhibited HeLa cell proliferation by inducing a sustained mitotic block at the metaphase/anaphase boundary. Half-maximal inhibition of cell proliferation occurred at 8 nM taxol, and mitosis was half-maximally blocked at 8 nM taxol. Inhibition of mitosis was associated with formation of an incomplete metaphase plate of chromosomes and an altered arrangement of spindle microtubules that strongly resembled the abnormal organization that occurs with low concentrations of vinblastine and other antimitotic compounds. No increase in microtubule polymer mass occurred below 10 nM taxol. The mass of microtubules increased half-maximally at 80 nM taxol and attained maximal levels (5 times normal) at 330 nM taxol. At submicromolar concentrations, taxol suppressed growing and shortening at the ends of microtubules reassembled in vitro from bovine brain tubulin in a manner that resembled suppression by vinblastine. Taxol was concentrated in HeLa cells several hundredfold to levels that were similar to those which suppressed dynamic instability in vitro. The results indicate that taxol shares a common antiproliferative mechanism with vinblastine. At its lowest effective concentrations, taxol appears to block mitosis by kinetically stabilizing spindle microtubules and not by changing the mass of polymerized microtubules.

Phylogenetic diversity of aggregate-attached vs. free-living marine bacterial assemblages

Abstract: The phylogenetic diversity of macroaggregate-attached vs. free-living marine bacteria, co-occurring in the same water mass, was compared. Bacterial diversity and phylogcnetic identity were inferred by analyzing polymerase chain reaction (PCR) amplified, cloned ribosomal RNA (rRNA) genes. Ribosomal RNA genes from macroaggregatc-associated bacteria were fundamentally different from those of free-living bacterioplankton. Most rRNA types recovered from the free-living bacterioplankton were closely related to a phenotypically undcscribcd (Y Proteobacteria group, previously detected in surface waters of North Pacific and Atlantic central ocean gyres. The results suggest that members of this phylogenetically distinct, (Y proteobacterial group are abundant free-living bactcrioplankters in coastal, as well as open-ocean habitats. In contrast, most macroaggregate-associated rRNA clones were closely related to Cytophuga, Planctomyce.s, or y Proteobacteria, within the domain Bacteria. These data indicate that specific bacterial populations, different from those which predominate in free-living bacterioplankton, develop on marine phytodetrital aggregates. The inferred properties of attached bacterial assemblages have significant implications for models of microbially mediated transformation of particulate organic material.

Coherent control of quantum dynamics: the dream is alive.

Abstract: Current experimental and theoretical progress toward the goal of controlling quantum dynamics is summarized. Two key developments have now revitalized the field. First, appropriate ultrafast laser pulse shaping capabilities have only recently become practical. Second, the introduction of engineering control concepts has put the required theoretical framework on a rigorous foundation. Extrapolations to determine what is realistically possible are presented.

The abundance and significance of a class of large, transparent organic particles in the ocean

Abstract: Polysaccharide-specific staining techniques reveal the existence and high abundance of a class of large, discrete, transparent particles in seawater and diatom cultures formed from dissolved exopolymers exuded by phytoplankton and bacteria. Transparent exopolymer particles (TEP), ranged from 28 to 5000 particles ml−1 and 3 to 100s μm in longest dimension at five coastal stations off California. A high percentage of seemingly free-living bacteria (28–68%) were attached to these transparent sheets and films, suggesting that they may alter the distributions and microenvironments of marine microbes in nature. Preliminary coagulation experiments demonstrated that TEP are major agents in the aggregation of diatoms and in the formation of marine snow. The existence of microbial exudates acting as large, discrete particles, rather than as dissolved molecules or as coating on other particles, suggests that the transformation of dissolved organic matter into particulate form in the sea can occur via a rapid abiotic pathway as well as through conventional microbial uptake. The existence of these particles has far reaching implications for food web structure, microbial processes, carbon cycling and particulate flux in the ocean.

