Showing papers by "Stanford University published in 1971"

Mental Rotation of Three-Dimensional Objects

Abstract: The time required to recognize that two perspective drawings portray objects of the same three-dimensional shape is found to be (i) a linearly increasing function of the angular difference in the portrayed orientations of the two objects and (ii) no shorter for differences corresponding simply to a rigid rotation of one of the two-dimensional drawings in its own picture plane than for differences corresponding to a rotation of the three-dimensional object in depth.

An introduction to observers

Abstract: Observers which approximately reconstruct missing state-variable information necessary for control are presented in an introductory manner. The special topics of the identity observer, a reduced-order observer, linear functional observers, stability properties, and dual observers are discussed.

Aspirin selectively inhibits prostaglandin production in human platelets

Abstract: Platelets in the blood of volunteers who have taken aspirin can no longer produce prostaglandins.

Microfilaments in Cellular and Developmental Processes

Abstract: In our opinion, all of the phenomena that are inhibited by cytochalasin can be thought of as resulting from contractile activity of cellular organelles. Smooth muscle contraction, clot retraction, beat of heart cells, and shortening of the tadpole tail are all cases in which no argument of substance for alternative causes can be offered. The morphogenetic processes in epithelia, contractile ring function during cytokinesis, migration of cells on a substratum, and streaming in plant cells can be explained most simply on the basis of contractility being the causal event in each process. The many similarities between the latter cases and the former ones in which contraction is certain argue for that conclusion. For instance, platelets probably contract, possess a microfilament network, and behave like undulating membrane organelles. Migrating cells possess undulating membranes and contain a similar network. It is very likely, therefore, that their network is also contractile. In all of the cases that have been examined so far, microfilaments of some type are observed in the cells; furthermore, those filaments are at points where contractility could cause the respective phenomenon. The correlations from the cytochalasin experiments greatly strengthen the case; microfilaments are present in control and "recovered" cells and respective biological phenomena take place in such cells; microfilaments are absent or altered in treated cells and the phenomena do not occur. The evidence seems overwhelming that microfilaments are the contractile machinery of nonmuscle cells. The argument is further strengthened if we reconsider the list of processes insensitive to cytochalasin (Table 2). Microtubules and their sidearms, plasma membrane, or synthetic machinery of cells are presumed to be responsible for such processes, and colchicine, membrane-active drugs, or inhibitors of protein synthesis are effective at inhibiting the respective phenomena. These chemical agents would not necessarily be expected to affect contractile apparatuses over short periods of time, they either do not or only secondarily interfere with the processes sensitive to cytochalasin (Table 1). It is particularly noteworthy in this context that microtubules are classed as being insensitive to cytochalasin and so are not considered as members of the "contractile microfilament" family. The overall conclusion is that a broad spectrum of cellular and developmental processes are caused by contractile apparatuses that have at least the common feature of being sensitive to cytochalasin. Schroeder's important insight (3) has, then, led to the use of cytochalasin as a diagnostic tool for such contracile activity: the prediction is that sensitivity to the drug implies presence of some type of contractile microfilament system. Only further work will define the limits of confidence to be placed upon such diagnoses. The basis of contraction in microfilament systems is still hypothetical. Contraction of glycerol-extracted cells in response to adenosine triphosphate (53), extraction of actin-like or actomyosin-like proteins from cells other than muscle cells (54), and identification of activity resembling that of the actomyosin-adenosine triphosphatase system in a variety of nonmuscle tissues (40, 54) are consistent with the idea that portions of the complex, striated muscle contractile system may be present in more primitive contractile machinery. In the case of the egg cortex, calcium-activated contractions can be inhibited by cytochalasin. If, as seems likely, microfilaments are the agents activated by calcium, then it will be clear that they have the same calcium requirement as muscle. Biochemical analyses of primitive contractile systems are difficult to interpret. Ishikawa's important observation (31), that heavy meromyosin complexes with fine filaments oriented parallel to the surface of chondrocytes and perpendicular to the surface of intestinal epithelial cells, implies that both types of filaments are "actin-like" in this one respect. Yet, it is very likely that these actin-like filaments correspond respectively to the cytochalasin-insensitive sheath of glial and heart fibroblasts and the core filaments of oviduct microvilli. No evidence from our studies links contractility directly to these meromyosin-binding filaments. Apart from this problem, activity resembling that of the