Evidence for velocity-tuned motion-sensitive descending neurons in the honeybee.

Behavioural experiments suggest the existence of two functionally distinct movement-sensitive pathways in honeybees: one mediates optomotor behaviour, consisting of reflexive turning responses preventing deviations from course, and the other controls flight speed. The first consists of direction-selective neurons responding optimally to a particular temporal frequency of motion, regardless of the pattern's spatial structure. The temporal frequency dependence matches the temporal tuning of the optomotor output. Behavioural experiments suggest the second pathway contains velocity-tuned cells, which generate equal-sized responses for any given image velocity, for patterns with a range of spatial structures. Here, recordings were made from direction-selective neurons in the honeybee's ventral nerve cord. Neurons were tested for responses to motion at velocities of 40-1000 deg s(-1) using four gratings with spatial periods of 11-76 degrees. In addition to temporal frequency-dependent optomotor neurons, direction-selective cells were found that had the same shaped velocity-response functions for all four patterns. The velocity-tuning properties of these cells suggest a possible role in monitoring flight speed because their velocity tuning matches the image velocities encountered during free flight and landing behaviour.     

Cortical Surround Interactions and Perceptual Salience via Natural Scene Statistics

Spatial context in images induces perceptual phenomena associated with salience and modulates the responses of neurons in primary visual cortex (V1). However, the computational and ecological principles underlying contextual effects are incompletely understood. We introduce a model of natural images that includes grouping and segmentation of neighboring features based on their joint statistics, and we interpret the firing rates of V1 neurons as performing optimal recognition in this model. We show that this leads to a substantial generalization of divisive normalization, a computation that is ubiquitous in many neural areas and systems. A main novelty in our model is that the influence of the context on a target stimulus is determined by their degree of statistical dependence. We optimized the parameters of the model on natural image patches, and then simulated neural and perceptual responses on stimuli used in classical experiments. The model reproduces some rich and complex response patterns observed in V1, such as the contrast dependence, orientation tuning and spatial asymmetry of surround suppression, while also allowing for surround facilitation under conditions of weak stimulation. It also mimics the perceptual salience produced by simple displays, and leads to readily testable predictions. Our results provide a principled account of orientation-based contextual modulation in early vision and its sensitivity to the homogeneity and spatial arrangement of inputs, and lends statistical support to the theory that V1 computes visual salience.     

Genetically augmenting tau levels does not modulate the onset or progression of Abeta pathology in transgenic mice

The two hallmark pathologies of Alzheimer's disease (AD) are amyloid plaques, composed of the small amyloid-beta (Abeta) peptide, and neurofibrillary tangles, comprised aggregates of the microtubule binding protein, tau. The molecular linkage between these two lesions, however, remains unknown. Based on human and mouse studies, it is clear that the development of Abeta pathology can trigger tau pathology, either directly or indirectly. However, it remains to be established if the interaction between Abeta and tau is bidirectional and whether the modulation of tau will influence Abeta pathology. To address this question, we used the 3xTg-AD mouse model, which is characterized by the age-dependent buildup of both plaques and tangles. Here we show that genetically augmenting tau levels and hyperphosphorylation in the 3xTg-AD mice has no effect on the onset and progression of Abeta pathology. These data suggest that the link between Abeta and tau is predominantly if not exclusively unidirectional, which is consistent with the Abeta cascade hypothesis and may explain why tauopathy-only disorders are devoid of any Abeta pathology.     

