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At least 865 records · Page 48Linked to original sources
PubMed2001

Fine-scale spatial genetic structure of eight tropical tree species as analysed by RAPDs.

The fine-scale spatial genetic structure of eight tropical tree species (Chrysophyllum sanguinolentum, Carapa procera, Dicorynia guianensis, Eperua grandiflora, Moronobea coccinea, Symphonia globulifera, Virola michelii, Vouacapoua americana) was studied in populations that were part of a silvicultural trial in French Guiana. The species analysed have different spatial distribution, sexual system, pollen and seed dispersal agents, flowering phenology and environmental demands. The spatial position of trees and a RAPD data set for each species were combined using a multivariate genetic distance method to estimate spatial genetic structure. A significant spatial genetic structure was found for four of the eight species. In contrast to most observations in temperate forests, where spatial structure is not usually detected at distances greater than 50 m, significant genetic structure was found at distances up to 300 m. The relationships between spatial genetic structure and life history characteristics are discussed.

DNA, Plant↗
PubMed2002

Spatial autocorrelation and linkage of Mendelian RAPD markers in a population of Picea abies Karst.

The spatial clustering of single- and di-locus genotypes in a natural, continuous population of Norway spruce was investigated using 69 Mendelian Random Amplified Polymorphic DNA (RAPD) markers that covered about 15 of the species genome, and whose linkage relationships were known. Spatial autocorrelation techniques and randomization tests, applied to both single- and di-locus genotypes, revealed a weak, though significant, spatial structure at the scale 0-200 m (5 of single-locus and 7 of di-locus genotypes). To assess the relative importance of isolation by distance and linkage between markers on their spatial genetic structuring, we grouped joins between sampled trees into equivalence categories expected to show similar, specific patterns of spatial distribution under isolation by distance. Results from both single- and di-locus analyses were consistent with the existence of patches of like homozygotes (about 8 and 11 of loci at the single- and di-locus level, respectively) surrounded by a mix of like heterozygotes. Similar structuring has been predicted by simulation models under isolation by distance and selective neutrality. Overall, linkage between markers accounted for an increase of spatial clumping of di-locus genotypes involving tightly linked loci with recombination fractions up to 0.1, a consequence of limited, stochastic spread of single-locus genotypes in space. Our results support the hypothesis that isolation by distance and linkage have a small, though significant, effect even within continuous forest tree populations. In general, the spatial distribution of multilocus genotypes within populations should be interpreted with caution when linkage relationships among the markers used are unknown.

DNA↗
PubMed2001

What does the study of the spatial patterns of pathological lesions tell us about the pathogenesis of neurodegenerative disorders?

Discrete pathological lesions, which include extracellular protein deposits, intracellular inclusions and changes in cell morphology, occur in the brain in the majority of neurodegenerative disorders. These lesions are not randomly distributed in the brain but exhibit a spatial pattern, that is, a departure from randomness towards regularity or clustering. The spatial pattern of a lesion may reflect pathological processes affecting particular neuroanatomical structures and, therefore, studies of spatial pattern may help to elucidate the pathogenesis of a lesion and of the disorders themselves. The present article reviews first, the statistical methods used to detect spatial patterns and second, the types of spatial patterns exhibited by pathological lesions in a variety of disorders which include Alzheimer's disease, Down syndrome, dementia with Lewy bodies, Creutzfeldt-Jakob disease, Pick's disease and corticobasal degeneration. These studies suggest that despite the morphological and molecular diversity of brain lesions, they often exhibit a common type of spatial pattern (i.e. aggregation into clusters that are regularly distributed in the tissue). The pathogenic implications of spatial pattern analysis are discussed with reference to the individual disorders and to studies of neurodegeneration as a whole.

Brain↗
PubMed1998

Spatial-frequency tuning and geniculocortical projections in the visual cortex (areas 17 and 18) of the pigmented ferret.

