When can refuges mediate the genetic effects of fire regimes? A simulation study of the effects of topography and weather on neutral and adaptive genetic diversity in fire‐prone landscapes

Understanding how landscape heterogeneity mediates the effects of fire on biodiversity is increasingly important under global changes in fire regimes. We used a simulation experiment to investigate how fire regimes interact with topography and weather to shape neutral and selection‐driven genetic diversity under alternative dispersal scenarios, and to explore the conditions under which microrefuges can maintain genetic diversity of populations exposed to recurrent fire. Spatial heterogeneity in simulated fire frequency occurred in topographically complex landscapes, with fire refuges and fire‐prone “hotspots” apparent. Interannual weather variability reduced the effect of topography on fire patterns, with refuges less apparent under high weather variability. Neutral genetic diversity was correlated with long‐term fire frequency under spatially heterogeneous fire regimes, being higher in fire refuges than fire‐prone areas, except under high dispersal or low fire severity (low mortality). This generated different spatial genetic structures in fire‐prone and fire‐refuge components of the landscape, despite similar dispersal. In contrast, genetic diversity was only associated with time since the most recent fire in flat landscapes without predictable refuges and hotspots. Genetic effects of selection driven by fire‐related conditions depended on selection pressure, migration distance and spatial heterogeneity in fire regimes. Allele frequencies at a locus conferring higher fitness under successional environmental conditions followed a pattern of “temporal adaptation” to contemporary conditions under strong selection pressure and high migration. However, selected allele frequencies were correlated with spatial variation in long‐term mean fire frequency (relating to environmental predictability) under weak dispersal, low selection pressure and strong spatial heterogeneity in fire regimes.     

Factors affecting transfer frequency of pAM beta 1 from Streptococcus faecalis to Lactobacillus plantarum.

Conjugal transfer of streptococcal plasmid pAM beta 1 to Lactobacillus plantarum by a filter mating method was examined under various conditions. The transfer frequency depended greatly on the type, pore size, and side (front or back) of the membrane filter. The passage of sterilized water through the membrane under reduced pressure after donor and recipient cells were trapped on it increased the transfer frequency about 10-fold.     

Spatiotemporal frequency responses of cat retinal ganglion cells

Spatiotemporal frequency responses were measured at different levels of light adaptation for cat X and Y retinal ganglion cells. Stationary sinusoidal luminance gratings whose contrast was modulated sinusoidally in time or drifting gratings were used as stimuli. Under photopic illumination, when the spatial frequency was held constant at or above its optimum value, an X cell's responsivity was essentially constant as the temporal frequency was changed from 1.5 to 30 Hz. At lower temporal frequencies, responsivity rolled off gradually, and at higher ones it rolled off rapidly. In contrast, when the spatial frequency was held constant at a low value, an X cell's responsivity increased continuously with temporal frequency from a very low value at 0.1 Hz to substantial values at temporal frequencies higher than 30 Hz, from which responsivity rolled off again. Thus, 0 cycles X deg-1 became the optimal spatial frequency above 30 Hz. For Y cells under photopic illumination, the spatiotemporal interaction was even more complex. When the spatial frequency was held constant at or above its optimal value, the temporal frequency range over which responsivity was constant was shorter than that of X cells. At lower spatial frequencies, this range was not appreciably different. As for X cells, 0 cycles X deg-1 was the optimal spatial frequency above 30 Hz. Temporal resolution (defined as the high temporal frequency at which responsivity had fallen to 10 impulses X s-1) for a uniform field was approximately 95 Hz for X cells and approximately 120 Hz for Y cells under photopic illumination. Temporal resolution was lower at lower adaptation levels. The results were interpreted in terms of a Gaussian center-surround model. For X cells, the surround and center strengths were nearly equal at low and moderate temporal frequencies, but the surround strength exceeded the center strength above 30 Hz. Thus, the response to a spatially uniform stimulus at high temporal frequencies was dominated by the surround. In addition, at temporal frequencies above 30 Hz, the center radius increased.     

