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Results for “spatial dependency”

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At least 757 records · Page 42Linked to original sources

PubMed2004

Noise reduction in MR angiography with nonlinear anisotropic filtering.

PURPOSE: To evaluate three-dimensional nonlinear anisotropic filtering in suppressing image noise in high spatial resolution magnetic resonance angiograms (MRA) acquired with hybrid undersampled projection reconstruction and phase contrast vastly undersampled isotropic projection reconstruction (PC-VIPR). MATERIALS AND METHODS: Three-dimensional nonlinear anisotropic filtering was quantitatively analyzed and evaluated through the measurement of contrast to noise ratio (CNR) in PC-VIPR images and contrast enhanced peripheral MRA images. To filter MRA images with ultra-high spatial resolution and poor CNR, a spatial frequency dependent nonlinear anisotropic filtering algorithm was proposed that uses two-step processing to filter the whole spatial frequency data. RESULTS: Three-dimensional nonlinear anisotropic filtering was shown to be effective in suppressing noise and improving CNR in MRA with isotropic spatial resolution. Higher CNR was achieved using spatial frequency dependent nonlinear anisotropic filtering. A typical CNR gain of between 50-100% was shown in our studies. CONCLUSION: Three-dimensional nonlinear anisotropic filtering significantly improved CNR in MRA images with isotropic spatial resolution. Spatial frequency dependent nonlinear anisotropic filtering further improved CNR for MRA images with ultra-high spatial resolution and low CNR.

Anisotropy↗

PubMed2004

Spatially explicit analyses unveil density dependence.

Density-dependent processes are fundamental in the understanding of species population dynamics. Whereas the benefits of considering the spatial dimension in population biology are widely acknowledged, the implications of doing so for the statistical detection of spatial density dependence have not been examined. The outcome of traditional tests may therefore differ from those that include ecologically relevant locational information on both the prey species and natural enemy. Here, we explicitly incorporate spatial information on individual counts when testing for density dependence between an insect herbivore and its parasitoids. The spatially explicit approach used identified significant density dependence more frequently and in different instances than traditional methods. The form of density dependence detected also differed between methods. These results demonstrate that the explicit consideration of patch location in density-dependence analyses is likely to significantly alter current understanding of the prevalence and form of spatial density dependence in natural populations.

Animals↗

PubMed2002

Inhibitory effect of AP-1 complex on 5-aminolevulinate synthase gene expression through sequestration of cAMP-response element protein (CRE)-binding protein (CBP) coactivator.

Activation protein-1 (AP-1) transcription factors are early response genes involved in a diverse set of transcriptional regulatory processes. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is often used to induce AP-1 activity. The purpose of this work was to explore the molecular mechanisms involved in the TPA regulation of ubiquitous 5-aminolevulinate synthase (ALAS) gene expression, the first and rate-controlling step of the heme biosynthesis. Previous analysis of the 5'-flanking sequence of ALAS revealed the existence of two cAMP-response elements (CRE) required for basal and cAMP-stimulated expression. The fragment -833 to +42 in the 5'-flanking region of rat ALAS gene was subcloned into a chloramphenicol acetyltransferase (CAT) reporter vector. The expression vector pALAS/CAT produced a significant CAT activity in transiently transfected HepG2 human hepatoma cells, which was repressed by TPA. Sequence and deletion analysis detected a TPA response element (TRE), located between -261 and -255 (TRE-ALAS), that was critical for TPA regulation. We demonstrated that c-Fos, c-Jun, and JunD are involved in TPA inhibitory effect due to their ability to bind TRE-ALAS, evidenced by supershift analysis and their capacity to repress promoter activity in transfection assays. Repression of ALAS promoter activity by TPA treatment or Fos/Jun overexpression was largely relieved when CRE protein-binding protein or p300 was ectopically expressed. When the TRE site was placed in a different context with respect to CRE sites, it appeared to act as a transcriptional enhancer. We propose that the decrease in ALAS basal activity observed in the presence of TPA may reflect a lower ability of this promoter to assemble the productive pre-initiation complex due to CRE protein-binding protein sequestration. We also suggest that the transcriptional properties of this AP-1 site would depend on a spatial-disposition-dependent manner with respect to the CRE sites and to the transcription initiation site.