Network studies of social influence

Abstract: Network analysts interested in social influence examine the social foundations for influence—the social relations that provide a basis for the alteration of an attitude or behavior by one network actor in response to another. This article contrasts two empirical accounts of social influence (structural cohesion and equivalence) and describes the social processes (e.g., identification, competition, and authority) presumed to undergird them. It then reviews mathematical models of influence processes involving networks and related statistical models used in data analysis. Particular attention is given to the “network effects” model. A number of empirical studies of social influence are reviewed. The article concludes by identifying several problems of specification, research design, and measurement and suggesting some research that would help to resolve these problems.

Microscale Heat Conduction in Dielectric Thin Films

Abstract: Heat conduction in dielectric thin films is a critical issue in the design of electronic devices and packages. Depending on the material properties, there exists a range of film thickness where the Fourier law, used for macroscale heat conduction, cannot be applied. In this microscale regime, heat transport by lattice vibrations or phonons can be analyzed as a radiative transfer problem. Based on Boltzmann transport theory, an equation of phonon radiative transfer (EPRT) is developed. In the acoustically thick limit, ξ L >>1, or the macroscale regime, where the film thickness is much larger than the phonon-scattering mean free path, the EPRT reduces to the Fourier law

Classical equations for quantum systems

Abstract: The origin of the phenomenological deterministic laws that approximately govern the quasiclassical domain of familiar experience is considered in the context of the quantum mechanics of closed systems such as the universe as a whole. A formulation of quantum mechanics is used that predicts probabilities for the individual members of a set of alternative coarse-grained histories that decohere, which means that there is negligible quantum interference between the individual histories in the set. We investigate the requirements for coarse grainings to yield decoherent sets of histories that are quasiclassical, i.e., such that the individual histories obey, with high probability, effective classical equations of motion interrupted continually by small fluctuations and occasionally by large ones. We discuss these requirements generally but study them specifically for coarse grainings of the type that follows a distinguished subset of a complete set of variables while ignoring the rest. More coarse graining is needed to achieve decoherence than would be suggested by naive arguments based on the uncertainty principle. Even coarser graining is required in the distinguished variables for them to have the necessary inertia to approach classical predictability in the presence of the noise consisting of the fluctuations that typical mechanisms of decoherence produce. We describe the derivation of phenomenological equations of motion explicitly for a particular class of models. Those models assume configuration space and a fundamental Lagrangian that is the difference between a kinetic energy quadratic in the velocities and a potential energy. The distinguished variables are taken to be a fixed subset of coordinates of configuration space. The initial density matrix of the closed system is assumed to factor into a product of a density matrix in the distinguished subset and another in the rest of the coordinates. With these restrictions, we improve the derivation from quantum mechanics of the phenomenological equations of motion governing a quasiclassical domain in the following respects: Probabilities of the correlations in time that define equations of motion are explicitly considered. Fully nonlinear cases are studied. Methods are exhibited for finding the form of the phenomenological equations of motion even when these are only distantly related to those of the fundamental action. The demonstration of the connection between quantum-mechanical causality and causality in classical phenomenological equations of motion is generalized. The connections among decoherence, noise, dissipation, and the amount of coarse graining necessary to achieve classical predictability are investigated quantitatively. Routes to removing the restrictions on the models in order to deal with more realistic coarse grainings are described.

Low-temperature transport through a quantum dot: The Anderson model out of equilibrium.

Abstract: The infinite-U Anderson model is applied to nonequilibrium transport through a quantum dot containing two spin levels weakly coupled to two leads. At low temperatures, the Kondo peak in the equilibrium density of states is split upon the application of a voltage bias. The split peaks, one at the chemical potential of each lead, are suppressed by nonequilibrium dissipation. In a magnetic field, the Kondo peaks shift away from the chemical potentials by the Zeeman energy, leading to an observable peak in the differential conductance when the nonequilibrium bias equals the Zeeman energy.

The distribution and nature of colour vision among the mammals.

