A motion illusion reveals mechanisms of perceptual stabilization

Visual illusions are valuable tools for the scientific examination of the mechanisms underlying perception. In the peripheral drift illusion special drift patterns appear to move although they are static. During fixation small involuntary eye movements generate retinal image slips which need to be suppressed for stable perception. Here we show that the peripheral drift illusion reveals the mechanisms of perceptual stabilization associated with these micromovements. In a series of experiments we found that illusory motion was only observed in the peripheral visual field. The strength of illusory motion varied with the degree of micromovements. However, drift patterns presented in the central (but not the peripheral) visual field modulated the strength of illusory peripheral motion. Moreover, although central drift patterns were not perceived as moving, they elicited illusory motion of neutral peripheral patterns. Central drift patterns modulated illusory peripheral motion even when micromovements remained constant. Interestingly, perceptual stabilization was only affected by static drift patterns, but not by real motion signals. Our findings suggest that perceptual instabilities caused by fixational eye movements are corrected by a mechanism that relies on visual rather than extraretinal (proprioceptive or motor) signals, and that drift patterns systematically bias this compensatory mechanism. These mechanisms may be revealed by utilizing static visual patterns that give rise to the peripheral drift illusion, but remain undetected with other patterns. Accordingly, the peripheral drift illusion is of unique value for examining processes of perceptual stabilization.     

A simulation test of the resource‐averaging hypothesis of ecotone formation

Abstract. The pattern at an ecotone may indicate the processes that created that ecotone. Such patterns may in turn affect the responses of ecotones to environmental change. The resource averaging hypothesis suggests a process for the development of tree lines that should produce patterns that are modifications of patterns in soil resources. A computer simulation model that embodies the resource averaging hypothesis is used to generate tree‐line patterns. Different spatial patterns in the variation of soil resources are represented in the model. The patterns of tree line computed by the simulation closely correspond to the patterns of soil resources that were input. These patterns are compared to patterns recorded in the field and by aerial photography. For the patterns of soil resources observed at some alpine tree lines, the model cannot produce the kinds of patterns of vegetation observed. Resource averaging alone cannot be an explanation of such tree lines.     

A model for photoreceptor-based magnetoreception in birds

A large variety of animals has the ability to sense the geomagnetic field and utilize it as a source of directional (compass) information. It is not known by which biophysical mechanism this magnetoreception is achieved. We investigate the possibility that magnetoreception involves radical-pair processes that are governed by anisotropic hyperfine coupling between (unpaired) electron and nuclear spins. We will show theoretically that fields of geomagnetic field strength and weaker can produce significantly different reaction yields for different alignments of the radical pairs with the magnetic field. As a model for a magnetic sensory organ we propose a system of radical pairs being 1) orientationally ordered in a molecular substrate and 2) exhibiting changes in the reaction yields that affect the visual transduction pathway. We evaluate three-dimensional visual modulation patterns that can arise from the influence of the geomagnetic field on radical-pair systems. The variations of these patterns with orientation and field strength can furnish the magnetic compass ability of birds with the same characteristics as observed in behavioral experiments. We propose that the recently discovered photoreceptor cryptochrome is part of the magnetoreception system and suggest further studies to prove or disprove this hypothesis.     

The relationship between the golden spiny mouse circadian system and its diurnal activity: an experimental field enclosures and laboratory study

Examples of animals that switch activity times between nocturnality and diurnality in nature are relatively infrequent. Furthermore, the mechanism for switching activity time is not clear: does a complete inversion of the circadian system occur in conjunction with activity pattern? Are there switching centers downstream from the internal clock that interpret the clock differently? Or does the switch reflect a masking effect? Answering these key questions may shed light on the mechanisms regulating activity patterns and their evolution. The golden spiny mouse (Acomys russatus) can switch between nocturnal and diurnal activity. This study investigated the relationship between its internal circadian clock and its diurnal activity pattern observed in the field. The goal is to understand the mechanisms underlying species rhythm shifts in order to gain insight into the evolution of activity patterns. All golden spiny mice had opposite activity patterns in the field than those under controlled continuous dark conditions in the laboratory. Activity and body temperature patterns in the field were diurnal, while in the laboratory all individuals immediately showed a free-running rhythm starting with a nocturnal pattern. No phase transients were found toward the preferred nocturnal activity pattern, as would be expected in the case of true entrainment. Moreover, the fact that the free-running activity patterns began from the individuals' subjective night suggests that golden spiny mice are nocturnal and that their diurnality in their natural habitat in the field results from a change that is downstream to the internal clock or reflects a masking effect.     