We have examined the spatial-frequency selectivity of neurons in areas 17 and 18 of the adult pigmented ferret, by measuring how the amplitude of response depends on the spatial-frequency of moving sinusoidal gratings of optimal orientation and fixed contrast. Neurons in area 17 of the ferret respond optimally to low spatial frequencies [average 0.25 cycles per degree (c/deg)], much lower than the optima for cat area 17. The tuning curves are of the same form as those found in cat and monkey: unimodal with bandwidths in the range 0.8-3.5 octaves. Neurons in area 18 of the ferret respond optimally to even lower spatial frequencies (average 0.087 c/deg) than area 17 neurons, and the distributions of optimal spatial frequency for areas 17 and 18 hardly overlap. In both cortical areas, the bandwidth of the tuning curves is inversely correlated with optimal spatial frequency. This marked difference in tuning between the two cortical areas is probably attributable to differential geniculo-cortical projections. Small injections of fluorescent latex microspheres or horseradish peroxidase (HRP) were made into area 17 or area 18 in order to investigate the populations of geniculate neurons projecting to the two cortical areas. After injections into area 17, labelled neurons are found predominantly in the geniculate A layers, with a few neurons labelled in the C layers. Conversely, after an area 18 injection, similar numbers of labelled neurons are found in the C layers as in the A layers. Soma-size analysis of the neurons in the A-layers suggests the existence of two populations of relay neurons, which project differentially to areas 17 and 18. The different geniculate inputs and the different spatial-frequency tuning in areas 17 and 18 may imply that the two cortical areas process visual information more in parallel than in series.

Animals↗
PubMed1980

Adaptation to a "spatial-frequency doubled" stimulus.

The perceived spatial frequency of low-frequency sinusoidal gratings is elevated when the gratings are temporally modulated sufficiently rapidly. Although this phenomenon is usually called "spatial-frequency doubling", the magnitude of this spatial-frequency elevation depends on the spatial and temporal parameters of the grating and may be less than or greater than doubling. Adaptation to such a pattern produces contrast-threshold elevation whose spatial-frequency tuning is centered upon the grating's real rather than perceived spatial frequency. These results suggest that the spatial-frequency elevation occurs later in the visual system than adaptation or that these two processes are independent of one another, at least under the conditions of these experiments.

Adaptation, Ocular↗
PubMed1992

Temporal integration of spatially filtered visual images.

Factors which govern the temporal integration of spatial information were examined in a group of five experiments. A series of high-pass and low-pass spatially filtered versions of a visual scene were generated. Observers' ratings of these filtered versions of the scene for perceived image quality indicated that quality was determined both by the bandwidth of spatial information and the presence of high-spatial-frequency edge information. When sequences of three different versions of the scene were presented over an interval of 120 ms the perceived quality of the resulting composite image was determined both from the ratings of the individual components of that sequence and from the order in which these components were presented. When the order of spatial information in a sequence moved from coarse to fine detail the perceived quality of the composite image was significantly better than when the order moved from fine to coarse. This evidence of a coarse-to-fine bias in pattern integration was further investigated with a detection paradigm. The pattern of errors once again indicated that temporal integration of spatial information was superior when a coarse-to-fine mode of information delivery was employed. Taken together the data indicate that the pattern-integration mechanism has an inherent order bias and does not accumulate spatial information so efficiently when the 'natural' coarse-to-fine order is violated.

Attention↗
PubMed1998

Spatial-frequency discrimination, brain lateralisation, and acute intake of alcohol.

The effect of alcohol (breath-alcohol level of 0.1%) on perceptual discrimination of low (1.5 cycles deg-1) and high (8 cycles deg-1) spatial frequencies in the left and right visual field was measured in eighteen right-handed males, in a double-blind, balanced placebo design. Discrimination thresholds for briefly (180 ms) presented sinusoidal gratings were determined by two-alternative forced-choice judgments with four interleaving psychophysical staircases providing random trial-to-trial variation of reference spatial frequency and visual field, in addition to a random (+/- 10%) jitter of reference spatial frequency. Alcohol produced overall higher discrimination thresholds but did not alter the visual-field balance: no main effect of visual field was observed, but in both placebo and alcohol conditions spatial frequency interacted with visual field in the direction predicted by the spatial-frequency hypothesis of hemispheric asymmetry in visual-information processing, with left-visual-field/right-hemisphere superiority in discrimination of low spatial frequencies and right-visual-field/left-hemisphere superiority in discrimination of high spatial frequencies.

Adult↗
PubMed1998

Stimulus eccentricity and spatial frequency interact to determine circular vection.

