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At least 1,207 records · Page 67Linked to original sources
PubMed1997

Scale selection for second-order (non-linear) stereopsis.

In addition to the conventional luminance spatial frequency-dependent, disparity processing mode, there is a second-order luminance spatial frequency-independent type of processing available to the stereoscopic system. Here we use gaussian-enveloped, amplitude-modulated grating patches to determine how the stereoscopic system responds to the presence of two sources of second-order disparity information at different scales when there is no disparity information available via the conventional luminance-based system. In the first experiment we show that the stereoscopic system uses the disparity signal provided by the stimulus envelope, even though it is at a coarser scale than that provided by the amplitude modulation (AM). We then demonstrate that if the stimulus envelope is degraded via blurring, or if it is fixed at zero disparity, then performance depends on the finer-scale AM disparity signal. To show that the stereoscopic system uses the disparity signal provided by the AM we extend the carrier grating outside the borders of the AM stimulus, thereby making the boundary of the patch less discernible. Results obtained using this stimulus suggest that when two sources of second-order disparity information are present within the same stimulus (i.e., with no reliable luminance-based disparity signal available), the disparity signal provided by the coarser-scale contrast envelope vetos the finer-scale disparity signal. The coarse-scale disparity information dominates as long at it provides an adequate disparity signal. When it is degraded, however, the finer-scale signal takes precedence.

Depth Perception↗
PubMed2001

Spatial population structure in a patchily distributed beetle.

The dynamics and evolution of populations will critically depend on their spatial structure. Hence, a recent emphasis on one particular type of structure--the metapopulation concept of Levins--can only be justified by empirical assessment of spatial population structures in a wide range of organisms. This paper focuses on Aphodius fossor, a dung beetle specialized on cattle pastures. An agricultural database was used to locate nearly 50 000 local populations of A. fossor in Finland. Several independent methods were then used to quantify key processes in this vast population system. Allozyme markers and mitochondrial DNA (mtDNA) sequences were applied to examine genetic differentiation of local populations and to derive indirect estimates of gene flow. These estimates were compared to values expected on the basis of direct observations of dispersing individuals and assessments of local effective population size. Molecular markers revealed striking genetic homogeneity in A. fossor. Differentiation was only evident in mtDNA haplotype frequencies between the isolated Aland islands and the Finnish mainland. Thus, indirect estimates of gene flow agreed with direct observations that local effective population size in A. fossor is large (hundreds of individuals), and that in each generation, a substantial fraction (approximately one-fifth) of the individuals move between populations. Large local population size, extreme haplotype diversity and a high regional incidence of A. fossor all testify against recurrent population turnover. Taken together, these results provide strong evidence that the whole mainland population of A. fossor is better described as one large 'patchy population', with substantial movement between relatively persistent local populations, than as a classical metapopulation.

Animals↗
PubMed2004

Spatial interactions within modular organisms: genetic heterogeneity and organism fitness.

Modular organisms are composed of iterated units of construction that vary in their spatial arrangement. This variation is expected to affect the fitness of modular organisms due to interactions among neighboring modules and the potential for such organisms to be genetically heterogeneous. We devise a spatially explicit model to investigate how spatial interactions among neighboring modules affect organism fitness. We show that fitness is strongly dependent on the spatial arrangement of modules in both genetically homogeneous and heterogeneous organisms, and that the magnitude of the variation is dependent on the strength of interactions among modules. Organism fitness is more variable with interactions among modules that are symmetrical (each affects each other in the same directions) than with asymmetrical interactions (neighbors affect each other in different directions). We conclude by discussing potential extension of the present framework to a general dynamic model of spatially structured organism development.

Adaptation, Physiological↗
PubMed1990

Orientation bias of neurons in the lateral geniculate nucleus of macaque monkeys.