myosin-adenosine triphosphatase has been associated with the microtubule systems of sperm tails and cilia (55), but those organelles are insensitive to cytochalasin in structure and function. Clearly, a means must be found to distinguish between enzymatic activities associated with microfilament networks, microfilament bundles, microtubules, and the sheath filaments of migratory cells. Until such distinctions are possible, little of substance can be said about the molecular bases of primitive contractile systems. Three variables are important for the control of cellular processes dependent upon microfilaments: (i) which cells of a population shall manufacture and assemble the filaments; (ii) where filaments shall be assembled in cells; and (iii) when contractility shall occur. With respect to distribution among cells, the networks involved in cell locomotion are presumed to be present in all cells that have the potential to move in cell culture. In this respect, the networks can be regarded as a common cellular organelle in the sense that cytoplasmic microtubules are so regarded. In some developing systems, all cells of an epithelium possess microfilament bundles (7, 13), whereas, in others, only discrete subpopulations possess the bundles (5, 6). In these cases the filaments can be regarded as being differentiation products associated only with certain cell types. These considerations may be related to the fact that microfilament networks are associated with behavior of individual cells (such as migration, wound healing, and cytokinesis), whereas the bundles are present in cells that participate in coordinated changes in shape of cell populations. With respect to placement in cells, two alternatives are apparent, namely, localized or ubiquitous association with the plasma membrane. Microfilament bundles of epithelial cells are only found extending across the luminal and basal ends of cells. In this respect they contrast with desmosomal tonofilaments and with microtubules, each of which can curve in a variety of directions through the cell. The strict localization of microfilament bundles probably rests upon their association with special junctional complex insertion regions that are only located near the ends of cells. In the case of mitotically active cells, the orientation of the spindle apparatus may determine the site at which the contractile ring of microfilaments will form (4, 56); this raises the question of what sorts of cytoplasmic factors can influence the process of association between filament systems and plasma membranes. In contrast to such cases of localized distribution, contractile networks responsible for cell locomotion are probably found beneath all of the plasma membrane, just as the network of thrombosthenin may extend to all portions of the periphery of a blood platelet. This ubiquitous distribution probably accounts for the ability of a fibroblast or glial cell to establish an undulating membrane at any point on its edge, or of an axon to form lateral microspikes along its length. The third crucial aspect of control of these contractile apparatuses involves the choice of when contraction shall occur (and as a corollary the degree or strength of contraction that will occur). In the simplest situation, contraction would follow automatically upon assembly of the microfilament bundles or networks. In cleavage furrows of marine embryos (4), for instance, microfilaments are seen beneath the central cleavage furrow and at its ends, but not beyond, under the portion of plasma membrane that will subsequently become part of the furrow. This implies that the furrow forms very soon after the contractile filaments are assembled in the egg cortex. In other cases, microfilaments are apparently assembled but not in a state of (maximal?) contraction. Thus, networks are seen along the sides of migratory cells, although such regions are not then active as undulating membrane organelles. Similarly, microfilament bundles occur in all epithelial cells of the salivary gland (13), or pancreatic anlage (7), although only the ones at discrete points are thought to generate morphogenetic tissue movements. Likewise, bundles begin to appear as early as 12 hours after estrogen administration to oviduct, although visible tubular gland formation does not start until 24 to 30 hours. Finally, streaming in plant cells can wax and wane, depending upon external factors such as auxin (57). All of these cases imply a control mechanism other than mere assembly of the microfilament systems and even raise the possibility that within one cell some filaments may be contracting while others are not. In discussing this problem, it must be emphasized that different degrees of contraction or relaxation cannot as yet be recognized with the electron microscope. In fact, every one of the cases cited above could be explained by contraction following immediately upon some subtle sort of "assembly." Inclusive in the latter term are relations between individual filaments, relations of the filaments and their insertion points on plasma membrane, and quantitative alterations in filament systems. Furthermore, the critical role of calcium and high-energy compounds in muscle contraction suggest that equivalent factors may be part of primitive, cytochalasinsensitive systems. The finding that calcium-induced contraction in the cortex of eggs is sensitive to cytochalasin strengthens that supposition and emphasizes the importance of compartmentalization of cofactors as a means of controlling microfilaments in cells.