The relationship between the Gabor elementary function and a stochastic model of the inter-spike interval distribution in the responses of visual cortex neurons

In a previously reported study (Berger et al. 1990) we analyzed distributions of interspike intervals recorded extracellularly from cat visual cortex under four stimulus conditions. Stimuli were gratings differing in orientation and spatial frequency. The probability density function of first passage time for a random walk with drift process, which is defined by its barrier height and drift coefficient, was used to characterize the generating process of axonal discharge under resting and stimulus conditions. Drift coefficient and barrier height were derived from the sample mean and standard deviation of the measured inter-spike intervals. For cells with simple receptive fields, variations in spatial frequency produced changes only in drift coefficient. Variations in barrier height were produced only by changes in orientation of the stimulus. Currently, the method used to analyze these data was implemented in a simulation which displayed the relationship between the interval distribution of impulses, the random walk which represents the time series characteristic of the spike train model and the Gabor filter function which represents the geometry of the receptive field process.     

Effects of Spatial Frequency Similarity and Dissimilarity on Contour Integration

We examined the effects of spatial frequency similarity and dissimilarity on human contour integration under various conditions of uncertainty. Participants performed a temporal 2AFC contour detection task. Spatial frequency jitter up to 3.0 octaves was applied either to background elements, or to contour and background elements, or to none of both. Results converge on four major findings. (1) Contours defined by spatial frequency similarity alone are only scarcely visible, suggesting the absence of specialized cortical routines for shape detection based on spatial frequency similarity. (2) When orientation collinearity and spatial frequency similarity are combined along a contour, performance amplifies far beyond probability summation when compared to the fully heterogenous condition but only to a margin compatible with probability summation when compared to the fully homogenous case. (3) Psychometric functions are steeper but not shifted for homogenous contours in heterogenous backgrounds indicating an advantageous signal-to-noise ratio. The additional similarity cue therefore not so much improves contour detection performance but primarily reduces observer uncertainty about whether a potential candidate is a contour or just a false positive. (4) Contour integration is a broadband mechanism which is only moderately impaired by spatial frequency dissimilarity.     

Two spatio-temporal filters in human vision

We have used the psychophysical methods described in the first paper of this series (Holliday and Ruddock, 1983) to determine selected spatial and temporal response characteristics of the ST1 and ST2 filters for subjects suffering visual defects. Data are given for 19 amblyopes, an albino and a hemianope, and comparison data are also given for a number of subjects with normal vision. The ST1 spatial responses for both the "normal" and "amblyopic" eyes of 12 convergent strabismic amblyopes are displaced to low spatial frequencies compared to the normal curve, which implies that there is a loss of fine spatial tuning. In all but one subject, the curve for the "amblyopic" eye peaks at a spatial frequency lower than that for the "normal" eye, thus the former deviates further from the normal pattern than the latter. The ST1 spatial responses of 6 refractive amblyopes are also displaced to the low frequency side of the normal curve, although on average the shift is smaller than in the case of the strabismic amblyopes. For each subject, the response curve of the "amblyopic" eye peaks at a lower spatial frequency than does that for the "normal" eye. ST1 spatial responses were measured for targets located up to 30 degrees off-axis along the horizontal meridian and sample data are given for one strabismic and one refractive amblyope and for two normal subjects. It is concluded from these data that the changes in the spatial responses associated with amblyopia do not simply reflect eccentric fixation of the target. The ST2 spatial response was measured for the "normal" and "amblyopic" eyes of 9 amblyopes (7 strabismic and 2 refractive). There is no significant difference between the average amblyopic response and that of normal subjects, and only in one case does the response for an "amblyopic" eye peak at a frequency lower than the peak frequency for normal vision. The ST2 temporal response for 9 amblyopes shows no systematic deviations from the normal response. For the albino, both the ST1 and ST2 spatial responses peak at around 0.3 cycles deg-1, and both curves are displaced considerably to the low spatial frequency side of the normal ST2 spatial response. The albino's ST2 temporal response is essentially normal. Measurements for the hemianope's "blind" hemifield under conditions appropriate to the isolation of the ST1 and ST2 spatial responses reveal no tuning curves. The ST2 temporal response for the "blind" hemifield, however, is of large amplitude, with a peak at 2 Hz, well below the normal frequency response peak. It is argued that the loss of fine spatial tuning which occurs in the ST1, but not the ST2, spatial responses of the amblyopes is consistent with the sequential organisation of these two filter classes proposed by Holliday and Ruddock (1983). Further, for the only two subjects whose ST2 spatial response curves are displaced to abnormally low frequencies (the albino and a strabismic amblyope) the ST1 spatial response is shifted to low spatial frequencies compared to the normal ST2 curve...     

