Tensotaxis--a collective movement of embryonic cells up along the gradients of mechanical tensions]

We have examined the active collective movement of ectodermal cells from early gastrula of Xenopus laevis towards the point source of stretching, using techniques of videomicroscopy and scanning electron microscopy. We define this mode of cell movement as tensotaxis. This movement begins near the source of tension 5-10 min after the beginning of stretching and is spread in a relay fashion to more distant cells. As a result, a considerable fraction of observed cells more towards the source of stretching over a considerable territory at a rate of 0.6-3 mu/min. Subsequently, these movements are replaced by cell intercalation roughly oriented in the direction transverse to that of tissue stretching. It is proposed that tensotaxis is initiated by asymmetric deformation of the embryonic tissue due to the concentration (focusing) of a stretching force and contains both passive and active components. Data are presented supporting the view that, during normal development, tensotaxis may determine the movement of embryonic cells towards the blastopore and can also participate in other morphogenetic processes.     

Air movement preferences observed in office buildings

Office workers’ preferences for air movement have been extracted from a database of indoor environmental quality surveys performed in over 200 buildings. Dissatisfaction with the amount of air motion is very common, with too little air movement cited far more commonly than too much air movement. Workers were also surveyed in a detailed two-season study of a single naturally ventilated building. About one-half the building’s population wanted more air movement and only 4% wanted less. This same ratio applied when the air movement in workspaces was higher than 0.2 m/s, the de facto draft limit in the current ASHRAE and ISO thermal environment standards. Preference for “less air motion” exceeded that for “more” only at thermal sensations of −2 (cool) or colder. These results raise questions about the consequences of the ASHRAE and ISO standards’ restrictions on air movement, especially for neutral and warm conditions.     

Redefining the X Axis: “Professionals,” “Amateurs” and the Making of Mid-Victorian Biology – A Progress Report

A summary of revisionist accounts of the contextual meaning of`“professional” and “amateur,” as applied to the mid-Victorian X Club, is followed by an analysis of the liberal goals and inner tensions of this coalition of gentlemen specialists and government teachers. The changing status of amateurs is appraised, as are the new sites for the emerging laboratory discipline of “biology.” Various historiographical strategies for recovering the women’s role are considered. The relationship of science journalism to professionalization, and the constructive engagement of X Club publicists with their empowering audiences, are discussed. Finally, the article assesses how far the content and boundary closure of ``biology,' forged by Thomas Henry Huxley, were related to `professional' and political goals. Purebiology’s social and medical roots are examined, and the way inter-professional and wider Darwinian conflicts resulted in anew lexicon of words for the X Clubbers around 1870, including“evolution” and “agnosticism,” as well as “biology.” Biology’srole in the forging of British national identity is discussed, as are its relationship to the social strategies of liberal, Dissenting, and industrial groups in the country, whose authority sustained the new laboratory rhetoric. This revised version was published online in July 2006 with corrections to the Cover Date.     

Movement curvature planning through force field internal models

Human motion studies have focused primarily on modeling straight point-to-point reaching movements. However, many goal-directed reaching movements, such as movements directed towards oneself, are not straight but rather follow highly curved trajectories. These movements are particularly interesting to study since they are essential in our everyday life, appear early in development and are routinely used to assess movement deficits following brain lesions. We argue that curved and straight-line reaching movements are generated by a unique neural controller and that the observed curvature of the movement is the result of an active control strategy that follows the geometry of one’s body, for instance to avoid trajectories that would hit the body or yield postures close to the joint limits. We present a mathematical model that accounts for such an active control strategy and show that the model reproduces with high accuracy the kinematic features of human data during unconstrained reaching movements directed toward the head. The model consists of a nonlinear dynamical system with a single stable attractor at the target. Embodiment-related task constraints are expressed as a force field that acts on the dynamical system. Finally, we discuss the biological plausibility and neural correlates of the model’s parameters and suggest that embodiment should be considered as a main cause for movement trajectory curvature.     