5-Aminolevulinate Synthetase↗

PubMed2004

Crowding is unlike ordinary masking: distinguishing feature integration from detection.

A letter in the peripheral visual field is much harder to identify in the presence of nearby letters. This is "crowding." Both crowding and ordinary masking are special cases of "masking," which, in general, refers to any effect of a "mask" pattern on the discriminability of a signal. Here we characterize crowding, and propose a diagnostic test to distinguish it from ordinary masking. In ordinary masking, the signal disappears. In crowding, it remains visible, but is ambiguous, jumbled with its neighbors. Masks are usually effective only if they overlap the signal, but the crowding effect extends over a large region. The width of that region is proportional to signal eccentricity from the fovea and independent of signal size, mask size, mask contrast, signal and mask font, and number of masks. At 4 deg eccentricity, the threshold contrast for identification of a 0.32 deg signal letter is elevated (up to six-fold) by mask letters anywhere in a 2.3 deg region, 7 times wider than the signal. In ordinary masking, threshold contrast rises as a power function of mask contrast, with a shallow log-log slope of 0.5 to 1, whereas, in crowding, threshold is a sigmoidal function of mask contrast, with a steep log-log slope of 2 at close spacing. Most remarkably, although the threshold elevation decreases exponentially with spacing, the threshold and saturation contrasts of crowding are independent of spacing. Finally, ordinary masking is similar for detection and identification, but crowding occurs only for identification, not detection. More precisely, crowding occurs only in tasks that cannot be done based on a single detection by coarsely coded feature detectors. These results (and observers' introspections) suggest that ordinary masking blocks feature detection, so the signal disappears, while crowding (like "illusory conjunction") is excessive feature integration - detected features are integrated over an inappropriately large area because there are no smaller integration fields - so the integrated signal is ambiguous, jumbled with the mask. In illusory conjunction, observers see an object that is not there made up of features that are. A survey of the illusory conjunction literature finds that most of the illusory conjunction results are consistent with the spatial crowding described here, which depends on spatial proximity, independent of time pressure. The rest seem to arise through a distinct phenomenon that one might call "temporal crowding," which depends on time pressure ("overloading attention"), independent of spatial proximity.

Contrast Sensitivity↗

PubMed1996

Masking of spatial frequency in visual memory depends on distal, not retinal, frequency.

Spatial frequency discrimination thresholds measured in a two-interval forced choice paradigm are virtually constant across inter-stimulus intervals ranging from 400 to 30,000 msec, demonstrating that an accurate representation of spatial frequency is maintained in short-term memory. This representation can be degraded by briefly flashing a grating during the retention interval. Moreover, this memory masking effect varies with the spatial frequency of the mask, suggesting that the mechanisms used to store spatial frequency in memory are similar to low-level visual filters. In this paper we replicate those previous findings and extend them by showing (1) that accurate memory for spatial frequency lasts as long as 1 min; (2) that memory masking is based on distal (c/cm), not retinal (c/deg), spatial frequency.

Differential Threshold↗

PubMed1996

The essential role of hippocampal CA1 NMDA receptor-dependent synaptic plasticity in spatial memory.

We have produced a mouse strain in which the deletion of the NMDAR1 gene is restricted to the CA1 pyramidal cells of the hippocampus by using a new and general method that allows CA1-restricted gene knockout. The mutant mice grow into adulthood without obvious abnormalities. Adult mice lack NMDA receptor-mediated synaptic currents and long-term potentiation in the CA1 synapses and exhibit impaired spatial memory but unimpaired nonspatial learning. Our results strongly suggest that activity-dependent modifications of CA1 synapses, mediated by NMDA receptors, play an essential role in the acquisition of spatial memories.

Animals↗

PubMed2002

Spatial frequency, phase, and the contrast of natural images.