Mach bands in the lateral eye of Limulus

Patterns of optic nerve activity were computed for stationary step patterns of illumination from theoretical models of lateral inhibiton based on revised Hartlin-Ratliff equations. The computed response patterns contain well-defined Mach bands which match closely in amplitude and shape those recorded from single optic nerve fibers of the Limulus lateral eye. Theory and experiment show that the amplitude of the Mach bands is reduced by in inhibitory nonlinearity, the width of the Mach bands is approximately equal to the lateral dimension of the inhibitory field, but the shapes of the Mach bands are poor indices of the precise configuration of the inhibitory field. Theorems are proved establishing the equivalence of Mach-band patterns for models of different dimensions and a uniqueness condition for solutions of the piecewise linear model.     

Collective Almost Synchronisation in Complex Networks

This work introduces the phenomenon of Collective Almost Synchronisation (CAS), which describes a universal way of how patterns can appear in complex networks for small coupling strengths. The CAS phenomenon appears due to the existence of an approximately constant local mean field and is characterised by having nodes with trajectories evolving around periodic stable orbits. Common notion based on statistical knowledge would lead one to interpret the appearance of a local constant mean field as a consequence of the fact that the behaviour of each node is not correlated to the behaviours of the others. Contrary to this common notion, we show that various well known weaker forms of synchronisation (almost, time-lag, phase synchronisation, and generalised synchronisation) appear as a result of the onset of an almost constant local mean field. If the memory is formed in a brain by minimising the coupling strength among neurons and maximising the number of possible patterns, then the CAS phenomenon is a plausible explanation for it.     

Intraspecific facilitation: a missing process along increasing stress gradients – insights from simulated shrub populations

In recent years many field studies have been conducted to assess the relative importance of facilitation and competition in structuring vegetation communities in different environments. Herein, we present a simulation model which systematically explores the relative importance of intra‐specific facilitation and competition between adult shrubs and seedlings for spatial pattern formation. A grid‐based simulation model was constructed and calibrated using data collected in the field from Sarcopoterium spinosum populations in Israel to simulate population dynamics along a rainfall gradient. A series of simulation experiments was conducted in which manipulations of seedling survival probabilities were carried out to assess the relative importance of these processes in generating spatial patterns. Increased survival probabilities of first‐year shrubs in open areas were used to simulate competition effects, while increased survival probabilities in the vicinity of shrubs were used to simulate facilitation effects. Simulation results were then compared to shrub spatial patterns observed in the field. The results indicate that facilitation is not an important process in generating intra‐specific spatial patterns. Rather, in mesic environments with high precipitation, competition is the dominant process generating spatial patterns, resulting in regular spacing of shrubs, similarly to the patterns observed in the field (L(h) values<0). In arid sites, where precipitation values are lower, and stress conditions are higher, the dominant process generating spatial patterns was random mortality due to drought conditions. The resulting spatial patterns in this case are random (L(h)～0), whereas observed field populations exhibited clumped patterns (L(h)>0). We conclude that as stress conditions increase, the importance of intraspecific neighborhood interactions decrease whereas the importance of environmental factors increase in dictating intra‐specific spatial pattern formation. Consequently in mesic environments intra‐specific competition among adults determines the emerging patterns, while intraspecific facilitation is a negligible process.     

Spatial and temporal patterns of onion maggot adult activity and oviposition within onion fields that vary in bordering habitat

In New York, onion [ Allium cepa L. (Alliaceae)] fields often border woods or other vegetable fields. Because onion maggot adults, Delia antiqua (Meigen) (Diptera: Anthomyiidae), spend a significant portion of their time outside of onion fields, surrounding habitat may affect patterns of fly activity and oviposition within onion fields. To better understand these patterns throughout the onion-growing season, first-, second-, and third-flights maggot adult (male and female) activity was monitored using yellow sticky cards. Half of the monitored fields bordered woods, whereas the other half bordered other onion or vegetable fields. Within all fields, yellow sticky cards were placed at five distances along a transect beginning at the onion field edge extending into the field center. First-flight male and female adult activity was greatest along onion field edges and was especially high along edges bordering woods. This pattern diminished during the second flight and was absent during the third. To determine if spatial patterns of onion maggot oviposition by first-flight onion maggots were similar to first-flight adult activity patterns, potted onion plants were placed in onion fields that bordered or did not border woods in late May and early June 2003. The number of eggs laid in the soil at the base of each plant was recorded. Unlike spatial patterns of first-flight adult activity, oviposition patterns were not affected by bordering habitat or distance from the field edge. Based on the activity of onion maggot adults in onion fields, future and existing control strategies should consider targeting first-flight adults along field edges rather than across entire fields, especially in fields that border woods. In contrast, based on spatial patterns of oviposition within onion fields, controls targeting onion maggot larvae should be applied on a fieldwide basis.     

