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PubMed2002

Metapopulation models for extinction threshold in spatially correlated landscapes.

Simple analytical models assuming homogeneous space have been used to examine the effects of habitat loss and fragmentation on metapopulation size. The models predict an extinction threshold, a critical amount of suitable habitat below which the metapopulation goes deterministically extinct. The consequences of non-random loss of habitat for species with localized dispersal have been studied mainly numerically. In this paper, we present two analytical approaches to the study of habitat loss and its metapopulation dynamic consequences incorporating spatial correlation in both metapopulation dynamics as well as in the pattern of habitat destruction. One approach is based on a measure called metapopulation capacity, given by the dominant eigenvalue of a "landscape" matrix, which encapsulates the effects of landscape structure on population extinctions and colonizations. The other approach is based on pair approximation. These models allow us to examine analytically the effects of spatial structure in habitat loss on the equilibrium metapopulation size and the threshold condition for persistence. In contrast to the pair approximation based approaches, the metapopulation capacity based approach allows us to consider species with long as well as short dispersal range and landscapes with spatial correlation at different scales. The two methods make dissimilar assumptions, but the broad conclusions concerning the consequences of spatial correlation in the landscape structure are the same. Our results show that increasing correlation in the spatial arrangement of the remaining habitat increases patch occupancy, that this increase is more evident for species with short-range than long-range dispersal, and that to be most beneficial for metapopulation size, the range of spatial correlation in landscape structure should be at least a few times greater than the dispersal range of the species.

Algorithms↗
PubMed2003

Dynamics and genealogy of strains in spatially extended host-pathogen models.

We examine the dynamics of evolution in a generic spatial model of a pathogen infecting a population of hosts, or an analogous predator-prey system. Previous studies of this model have found a range of interesting phenomena that differ from the well-mixed version. We extend these studies by examining the spatial and temporal dynamics of strains using genealogical tracing. When transmissibility can evolve by mutation, strains of intermediate transmissibility dominate even though high-transmissibility mutants have a short-term reproductive advantage. Mutant strains continually arise and grow rapidly for many generations but eventually go extinct before dominating the system. We find that, after a number of generations, the mutant pathogen characteristics strongly impact the spatial distribution of their local host environment, even when there are diverse types coexisting. Extinction is due to the depletion of susceptibles in the local environment of these mutant strains. Studies of spatial and genealogical relatedness reveal the self-organized spatial clustering of strains that enables their impact on the local environment. Thus, we find that selection acts against the high-transmissibility strains on long time-scales as a result of the feedback due to environmental change. Our study shows that averages over space or time should not be assumed to adequately describe the evolutionary dynamics of spatially distributed host-pathogen systems.

Animals↗
PubMed2000

An fMRI investigation of cortical contributions to spatial and nonspatial visual working memory.

The experiments presented in this report were designed to test the hypothesis that visual working memory for spatial stimuli and for object stimuli recruits separate neuronal networks in prefrontal cortex. We acquired BOLD fMRI data from subjects while they compared each serially presented stimulus to the one that had appeared two or three stimuli previously. Three experiments failed to reject the null hypothesis that prefrontal cortical activity associated with spatial working memory performance cannot be dissociated from prefrontal cortical activity associated with nonspatial working memory performance. Polymodal regions of parietal cortex (inferior and superior parietal lobules), as well as cortex surrounding the superior frontal sulcus (and encompassing the frontal eye fields), also demonstrated equivalent levels of activation in the spatial and object conditions. Posterior cortical regions associated with the ventral visual processing stream (portions of lingual, fusiform, and inferior temporal gyri), however, demonstrated greater object than spatial working memory-related activity, particularly when stimuli varied only along spatial or featural dimensions. These experiments, representing fMRI studies of spatial and object working memory in which the testing procedure and the stimuli were identical in the two conditions, suggest that domain-specific visual working memory processing may be mediated by posterior regions associated with domain-specific sensory processing.

Adolescent↗
PubMed2000

Extrastriatal mean regional uptake of fluorine-18-FDOPA in the normal aged brain--an approach using MRI-aided spatial normalization.