High-fat diet increases tau expression in the brain of T2DM and AD mice independently of peripheral metabolic status

Alzheimer’s disease and type 2 diabetes mellitus are risk factors for each other. To investigate the effects of both genetic and high-fat-induced diabetic phenotype on the expression and exon 10 splicing of tau, we used the Alzheimer’s disease mouse model (APdE9) cross-bred with the type 2 diabetes mouse model over-expressing insulin-like growth factor 2 in the pancreas. High-fat diet, regardless of the genotype, significantly induced the expression of four repeat tau mRNA and protein in the temporal cortex of female mice. The mRNA levels of three repeat tau were also significantly increased by high-fat diet in the temporal cortex, although three repeat tau expression was considerably lower as compared to four repeat tau. Moreover, high-fat diet significantly increased the mRNA ratio of four repeat tau vs. three repeat tau in the temporal cortex of these mice. All of these effects were independent of the peripheral hyperglycemia, hyperinsulinemia and insulin resistance. Increased four repeat tau and three repeat tau levels significantly associated with impaired memory and reduced rearing in the female mice. High-fat diet did not affect neuroinflammation, Akt/GSK3β signaling pathway or the expression of tau exon 10 splicing enhancers in the temporal cortex. Our study suggests that the high-fat diet independently of type 2 diabetes or Alzheimer’s disease background induces the expression and exon 10 inclusion of tau in the brain of female mice.     

Correction for missed events based on a realistic model of a detector.

Quantitative patch-clamp analysis based on dwell-time histograms has to deal with the problem of missed events. The correction of the evaluated time constants has to take into account the characteristics of the detector used for the reconstruction of the time series. In previous approaches a simple model of the detector has been used, which is based on the assumption that all events shorter than the temporal resolution tres were missed, irrespective of the preceding events. Rather than the standard assumption of a fixed dead time, we introduce a more realistic model of a detector by a continuous-time version of the Hinkley detector. The combined state of the channel and the detector obeys a Markov model, which is governed by a Fokker-Planck-Kolmogorov partial differential equation. The steady-state solution leads to the determination of the apparent time constants tau o and tau c depending on the true rate constants koc and kco and the temporal resolution tres of the detector. Simulations with different kinds of detectors, including the Bessel filter with half-amplitude threshold detection, are performed. They show that our new equation predicts the dependence of tau c and tau o on koc, kco, and tres better than the standard equation used until now.     

Rethinking the conceptual foundations of habitat fragmentation research

The conceptual foundations of habitat fragmentation research have not kept pace with empirical advances in our understanding of species responses to landscape change, nor with theoretical advances in the wider disciplines of ecology. There is now real debate whether explicit recognition of ‘habitat fragmentation’ as an over‐arching conceptual domain will stimulate or hinder further progress toward understanding and mitigating the effects of landscape change. In this paper, we critically challenge the conceptual foundations of the discipline, and attempt to derive an integrated perspective on the best way to advance mechanistic understanding of fragmentation processes. We depict the inherent assumptions underlying the discipline as a ‘conceptual phase space’ of contrasting false dichotomies in fragmentation ‘problem space’. In our opinion, the key determinant of whether ‘habitat fragmentation’ can remain a cohesive framework lies in the concept of ‘interdependence’: 1) interdependence of landscape effects on species and 2) interdependence of species responses to landscape change. If there is non‐trivial interdependence among the various sub‐components of habitat fragmentation, or non‐trivial interdependence among species responses to landscape change, then there will be real heuristic value in ‘habitat fragmentation’ as a single conceptual domain. At present, the current paradigms entrenched in the fragmentation literature are implicitly founded on strict independence of landscape effects (e.g. the debate about the independent effects of habitat loss versus fragmentation per se) and strict independence of species responses (e.g. the individualistic species response models underpinning landscape continuum models), despite compelling evidence for interdependence in both effects and responses to fragmentation. We discuss how strong ‘interdependence’ of effects and responses challenges us to rethink long‐held views, and re‐cast the conceptual foundations of habitat fragmentation in terms of spatial context‐dependence in the effects of multiple interacting spatial components of fragmentation, and community context‐dependence in the responses of multiple interacting species to landscape change.     

Ratios of template responses as the basis of semivision.