Coexistence patterns of two invasive thistle species, Carduus nutans and C. acanthoides, at three spatial scales

To better understand the competitive processes involved in invasion by congeners, we examine coexistence patterns of two invasive species, Carduus nutans and C. acanthoides, at three spatial scales. A roadside survey of 5 × 5 km blocks in a previously identified overlap zone provided information about the regional scale. At smaller scales, we surveyed four fields of natural co-occurrence, quantifying the spatial patterns at the field scale by randomly placed 1 × 1 m quadrats and at the smallest scale by detailing plant position within the quadrats. The patterns observed are strikingly different at the different scales. At the regional scale, there is positive local autocorrelation in both species but negative cross-correlation between them, consistent with previous surveys. However, at the field scale, there is positive local autocorrelation in both species, and we generally see a positive association between the two species. At the plot scale, when excluding areas of joint absence, there is again a negative association between the two species. This pattern can also be seen at the field scale when excluding plots with joint absence. These results suggest that, at the scale of a field, the strongest factor determining location is aggregation in favorable habitats, which is a stronger force than the competition-induced segregation evidenced at small scales. Lottery competition for spatially aggregated safe sites thus appears to drive the patterns observed at the field scale, while the regional scale pattern may be a result of restricted natural dispersal and invasion history.     

On the effects of topography on wind and the generation of currents in a large multi-basin lake

The wind field over a lake surface is the key element in driving the exchange of momentum and energy at the free surface. It is rarely uniform, having a considerable degree of spatial variability, both on a synoptic and a local scale. Topographic features are one of the most common contributors to this variability. The aerodynamic effects of topography on the windfield, and its influence on the circulation patterns and interbasin exchange rates in a large multi-basin lake are documented. The complex landscape that surrounds this lake is dominated by the presence of a mountain peak rising over 900m above the lake level. The circulation patterns in those basins of the lake located in the leeward side of the mountain reflect the spatial patterns of the wind field generated by the surrounding topography. The spatial variations in the wind field are shown to significantly alter the residence times of each basin and the exchange rates between basins, isolating one from the other two basins of the lake. The inter-basin exchange rates determined with wind field variability are consistent with annual contaminant loadings estimated for each of the three basins, which suggests a close link between chemical and hydrodynamic behavior.     

The Relationship between the Golden Spiny Mouse Circadian System and Its Diurnal Activity: An Experimental Field Enclosures and Laboratory Study

Examples of animals that switch activity times between nocturnality and diurnality in nature are relatively infrequent. Furthermore, the mechanism for switching activity time is not clear: does a complete inversion of the circadian system occur in conjunction with activity pattern? Are there switching centers downstream from the internal clock that interpret the clock differently? Or does the switch reflect a masking effect? Answering these key questions may shed light on the mechanisms regulating activity patterns and their evolution. The golden spiny mouse (Acomys russatus) can switch between nocturnal and diurnal activity. This study investigated the relationship between its internal circadian clock and its diurnal activity pattern observed in the field. The goal is to understand the mechanisms underlying species rhythm shifts in order to gain insight into the evolution of activity patterns. All golden spiny mice had opposite activity patterns in the field than those under controlled continuous dark conditions in the laboratory. Activity and body temperature patterns in the field were diurnal, while in the laboratory all individuals immediately showed a free‐running rhythm starting with a nocturnal pattern. No phase transients were found toward the preferred nocturnal activity pattern, as would be expected in the case of true entrainment. Moreover, the fact that the free‐running activity patterns began from the individuals' subjective night suggests that golden spiny mice are nocturnal and that their diurnality in their natural habitat in the field results from a change that is downstream to the internal clock or reflects a masking effect.     