We examined contrast sensitivity and suprathreshold apparent contrast with natural images. The spatial-frequency components within single octaves of the images were removed (notch filtered), their phases were randomized, or the polarity of the images was inverted. Of Michelson contrast, root-mean-square (RMS) contrast, and band-limited contrast, RMS contrast was the best index of detectability. Negative images had lower apparent contrast than their positives. Contrast detection thresholds showed spatial-frequency-dependent elevation following both notch filtering and phase randomization. The peak of the spatial-frequency tuning function was approximately 0.5-2 cycles per degree (c/deg). Suprathreshold contrast matching functions also showed spatial-frequency-dependent contrast loss for both notch-filtered and phase-randomized images. The peak of the spatial-frequency tuning function was approximately 1-3 c/deg. There was no detectable difference between the effects of phase randomization and notch filtering on contrast sensitivity. We argue that these observations are consistent with changes in the activity within spatial-frequency channels caused by the higher-order phase structure of natural images that is responsible for the presence of edges and specularities.

Contrast Sensitivity↗

PubMed2001

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe.

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO2 tip on a Si cantilever was successfully fabricated by selective etching of the SiO2 tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10-100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.

Journal Article↗

PubMed1996

No prevalence of right-left over top-bottom spatial codes.

Reaction time is known to depend on spatial stimulus-response compatibility in both the horizontal and the vertical dimensions. However, if both dimensions are varied in the same task, horizontal but not vertical compatibility affects performance, even if subjects are instructed to attend to the vertical dimension only (Nicoletti & Umiltà, 1984). Experiment 1 compared the effect of horizontal and vertical instructions in a task with left-versus right-handed key responses placed at a top versus bottom location. A horizontal-prevalence effect was observed only with a horizontal, and not with a vertical, instruction. This suggests that subjects might not have heeded the vertical instruction in the Nicoletti and Umiltà study but instead attended to the horizontal dimension. Experiment 2 did not yield any horizontal prevalence with one-handed responses (joystick movements). This indicates that top-bottom codes become ineffective only if the responses suggest an exclusively horizontal response coding. In sum, results demonstrated multiple spatial coding of stimuli and responses under appropriate conditions and suggest that the right-left codes do not dominate the top-bottom spatial codes.

Adult↗

PubMed1999

Discourse deficits following right hemisphere damage in deaf signers.

Previous findings have demonstrated that hemispheric organization in deaf users of American Sign Language (ASL) parallels that of the hearing population, with the left hemisphere showing dominance for grammatical linguistic functions and the right hemisphere showing specialization for non-linguistic spatial functions. The present study addresses two further questions: first, do extra-grammatical discourse functions in deaf signers show the same right-hemisphere dominance observed for discourse functions in hearing subjects; and second, do discourse functions in ASL that employ spatial relations depend upon more general intact spatial cognitive abilities? We report findings from two right-hemisphere damaged deaf signers, both of whom show disruption of discourse functions in absence of any disruption of grammatical functions. The exact nature of the disruption differs for the two subjects, however. Subject AR shows difficulty in maintaining topical coherence, while SJ shows difficulty in employing spatial discourse devices. Further, the two subjects are equally impaired on non-linguistic spatial tasks, indicating that spared spatial discourse functions can occur even when more general spatial cognition is disrupted. We conclude that, as in the hearing population, discourse functions involve the right hemisphere; that distinct discourse functions can be dissociated from one another in ASL; and that brain organization for linguistic spatial devices is driven by its functional role in language processing, rather than by its surface, spatial characteristics.

Aged↗

PubMed2000

Characterisation of the signal and noise transfer of CCD cameras for electron detection.

Methods to characterise the performance of CCD cameras for electron detection are investigated with particular emphasis on the difference between the transfer of signal and noise. Similar to the Modulation Transfer Function MTF, which describes the spatial frequency dependent attenuation of contrast in the image, we introduce a Noise Transfer Function NTF that describes the transfer of the Poisson noise that is inevitably present in any electron image. A general model for signal and noise transfer by an image converter is provided. This allows the calculation of MTF and NTF from Monte-Carlo simulations of the trajectories of electrons and photons in the scintillator and the optical coupling of the camera. Furthermore, accurate methods to measure the modulation and noise transfer functions experimentally are presented. The spatial-frequency dependent Detection Quantum Efficiency DQE, an important figure of merit of the camera which has so far not been measured experimentally, can be obtained from the measured MTF and NTF. The experimental results are in good agreement with the simulations and show that the NTF at high spatial frequencies is in some cases by a factor of four higher than the MTF. This implies that the noise method, which is frequently used to measure the MTF, but in fact measures the NTF, gives over-optimistic results. Furthermore, the spatial frequency dependent DQE is lower than previously assumed.