Abstract: 1. An oft-cited view, derived principally from the writings of Gordon L. Walls, is that relatively few mammalian species have a capacity for colour vision. This review has evaluated that proposition in the light of recent research on colour vision and its mechanisms in mammals. 2. To yield colour vision a retina must contain two or more spectrally discrete types of photopigment. While this is a necessary condition, it is not a sufficient one. This means, in particular, that inferences about the presence of colour vision drawn from studies of photopigments, the precursors of photopigments, or from nervous system signals must be accepted with due caution. 3. Conjoint signals from rods and cones may be exploited by mammalian nervous systems to yield behavioural discriminations consistent with the formal definition of colour vision. Many mammalian retinas are relatively cone-poor, and thus there are abundant opportunities for such rod/cone interactions. Several instances were cited in which animals having (apparently) only one type of cone photopigment succeed at colour discriminations using such a mechanism. it is suggested that the exploitation of such a mechanism may not be uncommon among mammals. 4. Based on ideas drawn from natural history, Walls (1942) proposed that the receptors and photopigments necessary to support colour vision were lost during the nocturnal phase of mammalian history and then re-acquired during the subsequent mammalian radiations. Contemporary examination of photopigment genes along with the utilization of better techniques for identifying rods and cones suggest a different view, that the earliest mammals had retinas containing some cones and two types of cone photopigment. Thus the baseline mammalian colour vision is argued to be dichromacy. 5. A consideration of the broad range of mammalian niches and activity cycles suggests that many mammals are active during photic periods that would make a colour vision capacity potentially useful. 6. A systematic survey was presented that summarized the evidence for colour vision in mammals. Indications of the presence and nature of colour vision were drawn both from direct studies of colour vision and from studies of those retinal mechanisms that are most closely associated with the possession of colour vision. Information about colour vision can be adduced for species drawn from nine mammalian orders.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonvisual navigation by blind and sighted: assessment of path integration ability

Abstract: Blindfolded sighted, adventitiously blind, and congenitally blind subjects performed a set of navigation tasks. The more complex tasks involved spatial inference and included retracing a multisegment route in reverse, returning directly to an origin after being led over linear segments, and pointing to targets after locomotion. As a group, subjects responded systematically to route manipulations in the complex tasks, but performance was poor. Patterns of error and response latency are informative about the internal representation used; in particular, they do not support the hypothesis that only a representation of the origin of locomotion is maintained. The slight performance differences between groups varying in visual experience were neither large nor consistent across tasks. Results provide little indication that spatial competence strongly depends on prior visual experience.

Possible gravitational microlensing of a star in the Large Magellanic Cloud

Abstract: THERE is now abundant evidence for the presence of large quantities of unseen matter surrounding normal galaxies, including our own1,2. The nature of this ’dark matter‘ is unknown, except that it cannot be made of normal stars, dust or gas, as they would be easily detected. Exotic particles such as axions, massive neutrinos or other weakly interacting massive particles (collectively known as WIMPs) have been proposed3,4, but have yet to be detected. A less exotic alternative is normal matter in the form of bodies with masses ranging from that of a large planet to a few solar masses. Such objects, known collectively as massive compact halo objects5 (MACHOs), might be brown dwarfs or ‘jupiters’ (bodies too small to produce their own energy by fusion), neutron stars, old white dwarfs or black holes. Paczynski6 suggested that MACHOs might act as gravitational microlenses, temporarily amplifying the apparent brightness of background stars in nearby galaxies. We are conducting a microlensing experiment to determine whether the dark matter halo of our Galaxy is made up of MACHOs. Here we report a candidate for such a microlensing event, detected by monitoring the light curves of 1.8 million stars in the Large Magellanic Cloud for one year. The light curve shows no variation for most of the year of data taking, and an upward excursion lasting over 1 month, with a maximum increase of ∼2 mag. The most probable lens mass, inferred from the duration of the candidate lensing event, is ∼0.1 solar mass.

