Which geometric model for the curvature of 2-D shape contours?

We investigated the geometric representations underlying the perception of 2-D contour curvature. 88 arcs representing lower and upper halves of concentric circles, or halves of ellipses derived mathematically through planar projection by affinity with the circles, a special case of Newton's transform, were generated to produce curved line segments with negative and positive curvature and varying sagitta (sag) and/or aspect ratio. Aspect ratio is defined here as the ratio between the sagitta and the chord-length of a given arc. The geometric properties of the arcs suggest a regrouping into four structural models. The 88 stimuli were presented in random order to 16 observers eight of whom were experienced in the mathematical and visual analysis of 2-D curvature ('expert observers'), and eight of whom were not ('non-expert observers'). Observers had to give a number, on a psychophysical scale from 0 to 10, that was to reflect the magnitude of curvature they perceived in a given arc. The results show that the subjective magnitude of curvature increases exponentially with the aspect ratio and linearly with the sagitta of the arcs for both experts and nonexperts. Statistical analysis of the correlation coefficients of linear fits to individual data represented on a logarithmic scale reveals significantly higher correlation coefficients for aspect ratio than for sagitta. The difference is not significant when curves with the longest chords only (7 degrees -10 degrees ) are considered. The geometric model that produces the best psychometric functions is described by a combination of arcs of vertically and horizontally oriented ellipses, indicating that perceptual sensations of 2-D contour curvature are based on geometric representations that suggest properties of 3-D structures. A 'buckled bar model' is shown to optimally account for the perceptual data of all observers with the exception of one expert. His perceptual data can be linked to a more analytical, less 'naturalistic' representation originating from a specific perceptual experience, which is discussed. It is concluded that the structural properties of 'real' objects are likely to determine even the most basic geometric representations underlying the perception of curvature in 2-D images. A specific perceptual learning experience may engender changes in such representations.     

Influence of increasing D/A ratio on geometric and electronic properties of D–A-type copolymers

A series of D–A-type copolymers was designed and studied systematically for the purpose of gaining a deeper understanding of how the D/A ratio may influence the geometric and electronic properties when it varies from 2:1 to 6:1 by using DFT method. The results show that it has a significant effect on the bond length alternation, band gap, bandwidth and effective mass of carriers of the designed D–A-type copolymers. But its influence on the geometric and electronic properties shows something very different in degree when it increases from 2:1 to 4:1 and then increases further from 4:1 to 6:1. It is found that the effects of increasing D/A ratio on geometric and electronic properties are much stronger when the D/A ratio increases from 2:1 to 4:1 than when it further increases from 4:1 to 6:1. The theoretical results show that the polymers with a D/A ratio of 2:1 have much smaller effective mass of carriers and much wider bandwidth compared to those polymers with the D/A ratio of 4:1 and 6:1. Therefore, the designed D–A-type copolymers with a D/A ratio of 2:1 may actually be the better candidates for intrinsic conduction materials.     

Evolving coordinated group behaviours through maximisation of mean mutual information

A well known problem in the design of the control system for a swarm of robots concerns the definition of suitable individual rules that result in the desired coordinated behaviour. A possible solution to this problem is given by the automatic synthesis of the individual controllers through evolutionary or learning processes. These processes offer the possibility to freely search the space of the possible solutions for a given task, under the guidance of a user-defined utility function. Nonetheless, there exist no general principles to follow in the definition of such a utility function in order to reward coordinated group behaviours. As a consequence, task dependent functions must be devised each time a new coordination problem is under study. In this paper, we propose the use of measures developed in Information Theory as task-independent, implicit utility functions. We present two experiments in which three robots are trained to produce generic coordinated behaviours. Each robot is provided with rich sensory and motor apparatus, which can be exploited to explore the environment and to communicate with other robots. We show how coordinated behaviours can be synthesised through a simple evolutionary process. The only criteria used to evaluate the performance of the robotic group is the estimate of mutual information between the motor states of the robots.     

Estimation of the mean floristic similarity between and within sets of vegetational relevés

Various possibilities of usingSörensen's coefficient of floristic similarity in plant sociology are summarized. A new formula (11), derived from that bySörensen and based on constancy values, is suggested in order to calculate the mean floristic similarity within a set of relevés.     

Estimation of the mean floristic similarity between and within sets of vegetational relevés

Various possibilities of usingSörensen’s coefficient of floristic similarity in plant sociology are summarized. A new formula (11). derived from that bySörensen and based on constancy values, is suggested in order to calculate the mean floristic similarity within a set of relevés.     

Does disappearance mean extirpation? The case of right whales off Namibia

Right whales off Namibia were severely depleted by early 19th century whaling, and rarely featured in modern whaling catches in the 1920s. Aerial surveys of the Namibian coastline from 1978 and onwards revealed increasing numbers of right whales, but few cow‐calf pairs. Aerial surveys off South Africa since 2009 showed a major decline in the availability of animals without calves. Twenty individual matches were made between 94 whales photographed off Namibia/Northern Cape in 2003–2012 and 1,677 photographed off South Africa in 1979–2012. Eight were adult females that calved in South African waters, but only one was also seen with a calf off Namibia. Twelve out of 13 individuals off Namibia with distinctive dorsal pigmentation were first seen as calves off South Africa. These results strongly indicate connectivity between the two regions, while the presence off Namibia of three adult females from the South African population in the season in which they are believed to conceive suggests that there is unlikely to be any genetic differentiation between the two areas. We conclude that the reappearance of right whales off Namibia represents range expansion from South Africa rather than the survival of a few remnants of an originally separate stock.     