咸水滴灌下塔克拉玛干沙漠腹地人工绿地土壤水分三维时空动态

通过对咸水滴灌下塔克拉玛干沙漠腹地人工绿地的土壤水分进行实时监测,研究了4—7月各灌水周期内土壤水平0～60 cm和垂直0～120 cm空间距离范围内的水分三维时空变化.结果表明: 咸水滴灌下绿地土壤水分表现出周期性的动态规律,且在忽略降雨时,这种规律随时间变化未表现出明显差异;非降雨与降雨条件下的土壤水分时空变化差异显著.土壤含水量在4月偏高,7月偏低,在有明显降雨的6月最高;土壤水分在不同时间尺度上的变化特征不同,但整体随月变化(4—7月)呈下降趋势,随日变化(1~15 d)呈幂函数递减规律,且在周期内先后经历3个不同变化阶段.土壤水分在水平空间距离上呈一元线性递减,在垂直空间上近似表现为双峰曲线,且在20 cm土层处有一个显著峰值;土壤水分空间分布特征取决于土壤自身的物理性质.     

See globally, spike locally: oscillations in a retinal model encode large visual features

We show that coherent oscillations among neighboring ganglion cells in a retinal model encode global topological properties, such as size, that cannot be deduced unambiguously from their local, time-averaged firing rates. Whereas ganglion cells may fire similar numbers of spikes in response to both small and large spots, only large spots evoke coherent high frequency oscillations, potentially allowing downstream neurons to infer global stimulus properties from their local afferents. To determine whether such information might be extracted over physiologically realistic spatial and temporal scales, we analyzed artificial spike trains whose oscillatory correlations were similar to those measured experimentally. Oscillatory power in the upper gamma band, extracted on single-trials from multi-unit spike trains, supported good to excellent size discrimination between small and large spots, with performance improving as the number of cells and/or duration of the analysis window was increased. By using Poisson distributed spikes to normalize the firing rate across stimulus conditions, we further found that coincidence detection, or synchrony, yielded substantially poorer performance on identical size discrimination tasks. To determine whether size encoding depended on contiguity independent of object shape, we examined the total oscillatory activity across the entire model retina in response to random binary images. As the ON-pixel probability crossed the percolation threshold, which marks the sudden emergence of large connected clusters, the total gamma-band activity exhibited a sharp transition, a phenomena that may be experimentally observable. Finally, a reanalysis of previously published oscillatory responses from cat ganglion cells revealed size encoding consistent with that predicted by the retinal model.     

生物的时间地带性及其农学涵义

生物的时间地带性可表述为生物有机体为谋求生存阈限的扩展,突破地理地带性（暂称为空间地带性）的局限,对地理地带性加以剪裁,连缀,排除不适宜自身生存的环境时段,截取适宜自身生存的环境时段所构建的生存时空体;生物的时间地带性是生物对地球运动（自转与公转）造成的节律适应的结果,时间地带性依赖于空间地带性,不同的空间地带内,时间带谱的基带不同,并在空间地带性的基础上呈现为螺旋状往复循环,是生态要素的时间周期性变化规律的体现;时间地带性与空间地带性一样,都是依赖于尺度（时间尺度与空间尺工）的生态要素分布过程,带谱的成分,范围均随着尺度的变化而变化,生物通过不同的生态策略来适应这种变化;人类农业活动已经取得一些成效,但只能在小尺度上调节时间地带性,不能改变空间地带性。     

Spatiotemporal organization of simple-cell receptive fields in area 18 of cat’s cortex