The aim of this study was to define the mean regional 6-[(18)F]fluoro-l-dopa (FDOPA) uptake rate constant (K(i)) values in the striatal and extrastriatal regions of the brain of normal subjects using magnetic resonance imaging (MRI)-aided spatial normalization of the FDOPA K(i) image and using automatic region of interest (ROI) analysis. Dynamic three-dimensional FDOPA positron emission tomography (PET) and three-dimensional magnetic resonance (MR) images were acquired in 13 aged normal subjects. The FDOPA add image and the K(i) image of each subject were transformed into standard stereotactic space with the aid of individual coregistered MR image. The mean regional K(i) values of the striatal and extrastriatal regions before normalization were compared with the respective values after normalization. Then automatic ROI analysis was performed on the MRI-aided spatially normalized K(i) images of the 13 normal subjects. The K(i) values on original images and those on spatially normalized images were in good agreement, indicating that the spatial normalization technique did not change the regional K(i) values appreciably. Automatic ROI analysis of the spatially normalized FDOPA K(i) images of the normal subjects, showed high K(i) values in ventral and dorsal regions of the midbrain, amygdala, hippocampus, and medial prefrontal cortex, in addition to caudate nucleus and putamen, which correspond to the dopaminergic projections in the brain. Spatial normalization technique helped to establish a database of FDOPA K(i) images of normal subjects and high K(i) values were observed widely besides striatal regions corresponding to the dopaminergic projections in the brain.

Aged↗
PubMed2002

A parametric study of mental spatial transformations of bodies.

TWO CLASSES OF MENTAL SPATIAL TRANSFORMATION CAN BE DISTINGUISHED: Object-based spatial transformations are imagined movements of objects; and egocentric perspective transformations are imagined movements of one's point of view. The hypothesis that multiple neural systems contribute to these mental imagery operations was tested with functional MRI. Participants made spatial judgments about pictures of human bodies, and brain activity was analyzed as a function of the judgment required and the time taken to respond. Areas in right temporal, occipital and parietal cortex and the medial superior cerebellum appear to be differentially involved in object-based spatial transformations. Additionally, midline structures and lateral parietal cortex were found to decrease in activity during the spatial reasoning tasks, independently of the judgment required or of the latency of response. The results are discussed in terms of a model of spatial reasoning that postulates specialized subsystems for performing object-based and egocentric perspective image transformations.

Adult↗
PubMed1998

The effects of muscarinic cholinergic receptor blockade in the rat anterior cingulate and Prelimbic/Infralimbic cortices on spatial working memory.

Previous findings indicate that cholinergic input to the medial prefrontal cortex may modulate mnemonic processes. The present experiment determined whether blockade of muscarinic cholinergic receptors in the rodent anterior cingulate and prelimbic/infralimbic cortices impairs spatial working memory. In a 12-arm radial maze, a working memory for spatial locations task was employed using a continuous recognition go/no-go procedure. Rats were allowed to enter 12 arms for a reinforcement. Of the 12 arm presentations, 3 or 4 arms were presented for a second time in a session that did not contain a reinforcement. The number of trials between the first and second presentations of an arm ranged from 0 to 6 (lags). Infusions of scopolamine (1, 5, and 10 microgram), a muscarinic cholinergic antagonist, into the prelimbic/infralimbic cortices, but not the anterior cingulate cortex, significantly impaired spatial working memory in a lag- and dose-dependent manner. The deficit induced by scopolamine (10 microgram) was attenuated by concomitant intraprelimbic/infralimbic injections of oxotremorine (2 microgram) a muscarinic cholinergic agonist. A separate group of rats was tested on a successive spatial discrimination task. Injections of scopolamine (1, 5, and 10 microgram) into the prelimbic/infralimbic cortices did not impair performance on the spatial discrimination task. These findings suggest that muscarinic transmission in the prelimbic/infralimbic cortices, but not the anterior cingulate cortex, is important for spatial working memory.

Analysis of Variance↗
PubMed1996

Competition in a spatially heterogeneous environment: modelling the risk of spread of a genetically engineered population.