The template model starts with a layer of receptors that in the case of vision are leaky detectors or counters of photons. In many animals, the ratio of the responses of a few spectral types is the basis of colour vision irrespective of intensity. Ratios of template responses are now introduced as the basis of form discrimination. In insects, the second-order neurons on the visual pathway appear to detect temporal contrast at the spatial resolution of the retina. At the next level, in the optic medulla, we find a large number of small local neurons in a column on each visual axis. The template theory is a hypothesis about how the above system functions. All possible combinations of positive, indeterminate or negative temporal contrast are considered, at two adjacent visual axes at two successive instants, giving 81 possible local templates. These templates are therefore phasic detectors of all the possible spatiotemporal contrast combinations. Some of the template responses indicate polarity of edge, flicker, or direction of motion and other abstracted features of the stimulus pattern with the maximum spatial and temporal resolution. The ratios of numbers of template responses, in higher fields at a higher level, yield quantitative measures of the qualities of edges independently of the number of edges, but taking ratios causes a corresponding loss of the spatiotemporal resolution and the pattern within each field. Templates respond to transients without computation, are readily modified or selected in evolution and can be simulated in artificial vision.     

The Audiovisual Tau Effect in Infancy

Background

Perceived spatial intervals between successive flashes can be distorted by varying the temporal intervals between them (the “tau effect”). A previous study showed that a tau effect for visual flashes could be induced when they were accompanied by auditory beeps with varied temporal intervals (an audiovisual tau effect).

Methodology/Principal Findings

We conducted two experiments to investigate whether the audiovisual tau effect occurs in infancy. Forty-eight infants aged 5–8 months took part in this study. In Experiment 1, infants were familiarized with audiovisual stimuli consisting of three pairs of two flashes and three beeps. The onsets of the first and third pairs of flashes were respectively matched to those of the first and third beeps. The onset of the second pair of flashes was separated from that of the second beep by 150 ms. Following the familiarization phase, infants were exposed to a test stimulus composed of two vertical arrays of three static flashes with different spatial intervals. We hypothesized that if the audiovisual tau effect occurred in infancy then infants would preferentially look at the flash array with spatial intervals that would be expected to be different from the perceived spatial intervals between flashes they were exposed to in the familiarization phase. The results of Experiment 1 supported this hypothesis. In Experiment 2, the first and third beeps were removed from the familiarization stimuli, resulting in the disappearance of the audiovisual tau effect. This indicates that the modulation of temporal intervals among flashes by beeps was essential for the audiovisual tau effect to occur (Experiment 2).

Conclusions/Significance

These results suggest that the cross-modal processing that underlies the audiovisual tau effect occurs even in early infancy. In particular, the results indicate that audiovisual modulation of temporal intervals emerges by 5–8 months of age.     

Dissociable spatial and temporal effects of inhibition of return

Inhibition of return (IOR) refers to the relative suppression of processing at locations that have recently been attended. It is frequently explored using a spatial cueing paradigm and is characterized by slower responses to cued than to uncued locations. The current study investigates the impact of IOR on overt visual orienting involving saccadic eye movements. Using a spatial cueing paradigm, our experiments have demonstrated that at a cue-target onset asynchrony (CTOA) of 400 ms saccades to the vicinity of cued locations are not only delayed (temporal cost) but also biased away (spatial effect). Both of these effects are basically no longer present at a CTOA of 1200 ms. At a shorter 200 ms CTOA, the spatial effect becomes stronger while the temporal cost is replaced by a temporal benefit. These findings suggest that IOR has a spatial effect that is dissociable from its temporal effect. Simulations using a neural field model of the superior colliculus (SC) revealed that a theory relying on short-term depression (STD) of the input pathway can explain most, but not all, temporal and spatial effects of IOR.     

Testing for Independence of Binary Responses

In the context of experiments involving visual inspection of random dot patterns the problem of testing the null hypothesis of independence of binary responses is considered. A flexible model for dependence between binary responses is proposed. Two tests, optimal under different versions of the model, are derived. These two tests turn out to involve the same computations as the Wilcoxon two sample test and the runs test respectively.     