Application of association coefficients for estimating the mean similarity between sets of vegetational relevés

General rules for the use of all kinds of association coefficients for the direct computation of mean similarity between sets of vegetational relevés are given in the present paper. In the table the main kinds of association coefficients used in plant sociology are listed according toSokal andSneath (1963) and the corresponding formulas for direct computation of the mean similarity between and within sets of relevés are given.     

Which measures of diaphyseal robusticity are robust? A comparison of external methods of quantifying the strength of long bone diaphyses to cross‐sectional geometric properties

Measures of diaphyseal robusticity have commonly been used to investigate differences in bone strength related to body size, behavior, climate, and other factors. The most common methods of quantifying robusticity involve external diameters, or cross-sectional geometry. The data derived from these different methods are often used to address similar research questions, yet the compatibility of the resulting data has not been thoroughly tested. This study provides the first systematic comparison of externally derived measures of postcranial robusticity, with those based upon cross-sectional geometry. It includes sections taken throughout the skeleton, comparisons of prediction errors associated with different measurements, and analysis of the implications of different methods of body size standardization on the prediction of relative bone strength. While the results show reasonable correlations between diaphyseal diameters and strengths derived from cross-sectional geometry, considerable prediction errors are found in many cases. A new approach to externally based quantification of diaphyseal robusticity based upon moulding of sub-periosteal contours is proposed. This method maximizes correlation with cross-sectional geometry (r(2) = .998) and minimizes prediction errors in all cases. The results underscore the importance of accurate periosteal measurement in the quantification of bone strength, and suggest that, regardless of theoretical scaling predictions, external area based robusticity estimates involving the product of diaphyseal diameters are most directly comparable to cross-sectional geometric properties when they are standardized using the product of body mass and bone length.     

Meiosis VII: “Detorsive bending” as a basis for geometric shapes of late prophase bivalents

A new model depending on mechanical properties of chromosomes is adduced as a basis for diplotene opening-out and for curvature occurring in grasshopper bivalents, during and subsequent to diplotene. Conditions underlying the model are: (1) rigid physical binding exists between sister chromatids, (2) each chromatid remains free of torsional strain if its pairing face is straight, i.e. the chromatid is bilaterally symmetrical, (3) reciprocal exchange together with stiff binding between sisters produces twist in each chromated before diplotene begins, (4) stiffening of the bivalent during late meiotic prophase removes the twist resulting from reciprocal exchange, (5) since sister binding prevents untwisting of chromatids about their long axes, untwisting would be achieved only in conjunction withbending of each chromated. It is shown that this bending, called detorsive bending, automatically produces opening out, not only in bivalents with one chiasma but also in those with more than one, especially if the chiasmata are interstitial.In bivalonts with two chiasmata, classes of curvature resulting when both chiasmata are interstitial (II), when one is interstitial and one terminal (IT) and when both are terminal (TT) are attributed to differences in strength of opening out at interstitial and at terminal chiasmata respectively. It is postulated that mechanisms responsible for opening out at terminal chiasmata are basically different from those at interstitial chiasmata.A theoretical basis of a method for cytological detection of chromatid interference is outlined and arguments are presented against the electrostatic hypothesis.     

Universality in neural networks: the importance of the ‘mean firing rate’

We present a general analysis of highly connected recurrent neural networks which are able to learn and retrieve a finite number of static patterns. The arguments are based on spike trains and their interval distribution and require no specific model of a neuron. In particular, they apply to formal two-state neurons as well as to more refined models like the integrate-and-fire neuron or the Hodgkin-Huxley equations. We show that the mean firing rate defined as the inverse of the mean interval length is the only relevant parameter (apart from the synaptic weights) that determines the existence of retrieval solutions with a large overlap with one of the learnt patterns. The statistics of the spiking noise (Gaussian, Poisson or other) and hence the shape of the interval distribution does not matter. Thus our unifying approach explains why, and when, all the different associative networks which treat static patterns yield basically the same results, i.e., belong to the same universality class.     

Do you see what I mean? Corticospinal excitability during observation of culture-specific gestures

People all over the world use their hands to communicate expressively. Autonomous gestures, also known as emblems, are highly social in nature, and convey conventionalized meaning without accompanying speech. To study the neural bases of cross-cultural social communication, we used single pulse transcranial magnetic stimulation (TMS) to measure corticospinal excitability (CSE) during observation of culture-specific emblems. Foreign Nicaraguan and familiar American emblems as well as meaningless control gestures were performed by both a Euro-American and a Nicaraguan actor. Euro-American participants demonstrated higher CSE during observation of the American compared to the Nicaraguan actor. This motor resonance phenomenon may reflect ethnic and cultural ingroup familiarity effects. However, participants also demonstrated a nearly significant (p = 0.053) actor by emblem interaction whereby both Nicaraguan and American emblems performed by the American actor elicited similar CSE, whereas Nicaraguan emblems performed by the Nicaraguan actor yielded higher CSE than American emblems. The latter result cannot be interpreted simply as an effect of ethnic ingroup familiarity. Thus, a likely explanation of these findings is that motor resonance is modulated by interacting biological and cultural factors.     

What does 'chromatin remodeling' mean?

The regulated alteration of chromatin structure, termed 'chromatin remodeling', can be accomplished by covalent modification of histones or by the action of ATP-dependent remodeling complexes. A variety of mechanisms can be used to remodel chromatin; some act locally on a single nucleosome and others act more broadly. It is critical to establish a direct connection between the remodeling events observed in vivo and the mechanistic capabilities of remodeling complexes in vitro.