In recent years regulations have been developed to address the risks of releasing genetically engineered organisms into the natural environment. These risks are generally considered to be proportional to the exposure multiplied by the hazard. Exposure is, in part, determined by the spatial spread of the organisms, a component of risk suited to mathematical analysis. In this paper we exampine a mathematical model describing the spread of organisms introduced into a hetereogeneous environment, focusing on the risk of spread and plausibility of containment strategies. Two competing populations are assumed, one the natural species and the other an engineered species or strain, both of which move randomly in a spatially heterogenous environment consisting of alternating favourable and unfavourable patches. The classical Lotka-Volterra competition model with diffusion is used. Analyses of the possible spread and invasion of engineered organisms are thus reduced to finding periodic travelling wave solutions to the model equations. We focus on whether a very small number of engineered organisms can spatially invade a natural population. Initially we investigate the problem for spatially periodic diffusion coefficients and demonstrate that, under the right circumstances and a large enough unfavourable patch, invasion does not succeed. However, if spatially periodic carrying capacities are assumed along with spatially varying diffusion rates, the situation is far more complex. In this case containment of the engineered species is no longer only a simple function of the unfavourable patch length. By using perturbation solutions to the nonuniform steady states, approximate invasion conditions are obtained.

DNA, Recombinant↗
PubMed1992

Pattern electroretinogram as a function of spatial frequency after retrobulbar optic neuritis.

Steady-state (8-Hz) pattern electroretinograms in response to counterphased sinusoidal gratings of variable spatial frequency (0.6-4.8 c/deg) were recorded in 17 patients who had had retrobulbar optic neuritis in one or both eyes (23 eyes with a clinical history of optic neuritis) and in 21 age-matched normal subjects. Amplitude and phase of the Fourier-analyzed pattern electroretinogram second harmonic were measured. The mean pattern electroretinogram amplitude of patients was significantly reduced compared with that of controls. Amplitude reductions were more marked at intermediate (1-1.4 c/deg) than at lower or higher spatial frequencies. Therefore, the average amplitude versus spatial frequency response function differed significantly in patients compared with controls, displaying a lowpass instead of a band-pass shape. No significant differences in the mean pattern electroretinogram phase were observed between groups at any spatial frequency. These results indicate spatial frequency-dependent abnormalities in the pattern electroretinogram amplitude after optic neuritis, suggesting a specific loss of retinal neurons sensitive to stimuli of intermediate spatial frequencies.

Adult↗
PubMed1996

Spatial learning in deer mice: sex differences and the effects of endogenous opioids and 60 Hz magnetic fields.

We examined the effects of brief exposure to weak 60 Hz extremely low frequency (ELF) magnetic fields and opioid systems on spatial behavior and learning in reproductive adult male and female deer mice, Peromyscus maniculatus. Sex differences were evident in spatial performance, with male deer mice displaying significantly better performance than female mice in the Morris water maze, whereby animals had to acquire and retain the location of a submerged hidden platform. Brief (maximum 5 min) exposure to weak (100 microT) 60 Hz magnetic fields during task acquisition significantly improved female performance, eliminating the sex differences in acquisition. The opiate antagonist, naltrexone, also improved female acquisition, though significantly less than the magnetic fields. These facilitatory effects involved alterations of "non-spatial" (task familiarization and reduction of related anxiety/aversive related behaviors) and possibly "spatial" aspects of the task. Enhancement of enkephalin activity with the enkephalinase inhibitor, SCH 34826, significantly reduced task performance by male deer mice. Both naltrexone and the 60 Hz magnetic fields attenuated the enkephalin mediated reductions of spatial performance. These findings indicate that brief exposure to 60 Hz magnetic fields can enhance water maze task acquisition by deer mice and suggest that these facilitatory effects on spatial performance involve alterations in opioid activity.

Analgesics↗
PubMed1977

Spatial frequency and orientation tuning curves of visual neurones in the cat: effects of mean luminance.