Transient response of retinal rod outer segment phosphodiesterase to actinic light pulses. I. Simple quantitative kinetic model

We present a quantitative kinetic model for the transient velocity (microM of cGMP hydrolyzed/s) response of retinal rod outer segment (ROS) cGMP phosphodiesterase (v(t) versus t) to a stimulating light pulse in the linear response range. The model gives an excellent fit to experimental v(t) versus t data for ROS suspensions at different concentrations of GTP and GDP and clarifies experimental results which are difficult to understand in the absence of such a model. It contains the minimum number of steps required to fit our experimental data and consists of one rate-limiting step with specific rate kL for the production of active phosphodiesterase (PDE), PDE*, by photoactivated rhodopsin, R*, and deactivation processes for R* and PDE* with lifetimes tau R and tau P, respectively. The experimental graphs of v(t) versus t at each concentration of GTP and GDP are characterized by a fast rise to a peak value, vpeak, followed by a slow decay to zero level. The minimal kinetic model allows us to characterized completely the effects of GTP and GDP, and any other pertinent species, in terms of their effects on the parameters kL, tau R, and tau P. Our kinetic model indicates that for "washed" ROS preparations (a) the risetime of v(t) is determined by tau P which has a value of about 2 s and is insensitive to [GTP]. (b) The decay of v(t) is determined by tau R which decreases with [GTP] and has a value greater than 300 s at low [GTP] and a limiting value of 50 s at high [GTP]. We attribute the greater than 300 s lifetime to the complex R*G (where G is ROS G protein) and the 50-s lifetime to free R*. (c) The rate kL increases hyperbolically with [GTP] with a half-maximal value of 56 microM and kL.max = 22-45 s-1. (d) Peak velocity is given by the expression vpeak alpha kL tau P which is consistent with the dependence of kL on [GTP] and the experimental finding that vpeak varies hyperbolically with [GTP]. The minimal model has also allowed us to (a) develop clear definitions of amplification for the light-triggered enzymatic cascade and (b) clarify experimental methods for measuring gain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Importance of spatio–temporal connectivity to maintain species experiencing range shifts

Climate change can affect the habitat resources available to species by changing habitat quantity, suitability and spatial configuration, which largely determine population persistence in the landscape. In this context, dispersal is a central process for species to track their niche. Assessments of the amount of reachable habitat (ARH) using static snap-shots do not account, however, for the temporal overlap of habitat patches that may enhance stepping-stone effects. Here, we quantified the impacts of climate change on the ARH using a spatio–temporal connectivity model. We first explored the importance of spatio–temporal connectivity relative to purely spatial connectivity in a changing climate by generating virtual species distributions and analyzed the relative effects of changes in habitat quantity, suitability and configuration. Then, we studied the importance of spatio–temporal connectivity in three vertebrate species with divergent responses to climate change in North America (grey wolf, Canadian lynx and white-tailed deer). We found that the spatio–temporal connectivity could enhance the stepping-stone effect for species predicted to experience range contractions, and the relative importance of the spatio–temporal connectivity increased with the reduction in habitat quantity and suitability. Conversely, for species that are likely to expand their ranges, spatio–temporal connectivity had no additional contribution to improve the ARH. We also found that changes in habitat amount (quantity and suitability) were more influential than changes in habitat configuration in determining the relative importance of spatio–temporal connectivity. We conclude that spatio–temporal connectivity may provide less biased and more realistic estimates of habitat connectivity than purely spatial connectivity.     