Examining the link between competition and negative co‐occurrence patterns

Negative species co‐occurrence patterns have long intrigued ecologists because of their potential link to competition. Although manipulative field experiments have consistently revealed evidence of competition in natural communities, there is little evidence that this competition produces negative co‐occurrence patterns. Evidence does suggest that abiotic variation, dispersal limitation and herbivory can contribute to patterns of negative co‐occurrence among species; it is possible these influences have obscured a link with competition. Here, we test for a connection between negative co‐occurrence and competition by examining a small‐scale, relatively homogeneous old‐field plant community where the influence of abiotic variation was likely to be minimal and we accounted for the impact of herbivory with an herbivore exclosure treatment. Using three years of data (two biennial periods), we tested whether negatively co‐occurring pairs of species, when occasionally found together, experienced asymmetric abundance decline more frequently than positively co‐occurring pairs, for which there is no such expectation. We found no evidence that negatively co‐occurring pairs consistently suffered asymmetric abundance decline more frequently than positively co‐occurring pairs, providing no evidence that competition is a primary driver of negative co‐occurrence patterns in this community. Our results were consistent across control and herbivore exclosure treatments, suggesting that herbivores are not driving patterns of negative species co‐occurrence in this community. Any influence of competition or herbivory on co‐occurrence patterns is small enough that it is obscured by other factors such as substrate heterogeneity, dispersal and differential species responses to climatic variation through time. We interpret our results as providing evidence that competition is not responsible for producing negative co‐occurrence patterns in our study community and suggest that this may be the case more broadly.     

The Turing-Child energy field as a driver of early mammalian development

The equivalence of the early mammalian cells, of importance in assisted reproductive technologies (ART), is considered. It is suggested that this controversial topic can be settled by finding whether the cells are distinguished by the Turing-Child (TC) field, as expressed for example by patterns of mitochondrial activity. The division of the pronuclear embryo is driven by a symmetrical bipolar TC pattern whose experimental shape and chemical nature is predicted by TC theory. This bipolar pattern drives the subsequent cell divisions too, and according to present experimental results all cells are equivalent until compaction since they are not distinguished by the TC field in normal development. Interphase cells exhibit homogeneous mitochondrial activity, or perinuclear, or perinuclear and cortical activity, and these patterns too and the rotational symmetry observed are predicted by TC theory. The first differentiation, into an inner mass cell and the trophectoderm, as well as the formation of cell polarity in the trophectoderm are considered. It is suggested that these two events are driven by a peripheral spherical shell of high energy metabolism in the morula; such a shell is predicted by TC theory in a compacted multicellular sphere whose cells are connected by gap junctions. The experimental patterns of mitochondrial activity in unfertilized oocytes exhibit rotational symmetry or polarity. The shape and the chemical nature of these patterns also are predicted and explained by TC theory in a sphere. The change in the spatial pattern of mitochondrial activity with development is attributed to a change in the spatial pattern of mitochondrial activity and not to physical translocation of mitochondria. The experimental finding that these spatial patterns of mitochondrial activity are observed only in live and not in dead biological material is explained by the TC pattern being biology's unique and universal dissipative structure that requires ongoing specific biochemical reactions and energy dissipation.     

Cephalopod dynamic camouflage: bridging the continuum between background matching and disruptive coloration

Individual cuttlefish, octopus and squid have the versatile capability to use body patterns for background matching and disruptive coloration. We define—qualitatively and quantitatively—the chief characteristics of the three major body pattern types used for camouflage by cephalopods: uniform and mottle patterns for background matching, and disruptive patterns that primarily enhance disruptiveness but aid background matching as well. There is great variation within each of the three body pattern types, but by defining their chief characteristics we lay the groundwork to test camouflage concepts by correlating background statistics with those of the body pattern. We describe at least three ways in which background matching can be achieved in cephalopods. Disruptive patterns in cuttlefish possess all four of the basic components of ‘disruptiveness’, supporting Cott''s hypotheses, and we provide field examples of disruptive coloration in which the body pattern contrast exceeds that of the immediate surrounds. Based upon laboratory testing as well as thousands of images of camouflaged cephalopods in the field (a sample is provided on a web archive), we note that size, contrast and edges of background objects are key visual cues that guide cephalopod camouflage patterning. Mottle and disruptive patterns are frequently mixed, suggesting that background matching and disruptive mechanisms are often used in the same pattern.     

Contrasting diversity patterns of soil mites and nematodes in secondary succession