String-rewriting systems

Abstract: In this chapter we introduce the string-rewriting systems and study their basic properties. Such systems are the primary subject of this work. We provide formal definitions of string-rewriting systems and their induced reduction relations and Thue congruences. Some of the basic ideas that occur in the study of term-rewriting systems are considered. We rely on Section 1.4 for basic definitions and notation for strings, and we rely on Section 1.1 for basic definitions and results on notions such as reduction, confluence, the Church-Rosser property, and so forth.

Some general comments on the evolution and design of animal communication systems

Abstract: Animal communication systems have evolved so that individuals can make decisions based upon the behaviour, physiology or morphology of others. Receiving mechanisms probably evolve to increase the efficiency and reliability of information reception whereas signals probably evolve to increase the efficiency of communication and reliability of manipulation of the receiving individual to the benefit of the emitter. The minimum requirement for clear reception suggests that any study of the evolution and design of communication systems must consider the factors that affect the quality of the received and processed signal. Critical information is needed about how the signal is generated and emitted, how it fares during transmission through air, water or substrate, how it is received and processed by the receiver's sensory and cognitive systems, and the factors which affect the fitness consequences of alternative ways of reacting to the information contained in the signal. These should allow predictions about the kinds and forms of signals used by animals signalling under known conditions. Phylogenetic history, and the geological time a clade spends in different signalling environments, will also affect signal evolution, and hence the success of predictions about signal design. We need to use methods of many different biological fields to understand the design and evolution of signals and signalling systems.

Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings

Abstract: The authors have recently demonstrated 57 nm of tuning in a monolithic semiconductor laser using conventional distributed-Bragg-reflector (DBR) technology with grating elements removed in a periodic fashion. They describe the theory and design of these so-called sampled-grating tunable lasers. They calculate sample-grating reflectivity, and present normalized design curves which quantify tradeoffs involved in a sampled-grating DBR laser with two mismatched sampled-grating mirrors. These results are applied to a design example in the InP-InGaAsP system. The design provides 70-nm tuning while maintaining >30-dB mode suppression ratio (MSR), with fractional index change Delta mu / mu >

Carbon dioxide limitation of marine phytoplankton growth rates

Abstract: THE supply of dissolved inorganic carbon (DIC) is not considered to limit oceanic primary productivity1, as its concentration in sea water exceeds that of other plant macronutrients such as nitrate and phosphate by two and three orders of magnitude, respectively. But the bulk of oceanic new production2 and a major fraction of vertical carbon flux is mediated by a few diatom genera whose ability to use DIG components other than CO2, which comprises < 1% of total DIC3, is unknown4. Here we show that under optimal light and nutrient conditions, diatom growth rate can in fact be limited by the supply of CO2. The doubling in surface water pCO2 levels since the last glaciation from 180 to 355 p.p.m.5,6 could therefore have stimulated marine productivity, thereby increasing oceanic carbon sequestration by the biological pump.

Equal Opportunity Law and the Construction of Internal Labor Markets

Abstract: Internal labor markets have been explained with efficiency and control arguments; however, retrospective event-history data from 279 organizations suggest that federal Equal Employment Opportunity (EEO) law was the force behind the spread of formal promotion mechanisms after 1964. The findings highlight the way in which American public policy, with its broad outcome-oriented guidelines for organizations, stimulates managers to experiment with compliance mechanisms with an eye to judicial sanction. In response to EEO legislation and case law, personnel managers devised and diffused employment practices that treat all classes of workers as ambitious and achievement oriented in the process of formalizing and rationalizing promotion decisions.

A Satellite-derived Climatology of the ITCZ

Abstract: This paper presents fundamental climatological characteristics of the intertropical convergence zone (ITCZ) in a simple concise manner using the highly reflective cloud (HRC) dataset. This satellite-derived dataset uses both visible and infrared observations to measure the frequency of occurrence of large-scale convective systems over the global tropics at a 1 deg spatial resolution. These dataset characteristics make the HRC particularly well suited for climatological analysis of the ITCZ because the dataset is based on estimates of organized deep convective cloud systems rather than observations of clouds as a whole, and it provides the spatial resolution needed to identify these large-scale convective structures. Furthermore, the dataset covers a time period extending nearly two decades, which provides for a fairly robust climatology and the opportunity to examine seasonal and interannual variability of both the convection and the latitude of the ITCZ.

