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PubMed2006

Phenomenological model of bone remodeling cycle containing osteocyte regulation loop.

Biological parameters, such as bone resorption and formation constants, are important variables to achieve optimised hard tissue scaffolds design. To help to understand the modelling process that occurs when a scaffold is implanted it is vital to understand the rather complex bone remodeling process prevalent in native bone. One approach to developing a mathematical model that predicts osteoactivity both in scaffolds, as well as in bone in vivo, is based on a bio-cybernetic vision of basic multicellular unit (BMU) action -. In the case of the model presented in this paper, an additional loop of regulation based on osteocyte activity has been added. This approach has resulted in a four-dimensional system, which shows steady-quasi-cyclic behaviour using a particular range of constants with real biological meaning. The initial findings suggesting that the basic steady-state appears as a torus in multidimensional phase space have been discussed. The existence of this surface in the osteoclasts-osteoblasts-osteocytes-bone subspace indicates that there is a first integral for this dynamic system. Biological and physical interpretation of this integral as a conservative value has been proposed. It is possible to draw an analogy between this conservative value, as a kind of substrate-energy regenerative potential of the bone remodeling system with a molecular nature, to the classical physical value (energy). There are clear indications that there is recovering potential within the BMU that results in a steady operating genetically predominated bone remodeling process. This recovering potential is directed against both mechanical and biomechanical damage to the bone. The current model has credibility when compared to the normal bone remodeling process. However, additional work is required to study a wider range of constants.

Animals↗
PubMed2006

On the legacy of W.S. McCulloch.

In this paper, we review McCulloch's legacy, from his early work in neurophysiology, and its relationship to his philosophical quest for an 'experimental epistemology' to his role in the cybernetics movement during the 1940s and 1950s and his contributions to the development of computer science and communication theory. There are three parts in chronological sequence. First, the period up to his work at Yale University with Dusser de Barenne, where he concentrated on the experimental study of the functional organization of sensory cortex. Second, the time of his Psychiatric Chair at the University of Chicago and the organization of the Macy Foundation Conferences. To this period corresponds the genesis and publication of the most influential and quoted work by McCulloch and Pitts: A Logical Calculus of the Ideas Immanent in Neurons Activity. Third, the period of his research activity at the Massachussetts Institute of Technology where he, Lettvin, Maturana and Pitts produced epochmaking papers on epistemological neurophysiology, the modelling of the reticular formation and other work with da Fonseca and Moreno-Díaz. We finally refer to the International Conference that took place in McCulloch's memory at the 25th anniversary of his death. Our main conclusion is that McCulloch's writings are still a source of inspiration from neurophysiology to artificial intelligence and robotics.

Artificial Intelligence↗
PubMed2006

On allosteric control model of bone turnover cycle containing osteocyte regulation loop.

One approach to developing a mathematical model that predicts osteoactivity both in bio-scaffolds, as well as the in bone tissue in vivo, is based on a bio-cybernetic vision of basic multicellular unit (BMU) action. In the case of the model presented in this paper, some of the loops of regulation have been modified to reflect the range of allosteric control mechanisms: Michaelis-Menten, Hill, Adair, Koshland-Nemethy-Filmer (KNF), Monod-Wyman-Changeux (MWC). This approach has resulted in a four-dimensional system that shows steady cyclic behaviour using a range of constants with clear biological meaning. The initial findings suggesting that a steady state appears as a cycle in multidimensional phase space and this is discussed in this paper. The existence of this cycle in the osteoclasts-osteoblasts-osteocytes-bone subspace indicates that there is a conservative value along steady trajectories for this dynamic system. Biophysical interpretation of this conservative value has been proposed as a kind of substrate-energy regenerative potential of the bone remodelling system with a similarity to the classical physical value-energy. Such a recovery "potential" is directed against both mechanical and biomechanical damage to the bone. The current model has credibility when compared to the normal bone remodelling process. In the framework of widely recognised Michaelis-Menten mechanisms of allosteric regulation the cyclic attractor, described formerly for a pure cellular model, prevails for different forms of feedback control. This finding demonstrates the viability of the suggestion of the subsistence of conservative value (analogous to energy) that characterises the recovery potential of the bone remodelling cycle. The results indicate that the robust behaviour of the model is maintained from the simple cellular level to the molecular biochemical level of regulation.