Spatiotemporal structures of receptive-fields (RF) have been studied for simple cells in area 18 of eat by measuring the temporal transfer function (TTF) over different locations (subregions) within the RF. The temporal characteristics of different subregions differed from each other in the absolute phase shift (APS) to visual stimuli. Two types of relationships can be seen: (i)The APS varied continuously from one subregion to the next: (ii) A 180°-phase jump was seen as the stimulus position changed somewhere within the receptive field. Spatiotemporal receptive field profiles have been determined by applying reverse Fourier analysis to responses in the frequency domain. For the continuous type, spatial and temporal characteristics cannot be dissociated (space time inseparable) and the spatiotemporal structure is oriented. On the contrary, the spatial and temporal characteristics for the jumping type can be dissociated (space-time separable) and the structure is not oriented in the space-time plane. Based on the APSs measured at different subregions, the optimal direction of motion and optimal spatial frequency of neurons can be predicted.     

Energy filters,motion uncertainty,and motion sensitive cells in the visual cortex: a mathematical analysis

Energy filters are tuned to space-time frequency orientations. In order to compute velocity it is necessary to use a collection of filters, each tuned to a different space-time frequency. Here we analyze, in a probabilistic framework, the properties of the motion uncertainty. Its lower bound, which can be explicitly computed through the Cramér-Rao inequality, will have different values depending on the filter parameters. We show for the Gabor filter that, in order to minimize the motion uncertainty, the spatial and temporal filter sizes cannot be arbitrarily chosen; they are only allowed to vary over a limited range of values such that the temporal filter bandwidth is larger than the spatial bandwidth. This property is shared by motion sensitive cells in the primary visual cortex of the cat, which are known to be direction selective and are tuned to spacetime frequency orientations. We conjecture that these cells have larger temporal bandwidth relative to their spatial bandwidth because they compute velocity with maximum efficiency, that is, with a minimum motion uncertainty.     

Spatiotemporal testing and modeling of catfish retinal neurons.

The responses of retinal neurons depend on the interaction of both temporal and spatial aspects of a light stimulus. We developed a linear spatiotemporal model of receptor and horizontal cell layers in the catfish retina based on reciprocal interactions between both layers and coupling within each. Horizontal cell transfer properties were measured experimentally using white-noise intensity modulated light spots of different diameters and were compared with analytical predictions based on the model. Good agreement was obtained with a reasonable choice of model space-constants and feedback parameters. Furthermore, the same set of parameter values determined from spot experiments enabled accurate prediction of experimental horizontal cell responses to traveling gratings. The proposed feedback connections from horizontal cells to receptors quicken the time-course of responses in both layers and sharpen receptive fields.     

A tunable fluorescent timer method for imaging spatial‐temporal protein dynamics using light‐driven photoconvertible protein

Cellular function is largely determined by protein behaviors occurring in both space and time. While regular fluorescent proteins can only report spatial locations of the target inside cells, fluorescent timers have emerged as an invaluable tool for revealing coupled spatial‐temporal protein dynamics. Existing fluorescent timers are all based on chemical maturation. Herein we propose a light‐driven timer concept that could report relative protein ages at specific sub‐cellular locations, by weakly but chronically illuminating photoconvertible fluorescent proteins inside cells. This new method exploits light, instead of oxygen, as the driving force. Therefore its timing speed is optically tunable by adjusting the photoconverting laser intensity. We characterized this light‐driven timer method both in vitro and in vivo and applied it to image spatiotemporal distributions of several proteins with different lifetimes. This novel timer method thus offers a flexible “ruler” for studying temporal hierarchy of spatially ordered processes with exquisite spatial‐temporal resolution. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Spatiotemporal organization of simple-cell receptive fields in area 18 of cat''s cortex