Predictive feedback control and Fitts–law

Fitts’ law is a well established empirical formula, known for encapsulating the “speed-accuracy trade-off”. For discrete, manual movements from a starting location to a target, Fitts’ law relates movement duration to the distance moved and target size. The widespread empirical success of the formula is suggestive of underlying principles of human movement control. There have been previous attempts to relate Fitts’ law to engineering-type control hypotheses and it has been shown that the law is exactly consistent with the closed-loop step-response of a time-delayed, first-order system. Assuming only the operation of closed-loop feedback, either continuous or intermittent, this paper asks whether such feedback should be predictive or not predictive to be consistent with Fitts law. Since Fitts’ law is equivalent to a time delay separated from a first-order system, known control theory implies that the controller must be predictive. A predictive controller moves the time-delay outside the feedback loop such that the closed-loop response can be separated into a time delay and rational function whereas a non- predictive controller retains a state delay within feedback loop which is not consistent with Fitts’ law. Using sufficient parameters, a high-order non-predictive controller could approximately reproduce Fitts’ law. However, such high-order, “non-parametric” controllers are essentially empirical in nature, without physical meaning, and therefore are conceptually inferior to the predictive controller. It is a new insight that using closed-loop feedback, prediction is required to physically explain Fitts’ law. The implication is that prediction is an inherent part of the “speed-accuracy trade-off”.     

Adjustment of the human arm viscoelastic properties to the direction of reaching

The viscoelastic properties of the human arm were measured by means of short force perturbations during fast reaching movements in two orthogonal directions. A linear spring model with time delay described the neuromuscular system of the human arm. The obtained viscoelastic parameters ensured movement stability in spite of the time delay of 50 ms. The stiffness and viscosity ellipses appeared to be predominantly orthogonal to the movement direction, which reduced the effect of force perturbation in the direction orthogonal to the reaching movement. Thus, it can be argued that the viscoelastic properties of the neuromuscular system of the human arm are adjusted to the direction of movement according to a “path preserving” strategy, which minimizes the deviation of the movement path from a straight line, when exposed to an unexpected external force.     

Characterising sclerophylly: some mechanical properties of leaves from heath and forest

Although sclerophylly is widespread through the world and is often the dominant leaf-form in mediterranean climates, the mechanical properties of sclerophyllous leaves are poorly understood. The term ”sclerophyllous” means hard-leaved, but biologists also use terms such as tough, stiff and leathery to describe sclerophyllous leaves. The latter term has no precise definition that allows quantification. However, each of the former terms is well-defined in materials engineering, although they may be difficult or sometimes inappropriate to measure in leaves because of their size, shape or composite and anisotropic nature. Two of the most appropriate and practically applicable mechanical properties of sclerophyllous leaves are ”strength” and ”toughness”, which in this study were applied using punching, tearing and shearing tests to 19 species of tree and shrub at Wilson’s Promontory, Australia. The results of these tests were compared with leaf specific mass (LSM) and a sclerophylly index derived from botanists’ ranks. Principal components analysis was used to reduce the set of mechanical properties to major axes of variation. Component 1 correlated strongly with the botanists’ ranks. Overall, leaves ranked as sclerophyllous by botanists were both tough and strong in terms of punching and tearing tests. In addition, tough and strong leaves typically had high toughness and strength per unit leaf thickness. There was also a significant correlation between component 1 and LSM. Although more detailed surveys are required, we argue that sclerophylly should be defined in terms of properties that have precise meanings and are measurable, such as toughness and strength, and that relate directly to mechanical properties as implicit in the term. Received: 4 March 1999 / Accepted: 22 November 1999     

Frits Went’s Atomic Age Greenhouse: The Changing Labscape on the Lab-Field Border