Soil biodiversity has been recognized as a key feature of ecosystem functioning and stability. However, soil biodiversity is strongly impaired by agriculture and relatively little is known on how and at what spatial and temporal scales soil biodiversity is restored after the human disturbances have come to an end. Here, a multi-scale approach was used to compare diversity patterns of soil mites and nematodes at four stages (early, mid, late, reference site) along a secondary succession chronosequence from abandoned arable land to heath land. In each field four soil samples were taken during four successive seasons. We determined soil diversity within samples (α-diversity), between samples (β-diversity) and within field sites (γ-diversity). The patterns of α- and γ-diversity developed similarly along the chronosequence for oribatid mites, but not for nematodes. Nematode α-diversity was highest in mid- and late-successional sites, while γ-diversity was constant along the chronosequence. Oribatid mite β-diversity was initially high, but decreased thereafter, whereas nematode β-diversity increased when succession proceeded; indicating that patterns of within-site heterogeneity diverged for oribatid mites and nematodes. The spatio-temporal diversity patterns after land abandonment suggest that oribatid mite community development depends predominantly on colonization of new taxa, whereas nematode community development depends on shifts in dominance patterns. This would imply that at old fields diversity patterns of oribatid mites are mainly controlled by dispersal, whereas diversity patterns of nematodes are mainly controlled by changing abiotic or biotic soil conditions. Our study shows that the restoration of soil biodiversity along secondary successional gradients can be both scale- and phylum-dependent.     

Contrasting responses of bee communities to coffee flowering at different spatial scales

While investigating biodiversity patterns on different spatial scales, ecological processes determining these patterns have been rarely analysed. Flower visitation by bees is an important ecological process that is related to floral resource availability. However, little is known about whether responses of bee communities to floral resource availability change at different spatial scales. We studied density and species richness of flower-visiting bees in relation to floral resource availability, provided by coffee, in traditional agroforestry systems on a field, shrub, and branch scale. On a field scale, mean bee density per shrub increased with decreasing proportion of flowering coffee shrubs per site, showing a dilution effect. Conversely, on shrub and branch scales bee density per shrub, or shrub part, increased with increasing number of inflorescences, showing a concentration effect. Additionally, bee density per shrub was higher on those that were only partly, rather than totally surrounded by other flowering coffee shrubs. Species richness of flower-visiting bees was positively affected by high resource availability on a shrub and a branch scale, expressed by a high number of inflorescences, but at the field scale the proportion of flowering shrubs per site did not have any effect on species richness. Our results show contrasting responses of the community of flower-visiting bees to floral resource availability, depending on the spatial scale considered. We conclude that patterns of flower-visiting bee communities of only one spatial scale can not be generalized, since the number of pollinators may be limited on a field scale, but not on smaller scales.     

The Alopex process: Visual receptive fields by response feedback

The determination of trigger features of single neurons in afferent pathways has been one of the central problems in sensory physiology. A novel method, called Alopex, has been developed, in which response feedback is used to construct visual patterns that optimize the responses. Data are presented which show the emergence of trigger features of cells monitored in frog visual tectum. The method is checked against results obtained by scanning the visual field with a small spot. Correlations between Alopex patterns and scan patterns are generally between 0.3 and 0.5 but may be as high as 0.9 when smoothing and/or averaging procedures are applied to the Alopex patterns. The dynamics of the Alopex process are discussed and details of the algorithms are presented. The series of experiments presented here has established the validity of the method and suggests that this approach should find wide application in receptive field studies. For that purpose data on the instrumentation and software are also presented.This research has been supported by the National Institutes of Health, under grant EY 01215     

The effect of spatial pattern on community dynamics; a comparison of simulated and field data

The effect of the spatial limits of dispersal and competition on plant community dynamics was studied using Monte-Carlo simulation. The model generates community point patterns, using life-table data, dispersion parameters and radii of competitive effects. These data have been estimated in a field situation, for the 11 most abundant weed species growing on the refuse soil dumps of a strip coal mine. In a simulation experiment, the patterns produced by two versions of the model were compared. The first was based on the field situation as much as possible; the other used the same input parameters except for dispersal, which was randomized in this case. We found considerable differences regarding the temporal changes of species abundances, the realized competitive abilities and the spatial patterns generated by the two versions. An important conclusion of this comparison is that the realized competitive effect (both intra-and interspecific) of a species is dependent not only on constant competition parameters, but on the abundance relations and on the spatial patterns of the competing populations as well. It is concluded that the spatial limits of dispersal and competition may result in the increased persistence of weak competitors, moderate the realized competitive effects of strong species, and shape the spatial coalition structure of the community.     

Diurnal Variations of Nitrogenase Activity in the Field

Nitrogenase activity was determined in situ by repeated assays at regular intervals to compare the patterns of diurnal variations occurring in a rice field, a grassland, and a peanut field. In the rice field only one peak occurred (mid-day), whereas the other systems exhibited typical two-peak patterns, which were suspected to be induced by climatic conditions or possibly by a specific rhythm of exudation. Observed data on diurnal variations in nitrogenase activity were fitted to different models of diurnal variation comprised of computed periodic curves.