Allosteric Site↗
PubMed2005

A reductionist's systems biology: opinion.

To tackle the complexity inherent in understanding large networks of interacting biomolecules, systems biology emphasizes cybernetic and systems theoretical approaches. The resulting focus on organization independent of physical manifestation threatens to throw away all that has been learned from molecular studies and ignores the reality that biologists are drawn together more by a shared interest in mechanism and structure than anything else. The field of reaction engineering suggests a reductionist approach to systems biology that fits easily within existing molecular paradigms but that can nonetheless be integrated into expansive physiological perspectives through the use of multi-scale modeling.

Animals↗
PubMed2004

Chance and necessity do not explain the origin of life.

Where and how did the complex genetic instruction set programmed into DNA come into existence? The genetic set may have arisen elsewhere and was transported to the Earth. If not, it arose on the Earth, and became the genetic code in a previous lifeless, physical-chemical world. Even if RNA or DNA were inserted into a lifeless world, they would not contain any genetic instructions unless each nucleotide selection in the sequence was programmed for function. Even then, a predetermined communication system would have had to be in place for any message to be understood at the destination. Transcription and translation would not necessarily have been needed in an RNA world. Ribozymes could have accomplished some of the simpler functions of current protein enzymes. Templating of single RNA strands followed by retemplating back to a sense strand could have occurred. But this process does not explain the derivation of "sense" in any strand. "Sense" means algorithmic function achieved through sequences of certain decision-node switch-settings. These particular primary structures determine secondary and tertiary structures. Each sequence determines minimum-free-energy folding propensities, binding site specificity, and function. Minimal metabolism would be needed for cells to be capable of growth and division. All known metabolism is cybernetic--that is, it is programmatically and algorithmically organized and controlled.

Animals↗
PubMed2004

Cycling in ecological networks: Finn's index revisited.

A chief cybernetic feature of natural living systems is the recycling of nutrients, which tends to enhance stability and is one of the principal causes of ecosystem complexity. In 1976, Finn proposed a simple and effective measure (later known as the Finn cycling index [FCI]) to assess the quantitative importance of cycles in ecosystems. This index was successfully applied as a gauge of ecosystem health and maturity in a wide variety of studies. It turns out, however, that FCI is biased as a measure of cycling in ecosystems, because it does not include all flows engaged in recycling. A new, more inclusive version of the index is possible. What is called the comprehensive cycling index (CCI) accounts for all of the fluxes generated by cycling. Computing the new measure requires a large amount of time, however, even with ad-hoc software. To obviate the necessity for such heavy computation, a linear transformation of the FCI into the CCI is proposed.

Algorithms↗
PubMed2005

Body sway during quiet standing: is it the residual chattering of an intermittent stabilization process?

This paper reviews different approaches for explaining body sway while quiet standing that directly address the instability of the human inverted pendulum. We argue that both stiffness control [Winter, D. A., Patla, A. E., Riedtyk, S., & Ishac, M. (2001). Ankle muscle stiffness in the control of balance during quiet standing. Journal of Neurophysiology, 85, 2630-2633] and continuous feedback control by means of a PID (Proportional, Integral, Derivative) mechanism [Peterka, R. J. (2000). Postural control model interpretation of stabilogram diffusion analysis. Biological Cybernetics, 83, 335-343.] can guarantee asymptotic stability of controlled posture at the expense of unrealistic assumptions: the level of intrinsic muscle stiffness in the former case, and the level of background noise in the latter, which also determines an unrealistic level of jerkiness in the sway. We show that the decomposition of the control action into a slow and a fast component (rambling and trembling, respectively, as proposed by [Zatsiorsky, V. M., & Duarte, M. (1999). Instant equilibrium point and its migration in standing tasks: Rambling and trembling components of the stabilogram. Motor Control, 4, 185-200; Zatsiorsky, V. M., & Duarte, M. (2000). Rambling and trembling in quiet standing. Motor Control, 4, 185-200.]) is useful but must be modified in order to take into account that rambling is not a stable equilibrium trajectory. We address the possibility of a form of stability weaker than asymptotic stability in light of the intermittent stabilization mechanism outlined by [Loram, I. D., & Lakie, M. (2002a). Human balancing of an inverted pendulum: position control by small, ballistic-like, throw and catch movements. Journal of Physiology, 540, 1111-1124.], and propose an indicator of intermittent stabilization that is related to the phase portrait of the human inverted pendulum. This indicator provides a further argument against the plausibility of PID-like control mechanisms. Finally, we draw attention to the sliding mode control theory that provides a useful theoretical framework for formulating realistic intermittent, stabilization models.