The production of turbulence near a smooth wall in a turbulent boundary layer

Abstract: The structure of the flat plate incompressible smooth-surface boundary layer in a low-speed water flow is examined using hydrogen-bubble measurements and also hot-wire measurements with dye visualization. Particular emphasis is placed on the details of the process of turbulence production near the wall. In the zone 0 < y+ < 100, the data show that essentially all turbulence production occurs during intermittent ‘bursting’ periods. ‘Bursts’ are described in some detail.The uncertainties in the bubble data are large, but they have the distinct advantage of providing velocity profiles as a function of time and the time sequences of events. These data show that the velocity profiles during bursting periods assume a shape which is qualitatively distinct from the well-known mean profiles. The observations are also used as the basis for a discussion of possible appropriate mathematical models for turbulence production.

Forcing a sequential experiment to be balanced

Abstract: Subjects arrive sequentially at an experimental site and must be assigned immediately to treatment or control groups. In order to avoid biasing the results of the experiment it is customary to make the assignments by independent flips of a fair coin, but in small-sized experiments this may result in a severe imbalance between the numbers of treatments and controls. This paper discusses a new method of assigning the subjects which tends to balance the experiment, but at the same time is not over vulnerable to various common forms of experimental bias.

Ultrastructure and function of growth cones and axons of cultured nerve cells

Abstract: Dorsal root ganglion nerve cells undergoing axon elongation in vitro have been analyzed ultrastructurally. The growth cone at the axonal tip contains smooth endoplasmic reticulum, vesicles, neurofilaments, occasional microtubules, and a network of 50-A in diameter microfilaments. The filamentous network fills the periphery of the growth cone and is the only structure found in microspikes. Elements of the network are oriented parallel to the axis of microspikes, but exhibit little orientation in the growth cone. Cytochalasin B causes rounding up of growth cones, retraction of microspikes, and cessation of axon elongation. The latter biological effect correlates with an ultrastructural alteration in the filamentous network of growth cones and microspikes. No other organelle appears to be affected by the drug. Removal of cytochalasin allows reinitiation of growth cone-microspike activity, and elongation begins anew. Such recovery will occur in the presence of the protein synthesis inhibitor cycloheximide, and in the absence of exogenous nerve growth factor. The neurofilaments and microtubules of axons are regularly spaced. Fine filaments indistinguishable from those in the growth cone interconnect neurofilaments, vesicles, microtubules, and plasma membrane. This filamentous network could provide the structural basis for the initiation of lateral microspikes and perhaps of collateral axons, besides playing a role in axonal transport.

An empirical study of FORTRAN programs

Abstract: : A sample of programs, written in FORTRAN by a wide variety of people for a wide variety of applications, was chosen 'at random' in an attempt to discover quantitatively 'what programmers really do.' Statistical results of this survey are presented here, together with some of their apparent implications for future work in compiler design. The principal conclusion which may be drawn is the importance of a program 'profile,' namely a table of frequency counts which record how often each statement is performed in a typical run; there are strong indications that profile-keeping should become a standard practice in all computer systems, for casual users as well as system programmers. The paper is the report of a three month study undertaken by the author and about a dozen students and representatives of the software industry during the summer 1970. It is hoped that a reader who studies the report will obtain a fairly clear conception of how FORTRAN is being used, and what compilers can do about it.

Simultaneous Numerical Solution of Differential-Algebraic Equations

Abstract: A unified method for handling the mixed differential and algebraic equations of the type that commonly occur in the transient analysis of large networks or in continuous system simulation is discussed. The first part of the paper is a brief review of existing techniques of handling initial value problems for stiff ordinary differential equations written in the standard form y' f(y, t) . In the second part one of these techniques is applied to the problem F(y, y', t)=0 . This may be either a differential or an algebraic equation as \partial F/ \partial y' is nonzero or zero. It will represent a mixed system when vectors F and y represent components of a system. The method lends itself to the use of sparse matrix techniques when the problem is sparse.