While early research suggested that peripheral vision dominates the perception of self-motion, subsequent studies found little or no effect of stimulus eccentricity. In contradiction to these broad notions of 'peripheral dominance' and 'eccentricity independence', the present experiments showed that the spatial frequency of optic flow interacts with its eccentricity to determine circular vection magnitude--central stimulation producing the most compelling vection for high-spatial-frequency stimuli and peripheral stimulation producing the most compelling vection for lower-spatial-frequency stimuli. This interaction appeared to be due, in part at least, to the effect that the higher-spatial-frequency moving pattern had on subjects' ability to organise optic flow into related motion about a single axis. For example, far-peripheral exposure to this high-spatial-frequency pattern caused many subjects to organise the optic flow into independent local regions of motion (a situation which clearly favoured the perception of object motion not self-motion). It is concluded that both high-spatial-frequency and low-spatial-frequency mechanisms are involved in the visual perception of self-motion--with their activities depending on the nature and eccentricity of the motion stimulation.

Adult↗
PubMed2001

Visual discomfort: the influence of spatial frequency.

The response of different visual discomfort groups to a range of spatial frequencies at threshold and suprathreshold was investigated. In experiment 1, a paired-comparison task was conducted. The high visual discomfort group judged a spatial frequency of 4 cycles deg-1 as the most perceptually distorted and somatically unpleasant to view. The moderate and low visual discomfort groups judged 8 and 12 cycles deg-1 as more perceptually and somatically unpleasant to view than lower spatial frequencies. In experiment 2, the spatial contrast-sensitivity function (CSF) for the high visual discomfort group was depressed for spatial frequencies between 1 and 12 cycles deg-1 in comparison with the moderate and low visual discomfort groups. When these same spatial frequencies were modulated at 6 Hz, CSFs were the same for all groups. These results are discussed in relation to a failure of inhibition across spatial-frequency channels in the high visual discomfort group. This may be explained by a more generalised parvocellular system processing deficit. Possible similarities between some forms of migraine and visual discomfort are highlighted.

Adolescent↗
PubMed2001

Amplitude and spatial-period discrimination in sinusoidal gratings by dynamic touch.

We measured discrimination thresholds for sinusoidal gratings using active dynamic touch. In the first experiment we measured the thresholds for amplitude discrimination as a function of amplitude and spatial period. Thresholds ranged between 10.8%, and 15.8% of the standard amplitude. We showed that amplitude differences as small as 2 microm can be detected. We found that Weber fractions for amplitude discrimination are constant over a range of amplitudes, but are influenced by the spatial period of the grating; discrimination improved with increasing spatial period. In the second experiment we determined the thresholds for spatial-period discrimination. We used the same design as in the first experiment. Weber fractions ranged from 6.4% to 11.8%. Amplitude was found to have no effect on the Weber fractions for spatial-period discrimination. However, the spatial period had an effect on the Weber fractions: larger spatial periods yielded lower Weber fractions.

Adult↗
PubMed2003

Spatial reference and scanning with the left and right hand.

Discrepant findings on performance by the two hands in spatial tasks make it difficult to infer spatial coding unambiguously. We tested the hypotheses (a) that the left hand is consistently better in haptic spatial tasks and (b) that adding spatial reference information produces more accurate coding in spatial tasks, independently task and hand effects. Instructions to use external cues from a surrounding frame as well as body-centred cues for reference produced highly significant increases in accuracy in haptic distance and location experiments. The distance experiments showed no hand differences. A small right-hand advantage with longer positioning movements in the recall of locations was significant when combined with left-hand scanning of the frame, but did not relate to reference conditions. Hand use interacted significantly with the location-versus-distance experiments, but showed no interaction with the spatial reference factor, which was highly significant in both experiments. The finding suggests that modes of coding need to be distinguished from cross-lateral effects of sensory input conditions. The study shows that varying reference information offers a potentially useful behavioural tool for distinguishing spatial coding from input and task conditions independently of hand performance. Methodological, practical, and theoretical implications are discussed.

Adolescent↗
PubMed2004

Spatial synchrony of local populations has increased in association with the recent Northern Hemisphere climate trend.