Adult↗
PubMed2004

Analysis of nonlinear systems to estimate intraocular lens position after cataract surgery.

PURPOSE: To compare the performance of neural networks with that of linear regression to predict the postoperative effective lens position (ELP) from preoperative biometry measurements. SETTING: Departments of Ophthalmology, Medical Cybernetics and Artificial Intelligence, and Medical Physics, Medical University of Vienna, Vienna, Austria. METHODS: The neural-network-type multilayer perceptron (MLP) and a linear regression technique were used to predict ELP. Suitable MLP models and variable input combinations were selected by extended-feature subset selection. Apart from the usual preoperative biometric variables, anterior chamber depth and lens thickness were measured with partial coherence interferometry and white-to-white measurements were used as input variables. RESULTS: Prediction of ELP could be improved from a correlation coefficient (Pearson) of 0.54 for linear regression to a coefficient of 0.68 for the MLP; however, this difference was not statistically significant. CONCLUSION: The prediction of postoperative ACD with the MLP was not significantly better than the prediction using linear regression.

Adult↗
PubMed2006

Homeostatic synaptic scaling in self-organizing maps.

Various forms of the self-organizing map (SOM) have been proposed as models of cortical development [Choe Y., Miikkulainen R., (2004). Contour integration and segmentation with self-organized lateral connections. Biological Cybernetics, 90, 75-88; Kohonen T., (2001). Self-organizing maps (3rd ed.). Springer; Sirosh J., Miikkulainen R., (1997). Topographic receptive fields and patterned lateral interaction in a self-organizing model of the primary visual cortex. Neural Computation, 9(3), 577-594]. Typically, these models use weight normalization to contain the weight growth associated with Hebbian learning. A more plausible mechanism for controlling the Hebbian process has recently emerged. Turrigiano and Nelson [Turrigiano G.G., Nelson S.B., (2004). Homeostatic plasticity in the developing nervous system. Nature Reviews Neuroscience, 5, 97-107] have shown that neurons in the cortex actively maintain an average firing rate by scaling their incoming weights. In this work, it is shown that this type of homeostatic synaptic scaling can replace the common, but unsupported, standard weight normalization. Organized maps still form and the output neurons are able to maintain an unsaturated firing rate, even in the face of large-scale cell proliferation or die-off. In addition, it is shown that in some cases synaptic scaling leads to networks that more accurately reflect the probability distribution of the input data.

Animals↗
PubMed2004

Sensitivity to curvatures in orientation-based texture segmentation.

Texture segregation has long been attributed to changes in the distribution of elementary features across the visual field [Nature 290 (12) (1981) 91; Biol. Cybernet. 54 (1986) 245]. The study of orientation, a conspicuous feature, has led to models of orientation-based texture segmentation (OBTS) that depend on the magnitude of one or two orientation gradients [Vis. Res. 31 (4) (1991) 679; Vis. Res. 31 (6) (1991) 1073] and influenced further by the relative configuration between the orientation textons and the global orientation edge [Percept. Psychophys. 52 (4) (1992) 255; Vis. Res. 35 (20) (1995) 2863]. Here we show that these models are at best partial and that the notion of orientation gradient has been incompletely used in the study of OBTS. To do so, we first study the behavior of orientation in orientation-defined texture patches. Geometrical analysis identifies two texture curvatures and reveals the incompleteness of previous stimuli. Psychophysical experimentation then demonstrates that segmentation is strongly affected by discontinuities in these curvatures. Importantly, we show that this sensitivity to curvature is independent of the orientation gradients and inconsistent with the simple configural considerations proposed in the past.

Discrimination, Psychological↗
PubMed2005

Interaction of luminance and higher-order statistics in texture discrimination.