Predation risk and the structure of freshwater zooplankton communities

Summary Many predators inflict substantial mortality on their prey. The prey respond to these selective pressures with changes in their spatial and temporal overlap with the predator (density risk responses), or with changes in their vulnerability to the predator (prey vulnerability responses). Here we develop a simple predation model that permits quantification of the basic response types of the prey in nature. We then test the hypothesis that prey response will be proportional to the intensity of the predation mortality relative to all other sources of mortality and decreased natality acting on the prey. A significant regression relationship is obtained for the prey vulnerability response but not for any of the density risk responses. The individual response values and regression statistics are used to interpret the relative importance of the different response types and to assess the predator's influence on prey community structure.Supported by Lehigh University Environmental Studies Center     

Relationship between Size Summation Properties,Contrast Sensitivity and Response Latency in the Dorsomedial and Middle Temporal Areas of the Primate Extrastriate Cortex

Analysis of the physiological properties of single neurons in visual cortex has demonstrated that both the extent of their receptive fields and the latency of their responses depend on stimulus contrast. Here, we explore the question of whether there are also systematic relationships between these response properties across different cells in a neuronal population. Single unit recordings were obtained from the middle temporal (MT) and dorsomedial (DM) extrastriate areas of anaesthetized marmoset monkeys. For each cell, spatial integration properties (length and width summation, as well as the presence of end- and side-inhibition within 15° of the receptive field centre) were determined using gratings of optimal direction of motion and spatial and temporal frequencies, at 60% contrast. Following this, contrast sensitivity was assessed using gratings of near-optimal length and width. In both areas, we found a relationship between spatial integration and contrast sensitivity properties: cells that summated over smaller areas of the visual field, and cells that displayed response inhibition at larger stimulus sizes, tended to show higher contrast sensitivity. In a sample of MT neurons, we found that cells showing longer latency responses also tended to summate over larger expanses of visual space in comparison with neurons that had shorter latencies. In addition, longer-latency neurons also tended to show less obvious surround inhibition. Interestingly, all of these effects were stronger and more consistent with respect to the selectivity for stimulus width and strength of side-inhibition than for length selectivity and end-inhibition. The results are partially consistent with a hierarchical model whereby more extensive receptive fields require convergence of information from larger pools of “feedforward” afferent neurons to reach near-optimal responses. They also suggest that a common gain normalization mechanism within MT and DM is involved, the spatial extent of which is more evident along the cell’s preferred axis of motion.     

Motion detection and adaptation in crayfish photoreceptors. A spatiotemporal analysis of linear movement sensitivity

Impulse and sine wave responses of crayfish photoreceptors were examined to establish the limits and the parameters of linear behavior. These receptors exhibit simple low pass behavior which is well described by the transfer function of a linear resistor-capacitor cascade of three to five stages, each with the same time constant (tau). Additionally, variations in mean light intensity modify tau twofold and the contrast sensitivity by fourfold. The angular sensitivity profile is Gaussian and the acceptance angle (phi) increases 3.2-fold with dark adaptation. The responses to moving stripes of positive and negative contrast were measured over a 100-fold velocity range. The amplitude, phase, and waveform of these responses were predicted from the convolution of the receptor's impulse response and angular sensitivity profile. A theoretical calculation based on the convolution of a linear impulse response and a Gaussian sensitivity profile indicates that the sensitivity to variations in stimulus velocity is determined by the ratio phi/tau. These two parameters are sufficient to predict the velocity of the half-maximal response over a wide range of ambient illumination levels. Because phi and tau vary in parallel during light adaptation, it is inferred that many arthropods can maintain approximately constant velocity sensitivity during large shifts in mean illumination and receptor time constant. The results are discussed relative to other arthropod and vertebrate receptors and the strategies that have evolved for movement detection in varying ambient illumination.     

A Bayesian spatiotemporal statistical analysis of out-of-hospital cardiac arrests

We propose a Bayesian spatiotemporal statistical model for predicting out-of-hospital cardiac arrests (OHCAs). Risk maps for Ticino, adjusted for demographic covariates, are built for explaining and forecasting the spatial distribution of OHCAs and their temporal dynamics. The occurrence intensity of the OHCA event in each area of interest, and the cardiac risk-based clustering of municipalities are efficiently estimated, through a statistical model that decomposes OHCA intensity into overall intensity, demographic fixed effects, spatially structured and unstructured random effects, time polynomial dependence, and spatiotemporal random effect. In the studied geography, time evolution and dependence on demographic features are robust over different categories of OHCAs, but with variability in their spatial and spatiotemporal structure. Two main OHCA incidence-based clusters of municipalities are identified.     