In Landscapes and Labscapes Robert Kohler emphasized the separation between laboratory and field cultures and the creation of new “hybrid” or mixed practices as field sciences matured in the early twentieth century. This article explores related changes in laboratory practices, especially novel designs for the analysis of organism–environment relations in the mid-twentieth century. American ecologist Victor Shelford argued in 1929 that technological improvements and indoor climate control should be applied to ecological laboratories, but his recommendations were too ambitious for the time. In the postwar period Frits W. Went, plant physiologist at the California Institute of Technology, created a new high-tech laboratory, dubbed a “phytotron”, in the hope that it would transform plant sciences by allowing for unprecedented control of environmental variables. Went’s aspirations, the research conducted in his laboratory, and its impact in initiating an international movement, are considered. Went’s laboratory can be seen as a “hybrid culture” evolving in the laboratory, complementing and intersecting with some of the field practices that Kohler describes. It was also a countercultural movement against the reductionist trends of molecular biology in the 1950s and 1960s. By considering the history of the laboratory in relation to field sciences, we can explore how new funding sources and cross-disciplinary relations affected the development of field sciences, especially in the postwar period.     

The investigation of locomotor activity control system in humans under the air-stepping conditions during passive and voluntary leg movements

The degree of activation of the central stepping program during passive leg movement was studied in healthy subjects under unloading conditions; the excitability of spinal motoneurons was studied during passive and voluntary stepping movements. Passive stepping movements with characteristics maximally close to those during voluntary stepping were accomplished by the experimenter. The bursts of muscular activity during voluntary and imposed stepping movements were compared. In addition, the influence on the leg movement of artificially created loading onto the foot was studied. The excitability of spinal motoneurons was estimated by the amplitude of modulation of the m. soleus H reflex. Changes in the H reflex (Hoffmann’s reflex) after fixation of the knee and hip joints were also studied. In most subjects, passive movements were accompanied by bursts of electromyographic (EMG) activity in the hip muscles (sometimes in shank muscles); the timing of the EMG burst during the step cycle coincided with the burst’s timing during voluntary stepping. In many cases, the bursts in EMG activity exceeded the activity of homonymous muscles during voluntary stepping. Simulation of foot loading influenced significantly the distal part of the moving extremity during both voluntary and passive movements, which was expressed in the appearance of movements in the ankle joint and an increase in the phasic EMG activity of the shank muscles. The excitability of motoneurons during passive movements was higher than during voluntary movements. Changes and modulation of the H reflex throughout the step cycle were similar without restriction of joint mobility and without hip joint mobility. Fixation of the knee joint was of great importance. It is supposed that imposed movements activate the same mechanisms of rhythm generation as supraspinal commands during voluntary movements. During passive movements, presynaptic inhibition depends mostly on the afferent influences from the moving leg rather than on the central commands. Under the conditions of “air-stepping,” the afferent influences from the foot pressure receptors are likely to interact actively with the central program of stepping and to determine the final activity pattern irrespective of the movement type (voluntary or passive).     

Cell Locomotion and Contact Guidance in Amphibian Gastrulation

Presumptive mesodermal cells in amphibian gastrulae migratefrom the blastopore toward the animal pole by using the innersurface of the ectodermal layer as their substratum. Duringmigration, the mesodermal cells form lamellipodia and filopodiapredominantly in a direction toward the animal pole. There isa network of the extracellular fibrils on the inner surfaceof the ectodermal layer. The fibrils seem to serve as an adequatesubstratum for attachment of the filopodia and locomotion ofthe mesodermal cells. A significant alignment of the fibrilnetwork along the blastopore—animal pole axis suggestsa hypothesis that it directs morphogenetic cell movements bycontact guidance in combination with contact inhibition of movement.New culture conditions allow the gastrula mesodermal cells tomove actively in vitro with a similar cell shape and at a similarrate as in vivo. Such culture conditions enabled an in vitroexperiment to test the hypothesis of contact guidance. Explantedectodermal layers deposit the fibril network on the surfaceof a cover slip. Dissociated gastrula mesodermal cells seededon such a conditioned surface attach to the surface and moveabout actively. A computer analysis of the time—lapsefilms shows that the cell trails are significantly aligned alongthe blastopore—animal pole axis of the ectodermal layerthat conditioned the surface. The deposited fibril network showsthe alignment along the same axis. There is also a tendencyof the mesodermal cells to move in a polarized fashion preferentiallytoward the animal pole. These results support the hypothesisof contact guidance of mesodermal cell migration in vivo byoriented extracellular fibrils     