The automatic integration of ordinary differential equations

Abstract: An integration technique for the automatic solution of an initial value problem for a set of ordinary differential equations is described. A criterion for the selection of the order of approximation is proposed. The objective of the criterion is to increase the step size so as to reduce solution time. An option permits the solution of “stiff” differential equations. A program embodying the techniques discussed appears in Algorithm 407.

An algebraic definition of simulation between programs

Abstract: A simulation relation between programs is defined which is quasi-ordering. Mutual simulation is then an equivalence relation, and by dividing out by it we abstract from a program such details as how the sequencing is controlled and how data is represented. The equivalence classes are approxiamtions to the algorithms which are realized, or expressed, by their member programs. A technique is given and illustrated for proving simulation and equivalence of programs; there is an analogy with Floyd''s technique for proving correctness of programs. Finally, necessary and sufficient conditions for simulation are given.

Relationship between the Hard-Sphere Fluid and Fluids with Realistic Repulsive Forces

Abstract: We consider the equilibrium statistical mechanics of classical fluids in which the potential energy is decomposable into repulsive pair interactions. A generalized cluster expansion is derived relating the thermodynamic and structural properties of such systems to those of the hard-sphere fluid. The expansion is ordered by a softness parameter $\ensuremath{\xi}$ which is essentially the range of intermolecular distances in which the difference between the Mayer $f$ functions for the repulsive potential and an appropriate reference hard-sphere potential is nonzero. The first (lowest-order) approximation generated by the expansion equates the free energy and $y(r)$ for the fluid to the respective functions appropriate to a system of hard spheres with diameter $d$. Here $y(r)=g(r) {e}^{+\ensuremath{\beta}u(r)}$, where $g(r)$ and $u(r)$ denote the radial distribution function and repulsive pair potential, respectively. A prescription is given for choosing a temperature- and density-dependent diameter $d$ in the reference hard-sphere fluid so that the first approximation for the free energy contains errors of order ${\ensuremath{\xi}}^{4}$ only, and the corrections to the first approximation for $g(r)$ are of order ${\ensuremath{\xi}}^{2}$. The method is used to calculate the properties of a fluid whose intermolecular potential varies as ${r}^{\ensuremath{-}12}$. The repulsive potential that produces the repulsive forces in the Lennard-Jones potential is also studied. Since the properties of the hard-sphere fluid are known from the results of computer calculations and conveniently summarized by analytic equations, the application of the first approximation is numerically very simple. With this approximation, the results obtained for both model systems agree closely with those obtained by Monte Carlo calculations.

Inference about the change-point from cumulative sum tests

Abstract: SUMMARY The point of change in mean in a sequence of normal random variables can be estimated from a cumulative sum test scheme. The asymptotic distribution of this estimate and associated test statistics are derived and numerical results given. The relation to likelihood inference is emphasized. Asymptotic results are compared with empirical sequential results, and some practical implications are discussed. The cumulative sum scheme for detecting distributional change in a sequence of random variables is a well-known technique in quality control, dating from the paper of Page (1954) to the recent expository account by van Dobben de Bruyn (1968). Throughout the literature on cumulative sum schemes the emphasis is placed on tests of departure from initial conditions. The purpose of this paper is to examine a secondary aspect: estimation of the index T in a sequence {xt}, where the departure from initial conditions has taken place. The work is closely related to an earlier paper by Hinkley (1970), in which maximum likelihood estimation and inference were discussed. We consider specifically sequences of normal random variables x1, ..., xT, say, where initially the mean 00 and the variance o2 are known. A cumulative sum, cusum, scheme is used to detect possible change in mean from 00, and for simplicity suppose that it is a one-sided scheme for detecting decrease in mean. Then the procedure is to compute the cumulative sums t