According to ecological theory, populations whose dynamics are entrained by environmental correlation face increased extinction risk as environmental conditions become more synchronized spatially. This prediction is highly relevant to the study of ecological consequences of climate change. Recent empirical studies have indicated, for example, that large-scale climate synchronizes trophic interactions and population dynamics over broad spatial scales in freshwater and terrestrial systems. Here, we present an analysis of century-scale, spatially replicated data on local weather and the population dynamics of caribou in Greenland. Our results indicate that spatial autocorrelation in local weather has increased with large-scale climatic warming. This increase in spatial synchrony of environmental conditions has been matched, in turn, by an increase in the spatial synchrony of local caribou populations toward the end of the 20th century. Our results indicate that spatial synchrony in environmental conditions and the populations influenced by them are highly variable through time and can increase with climatic warming. We suggest that if future warming can increase population synchrony, it may also increase extinction risk.

Animals↗
PubMed2006

An integrated approach for risk profiling and spatial prediction of Schistosoma mansoni-hookworm coinfection.

Multiple-species parasitic infections are pervasive in the developing world, yet resources for their control are scarce. We present an integrated approach for risk profiling and spatial prediction of coinfection with Schistosoma mansoni and hookworm for western Côte d'Ivoire. Our approach combines demographic, environmental, and socioeconomic data; incorporates them into a geographic information system; and employs spatial statistics. Demographic and socioeconomic data were obtained from education registries and from a questionnaire administered to schoolchildren. Environmental data were derived from remotely sensed satellite images and digitized ground maps. Parasitologic data, obtained from fecal examination by using two different diagnostic approaches, served as the outcome measure. Bayesian variogram models were used to assess risk factors and spatial variation of S. mansoni-hookworm coinfection in relation to demographic, environmental, and socioeconomic variables. Coinfections were found in 680 of 3,578 schoolchildren (19.0%) with complete data records. The prevalence of monoinfections with either hookworm or S. mansoni was 24.3% and 24.1%, respectively. Multinomial Bayesian spatial models showed that age, sex, socioeconomic status, and elevation were good predictors for the spatial distribution of S. mansoni-hookworm coinfection. We conclude that our integrated approach, employing a diversity of data sources, geographic information system and remote sensing technologies, and Bayesian spatial statistics, is a powerful tool for risk profiling and spatial prediction of S. mansoni-hookworm coinfection. More generally, this approach facilitates risk mapping and prediction of other parasite combinations and multiparasitism, and hence can guide integrated disease control programs in resource-constrained settings.

Adolescent↗
PubMed2000

Spatial correlation analysis of the pharmacological conversion of sustained atrial fibrillation in conscious goats by cibenzoline.

The nonlinear spatial redundancy and the linear spatial correlation function were used to investigate to what extent non-linearity was involved in the coupling of atrial regions and how organization in activation patterns of sustained atrial fibrillation (AF) had been modified by administration of the class IC agent cibenzoline in the experimental model of sustained AF in instrumented conscious goats. Electrograms were measured in five goats during sustained AF and when the fibrillation interval had been prolonged to about 25%, 50% and 85% (CIB25, CIB50, CIB85) with respect to control. The nonlinear association length and linear correlation length were estimated along the principal axes of two-dimensional correlation maps estimated from the spatial redundancy and the spatial correlation function, respectively. The estimated short axis association length in the right atrium increased already shortly after the start of infusion (CIB25, +61%), and remained significantly different from control during the experiment, including the effects of non-simultaneous interaction. At CIB85 the association length had almost become twice as long with respect to control (increase from 16 to 29 mm, 89%), while in the left atrium changes were less pronounced (increase from 9 to 12 mm, +32%). The linearized association length which was estimated using multivariate surrogate data increased more gradually and was less sensitive to changes in spatial organization. The results of the spatial correlation analysis suggest that the drug-induced nonlinearity in the spatio-temporal dynamics of sustained AF is related to activation patterns which are characterized by extended uniformly propagating fibrillation wavefronts (AF type I). We conclude that cibenzoline enhanced the spatial organization of sustained AF associated with a transition from type II to type I AF activation patterns. This may destabilize the perpetuation of AF since an increase in association length is equivalent to a reduction of atrial tissue mass available to support reentrant circuits. The results are consistent with the hypothesis that larger association lengths result from fewer and larger reentrant circuits. It is argued that effects of diminished curvature of fibrillation wavefronts are anti-arrhythmic under conditions of suppressed excitability imposed by cibenzoline. Termination of AF may be mediated by a mechanism resembling a bifurcation of the dynamics which sets in when the ends of fractionated wavefronts cannot sufficiently curve anymore to maintain a positive balance of newly generated wavelets needed to sustain AF.