Algorithms↗

PubMed1996

Programmable dynamic line phantom control: an initial assessment of quantitative potential as a gamma camera test tool.

The dynamic line phantom (DLP) produces various test patterns for the qualitative assessment of gamma camera performance, by stepper motor control of the translatory movement of a radioactive rigid line source. The addition of a V24 serial interface port has allowed external control via an RS-232 connection to a personal computer. Programmability has enabled the DLP to be used as a quantitative test tool. One such program produces a variable contrast 'hot' and 'cold' bar pattern. The addition of a Quick Basic computer program modifies the line source movement to compensate for variation in line source activity. This maintains a test pattern with fixed, absolute contrast levels, giving the potential for the measurement of variations in long-term imaging performance. A modulation transfer function (MTF) pattern has been designed which encompasses a sinusoidally varying intensifying function with a changing spatial frequency component. Repeated measurements on two gamma cameras demonstrated poor reproducibility (CV of up to 50% dependent upon spatial frequency) and significant differences between the MTF obtained from line spread function measurements (percentage deviation of up to 23%, dependent upon spatial frequency). Neither test pattern has been adopted for routine quantitative assessment of gamma camera imaging characteristics. The reproducibility of results is poor (dominated by counting statistic limitations), making the techniques insensitive to small changes in camera performance.

Gamma Cameras↗

PubMed1988

Large time behavior in a nonlinear age-dependent population dynamics problem with spatial diffusion.

In this work we analyze the large time behavior in a nonlinear model of population dynamics with age-dependence and spatial diffusion. We show that when t----+ infinity either the solution of our problem goes to 0 or it stabilizes to a nontrivial stationary solution. We give two typical examples where the stationary solutions can be evaluated upon solving very simple partial differential equations. As a by-product of the extinction case we find a necessary condition for a nontrivial periodic solution to exist. Numerical computations not described below show a rapid stabilization.

Aging↗

PubMed2005

Scale-dependent feedback and regular spatial patterns in young mussel beds.

In the past decade, theoretical ecologists have emphasized that local interactions between predators and prey may invoke emergent spatial patterning at larger spatial scales. However, empirical evidence for the occurrence of emergent spatial patterning is scarce, which questions the relevance of the proposed mechanisms to ecological theory. We report on regular spatial patterns in young mussel beds on soft sediments in the Wadden Sea. We propose that scale-dependent feedback, resulting from short-range facilitation by mutual protection from waves and currents and long-range competition for algae, induces spatial self-organization, thereby providing a possible explanation for the observed patterning. The emergent self-organization affects the functioning of mussel bed ecosystems by enhancing productivity and resilience against disturbance. Moreover, self-organization allows mussels to persist at algal concentrations that would not permit survival of mussels in a homogeneous bed. Our results emphasize the importance of self-organization in affecting the emergent properties of natural systems at larger spatial scales.

Animals↗

PubMed2003

Dependency of energy and spatial distributions of photons on edge of object in brain SPECT.

OBJECTIVES: Accurate mu maps are important for quantitative image reconstruction in SPECT. The Compton scatter energy window (CSW) technique has been proposed to define the outline of objects. In this technique, a lower energy window image is acquired in addition to the main photo-peak energy window. The image of the lower energy window is used to estimate the edge of the scanned object to produce a constant attenuation map. The aim of this study was to investigate the dependency of CSW on the spatial and energy distribution of radioisotope to predict the edges of objects. METHODS: Two particular cases of brain study were considered, namely uniform distribution and non-uniform distribution using Monte Carlo simulation and experiments with uniform cylindrical phantom and hotspot phantom. The phantoms were filled with water and a radioactive solution of 99mTc. For each phantom, 20%, 30%, 40% and 50% thresholds of the mean profile were applied to estimate Ewt, the energy window for minimum difference between the estimated and true edge of objects. RESULTS: The Ewt's were 100-120 keV with a 40% threshold and 92-114 keV with a 30% threshold for uniform and hotspot phantoms, respectively. CONCLUSIONS: Edge of the objects with CSW technique varies with energy window and thresholds. Careful setting of the energy window is required to use the CSW technique.