Spatiotemporal structures of receptive-fields (RF) have been studied for simple cells in area 18 of eat by measuring the temporal transfer function (TTF) over different locations (subregions) within the RF. The temporal characteristics of different subregions differed from each other in the absolute phase shift (APS) to visual stimuli. Two types of relationships can be seen: (i)The APS varied continuously from one subregion to the next: (ii) A 180°-phase jump was seen as the stimulus position changed somewhere within the receptive field. Spatiotemporal receptive field profiles have been determined by applying reverse Fourier analysis to responses in the frequency domain. For the continuous type, spatial and temporal characteristics cannot be dissociated (space time inseparable) and the spatiotemporal structure is oriented. On the contrary, the spatial and temporal characteristics for the jumping type can be dissociated (space-time separable) and the structure is not oriented in the space-time plane. Based on the APSs measured at different subregions, the optimal direction of motion and optimal spatial frequency of neurons can be predicted. Project supported by the National Natural Science Foundation of China (Grant Nos. 39570206, 39330110) and the Laboratory of Visual Information Processing, Chinese Academy of Sciences.     

Spatial and temporal dependencies of cross-orientation suppression in human vision

A well-known property of orientation-tuned neurons in the visual cortex is that they are suppressed by the superposition of an orthogonal mask. This phenomenon has been explained in terms of physiological constraints (synaptic depression), engineering solutions for components with poor dynamic range (contrast normalization) and fundamental coding strategies for natural images (redundancy reduction). A common but often tacit assumption is that the suppressive process is equally potent at different spatial and temporal scales of analysis. To determine whether it is so, we measured psychophysical cross-orientation masking (XOM) functions for flickering horizontal Gabor stimuli over wide ranges of spatio-temporal frequency and contrast. We found that orthogonal masks raised contrast detection thresholds substantially at low spatial frequencies and high temporal frequencies (high speeds), and that small and unexpected levels of facilitation were evident elsewhere. The data were well fit by a functional model of contrast gain control, where (i) the weight of suppression increased with the ratio of temporal to spatial frequency and (ii) the weight of facilitatory modulation was the same for all conditions, but outcompeted by suppression at higher contrasts. These results (i) provide new constraints for models of primary visual cortex, (ii) associate XOM and facilitation with the transient magno- and sustained parvostreams, respectively, and (iii) reconcile earlier conflicting psychophysical reports on XOM.     

Responses from Area 3b of Somatosensory Cortex to Textured Surfaces during Active Touch in Primate

(1) The purpose of this experiment was to characterize the responses of neurons in somatosensory cortex while the hand was actively moved (stroked) across a textured surface. Surfaces consisted of horizontal gratings that varied by spatial period or ridge-groove ratio (roughness). Surfaces were attached to rectangular blocks. TOP and BOTTOM halves of each block could contain surfaces of different roughness. (2) Velocity and force of the stroke were behaviorally constrained within certain limits and continuously measured and recorded during the stroke. (3) Response samples for each neuron were obtained for repeated presentations of each surface. Statistical analyses consisted of analysis of variance and t tests across surfaces on the data of each neuron, and summary statistics on groups of neurons with similar response characteristics. The interaction effects of behavioral variables (velocity and force) were examined and found not to be significant. (4) The sample mainly consisted of rapidly adapting neurons in area 3b of somatosensory area I (SI). Three main response types were found: (a) GRADED cells showed a monotonic increase in firing rate to increasingly rougher surfaces. This effect was seen in one-third of cells studied and is consistent with other reports. These cells seem to code roughness in the magnitude of their response, (b) In some cells, response to a BOTTOM surface depended on the roughness of the preceding TOP surface. This is analogous to contrast in the visual system. These CONTRAST cells are a novel finding in the somatosensory system, (c) Some cells only responded to surfaces that were completely smooth. These “OFF”-response-type cells were seen in proximity to other cells that responded in a reciprocal fashion to surfaces with ridges, but not to smooth surfaces. SMOOTH cells did not respond to punctate or passively applied stimuli, and therefore could not be classified by adaptation of the responses. (5) An increase in firing rate as spatial period (roughness) increases (with a constant ratio of ridge to groove) seems contrary to vibratory models of texture perception. As spatial period increases, temporal frequency decreases, and thus “tuned” cells should show a decreased response rate. Yet GRADED cells showed an increased response. In addition, response varied on surfaces with different groove size, where spatial period, and thus temporal period, was constant. This suggests that in rapidly adapting neurons, at least for these simple surfaces, texture is coded by the magnitude of the firing rates rather than by its temporal fidelity. Reduced response to smoother surfaces does not exclude increased phase locking, however, so that GRADED cells may still be the same cells that respond to vibration.     