Natural inflation: Particle physics models, power-law spectra for large-scale structure, and constraints from the Cosmic Background Explorer.

Abstract: We discuss the particle physics basis for models of natural inflation with pseudo Nambu-Goldstone bosons and study the consequences for large-scale structure of the nonscale-invariant density fluctuation spectra that arise in natural inflation and other models. A pseudo Nambu-Goldstone boson, with a potential of the form $V(\ensuremath{\varphi})={\ensuremath{\Lambda}}^{4}[1\ifmmode\pm\else\textpm\fi{}cos(\frac{\ensuremath{\varphi}}{f})]$, can naturally give rise to an epoch of inflation in the early Universe, if $f\ensuremath{\sim}{M}_{\mathrm{Pl}}$ and $\ensuremath{\Lambda}\ensuremath{\sim}{M}_{\mathrm{GUT}}$. Such mass scales arise in particle physics models with a gauge group that becomes strongly interacting at the grand unified theory scale. We work out a specific particle physics example based on the multiple gaugino condensation scenario in superstring theory. We then study the cosmological evolution of and constraints upon these inflation models numerically and analytically. To obtain sufficient inflation with a probability of order 1 and a high enough post-inflation reheat temperature for baryogenesis, we require $f\ensuremath{\gtrsim}0.3{M}_{\mathrm{Pl}}$. The primordial density fluctuation spectrum generated by quantum fluctuations in $\ensuremath{\varphi}$ is a non-scale-invariant power law $P(k)\ensuremath{\propto}{k}^{{n}_{s}}$, with ${n}_{s}\ensuremath{\simeq}1\ensuremath{-}(\frac{{M}_{\mathrm{Pl}}^{2}}{8\ensuremath{\pi}{f}^{2}})$ leading to more power on large length scales than the ${n}_{s}=1$ Harrison-Zeldovich spectrum. (For the reader primarily interested in large-scale structure, the discussion of this topic is presented in Sec. IV and is intended to be nearly self-contained.) We pay special attention to the prospects of using the enhanced power to explain the otherwise puzzling large-scale clustering of galaxies and clusters and their flows. We find that the standard cold dark matter (CDM) model with $0\ensuremath{\lesssim}{n}_{s}\ensuremath{\lesssim}0.6$ could in principle explain these data. However, the microwave background anisotropies recently detected by the Cosmic Background Explorer (COBE) imply such low primordial amplitudes for these CDM models (that is, bias factors ${b}_{8}\ensuremath{\gtrsim}2$ for ${n}_{s}\ensuremath{\lesssim}0.6$) that galaxy formation would occur too late to be viable and the large-scale galaxy velocities would be too small. In fact, combining the COBE results with the requirement of sufficiently early galaxy formation (${z}_{\mathrm{GF}}g2$) leads to the constraint ${n}_{s}\ensuremath{\gtrsim}0.63$, which corresponds to $f\ensuremath{\gtrsim}0.3{M}_{\mathrm{Pl}}$ for natural inflation (virtually the same as the sufficient reheating constraint). A comparable bound ${n}_{s}\ensuremath{\gtrsim}0.72$ arises by combining COBE with the inferred large-scale flows. For other inflation models, such as extended inflation and inflation with exponential potentials, which give rise to initial fluctuation spectra that are power laws through the 3 decades in wavelength probed by large-scale observations, gravity waves can produce a significant fraction of the COBE signal (while they are negligible for natural inflation); for these models, our corresponding COBE constraints on ${n}_{s}$ are therefore tighter, ${n}_{s}g0.76$ (from ${z}_{\mathrm{GF}}g2$) and ${n}_{s}g0.89$ (from large-scale flows). Combined with other constraints on the Brans-Dicke parameter (which effectively imply ${n}_{s}l0.77\ensuremath{-}0.84$), this leaves little or no room for most extended inflation models. Chaotic inflation models with power-law potentials have ${n}_{s}\ensuremath{\gtrsim}0.95$ over observable wavelengths and so are not affected. Although no single value of the spectral index ${n}_{s}$ in the standard cold dark matter model universally fits the data, a value ${n}_{s}\ensuremath{\le}1$ may be combined with other variations of the standard CDM framework to explain the large-scale structure. For example, if the baryon density is as high as ${\ensuremath{\Omega}}_{B}=0.1$ or the Hubble parameter as low as ${H}_{0}=40$ km/(sec/Mpc), then ${n}_{s}\ensuremath{\sim}0.7$ with CDM would be at least marginally consistent with the large-scale structure data [e.g., the automatic plate measuring (APM) survey angular correlation function], COBE, large-scale velocities, and the requirement of sufficiently early structure formation.