Experiments have been performed on unanaesthetized and paralysed cats. The tuning curves for spatial frequency of retinal, lateral geniculate and simple and complex cells of the cortex have been determined in response to sinusoidal gratings of various spatial frequencies at different levels of mean luminance. For all neurones, decreasing the mean luminance leads to a progressive loss of spatial resolution and contrast sensitivity. Retinal ganglion cells of type X show, for scotopic levels of luminance, a flattening of their spatial frequency tuning curves in the low spatial frequency range. For geniculate and cortical neurones, on the contrary, the spatial frequency characteristics at the various levels of luminance remain practically invariant in their bandwidth. On the average, complex cells still respond to mean luminances ten times lower than simple cells. The tuning curves for orientation of cortical cells maintain, to a first approximation, the same shape at the various levels of mean luminance. The results are discussed comparing the electrophysiological with psychophysical data.

Animals↗
PubMed1988

Solid perception mechanism by a shading pattern: spatial frequency components in a corrugated wave pattern.

Illumination allows solid object perception to be obtained and depicted by a shading pattern produced by lighting. The shading cue, as one of solid perception cues (Gibson 1979), was investigated in regard to a white corrugated wave shape, using computer graphic device: Tospix-2. The reason the corrugated wave was chosen, is that an alternately bright and dark pattern, produced by shading, can be conveniently analyzed into contained spatial frequencies. This paper reports spatial frequency properties contained in the shading pattern. The shading patterns, input into the computer graphic device, are analyzed by Fourier Transformation by the same device. After the filtration by various spatial frequency low and high pass filters, Inverse Fourier Transformation is carried out for the residual components. The result of the analysis indicates that the third through higher harmonics components are important in regard to presenting a solid reality feeling in solid perception. Sakata (1983) also reported that an edged pattern, superimposed onto a lower sinusoidal pattern, was important in solid perception. The third through higher harmonics components express the changing position of luminance on the pattern, and a slanted plane relating to the light direction. Detection of a solid shape, constructed with flat planes, is assumed to be on the bottom of the perfect curved solid perception mechanism. Apparent evidence for this assumption, in difficult visual conditions, is that a flat paneled solid is seen before the curved solid. This mechanism is explained by two spatial frequency neural network systems, assumed as having correspondence with higher spatial frequency detection and lower spatial frequency detection.

Computer Graphics↗
PubMed1976

Effects of scopolamine on variable intertrial interval spatial alternation and memory in the rat.

A repeated measures procedure, variable intertrial interval (ITI) spatial alternation, was used to assess scopolamine effects on memory, and to compare effects of the drug on discrimination processes with effects on storage. Rats learned in two stages to press left and right levers in alternation on discrete trials separated by 5 different ITI's ranging from 2.5 to 40 s and presented in random order during the experimental session. In the first stage, alternating discrimination, alternation was controlled by a light on over the correct lever at the time of the trial; in the second stage, variable ITI spatial alternation, a centrally located panel light signalled all trials and alternation was controlled by stimuli from prior trials ('memory'). Alternation response occurrence declined moderately (but significantly) with increasing ITI duration in both the alternating discrimination and variable ITI spatial alternating discrimination and variable ITI spatial alternation stages; response occurrence was also significantly decreased by scopolamine treatment in both stages. Accuracy of alternating discrimination performance was not significantly altered by either ITI duration or scopolamine treatment. Accuracy of variable ITI spatial alternation performance on a trial varied inversely with the duration of the ITI that preceded the trial. Scopolamine treatment significantly reduced accuracy of lever pressing in variable ITI spatial alternation but did not alter the slope of the curves relating accuracy to ITI duration. These effects indicate that the drug impaired discrimination processes but did not alter memory storage.

Animals↗
PubMedDate not supplied

The effects of intrahippocampal BDNF and NGF on spatial learning in aged Long Evans rats.

