Showing papers by "University of Southern California published in 1976"

Analysis of a Model for Excitation of Myelinated Nerve

Abstract: Excellent models have been presented in the literature which relate membrane potential to transverse membrane current and which describe the propagation of action potentials along the axon, for both myelinated and nonmyelinated fibers. There is not, however, an adequate model for nerve excitation which allows one to compute the threshold of a nerve fiber for pulses of finite duration using electrodes that are not in direct contact with the fiber. This paper considers this problem and presents a model of the electrical properties of myelinated nerve which describes the time course of events following stimulus application up to the initiation of the action potential. The time-varying current and potential at all nodes can be computed from the model, and the strength-duration curve can be determined for arbitrary electrode geometries, although only the case of a monopolar electrode is considered in this paper. It is shown that even when the stimulus is a constant-current pulse, the membrane current at the nodes varies considerably with time. The strength-duration curve calculated from the model is consistent with previously published experimental data, and the model provides a quantitative relationship between threshold and fiber diameter which shows there is less selectivity among fibers of large diameter than those of small diameter.

On the wall structure of the turbulent boundary layer

Abstract: The wall structure of the turbulent boundary layer was examined using hot-wire rakes and conditional sampling techniques. Instantaneous velocity measurements indicate a high degree of coherence over a considerable area in the direction normal to the wall. At y+ = 15, there is some evidence of large-scale correlation in the spanwise direction, but almost no indication of the streamwise streaks that exist in the lower regions of the boundary layer. Conditional sampling showed that the normal velocity is directed outwards in regions of strong stream-wise-momentum deficit, and inwards when the streamwise velocity exceeds its mean value. The conditionally averaged Reynolds shear stress was approximately an order of magnitude greater than its conventionally averaged value and decayed slowly downstream.

Bicycle-pedal model for the first step in the vision process.

Abstract: Computer simulation of the molecular dynamics of retinal during its photoisomerisation inside a restrictive active site gives a detailed model for the sequence of events in the first step of the vision process. It is proposed that the prelumirhodopsin intermediate contains a strained all-trans retinal molecule produced directly and rapidly from the 11-cis, 12-s-trans conformation in rhodopsin by a bicycle-pedal isomerisation. The model reproduces the main experimental observations and explains how the protein makes the photoisomerisation path unique.

Exact and Approximate Algorithms for Scheduling Nonidentical Processors

Abstract: Exact and approximate algorithms are presented for scheduling independent tasks in a multiprocessor environment in which the processors have different speeds. Dynamic programming type algorithms are presented which minimize finish time and weighted mean flow time on two processors. The generalization to m processors is direct. These algorithms have a worst-case complexity which is exponential in the number of tasks. Therefore approximation algorithms of low polynomial complexity are also obtained for the above problems. These algorithms are guaranteed to obtain solutions that are close to the optimal. For the case of minimizing mean flow time on m-processors an algorithm is given whose complexity is O(n log mn).

Singular value decompositions and digital image processing

Abstract: The use of singular value decomposition (SVD) techniques in digital image processing is of considerable interest for those facilities with large computing power and stringent imaging requirements. The SVD methods are useful for image as well as quite general point spread function (impulse response) representations. The methods represent simple extensions of the theory of linear filtering. Image enhancement examples will be developed illustrating these principles. The most interesting cases of image restoration are those which involve space variant imaging systems. The SVD, combined with pseudoinverse techniques, provides insight into these types of restorations. Illustrations of large scale N2× N2point spread function matrix representations are discussed along with separable space variant N2× N2point spread function matrix examples. Finally, analysis and methods for obtaining a pseudoinverse of separable space variant point spread functions (SVPSF's) are presented with a variety of object and imaging system dagradations.

Identifiability of linear and nonlinear dynamical systems

Abstract: This short paper considers the identification of dynamical systems from input-output data. The problem of parameter identifiability for such systems is approached by considering whether system outputs obtained with different parameter values can be distinguished one from another. The results are stated formally by defining the notion of "output distinguishability." Parameter identifiability is then defined precisely in terms of output distinguishability. Relationships have been developed with the other definitions such as least square identifiability and identifiability from the transfer function. Several results for linear and nonlinear systems are presented with examples.

Functional hypoparathyroidism and parathyroid hormone end‐organ resistance in human magnesium deficiency