Most studies of texture processing are based on textures in which individual pixel statistics are varied and spatial correlations are absent ("IID textures"), or textures in which spatial correlation structure is varied and luminance, or first-order, statistics are held constant. Here we jointly examine simple pixel statistics and fourth-order spatial correlation structure along the continuum of "even" and "odd" isodipole textures of Julesz, Gilbert and Victor [Julesz, B., Gilbert, E.N., & Victor, J.D. (1978). Biological Cybernetics, 31(3) 137-140], as well as their interactions. Absolute efficiency to detect either kind of statistical cue is low: approximately 0.05 for luminance statistics, and 0.004 for isodipole statistics. Above threshold, isodipole statistics must change by approximately four times the amount that pixel statistics must change to generate an equally salient texture. When pixel statistics and isodipole statistics are simultaneously varied, the two texture cues combine by probability summation and perceptual distances are approximately Euclidean. Superimposed on this picture are subtle foreground/background asymmetries that suggest properties of the visual mechanisms that are sensitive to these image statistics.

Adult↗
PubMed1996

Strategic planning for academic health centers.

The mission, indeed the very existence, of the traditional academic health center is under siege. Changes in the financing and delivery of health care threaten the clinical revenue used to subsidize the tripartite mission of education, research, and patient care. Market practices, driven by the growth of managed care, will intensify the impact of declining revenue to threaten the actual patient base necessary to sustain these endeavors. The survival of academic health centers depends on their ability to change. This change will not be easy, and the size of the collective academic medical establishment will decrease. Successful enterprises will be those that go beyond incremental, reactive adjustments. Nothing short of organizational redesign, creation of strategic partnerships, and adopting a cybernetic model of continuous measurement, improvement and adaptability will suffice. Using the elements of a strategic planning exercise, this paper reviews the background issues that have produced the current predicament and explores the strengths and weaknesses inherent in academic institutions. Elements of an "idealized" academic health center are postulated and, finally, specific strategies that might be considered in creating a relevant and secure future are proposed.

Academic Medical Centers↗
PubMed1985

The control of condylar growth: an experimental evaluation of the role of the lateral pterygoid muscle.

The present study used 21 male albino rats to test the hypothesis that lateral pterygoid traction regulates the growth of the mandibular condyle. The condyles, the rami, and the top of each glenoid fossa were marked with metallic implants, and, following bilateral section of the condylar neck, one lateral pterygoid muscle was extirpated. On the basis of the literal details of Petrovic's cybernetic model, it was assumed that the continued forward growth of the midface and the backward translation of the glenoid fossa would combine to produce a progressive disturbance in the buccal occlusion that would, in turn, generate a reflex contraction of the remaining lateral pterygoid muscle. Initially, however, growth of the isolated condyles would have little impact on the spatial position of the rest of the mandible. As a result, the condyles on the side with the intact lateral pterygoid should grow for a time at a maximal, open-circuit rate, whereas the experimental condyle, deprived of all muscle traction, should show only a minimal "commanded" rate of growth. The serial change in the position of the condylar and ramal implants was assessed cephalometrically for 6 weeks, and between-sides differences were analyzed by randomized block analysis of variance. The presence or absence of the lateral pterygoid muscle had no significant effect on the anteroposterior position of the condylar implants and only a slight, transitory effect on their vertical position. The translation of the ramal implants, however, was greatly affected by the condylotomy. On both control and experimental sides, the mandible collapsed upward and backward until contact between the growing condyle and ramus had been achieved, whereupon a downward and forward pattern of translatory growth was re-established. Although it could not be shown that lateral pterygoid traction per se is a significant factor in the growth of isolated condyles, it was concluded that the condyle is vitally important to the translatory growth of the mandible as a whole. On the basis of these data and the current literature, a simple hypothesis was advanced for the control of condylar growth by the ongoing pattern of functional loading and for the role of this growth in the normal downward and forward displacement of the rest of the mandible.

Animals↗
PubMed2000

A participant-observer study of ergonomics in engineering design: how constraints drive design process.

Too often, ergonomics is relegated to being a "post-design" evaluation, leaving ergonomists little opportunity to make significant and important design changes. One way to start attacking this problem is to study the process of design and, in particular, ergonomics in design. This article describes the findings from a four-month long participant-observer study of the relationship between ergonomics and engineering design. The study was conducted in the context of a large, interdisciplinary project consisting of design of a control room for a nuclear power plant. It was observed that designers and ergonomists must negotiate through a changing web of constraints from many sources. The impact that these constraints had on the course of the design was documented. A model is developed based on the abstraction hierarchy (Rasmussen, 1985, IEEE Trans. Systems Man Cybernet. SMC-15, 234-243; 1990, Int. J. Ind. Ergon. 5, 5-16) which shows the interaction of conflicting goals as ergonomists and other designers attempt to solve a complex design problem. This model leads to several insights: (1) locally optimal ergonomic designs may not be globally optimal, (2) ergonomists can improve their solutions by understanding the goals of other designers, and (3) future tools to aid ergonomists must be compatible with the constraint-rich environments in which they work.