The purpose of this investigation was to analyze the influence of stimulus orientation on the responses of individual neurons in the monkey's lateral geniculate nucleus (LGN). Our specific goals were to assess the prevalence and the degree of orientation tuning in the monkey LGN and to determine if the preferred stimulus orientations of LGN neurons varied as a function of receptive-field position. The primary motivation for this research was to gain insight into the receptive-field configuration of LGN neurons and consequently into the neural mechanisms which determine the spatial organization of LGN receptive fields in primates. In both the parvocellular and magnocellular layers, the responses of the majority of individual neurons to sine-wave gratings varied as a function of stimulus orientation. The influence of stimulus orientation was, however, highly dependent on the spatial characteristics of the stimulus; the greatest degree of orientation bias was observed for spatial frequencies higher than the cell's optimal spatial frequency. On a population basis, the degree of orientation bias was similar for all major classes of LGN neurons (e.g. ON vs. OFF center; parvocellular vs. magnocellular) and did not vary systematically with receptive-field eccentricity. At a given receptive-field location, LGN neurons, particularly cells in the parvocellular laminae, tended to prefer either radially oriented stimuli or stimuli oriented more horizontally than their polar axis. Our analyses of the orientation-dependent changes in spatial-frequency response functions, which was based on the Soodak et al., (1987; Soodak, 1986) two-dimensional, difference-of-Gaussian receptive-field model, suggested that the orientation bias in LGN neurons was due to an elongation of the receptive-field center mechanism which in some cases appeared to consist of multiple subunits. Direct comparisons of the orientation-tuning characteristics of LGN cells and their retinal inputs (S potentials) indicated that the orientation bias in the monkey LGN reflects primarily the functional properties of individual retinal ganglion cells. We conclude that orientation sensitivity is a significant property of subcortical neurons in the primate's geniculo-cortical pathway.

Action Potentials↗
PubMed2006

Spatial stochastic modelling of the phosphoenolpyruvate-dependent phosphotransferase (PTS) pathway in Escherichia coli.

MOTIVATION: Many biochemical networks involve reactions localized on the cell membrane. This can give rise to spatial gradients of the concentration of cytosolic species. Moreover, the number of membrane molecules can be small and stochastic effects can become relevant. Pathways usually consist of a complex interaction network and are characterized by a large set of parameters. The inclusion of spatial and stochastic effects is a major challenge in developing quantitative and dynamic models of pathways. RESULTS: We have developed a particle-based spatial stochastic method (GMP) to simulate biochemical networks in space, including fluctuations from the diffusion of particles and reactions. Gradients emerging from membrane reactions can be resolved. As case studies for the GMP method we used a simple gene expression system and the phosphoenolpyruvate:glucose phosphotransferase system pathway. AVAILABILITY: The source code for the GMP method is available at http://www.science.uva.nl/research/scs/CellMath/GMP.

Computer Simulation↗
PubMed1996

Spatial pooling of contrast in contrast gain control.

We report psychophysical measurements of spatial pooling functions for contrast gain control. We use a nulling technique to measure the dependence of contrast induction on the spatial frequency of a sinusoidal contrast modulation. This dependence on spatial frequency, when transformed, provides the profile of a spatial pooling function. The measured profiles are fitted well by Gaussians. We confirm earlier results that the area over which spatial pooling takes place depends on the scale of the modulated pattern. We also find that pooling functions are similar for achromatic and isoluminant stimuli.

Contrast Sensitivity↗
PubMed2005

Spatial analysis of spike-timing-dependent LTP and LTD in the CA1 area of hippocampal slices using optical imaging.

Spike-timing-dependent long-term potentiation (LTP) and long-term depression (LTD) were investigated in the CA1 area of hippocampal slices using optical imaging. A pair of electrical pulses were used to stimulate the Schaffer-commissural collateral and the stratum oriens with various sets of relative timing (tau) between the two stimuli. These sets of paired pulses gave rise to LTP/LTD, whose induction was closely related to tau, and their profiles were classified into two types depending on their layer specific location along the dendrite. One was characterized by a symmetric time window observed in the proximal region of the stratum radiatum (SR) and the other by an asymmetric time window in the distal region of the SR. Bath application of bicuculline (gamma-aminobutyric acid [GABA] receptor antagonist) to hippocampal slices revealed that GABAergic interneuron projections were responsible for the symmetry of a time window.

Action Potentials↗
PubMed2006

Asymmetry of the amplitude-time properties of directed saccades in monkeys depending on the complexity of the spatial scheme of visual stimulation.