Spatial learning rate was compared in cognitively impaired aged rats infused with either brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF). BDNF or NGF was infused into the dorsal hippocampus/third ventricle while animals were being trained on the Morris water maze. Training continued until all rats met a spatial learning criterion. Seven weeks later, they were tested for retention of the task, and sacrificed for assessment of hippocampal high-affinity choline uptake (HACU) or hypothalamic biogenic amine levels. NGF, but not BDNF, improved spatial learning rate in aged rats and increased hippocampal choline uptake weeks after withdrawal of NGF. Although BDNF did not improve spatial learning, it did induce a partial, long-term normalization of the elevated hypothalamic 5-HT levels observed in our aged rats. These data suggest that (1) intrahippocampal/intraventricular infusion of NGF can improve the learning rate of aged, spatial learning-impaired rats, and that this improvement in acquisition could be associated with increased hippocampal cholinergic activity, and (2) that the BDNF-induced normalization of hypothalamic 5-HT levels in aged rats was not sufficient to improve learning rate in aged, spatial learning-impaired rats.

Aging↗
PubMed2005

Spatial heterogeneity of DTPA-extractable zinc in cultivated soils induced by city pollution and land use.

The spatial heterogeneity of DTPA-extractable zinc in the cultivated soils of Shenyang suburbs in Liaoning Province of China was investigated, and its map was drawn by the methods of geostatistics combined with geographic information system. The data of soil DTPA-extractable zinc fitted normal distribution after logarithm transformation, and its semivariogram fitted a spherical model. The semivariogram indicated that the spatial dependence of soil DTPA-extractable zinc content was moderate, with the spatial dependence range of 1.69 km and the fractal dimension of 1.96. Stochastic factors contributed to 49.9% of the spatial variability, while structural factors contributed to 50.1% of it. The spatial heterogeneity of soil DTPA-extractable zinc shown by a kriged interpolation map was deeply influenced by stochastic factors such as city pollution, land use pattern and crop distributions. For example, the average content of Zn in vegetable garden soils was 2.5-4 times as much as in their originated soils, and was lower in paddy soils than in their originated soils. The areas with a higher content of soil DTPA-extractable zinc appeared in the near suburbs and the riverside along Hunhe River and the wastewater drainage of Xihe River, and the extremely high values in the near suburb of the city's residential area were a striking feature, indicating the key role of city pollution in the spatial heterogeneity of soil DTPA-extractable zinc. When recorded in the form of a soil pollution map, the results of such a survey make it possible to identify the unusually polluted areas, and to provide more information for precise agriculture and environmental control.

Agriculture↗
PubMed1997

Neglect as a deficit determined by an imbalance between multiple spatial representations.

A previous study on neglect suggested that at least two hand parameters are crucial in producing an amelioration of neglect: the hand (left or right) and the spatial position of the hand (left or right). The improvement observed in perceiving left targets when the left hand acts in the left space can be due either to proprioceptive or to visual cuing. The stimulated left hand located in the left space may act as a powerful visual cue for the enhancement of the left visuo-spatial representation, in the same way as any other visual stimulus presented in the periphery of the visual field. Alternatively, it may be that the perceived hand location (due to the activation of the proprioceptive system) acts as an attentional field able to enhance the representation of the left space. In order to disentangle these two hypotheses, in the present study a naming task was executed by a group of neglect patients and by a control group. The subjects had to name all the objects depicted on a sheet of paper which were reflected on a mirror that inverted right and left space. While doing the naming performance, the subjects passively moved either the right or the left hand, in the left or right space. Stimuli and hand were reflected in the mirror that inverted right and left space and direct view of the stimuli and of the stimulated hand was prevented by a board. The results show that patients were more accurate at naming stimuli reflected in the left side of the mirror when the left hand was located and moved on the left side. In this condition, however, the left hand was seen in the right side of the mirror. It is therefore clear that the better performance was not due to visuo-spatial cuing but to a proprioceptive cuing effect. The results are discussed in terms of the relevance of personal and peripersonal spatial activation in the modulation of extrapersonal visual neglect. The coactivation of different spatial representations seems to be very influential on stimulus coding, thus confirming that spatial awareness is strictly related to the joint activity of multiple brain maps.

Aged↗
PubMed2005

Combined D1/D2 receptor stimulation under conditions of dopamine depletion impairs spatial working memory performance in humans.