Reconciling the influence of global climate phenomena on macrofaunal temporal dynamics at a variety of spatial scales

Determining the relative importance of environmental forces on population dynamics is a fundamental question for ecologists. Growing concern over the ecological effects of climate change emphasizes the importance of defining whether broad-scale environmental forces uniformly act upon local populations (hierarchy theory) or cross-scale interactions influence local responses (multiscale theory). This study analyses 13 years of data on species abundances at six sites within a large harbour to determine the effect of the El Niño Southern Oscillation (ENSO). Environmental variables both directly and indirectly related to ENSO were observed to be important predictors of the temporal dynamics of abundance in many species, but the observed effects were not consistent across sites or species. While nearly all species were affected by large temporal and spatial scale variability, smaller temporal scale, location-specific environmental variables (such as wind-generated wave exposure and turbidity) were also generally important, increasing the variability explained by our models by up to 25%. As with many other broad-scale variables, generality of response to ENSO is affected by interactions across time and space with smaller scale heterogeneity. This study therefore suggests that the degree of interaction between broad-scale climatic factors, such as ENSO, with smaller scale variability, will determine the consistency of responses over large spatial scales, and control our ability to predict effects of climate change on coastal and estuarine communities.     

Quantification of curvature production in cylindrical organs, such as roots and hypocotyls

Differential growth curvature rate (DGCR), defined as the spatial derivative of the tropic speed, was derived as a measure of curvature production in cylindrical organs. Its relation to usual concepts, such as curvature (kappa), rate of curvature (dkappa/dt) and differential growth profiles, was determined. A root gravitropism model, testing the hypothesis of one and two motors, exemplified its capabilities.DGCR was derived using cylindrical geometry and its meaning was obtained through a curvature conservation equation. The root gravitropism model was solved using a discrete difference method on a computer.DGCR described curvature production independently of growth, and was superior to dkappa/dt, which underestimated production. Moreover, DGCR profiles were able to differ between one and two motors, while profiles of kappa and dkappa/dt were not.The choice of the measure of curvature production has a large impact on experimental results, in particular when spatial and temporal patterns of differential growth need to be determined. DGCR was shown to fulfill the accuracy needed in the quantification of curvature production and should thus serve as a helpful tool for measurements.     

Investigating sensitivity coefficients characterizing the response of a model of tau protein transport in an axon to model parameters

Evaluating the sensitivity of biological models to various model parameters is a critical step towards advancing our understanding of biological systems. In this paper, we investigated sensitivity coefficients for a model simulating transport of tau protein along the axon. This is an important problem due to the relevance of tau transport and agglomeration to Alzheimer’s disease and other tauopathies, such as some forms of parkinsonism. The sensitivity coefficients that we obtained characterize how strongly three observables (the tau concentration, average tau velocity, and the percentage of tau bound to microtubules) depend on model parameters. The fact that the observables strongly depend on a parameter characterizing tau transition from the retrograde to the anterograde kinetic states suggests the importance of motor-driven transport of tau. The observables are sensitive to kinetic constants characterizing tau concentration in the free (cytosolic) state only at small distances from the soma. Cytosolic tau can only be transported by diffusion, suggesting that diffusion-driven transport of tau only plays a role in the proximal axon. Our analysis also shows the location in the axon in which an observable has the greatest sensitivity to a certain parameter. For most parameters, this location is in the proximal axon. This could be useful for designing an experiment aimed at determining the value of this parameter. We also analyzed sensitivity of the average tau velocity, the total tau concentration, and the percentage of microtubule-bound tau to cytosolic diffusivity of tau and diffusivity of bound tau along the MT lattice. The model predicts that at small distances from the soma the effect of these two diffusion processes is comparable.