Abstract: Hypocalcaemia is a well-recognized manifestation of magnesium deficiency. We have studied seventeen patients with this syndrome in an attempt to determine the pathogenesis of the hypocalcaemia. Mean initial serum calcium concentration was 5-6 mg/dl and mean initial serum magnesium concentration was 0-75 mg/dl. Serum immunoreactive parathyroid hormone (IPTH) was measured in sixteen patients in the untreated state. Despite severe hypocalcaemia, serum IPTH was either undetectable (less than 150 pg/ml) or normal (less than 550 pg/ml) in all but two patients. Serial measurements made during the initial 4 days of magnesium therapy in four patients showed an increase in serum IPTH within 24h, but a delayed increase in serum calcium, which required approximately 4 days to reach normal values. The effect of the rapid normalization of serum magnesium on serum IPTH and serum calcium concentration was studied in three patients. Within 1 min after 144-300 mg of elemental magnesium was administered i.v., serum IPTH had risen from undetectable to 3600 pg/ml and 1725 pg/ml in two patients and from 425 pg/ml to 937 pg/ml in the third. Serum calcium concentrations were unchanged after 30-60 min. These data provide evidence for impaired parathyroid gland function in most of the magnesium deficient patients. The rapidity with which serum IPTH rose in response to magnesium therapy indicates that this may reflect a defect in parathyroid hormone (PTH) secretion rather than its biosynthesis. The failure of serum calcium concentration to increase during the initial days of magnesium repletion, at a time when serum IPTH concentrations were normal or elevated, suggests end-organ resistance to PTH in these patients. The renal response to PTH was examined in two magnesium deficient patients by measurement of urinary cyclic AMP excretion following administration of parathyroid extract. In both patients there was a minimal increase in urinary cyclic AMP concentrations. In contrast, when the hepatic response to glucagon was tested on the same patients by measurement of plasma cyclic AMP concentrations following administration of glucagon, normal increases were observed. These results suggest that adenylate cyclase systems of various organs may be affected differentially by a state of magnesium deficiency. It is suggested that magnesium deficiency may result in defective cyclic AMP generation in the parathyroid glands and in the PTH target organs. This could be the principal mechanism operative in both impaired PTH secretion and end-organ resistance to PTH which together contribute to the development of hypocalcaemia.

Singular Value Decomposition (SVD) Image Coding

Abstract: The numerical techniques of transform image coding are well known in the image bandwidth compression literature. This concise paper presents a new transform method in which the singular values and singular vectors of an image are computed and transmitted instead of transform coefficients. The singular value decomposition (SVD) method is known to be the deterministically optimal transform for energy compaction [2]. A systems implementation is hypothesized, and a variety of coding strategies is developed. Statistical properties of the SVD are discussed and a self adaptive set of experimental results is presented, Imagery compressed to 1, 1.5, and 2.5 bits per pixel with less than 1.6, 1, and 1/3 percent, respective mean-square error is displayed. Finally, additional image coding scenarios are postulated for further consideration.

The universality of conversational postulates

Abstract: Grice's analysis of conversational maxims and implicatures is examined in the light of Malagasy language and ways of speaking. A cultural contrast in primary assumptions is described. Grician analysis retains usefulness but within the perspective of a comparative typology in which locally valid systems may differ strikingly in what is marked and unmarked. An ethnographic base and ethnological comparison are required. The situation somewhat resembles the situation with regard to grammatical categories addressed by Boas (1911) and Sapir (1921). (Conversational postulates, ways of speaking; English (US), Malagasy (Madagascar)). (DH)

Large scales in the developing mixing layer

Abstract: A new experimental technique is described for the study of the interactions between the large-scale vortical features in the two-dimensional mixing layer. Detector probes above and below the mixing layer are used to monitor the large-scale structure. Conditional sampling is performed in a moderate Reynolds number developing flow, by using phase and amplitude information from these detector probes. It is shown that significant Reynolds-stress production is associated with the pairing interaction in which two vortical structures combine to form a single, larger vortical structure.

Topics on Multiphoton Processes in Atoms

Abstract: Publisher Summary The theory of multiphoton processes can be developed in a semiclassical or fully quantum mechanical formalism. It is not possible to separate multiphoton ionization from bound–bound transitions as the latter are involved in resonant multiphoton ionization. Thus, the study of ionization involves the study of multiphoton bound–bound transitions. Also, in multiphoton processes, the amplitude of the near-resonant level interferes with the background due to off-resonance contributions by the continuum as well as all other bound states. The momentum translation approximation (MTA) is a rearrangement of perturbation theory. The first term of this expansion contains the field to all orders, but so do the omitted terms. The space translation approximation (STA) represents an attempt to incorporate part of the strong field into the atomic Hamiltonian, the remaining part being treated as a perturbation. The part incorporated into the atomic potential is a time-independent quantity obtained after an expansion in terms of harmonics of the frequency of the applied external field A 0 sin ωt .

The Analysis and Design of Pneumatic Systems

Abstract: The analysis and design of pneumatic systems , The analysis and design of pneumatic systems , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

A Graph Model for Fault-Tolerant Computing Systems

Abstract: An approach to fault-tolerant design is described in which a computing system S and an algorithm A to be executed by S are both defined by graphs whose nodes represent computing facilities. A is executable by S if A is isomorphic to a subgraph of S.A k-fault is the removal of k nodes (facilities) from S.S is a k-fault tolerant (k-FT) realization of A if A can be executed by S with any k-fault present in S. The problem of designing optimal k-FT systems is considered where A is equated to a 0-FT system. Techniques are described for designing optimal k-FT realizations of single-loop systems; these techniques are related to results in Hamiltonian graph theory. The design of optimal k-FT realizations of certain types of tree systems is also examined. The advantages and disadvantages of the graph model are discussed.