Jendrassik’s maneuver creates conditions for triggering involuntary stepping

The influence of the Jendrassik maneuver on the generation of the locomotor rhythmicity was studied under conditions of the “suspension” of legs in a horizontal plane. It was shown that during Jendrassik’s maneuver, passive movements of one limb trigger stepping with the participation of both limbs. The Jendrassik’s maneuver also notably facilitates the vibration-induced locomotion. The kinematics of the evoked stepping movements did not differ from the kinematics of the voluntary “air-stepping,” reciprocal electromyographic activity being observed in antagonistic muscles. It seems likely that the generation of cyclic movements during the Jendrassik maneuver occurs due to the activation of the same stepping automatism as in normal stepping. The experimental results suggest that an increase in the level of tonic readiness of the tonogenic CNS structures, which participate in realization of the locomotor program, is a necessary condition for the activation of the spinal mechanisms of the stepping generation.     

Simulating a neural cross-talk model for between-hand interference during bimanual circle drawing

Studies on drawing circles with both hands in the horizontal plane have shown that this task is easy to perform across a wide range of movement frequencies under the symmetrical mode of coordination, whereas under the asymmetrical mode (both limbs moving clockwise or counterclockwise) increases in movement frequency have a disruptive effect on trajectory control and hand coordination. To account for these interference effects, we propose a simplified computer model for bimanual circle drawing based on the assumptions that (1) circular trajectories are generated from two orthogonal oscillations coupled with a phase delay, (2) the trajectories are organized on two levels, “intention” and “motor execution”, and (3) the motor systems controlling each hand are prone to neural cross-talk. The neural cross-talk consists in dispatching some fraction of any force command sent to one limb as a mirror image to the other limb. Assuming predominating coupling influences from the dominant to the nondominant limb, the simulations successfully reproduced the main characteristics of performance during asymmetrical bimanual circle drawing with increasing movement frequencies, including disruption of the circular form drawn with the nondominant hand, increasing dephasing of the hand movements, increasing variability of the phase difference, and occasional reversals of the movement direction in the nondominant limb. The implications of these results for current theories of bimanual coordination are discussed. Received: 23 June 1998 / Accepted in revised form: 20 April 1999     

The point of no return in planar hand movements: an indication of the existence of high level motion primitives

Previous psychophysical studies have sought to determine whether the processes of movement engagement and termination are dissociable, whether stopping an action is a generic process, and whether there is a point in time in which the generation of a planned action is inevitable (“point of no return”). It is not clear yet, however, whether the action of stopping is merely a manifestation of low level, dynamic constraints, or whether it is also subject to a high level, kinematic plan. In the present study, stopping performance was studied while nine subjects, who generated free scribbling movements looking for the location of an invisible circular target, were requested unexpectedly to impede movement. Temporal analysis of the data shows that in 87% of the movements subsequent to the ‘stop’ cue, the tangential motion velocity profile was not a decelerating function of the time but rather exhibited a complex pattern comprised of one or more velocity peaks, implying an unstoppable motion element. Furthermore, geometrical analysis shows that the figural properties of the path generated after the ‘stop’ cue were part of a repetitive geometrical pattern and that the probability of completing a pattern after the ‘stop’ cue was correlated with the relative advance in the geometrical plan rather than the amount of time that had elapsed from the pattern initiation. Altogether, these findings suggest that the “point of no return” phenomenon in humans may also reflect a high level kinematic plan and could serve as a new operative definition of motion primitives.     

Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study

 The large mass of the human upper trunk, its elevated position during erect stance, and the small area limited by the size of the feet, stress the importance of equilibrium control during trunk movements. The objective of the present study was to perform a biomechanical analysis of fast forward trunk movements in order to understand the coordination between movement and posture. The analysis is based on a comparison between experimentally observed bending and hypothetical “optimal bending” performed on an infinitely narrow support, as presented in a companion paper. The experimental data were obtained from 16 subjects who performed fast forward bending while standing on a wide platform or on a narrow beam. The analysis is performed by decomposition of the movement into three dynamically independent components, each representing a movement along one of the three eigenvectors of the motion equation. The eigenmovements are termed “hip”, “ankle”, and “knee” eigenmovements, according to the dominant joint. The experimentally observed movement is characterized mainly by the hip and ankle eigenmovements, whereas the knee eigenmovement is negligible. Similarly to the “optimal bending” the ankle eigenmovement starts earlier and lasts longer than the hip eigenmovement. An early forward acceleration of the center of gravity in the ankle eigenmovement is caused by anticipatory changes in the ankle joint torque. This clarifies the role of the early tibialis anterior burst and/or soleus inhibition usually observed in electromyographic recordings during forward bending. The results suggest that the hip and the ankle eigenmovements can be treated as independently controlled motion units aimed at functionally different behavioral goals: the bending per se and postural adjustment. It is proposed that the central nervous system has to control these motion units sequentially in order to perform the movement and maintain equilibrium. It is also suggested that the hip and ankle eigenmovements can be regarded as a biomechanical background for the hip and ankle strategies introduced by Horak and Nashner (1986) on the basis of electromyographic recordings and kinematic patterns in response to postural perturbations. Received: 1 July 1999 / Accepted in revised form: 23 October 2000     

Civic Biology and the Origin of the School Antievolution Movement

In discussing the origins of the antievolution movement in American high schools within the framework of science and religion, much is overlooked about the influence of educational trends in shaping this phenomenon. This was especially true in the years before the 1925 Scopes trial, the beginnings of the school antievolution movement. There was no sudden realization in the 1920’s – sixty years after the Origin of Species was published – that Darwinism conflicted with the Bible, but until evolution was being taught in the high schools, there was no impetus to outlaw it. The creation of “civic biology” curricula in the late 1910’s and early 20’s, spearheaded by a close-knit community of textbook authors, brought evolution into the high school classroom as part of a complete reshaping of “biology” as a school subject. It also incorporated progressive ideologies about the purposes of compulsory public education in shaping society, and civic biology was fundamentally focused on the applications of the life sciences to human life. Antievolution legislation was part of a broader response to the ideologies of the new biology field, and was a reaction not only to the content of the new subject, but to the increasingly centralized control and regulation of education. Viewing the early school antievolution movement through the science-religion conflict is an artifact of the Scopes trial’s re-creation of its origins. What largely caused support for␣the school antievolution movement in the South and particularly Tennessee were concerns over public education, which biology came to epitomize.     

Bhattacharyya’s distance measure as a precursor of genetic distance measures

In Population Genetics, two populations are distinguished from each other on the basis of the differences in the distributions of the alleles at the locus or loci under consideration. These differences are measured by a “genetic distance” between the two populations (not to be confused with genetic distance between two loci, which is based on recombination fractions) and they play a major role in inferences at the population level. Several measures of genetic distance have been proposed by different authors (Sanghvi 1953; Cavalli-Sforza and Edwards 1967; Jukes and Cantor 1969; Nei 1972; Kimura 1980; Reynoldset al 1983; reviews in Felsenstein 1991; Nei and Kumar 2000). Most of these measures are actually dissimilarity measures and not mathematically true distance measures (B-Rao and Majumdar 1999). Independently, and much before the geneticists, statisticians too were concerned with the idea of distinguishing between two (statistical) populations. In order to discriminate between two populations on the basis of one or more characters, divergence measures like “Mahalanobis’D ² statistic” or “Mahalanobis’ generalized distance” (1936) and “Bhattacharyya’s distance” (1943, 1946), Kullback-Leibler’s divergence measure (1951) etc. have been proposed by statisticians. Mukherjee and Chattopadhyaya (1986) have mentioned measures based on distances, association between two attributes and discrimination function. There are similarities between the distance measures defined by applied scientists and by theoreticians. Felsenstein (1985) shows that three of the allele frequency-based genetic distance measures were anticipated by Bhattacharyya (1946). Nei and Takezaki (1994) have also studied the effectiveness of several genetic distance measures in the context of phylogenetic analysis, including Bhattacharyya’s distance measure.     