Three Macaca rhesus monkeys were used for studies of the performance of visually evoked saccades in single-step changes in the position of a stimulus using standard schemes for presentation of GAP-OVERLAP stimuli. Two spatial schemes were used: presentation of stimuli along the horizontal meridian (one-dimensional) and presentation of stimuli within a rectangular area of the visual field (two-dimensional). Asymmetrical foci of short-and long-latency saccades were found in the visual field. Dispersion factor analysis demonstrated that the dimensionality factor (one-dimensional versus two-dimensional stimulation schemes) had greater effects on the latent period of saccades than the lateralization factor (presentation on the left or right sides of the gaze point). The precision of the performance of visually evoked saccades decreased with increases in its eccentricity in both spatial stimulation schemes.

Animals↗
PubMed1992

Genotype-dependent involvement of limbic areas in spatial learning and postlesion recovery.

Male C57BL/6 (C57) and DBA/2 (DBA) mice with hippocampal, amygdaloid, or sham lesions were tested in a radial eight-arm maze 1 or 4 weeks after surgery. The results show that the effect of the lesions varied according to the performance level of the strain considered. In the high-learner C57 strain, the two lesions impaired acquisition at both postlesion intervals. Conversely, in the low-learner DBA strain, only hippocampal lesions impaired acquisition 1 week but not 4 weeks after lesioning. It is hypothesized that if more limbic areas are involved in controlling spatial learning in C57 mice, these structures could be processing distinct but complementary memory attributes, thus contributing to a high baseline performance. This, however, also entails an increased sensitivity of C57 performance to brain damage with reduced possibilities of long-term recovery.

Amygdala↗
PubMed1993

NMDA receptors modulate long-term habituation to spatial novelty: dose- and genotype-dependent differential effects of posttrial MK-801 and CPP in rats.

To investigate the role N-methyl-D-aspartate (NMDA) receptors play in behavioral plasticity, adult male rats of the Naples high-(NHE) and low-excitability (NLE) lines, and of a random-bred Sprague-Dawley strain (NRB) received, the noncompetitive (MK-801:0.01 or 2.5 mg/kg) or the competitive (CPP: 0.01 or 5 mg/kg) NMDA receptor antagonists, or vehicle IP soon after a 10-min test in a Làt-maze. Retention was tested 1 week later. Habituation of activity and defecation score was monitored by the between-test decrement (LTH) in the frequency of corner-crossings (HA) and rearings (VA), with prevailing cognitive and noncognitive meaning, respectively, and of fecal boli. (i) In the NLE-rats, low and high doses of MK-801 facilitate LTH of HA, and a high dose of CPP facilitates LTH of HA. (ii) In the NRB-rats, MK-801 facilitates LTH of HA at a low dose and inhibits LTH of VA at a high dose, whereas CPP inhibits LTH of HA at a high dose only. In contrast, (iii) in the NHE-rats, high doses of MK-801 impair LTH of HA, and low doses of CPP facilitate LTH of HA. In conclusion, the dose- and genotype-dependent differential effects of allosteric and isosteric receptor blockade support the hypothesized modulatory role of NMDA receptors in behavioral plasticity; and the dissociation between retention of cognitive and noncognitive behavioral components suggests that NMDA receptors are involved in their parallel processing.

Animals↗
PubMed2005

The homeodomain transcription factor Irx5 establishes the mouse cardiac ventricular repolarization gradient.

Rhythmic cardiac contractions depend on the organized propagation of depolarizing and repolarizing wavefronts. Repolarization is spatially heterogeneous and depends largely on gradients of potassium currents. Gradient disruption in heart disease may underlie susceptibility to fatal arrhythmias, but it is not known how this gradient is established. We show that, in mice lacking the homeodomain transcription factor Irx5, the cardiac repolarization gradient is abolished due to increased Kv4.2 potassium-channel expression in endocardial myocardium, resulting in a selective increase of the major cardiac repolarization current, I(to,f), and increased susceptibility to arrhythmias. Myocardial Irx5 is expressed in a gradient opposite that of Kv4.2, and Irx5 represses Kv4.2 expression by recruiting mBop, a cardiac transcriptional repressor. Thus, an Irx5 repressor gradient negatively regulates potassium-channel-gene expression in the heart, forming an inverse I(to,f) gradient that ensures coordinated cardiac repolarization while also preventing arrhythmias.

Action Potentials↗
PubMed2005

Human sound localization at near-threshold levels.