Relation between physical and gravitational geometry.

Abstract: The appearance of two geometries in a single gravitational theory is familiar. Usually, as in the Brans-Dicke theory or in string theory, these are conformally related Riemannian geometries. Is this the most general relation between the two geometries allowed by physics? We study this question by supposing that the physical geometry on which matter dynamics takes place could be Finslerian rather than just Riemannian. An appeal to the weak equivalence principle and causality then leads us to the conclusion that the Finsler geometry has to reduce to a Riemann geometry whose metric, the physical metric, is related to the gravitational metric by a generalization of the conformal transformation involving a scalar field.

Black hole entropy and higher curvature interactions.

Abstract: A general formula for the entropy of stationary black holes in Lovelock higher-curvature gravity theories is obtained by integrating the first law of black hole mechanics, which is derived by Hamiltonian methods. The entropy is not simply one quarter of the surface area of the horizon, but also includes a sum of intrinsic curvature invariants integrated over a cross section of the horizon.

Timing of collision of the Sino-Korean and Yangtse cratons: U-Pb zircon dating of coesite-bearing eclogites

Abstract: The suture zone between the Sino-Korean and Yangtse cratons in central China is marked by a series of fault-bounded belts composed of crustal protoliths. In the Dabie Mountains some of these blocks contain coesite and diamond, indicating subduction to minimum depths of ∼120 km as a result of continental collision. The blocks range from the structurally lowest ultrahigh-pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest, low greenschist facies belt. Furthermore, the faults mark large contrasts in metamorphic grade. These features form a structure that is similar to metamorphic core complexes, indicating extension as part of the exhumation process. Timing of the continental collision has previously been inferred as Late Triassic, from regional geologic relations, or Caledonian, from 40 Ar/ 39 Ar dating of the adjacent and possibly related Qinling metamorphic belt. U-Pb dating of zircon from ultra-high-pressure eclogites yields a metamorphic age of 209 ±2 Ma. This age coincides with later stages of collision and is therefore a minimum estimate of the time of collision.

Comparing decision bound and exemplar models of categorization

Abstract: The performance of a decision bound model of categorization (Ashby, J992a; Ashby & Maddox, in press) is compared with the performance of two exemplar models. The first is the generalized context model (e.g., Nosofsky, 1986, 1992) and the second is a recently proposed deterministic exemplar model (Ashby & Maddox, in press), which contains the generalized context model as a special case. When the exemplars from each category were normally distributed and the optimal decision bound was linear, the deterministic exemplar model and the decision bound model provided roughly equivalent accounts of the data. When the optimal decision bound was nonlinear, the decision bound model provided a more accurate account of the data than did either exemplar model. When applied to categorization data collected by Nosofsky (1986, 1989), in which the category exemplars are not normally distributed, the decision bound model provided excellent accounts of the data, in many cases significantly outperforming the exemplar models. The decision bound model was found to be especially successful when(1) single subject analyses were performed, (2) each subject was given relatively extensive training, and (3) the subject's performance was characterized by complex suboptimalities. These results support the hypothesis that the decision bound is of fundamental importance in predicting asymptotic categorization performance and that the decision bound models provide a viable alternative to the currently popular exemplar models of categorization.