RATIONALE: The mesocortical dopamine system is regarded as an important modulator of working memory. While it has been established that stimulation of the D1/D2 receptor in primates can improve spatial working memory performance, findings in humans are less consistent. Recent studies in humans suggest that global depletion of dopamine via tyrosine/phenylalanine depletion may impair spatial working memory performance, although these results are also inconsistent, and it has been suggested that task differences may partly underlie the inconsistent findings. OBJECTIVES: This study had two aims: (1) to investigate the effects of acute tyrosine depletion (TPD) on a number of working memory tasks and (2) to examine whether stimulation of D1/D2 receptors under conditions of TPD can attenuate or "reverse" TPD-induced working memory impairments. METHODS: Eighteen healthy male participants performed a spatial working memory delayed-recognition task, non-spatial working memory task and spatial n-back task on three separate occasions, after TPD, TPD and pergolide (D1/D2 agonist), and placebo. RESULTS: TPD did not impair working memory performance on any of the tasks administered. However, stimulation of D1/D2 receptors under TPD conditions caused a subtle impairment in spatial working memory performance. CONCLUSIONS: The finding that D1/D2 stimulation under TPD conditions impairs working memory highlights the complexity of functional effects of augmenting dopaminergic transmission within a dopamine-depleted state. The lack of TPD-related effects on a range of working memory tasks questions the reliability of TPD as a modulator of dopamine function and working memory performance in humans.

Adolescent↗
PubMed2002

Exploring the syndrome of spatial unilateral neglect through an illusion of length.

Both a neuropsychological syndrome (unilateral spatial neglect) and a visual illusion of length (the Brentano version of the Müller-Lyer illusion) bring about a misjudgement of the subjective centre of a horizontal line, with a unilateral shift. In experiment 1 we investigated, in patients with left unilateral neglect, illusory effects of horizontal length, with the aim of exploring the functional and neural basis of horizontal space perception, and the role of visual processing in shaping the patients' bisection performance. Fourteen right-brain-damaged patients with left spatial unilateral neglect, seven with and seven without left visual half-field deficits (assessed by confrontation, perimetry, and visual event-related potentials), entered this study. Two conditions of manual line bisection were assessed: setting the mid-point of a horizontal line, and of the shaft of the Brentano-Müller-Lyer illusion, with either a left- or a right-sided expansion. Both groups of patients set the subjective midpoint to the right of the objective centre of the line, consistent with the presence of left neglect. Patients with neglect and left hemianopia showed no illusory effects and a greater bisection error. The effects of the illusion, by contrast, were fully present in neglect patients without hemianopia, in both illusory conditions, adding to, or subtracting from, the rightward bisection bias. Anatomoclinical correlations revealed an association of damage to the occipital regions with the lack of illusory effects. Conversely, more anterior damage, sparing these regions, did not disrupt the illusion, revealing a dissociation between visual and spatial processing of extension. These findings suggest that processing of the Müller-Lyer illusion of length is likely to occur in the occipital cortex, at a retinotopic level of representation. In neglect patients with left homonymous hemianopia the visual deficit adds to the spatial bias, yielding a greater error in line bisection, but not in other visual exploratory tasks, such as cancellation, where the contribution of retinotopic frames is likely to be comparatively minor. Experiment 2 showed preserved illusory effects in patients with homonymous visual field defects without spatial unilateral neglect, suggesting that preserved spatial processing may compensate for unilateral visual field defects.

Adult↗
PubMed2002

Temporal information for spatially constrained locomotion.

How is locomotion adapted to spatial environmental constraints? The control of this everyday behavior is claimed to be based on information that specifies either spatial or temporal properties of the actor-environment system. Although studies on open-loop locomotor pointing (i.e., the positioning of a foot on a target on the floor while walking) agree on the use of spatial information, studies on closed-loop locomotor pointing propose the use of temporal information. Here, we test the hypothesis of closed-loop locomotor pointing based on temporal information, by dissociating spatial and temporal information in a virtual reality setup (virtual environment connected to a treadmill). The results support this hypothesis and shed some light on the type of temporal information that is used. The performed dissociation between spatial and temporal information, however, does not rule out a control based on a continuous updating of spatial information. Therefore, our conclusion on the use of temporal information was moderated.

Adaptation, Physiological↗