Animals↗
PubMed2000

Spatial memory following prenatal alcohol exposure: more than a material specific memory deficit.

Spatial memory deficits have been reported following prenatal alcohol exposure and animal studies have demonstrated hippocampal vulnerability to alcohol. This study examined spatial memory in children diagnosed with fetal alcohol syndrome (FAS) or fetal alcohol effects (FAE) and matched controls. Spatial memory was examined with location recall measures. Since visual perceptual skills and nonspecific memory impairment could impact spatial memory, tasks assessing perception and verbal memory were administered. Analyses revealed group differences on perceptual and verbal and spatial memory tasks. There was no significant difference in spatial memory once perceptual and verbal memory task performance was taken into account, suggesting that differences in spatial memory were not solely attributable to a material specific memory deficit.

Adolescent↗
PubMed2005

Analysis of spatial patterns at a geographical scale over north-western Europe from point-referenced aphid count data.

The spatial analysis by distance indices (SADIE) technique was developed to evaluate the spatial pattern of point-referenced count data as well as the spatial association between two sets of data sharing the same point locations. This paper presents an analysis of spatial patterns in aphid count data and the association of these data with climate across north-west Europe. The paper tests the applicability of the technique to large geographical areas. Aggregation and cluster indices were calculated for the total annual abundance of the peach-potato aphid Myzus persicae (Sulzer) and for the annual mean rainfall and temperature at aphid monitoring sites. Association indices demonstrated the stability in time of aphid spatial structures and the correlation between aphid density and climate patterns. Groups of relatively large numbers of aphids, termed patches, and groups of relatively small numbers of aphids, termed gaps, were located and their mean size estimated. The aphid patterns were quite stable in time and the spatial patterns of temperature and rainfall were weakly associated with M. persicae annual abundance. Similarities were observed between the results of SADIE and those from the more widely used technique of spatial autocorrelation (SAC). However, the SADIE association index has the advantage of quantifying the possible associations between aphid data and the factors that determine population distribution. Thus, high temperature and low rainfall were identified as environmental factors that were positively associated with aphid abundance across north-west Europe.

Animals↗
PubMed2006

Differentiating between spatial and object-based working memory using complex stimuli: an erp study.

Event-related potential studies using simple low-level stimuli have associated early visual-spatial processing with P1 amplitude, and early visual object-based processing with N2 amplitude. Other studies have linked later spatial processing and spatial working memory with posterior slow wave activity. Using complex real world representations in a matched object and spatial processing task, P1 was found to reflect spatially based information, and N2 was found to reflect object-based information. Furthermore, object-based working memory was associated with larger slow wave amplitude compared with spatial working memory, raising the possibility that spatial and object-based working memory utilize common processing resources.

Adult↗
PubMed2001

Complexity effects in visuo-spatial working memory: implications for the role of long-term memory.

Several studies have shown that the capacity of visuo-spatial working memory is limited by complexity. Using a variant of the Corsi blocks task, this paper investigates the effect of complexity of the to-be-remembered path on visuo-spatial memory span. Redundancy was determined by three Gestalt principles: symmetry, repetition, and continuation. Experiment 1 revealed an effect of path complexity. The subsequent experiments explored whether the superiority for recall of structured over complex paths can be attributed solely to the operation of visuo-spatial working memory, or whether it also reflects the use of long-term knowledge. Experiment 2 demonstrated that the effect of complexity remained, even when the mechanisms for visuo-spatial coding were removed by a secondary visuo-spatial task. In Experiments 3 and 4 subjects were trained in the recall of complex paths. This led to the creation of long-term memory representations for these paths, as shown by an improvement in their span, and a concomitant lack of transfer to new paths. Finally, Experiment 5 showed that one prior repetition of a complex path was sufficient to produce specific and long-term learning effects. These results point to the involvement of long-term memory processes in the temporary retention of visuo-spatial material for which representations exist in long-term memory. They also suggest that the effect of complexity may provide a tractable technique for investigating the mechanisms underlying the limits of visuo-spatial short-term storage.

Humans↗