Self-assembly of neural networks viewed as swarm intelligence

While self-assembly is a fairly active area of research in swarm intelligence, relatively little attention has been paid to the issues surrounding the construction of network structures. In this paper we extend methods developed previously for controlling collective movements of agent teams to serve as the basis for self-assembly or “growth” of networks, using neural networks as a concrete application to evaluate our approach. Our central innovation is having network connections arise as persistent “trails” left behind moving agents, trails that are reminiscent of pheromone deposits made by agents in ant colony optimization models. The resulting network connections are thus essentially a record of agent movements. We demonstrate our model’s effectiveness by using it to produce two large networks that support subsequent learning of topographic and feature maps. Improvements produced by the incorporation of collective movements are also examined through computational experiments. These results indicate that methods for directing collective movements can be adopted to facilitate network self-assembly.     

“Good Mothering” or “Good Citizenship”? Conflicting Values in Choosing Whether to Donate or Store Umbilical Cord Blood

Umbilical cord blood banking is one of many biomedical innovations that confront pregnant women with new choices about what they should do to secure their own and their child’s best interests. Many mothers can now choose to donate their baby’s umbilical cord blood (UCB) to a public cord blood bank or pay to store it in a private cord blood bank. Donation to a public bank is widely regarded as an altruistic act of civic responsibility. Paying to store UCB may be regarded as a “unique opportunity” to provide “insurance” for the child’s future. This paper reports findings from a survey of Australian women that investigated the decision to either donate or store UCB. We conclude that mothers are faced with competing discourses that force them to choose between being a “good mother” and fulfilling their role as a “good citizen.” We discuss this finding with reference to the concept of value pluralism.     

Lentil root statoliths reach a stable state in microgravity

 The kinetics of the movement of statoliths in gravity-perceiving root cap cells of Lens culinaris L. and the force responsible for it have been analysed under 1 g and under microgravity conditions (S/MM-03 mission of Spacehab 1996). At the beginning of the experiment in space, the amyloplasts were grouped at the distal pole of the statocytes by a root-tip-directed 1-g centrifugal acceleration. The seedlings were then placed in microgravity for increasing periods of time (13, 29, 46 or 122 min) and chemically fixed. During the first 29 min of microgravity there were local displacements (mean velocity: 0.154 μm min⁻¹) of some amyloplasts (first at the front of the group and then at the rear). Nevertheless, the group of amyloplasts tended to reconstitute. After 122 min in microgravity the bulk of amyloplasts had almost reached the proximal pole where further movement was blocked by the nucleus. After a longer period in microgravity (4 h; experiment carried out 1994 during the IML 2 mission) the statoliths reached a stable position due to the fact that they were stopped by the nucleus. The position was similar to that observed in roots grown continuously in microgravity. Treatment with cytochalasin D (CD) did not stop the movement of the amyloplasts but slowed down the velocity of their displacement (0.019 μm min⁻¹). Initial movement patterns were the same as in control roots in water. Comparisons of mean velocities of amyloplast movements in roots in space and in inverted roots on earth showed that the force responsible for the movement in microgravity (F_c) was about 86% less (F_c = 0.016 pN) than the gravity force (F_g = 0.11 pN). Treatment with CD reduced F_c by two-thirds. The apparent viscosity of the statocyte cytoplasm was found to be 1 Pa s or 3.3 Pa s for control roots or CD treated roots, respectively. Brownian motion or elastic forces due to endoplasmic reticulum membranes do not cause the movement of the amyloplasts in microgravity. It is concluded that the force transporting the statoliths is caused by the actomyosin system. Received: 22 March 1999 / Accepted: 18 December 1999