Ergonomics↗
PubMed2000

A method for measuring endpoint stiffness during multi-joint arm movements.

Current methods for measuring stiffness during human arm movements are either limited to one-joint motions, or lead to systematic errors. The technique presented here enables a simple, accurate and unbiased measurement of endpoint stiffness during multi-joint movements. Using a computer-controlled mechanical interface, the hand is displaced relative to a prediction of the undisturbed trajectory. Stiffness is then computed as the ratio of restoring force to displacement amplitude. Because of the accuracy of the prediction (< 1 cm error after 200 ms) and the quality of the implementation, the movement is not disrupted by the perturbation. This technique requires only 13 as many trials to identify stiffness as the method of Gomi and Kawato (1997, Biological Cybernetics 76, 163-171) and may, therefore, be used to investigate the evolution of stiffness during motor adaptation.

Arm↗
PubMed2000

The contributions of slant and tilt to the detection of local surface orientation in structure from motion.

The contribution of slant and tilt to the detection of differences in local surface orientation was examined for structure-from-motion (SFM) displays of a complex sinusoidal surface. Observers judged whether an elliptical SFM gauge figure appeared to be lying on the surface or intersecting it. The gauge figure orientation either matched the local surface orientation or differed from it in slant, tilt, or both. Similar sensitivity was found for deviations in slant and tilt, but greater biases and variability were found when the gauge figure deviated from the local surface orientation in slant, depending on the sign of the difference between the gauge figure and local surface orientation and the position of the gauge figure. The results are consistent with Stevens' (Biological Cybernetics, 46 (1983) 183-195) discussion of the computational advantages of slant and tilt contributing independently to the detection of differences in local surface orientation. The effects of changes in perceived surface slant and tilt during rotation and of the misperception of surface depth on the detection of local orientation in dynamic images are discussed.

Female↗
PubMed2001

A model of visual-spatial memory across saccades.

This paper describes a neural network model that directs saccades back to targets after they disappear and other saccades intervene. This is a simple example of knowing where something is after it is no longer visible and the observer has moved. These tasks require a short-term memory that can store continuous values of spatial location. The model was generated by training a neural network with a recurrently connected hidden layer to specify memory-guided saccades. The trained network maintains stored locations accurately for a few seconds. It uses a leaky integrator mechanism in which there is a slow decay of the stored value to a small number of fixed point attractors. Similar mechanisms have been used to model oculomotor integration (Cannon, S., Robinson, D., & Shamma, S. (1983). A proposed neural network for the integrator of the oculomotor system. Biological Cybernetics, 49, 127-136; Seung, H. (1998). Continuous attractors and oculomotor control. Neural Networks, 11, 1253-1258). The mechanism is robust to parameters such as the input and output format and the constraints in training. However, the receptive field properties of the hidden units do depend on these parameters. It was possible to find biologically plausible parameters that produced hidden unit behavior similar to that of real neurons involved in saccade memory. In particular, training the model to simultaneously represent the target location in both eye- and head-based reference frames produces units similar to neurons in parietal saccade areas.

Humans↗
PubMed2001

What defines a view?

At a given instant we see only visible surfaces, not an object's complete 3D appearance. Thus, objects may be represented as discrete 'views' showing only those features visible from a limited range of viewpoints. We address how to define a view using Koenderink's (Koenderink & Van Doorn, Biol. Cybernet. 32 (1979) 211.) geometric method for enumerating complete sets of stable views as aspect graphs. Using objects with known aspect graphs, five experiments examined whether the perception of orientation is sensitive to the qualitative features that define aspect graphs. Highest sensitivity to viewpoint changes was observed at locations where the theory predicts qualitative transitions, although some transitions did not affect performance. Hypotheses about why humans ignore some transitions offer insights into mechanisms for object representation.

Discrimination, Psychological↗