Waveguides: characteristic modes of hollow rectangular dielectric waveguides.

Abstract: The field configurations and propagation constants of the normal modes of a hollow rectangular dielectric waveguide have been determined. In addition, the coupling coefficients of a Gaussian free-space mode into the normal modes of a square guide were calculated. The attenuation of each mode is found to be inversely proportional to the cube of the guide aperture 2alpha and proportional to the square of the free-space wavelength lambda. For a hollow dielectric square guide with 2alpha = 1 mm and lambda = 10.6 microm, an attenuation of 0.140 dB/m is predicted for SiO(2) and 0.032 dB/m for BeO. All modes are found to be hybrid modes, although they very closely approximate linearly polarized TEM modes.

Characterization of structural controllability

Abstract: A self-contained algebraic derivation of the necessary and sufficient conditions for a multiinput system with a fixed zero structure to be structurally controllable is given. In addition, a new recursive test for determining structural controllability which utilizes only Boolean operations is obtained.

Experiments on collisions between solitary waves

Abstract: Experiments on ‘head-on’ collisions between two solitary waves show that the waves reach a maximum amplitude greater than twice the initial wave amplitude and that they suffer a time delay during their interaction. These results are compared with available theories and found to be in qualitative but not quantitative agreement.

Polygraphic studies of normal infants during the first six months of life. I. Heart rate and variability as a function of state.

Abstract: This study examined spontaneous heart rate (HR) and variability as a function of age and sleep state in eight normal full term infants from birth to 6 months of age. Heart rates recorded during sleep were lower and less regular at 1 week (quiet sleep (QS) mean rate= 128, interquartile range = 6.4 beats/min; rapid eye movement (REM) = 134.5, 11.6) than at 1 month (QS = 138.6, 3.4; REM 139.6, 4.2). Rate decreased sharply from 1 to 3 months (QS = 118; REM 123.8) and decreased only slightly therafter (6-month QS = 113.5; REM 118.9). Variability decreased rapidly in REM from 2 to 4 months (from 11.4 to 9.1) and less quickly from 4 to 6 months (from 9.1 to 8.2), while QS variability decreased at 1 month (from 6.4 to 5.7) and became stable from that point (6.0 at 6 months). Waking heart rate and variability were both relatively low at 1 week (163, 11.2 beats/min) and increased from that age to 1 month (167.4, 14.3). Rate decreased rapidly in waking at 3 months (152 beats/min) and more slowly therafter (152 beats/min at 4 months, 149 beats/min at 6 months), whereas variability remained elevated until after 3 months, becoming stable at a lower level during later infancy (3 months = 14.8,6 months 11.7). Lowest values of rate and variability were found in QS and the highest values were found in waking at all ages, except at 1 week. Heart rates during REM closely approximated those in QS, whereas variability values more closely resembled those of waking.

Handedness in a Chinese Population: Biological, Social, and Pathological Factors

Abstract: A questionnaire survey of 4143 Chinese was conducted. Social pressure for right-handed writing and eating was effective on these two target skills but showed little indirect influence on hand use in other activities. Neither primiparous birth nor birth to older mothers affected handedness. Twinning, however, seemed to be associated with both decreased right-handedness and lower intelligence.

Raman-Induced Kerr Effect

Abstract: A new effect is demonstrated in which a laser pulse can be made to induce a Kerr effect only at Raman-shifted frequencies. This permitted the observation of a Raman spectrum with a single dye-laser pulse. The pulse power required is a small fraction of that required to produce stimulated instabilities. The spectral information emerges in a coherent beam, phase matching is not necessary, and spectra can be obtained at any scattering angle.

Cooperation and Group Size in the N-Person Prisoners' Dilemma

Abstract: This study explores the relationship between group size and member cooperation when individual and group interests conflict. Existing theories incorrectly imply that larger groups should experience more difficulty in providing “public goods” (goods whose benefits are not confined to the purchaser). A theoretically based typology that identifies nine systematic patterns associated with changes in group size and parameters of individual and group payoff structure is presented. The typology describes nine different ways in which group size can affect cooperation in the provision of public goods. In a computer-administered experiment using three of these nine types and group sizes of three, six, and nine, 90 college students were given sequences of opportunities to choose between “cooperation” (which benefited everyone) and “noncooperation” (which benefited the chooser but harmed everyone else). For one of these types, a counter-intuitive but theoretically predicted positive relationship between group size an...

Relativization of questions about log space computability

Abstract: A notion of log space Turing reducibility is introduced. It is used to define relative notions of log space,ℒ A , and nondeterministic log space, . These classes are compared with the classes and which were originally defined by Baker, Gill, and Solovay [BGS]. It is shown that there exists a computable setA such that . Furthermore, there exists a computable setA such that and . Also a notion of log space truth table reducibility is defined and shown to be equivalent to the notion of log space Turing reducibility.