Primitive Auditory Memory Is Correlated with Spatial Unmasking That Is Based on Direct-Reflection Integration

In reverberant rooms with multiple-people talking, spatial separation between speech sources improves recognition of attended speech, even though both the head-shadowing and interaural-interaction unmasking cues are limited by numerous reflections. It is the perceptual integration between the direct wave and its reflections that bridges the direct-reflection temporal gaps and results in the spatial unmasking under reverberant conditions. This study further investigated (1) the temporal dynamic of the direct-reflection-integration-based spatial unmasking as a function of the reflection delay, and (2) whether this temporal dynamic is correlated with the listeners’ auditory ability to temporally retain raw acoustic signals (i.e., the fast decaying primitive auditory memory, PAM). The results showed that recognition of the target speech against the speech-masker background is a descending exponential function of the delay of the simulated target reflection. In addition, the temporal extent of PAM is frequency dependent and markedly longer than that for perceptual fusion. More importantly, the temporal dynamic of the speech-recognition function is significantly correlated with the temporal extent of the PAM of low-frequency raw signals. Thus, we propose that a chain process, which links the earlier-stage PAM with the later-stage correlation computation, perceptual integration, and attention facilitation, plays a role in spatially unmasking target speech under reverberant conditions.     

Phylogenetic community structure: temporal variation in fish assemblage

Hypotheses about phylogenetic relationships among species allow inferences about the mechanisms that affect species coexistence. Nevertheless, most studies assume that phylogenetic patterns identified are stable over time. We used data on monthly samples of fish from a single lake over 10 years to show that the structure in phylogenetic assemblages varies over time and conclusions depend heavily on the time scale investigated. The data set was organized in guild structures and temporal scales (grouped at three temporal scales). Phylogenetic distance was measured as the mean pairwise distances (MPD) and as mean nearest‐neighbor distance (MNTD). Both distances were based on counts of nodes. We compared the observed values of MPD and MNTD with values that were generated randomly using null model independent swap. A serial runs test was used to assess the temporal independence of indices over time. The phylogenetic pattern in the whole assemblage and the functional groups varied widely over time. Conclusions about phylogenetic clustering or dispersion depended on the temporal scales. Conclusions about the frequency with which biotic processes and environmental filters affect the local assembly do not depend only on taxonomic grouping and spatial scales. While these analyzes allow the assertion that all proposed patterns apply to the fish assemblages in the floodplain, the assessment of the relative importance of these processes, and how they vary depending on the temporal scale and functional group studied, cannot be determined with the effort commonly used. It appears that, at least in the system that we studied, the assemblages are forming and breaking continuously, resulting in various phylogeny‐related structures that makes summarizing difficult.     

Evaluation of possible horizontal gene transfer from transgenic plants to the soil bacterium Acinetobacter calcoaceticus BD413

 The use of genetically engineered crop plants has raised concerns about the transfer of their engineered DNA to indigenous microbes in soil. We have evaluated possible horizontal gene transfer from transgenic plants by natural transformation to the soil bacterium Acinetobacter calcoaceticus BD413. The transformation frequencies with DNA from two sources of transgenic plant DNA and different forms of plasmid DNA with an inserted kanamycin resistance gene, nptII, were measured. Clear effects of homology were seen on transformation frequencies, and no transformants were ever detected after using transgenic plant DNA. This implied a transformation frequency of less than 10^-13 (transformants per recipient) under optimised conditions, which is expected to drop even further to a minimum of 10^-16 due to soil conditions and a lowered concentration of DNA available to cells. Previous studies have shown that chromosomal DNA released to soil is only available to A. calcoaceticus for limited period of time and that A. calcoaceticus does not maintain detectable competence in soil. Taken together, these results suggest that A. calcoaceticus does not take up non-homologous plant DNA at appreciable frequencies under natural conditions. Received: 1 November 1996 / Accepted: 18 April 1997