Algorithms↗

PubMed2003

The neural representation of speed in macaque area MT/V5.

Tuning for speed is one key feature of motion-selective neurons in the middle temporal visual area of the macaque cortex (MT, or V5). The present paper asks whether speed is coded in a way that is invariant to the shape of the moving stimulus, and if so, how. When tested with single sine-wave gratings of different spatial and temporal frequencies, MT neurons show a continuum in the degree to which preferred speed depends on spatial frequency. There is some dependence in 75% of MT neurons, and the other 25% maintain speed tuning despite changes in spatial frequency. When tested with stimuli constructed by adding two superimposed sine-wave gratings, the preferred speed of MT neurons becomes less dependent on spatial frequency. Analysis of these responses reveals a speed-tuning nonlinearity that selectively enhances the responses of the neuron when multiple spatial frequencies are present and moving at the same speed. Consistent with the presence of the nonlinearity, MT neurons show speed tuning that is close to form-invariant when the moving stimuli comprise square-wave gratings, which contain multiple spatial frequencies moving at the same speed. We conclude that the neural circuitry in and before MT makes no explicit attempt to render MT neurons speed-tuned for sine-wave gratings, which do not occur in natural scenes. Instead, MT neurons derive form-invariant speed tuning in a way that takes advantage of the multiple spatial frequencies that comprise moving objects in natural scenes.

Action Potentials↗

PubMed2002

Time-dependent hierarchical organization of spatial working memory: a transcranial magnetic stimulation study.

The performance of memory-guided saccades with two different delays (3 and 30 s of memorization) was studied in seven healthy subjects. Double-pulse transcranial magnetic stimulation (dTMS) with an interstimulus interval of 100 ms was applied over the right dorsolateral prefrontal cortex (DLPFC) early (1 s after target presentation) and late (28 s after target presentation). Early stimulation significantly increased in both delays the percentage of error in amplitude (PEA) of contralateral memory-guided saccades compared to the control experiment without stimulation. dTMS applied late in the delay had no significant effect on PEA. Furthermore, we found a significantly smaller effect of early stimulation in the long-delay paradigm. These results suggest a time-dependent hierarchical organization of the spatial working memory with a functional dominance of DLPFC during the early memorization, independent from the memorization delay. For a long memorization delay, however, working memory seems to have an additional, DLPFC-independent component.

Adult↗

PubMed1993

Spatial and gender-dependent variations in perioral pinprick sensitivity.

Twenty-eight right-handed young adults participated in a sensory testing experiment to evaluate pinprick sensitivity at ten spatially matched sites on the right and left sides of the face. Stimuli were provided by a sharp-pointed dental explorer on which a rubber eraser had been positioned to minimize variations in the extent to which the skin was indented. Sharpness was defined as the magnitude at which abrupt, localized pricking/stinging sensations were evoked. A magnitude matching procedure was used to reduce among-subject variability in the data. Specifically, each estimate of sharpness was adjusted (i.e., divided) by the subject's mean estimate of the brightness of a visual stimulus. Prior to data collection, subjects were carefully instructed on the use of a common numerical scale for assignment of values of sharpness and brightness. Repeated-measures analysis of variance of the adjusted estimates of sharpness revealed a non-significant effect of gender (p > 0.4), a highly significant effect of side (p < 0.0001), and a highly significant effect of test site (p < 0.0001). Pinprick percepts were sharper on the left side of the face than on the right. Moreover, the vermilion of the upper lip exhibited the greatest sensitivity to pinprick; the vermilion of the lower lip exhibited the least sensitivity. These results suggest that use of a patient's sensitivity to pinprick during clinical neurosensory examination must be undertaken in an informed manner. A conclusion of pathological alteration in sensation can be made only after consideration of the normal spatial variations in the percept of sharpness.

Adolescent↗