Discrete square root filtering: A survey of current techniques

Abstract: The conventional Kalman approach to discrete filtering involves propagation of a state estimate and an error covariance matrix from stage to stage. Alternate recursive relationships have been developed to propagate a state estimate and a square root error covariance instead. Although equivalent algebraically to the conventional approach, the square root filters exhibit improved numerical characteristics, particularly in ill-conditioned problems. In this paper, current techniques in square root filtering are surveyed and related by applying a duality association. Four efficient square root implementations are suggested, and compared with three common conventional implementations in terms of computational complexity and precision. The square root computational burden should not exceed the conventional by more than 50 percent in most practical problems. An examination of numerical conditioning predicts that the square root approach can yield twice the effective precision of the conventional filter in ill-conditioned problems. This prediction is verified in two examples. The excellent numerical characteristics and reasonable computation requirements of the square root approach make it a viable alternative to the conventional filter in many applications, particularly when computer word length is limited, or the estimation problem is badly conditioned.

An Operator Which Locates Edges in Digitized Pictures

Abstract: Because of the fundamental importance of edges as primitives of pictures, automatic edge finding is set as goal. A set of requirements which should be met by a local edge recognizer is formulated. Their main concerns are fast and reliable recognition in the presence of noise. A unique optimal solution for an edge operator results. The operator obtains the best fit of an ideal edge element to any empirically obtained edge element. Proof of this is given. A reliability assessment accompanies every recognition process.

An algorithm with guaranteed convergence for finding a zero of a function

Abstract: An algorithm is presented for finding a zero of a function which changes sign in a given interval. The algorithm combines linear interpolation and inverse quadratic interpolation with bisection. Convergence is usually superlinear, and is never much slower than for bisection. ALGOL 60 procedures are given. Comments Only the Abstract is given here. The full paper appeared as [1]. For similar material see [2, Chapter 4]. Related algorithms are described in [3, 4]. References [1] R. P. Brent, “An algorithm with guaranteed convergence for finding a zero of a function”, Computer J. 14 (1971), 422–425. MR 49#4234, Zbl 231.65046. rpb005. [2] R. P. Brent, Algorithms for Minimization without Derivatives, Prentice-Hall, Englewood Cliffs, New Jersey, 1973, 195 pp. MR 49#4251, CR 15#26544. rpb011. [3] T. J. Dekker, “Finding a zero by means of successive linear interpolation”, in Constructive Aspects of the Fundamental Theorem of Algebra (edited by B. Dejon and P. Henrici), Interscience, New York, 1969. [4] J. H. Wilkinson, Two Algorithms based on Successive Linear Interpolation, Technical Report CS 60, Computer Science Department, Stanford University, Stanford, California, 1967. Department of Computer Science, Stanford University, Stanford, CA 94305, USA 1991 Mathematics Subject Classification. Primary 65H05; Secondary 65-04, 65G05, 65H20, 68-04, 68Q25.

Alcohol Dependence Produced in Mice by Inhalation of Ethanol: Grading the Withdrawal Reaction

Abstract: Intoxicating blood levels of ethanol are maintained for several days in mice housed in an atmosphere of ethanol vapor. On removal from the alcohol, all the mice develop withdrawal signs. The signs can be graded to indicate the time course and intensity of the withdrawal reaction.

Microfilaments and cell locomotion.

Abstract: The role of microfilaments in generating cell locomotion has been investigated in glial cells migrating in vitro. Such cells are found to contain two types of microfilament systems: First, a sheath of 50–70-A in diameter filaments is present in the cytoplasm at the base of the cells, just inside the plasma membrane, and in cell processes. Second, a network of 50-A in diameter filaments is found just beneath the plasma membrane at the leading edge (undulating membrane locomotory organelle) and along the sides of the cell. The drug, cytochalasin B, causes a rapid cessation of migration and a disruption of the microfilament network. Other organelles, including the microfilament sheath and microtubules, are unaltered by the drug, and protein synthesis is not inhibited. Removal of cytochalasin results in complete recovery of migratory capabilities, even in the absence of virtually all protein synthesis. Colchicine, at levels sufficient to disrupt all microtubules, has no effect on undulating membrane activity, on net cell movement, or on microfilament integrity. The microfilament network is, therefore, indispensable for locomotion.