Physiological studies of spatial hearing show that the spatial receptive fields of cortical neurons typically are narrow at near-threshold levels, broadening at moderate levels. The apparent loss of neuronal spatial selectivity at increasing sound levels conflicts with the accurate performance of human subjects localizing at moderate sound levels. In the present study, human sound localization was evaluated across a wide range of sensation levels, extending down to the detection threshold. Listeners reported whether they heard each target sound and, if the target was audible, turned their heads to face the apparent source direction. Head orientation was tracked electromagnetically. At near-threshold levels, the lateral (left/right) components of responses were highly variable and slightly biased towards the midline, and front vertical components consistently exhibited a strong bias towards the horizontal plane. Stimulus levels were specified relative to the detection threshold for a front-positioned source, so low-level rear targets often were inaudible. As the sound level increased, first lateral and then vertical localization neared asymptotic levels. The improvement of localization over a range of increasing levels, in which neural spatial receptive fields presumably are broadening, indicates that sound localization does not depend on narrow spatial receptive fields of cortical neurons.

Adult↗
PubMed1999

Facilitation and inhibition arising from the exogenous orienting of covert attention depends on the temporal properties of spatial cues and targets.

On the covert orienting of visual attention task (COVAT), responses to targets appearing at the location indicated by a non-predictive spatial cue are faster than responses to targets appearing at uncued locations when stimulus onset asynchrony (SOA) is less than approximately 200 ms. For longer SOAs, this pattern reverses and RTs to targets appearing at uncued locations become faster than RTs to targets appearing at the cued location. This facilitation followed by inhibition has been termed the biphasic effect of non-predictive peripheral spatial cues. Currently, there is debate about whether these two processes are independent. This issue was addressed in a series of experiments where the temporal overlap between the peripheral cue and target was manipulated at both short and long SOAs. Results showed that facilitation was present only when the SOA was short and there was temporal overlap between cue and target. Conversely, inhibition occurred only when the SOA was long and there was no temporal overlap between cue and target. The biphasic effect, with an early facilitation followed by a later inhibition, occurred only when the cue duration was fixed such that there was temporal overlap between the cue and target at short but not long SOAs. In a final experiment, the duration of targets the temporal overlap between cue and target and the SOA were manipulated factorially. The results showed that facilitation occurred only when the SOA was short, there was temporal overlap between cue and target and the target remained visible until the subject responded. These results suggest that the facilitation and inhibition found on COVATs which use non-informative peripheral cues are independent processes and their presence and magnitude is related to the temporal properties of cues and targets.

Adult↗
PubMed2002

Characterization of soil organic matter content of two sample size populations along a climatic transect.

Soil organic matter (SOM) content was determined in two populations of soil samples that were taken from 0-2 soil depth. One population represented soil samples that were taken from a square of 25 cm2 in size (small-S population) and the other population represented soil samples that were taken from a square of 2500 cm2 in size (large-L population). The samples were collected on hillslopes in different climatic regions: Mediterranean (GIV), semi-arid (MAL), mildly-arid (MIS) and arid (KAL). The results of both S and L populations showed decreasing SOM mean and variance from the Mediterranean site to the arid site. Statistical and spatial characteristics of each population were compared between the climatic regions. In addition, comparison between the two populations was made for each site. The difference in sample size did not significantly affect the mean values of SOM of the two populations in sites GIV, MAL and KAL, but did affect the mean at site MIS. At all study sites, except for site MAL, the variance increased with decreasing sample size. At sites GIV and KAL the coefficient of variation of S population was higher (more than 1.5 times) than that of L population, whereas at sites MAL and MIS, the differences were negligible. The relationships between the values of S and L samples at the individual sampling points defined the background of the study sites, which reflects the effect of vegetation (type), grazing, biological crust and soil properties. It was found that at the extreme sites GIV and KAL the background was characterized by relatively low SOM content with small areas of high organic matter content. At site MIS the background was characterized by relatively high SOM with small areas of low organic matter content. At site MAL the background was not dominated by high values of SOM nor by low ones. The spatial pattern of L population became more simple with increasing aridity. At the relatively wet sites the spatial pattern did not depend on the sample size while in the more arid sites it was sample size dependent. It was indicated that the spatial structure of SOM at the semi-arid and mildly arid sites is anisotropic whereas at the Mediterranean and arid sites it is isotropic.

Climate↗