Intercalation Complexes of Lewis Bases and Layered Sulfides: A Large Class of New Superconductors

Abstract: Exploration of the generality of the recently discovered reaction whereby certain organic molecules can be inserted between the metallic layers of the superconductors tantalum disulfide and niobium disulfide revealed that a large variety of organic and inorganic molecules can penetrate between the crystalline layers of a number of transition metal dichalcogenides and that the resulting complexes are superconducting if the layered chalcogenide from which they are formed is superconducting. The critical temperatures of the 50 new superconductors we report depend on the nature of the intercalate but are insensitive to a separation of the superconducting planes of up to 57 angstroms.

Renormalization Group and Strong Interactions

Abstract: The renormalization-group method of Gell-Mann and Low is applied to field theories of strong interactions. It is assumed that renormalization-group equations exist for strong interactions which involve one or several momentum-dependent coupling constants. The further assumption that these coupling constants approach fixed values as the momentum goes to infinity is discussed in detail. However, an alternative is suggested, namely, that these coupling constants approach a limit cycle in the limit of large momenta. Some results of this paper are: (1) The e+−e− annihilation experiments above 1-GeV energy may distinguish a fixed point from a limit cycle or other asymptotic behavior. (2) If electrodynamics or weak interactions become strong above some large momentum Λ, then the renormalization group can be used (in principle) to determine the renormalized coupling constants of strong interactions, except for U(3)×U(3) symmetry-breaking parameters. (3) Mass terms in the Lagrangian of strong, weak, and electromagnetic interactions must break a symmetry of the combined interactions with zero mass. (4) The ΔI=12 rule in nonleptonic weak interactions can be understood assuming only that a renormalization group exists for strong interactions.

Decay Correlations of Heavy Leptons in e + + e − → l + + l −

Abstract: Assuming that leptons heavier than muons exist in nature, we consider their decay modes and the correlations between the decay products of ${l}^{+}$ and ${l}^{\ensuremath{-}}$ in the colliding-beam experiment: ${e}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{l}^{+}+{l}^{\ensuremath{-}}$. Far above the threshold, the helicities of ${l}^{+}$ and ${l}^{\ensuremath{-}}$ tend to be opposite to each other. Near the threshold the directions of spins of ${l}^{+}$ and ${l}^{\ensuremath{-}}$ prefer to be parallel to each other, and the sum of the two spins prefers to be either parallel or anti-parallel to the direction of the incident electron. Because the parity conservation is violated maximally in the decays of ${l}^{+}$ and ${l}^{\ensuremath{-}}$, the angular distributions of decay products depend strongly on the spin orientation of the heavy leptons. Since the spins of ${l}^{+}$ and ${l}^{\ensuremath{-}}$ are strongly correlated in the production, we found a strong correlation between the energy-angle distributions of the decay products of ${l}^{+}$ and ${l}^{\ensuremath{-}}$. The decay widths of ${l}^{\ensuremath{-}}$ into channels ${\ensuremath{ u}}_{l}{\overline{\ensuremath{ u}}}_{e}{e}^{\ensuremath{-}}$, ${\ensuremath{ u}}_{l}{\overline{\ensuremath{ u}}}_{\ensuremath{\mu}}{\ensuremath{\mu}}^{\ensuremath{-}}$, ${\ensuremath{ u}}_{l}{\ensuremath{\pi}}^{\ensuremath{-}}$, ${\ensuremath{ u}}_{l}{K}^{\ensuremath{-}}$, ${\ensuremath{ u}}_{l}{\ensuremath{\rho}}^{\ensuremath{-}}$, ${\ensuremath{ u}}_{l}{K}^{*}$, ${\ensuremath{ u}}_{l}{A}_{1}$, ${\ensuremath{ u}}_{l}Q$, and ${\ensuremath{ u}}_{l}+\mathrm{hadron}$ continuum as functions of the mass of ${l}^{\ensuremath{-}}$ are estimated.

On the basic theorem of complementarity

Abstract: Using a fixed point theorem of Browder, the basic existence theorem of Lemke in linear complementarity theory is generalized to the nonlinear case.