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PubMed1994

Nonperturbative methods in HZE propagation.

An analytical solution to the perturbative multiple collision series of a fragmenting HZE ion beam has limited usefulness since the first collision term has several hundred contributions, the second collision term has tens of thousands of contributions, and each successive collision term progresses to unwieldy computational proportions. Our previous work has revealed the multiple collision terms in the straight-ahead approximation to be simple products of a spatially dependent factor times a linear energy-dependent factor of limited domain and unit normalization. The properties of these forms allow the development of the nonperturbative summation of the series to all orders assuming energy-independent nuclear cross sections as matrix products of a scaled Green's function described herein. This nonperturbative Green's function with multiple scattering correction factors compares well with experiments using 670 MeV/u neon-20 ion beams in thick water targets.

Cosmic Radiation↗

PubMed2004

Transport lattice models of heat transport in skin with spatially heterogeneous, temperature-dependent perfusion.

BACKGROUND: Investigation of bioheat transfer problems requires the evaluation of temporal and spatial distributions of temperature. This class of problems has been traditionally addressed using the Pennes bioheat equation. Transport of heat by conduction, and by temperature-dependent, spatially heterogeneous blood perfusion is modeled here using a transport lattice approach. METHODS: We represent heat transport processes by using a lattice that represents the Pennes bioheat equation in perfused tissues, and diffusion in nonperfused regions. The three layer skin model has a nonperfused viable epidermis, and deeper regions of dermis and subcutaneous tissue with perfusion that is constant or temperature-dependent. Two cases are considered: (1) surface contact heating and (2) spatially distributed heating. The model is relevant to the prediction of the transient and steady state temperature rise for different methods of power deposition within the skin. Accumulated thermal damage is estimated by using an Arrhenius type rate equation at locations where viable tissue temperature exceeds 42 degrees C. Prediction of spatial temperature distributions is also illustrated with a two-dimensional model of skin created from a histological image. RESULTS: The transport lattice approach was validated by comparison with an analytical solution for a slab with homogeneous thermal properties and spatially distributed uniform sink held at constant temperatures at the ends. For typical transcutaneous blood gas sensing conditions the estimated damage is small, even with prolonged skin contact to a 45 degrees C surface. Spatial heterogeneity in skin thermal properties leads to a non-uniform temperature distribution during a 10 GHz electromagnetic field exposure. A realistic two-dimensional model of the skin shows that tissue heterogeneity does not lead to a significant local temperature increase when heated by a hot wire tip. CONCLUSIONS: The heat transport system model of the skin was solved by exploiting the mathematical analogy between local thermal models and local electrical (charge transport) models, thereby allowing robust, circuit simulation software to obtain solutions to Kirchhoff's laws for the system model. Transport lattices allow systematic introduction of realistic geometry and spatially heterogeneous heat transport mechanisms. Local representations for both simple, passive functions and more complex local models can be easily and intuitively included into the system model of a tissue.

Blood Flow Velocity↗

PubMed2004

Spatial frequency-dependent feedback of visual cortical area 21a modulating functional orientation column maps in areas 17 and 18 of the cat.

The feedback effect of activity of area 21a on orientation maps of areas 17 and 18 was investigated in cats using intrinsic signal optical imaging. A spatial frequency-dependent decrease in response amplitude of orientation maps to grating stimuli was observed in areas 17 and 18 when area 21a was inactivated by local injection of GABA, or by a lesion induced by liquid nitrogen freezing. The decrease in response amplitude of orientation maps of areas 17 and 18 after the area 21a inactivation paralleled the normal response without the inactivation. Application in area 21a of bicuculline, a GABAa receptor antagonist caused an increase in response amplitude of orientation maps of area 17. The results indicate a positive feedback from high-order visual cortical area 21a to lower-order areas underlying a spatial frequency-dependent mechanism.

Animals↗

PubMed2005

Spatial scale-dependent tracer diffusion in bulk polycarbonate studied by holographic relaxation.

The diffusion of a photochromic dye tracer in polycarbonate was studied by a holographic relaxation technique (forced Rayleigh scattering) at temperatures close to the glass transition temperature. By varying the holographic grating period the results could be interpreted via the spatial scale dependence of apparent diffusion coefficients within a two-state diffusion model. This indicates inhomogeneities on the scale of a few micrometers in the polymer glass.

Journal Article↗

PubMed1985

Context dependent processing in spatial hyperacuity.

The importance of comparison processes in spatial hyperacuity tasks was investigated by means of a new relative nonalignment discrimination task. Vernier-type configuations nonaligned to either left or right, were presented in succession, and the subject had to decide which configuation exhibited a larger nonalignment. The relative nonalignment thresholds were found to be more strongly dependent on the spatial arrangement of the two lines than the thresholds corresponding to "ordinary" spatial hyperacuity performances. This context dependency is attributed to the active role played in the processing by comparison mechanisms.

Discrimination, Psychological↗

PubMed1995

Dmax for stereopsis depends on size, not spatial frequency content.

Stereoacuity depends not only on the carrier frequency of Gabor stimuli, but also upon their size. To determine if this is also the case at large disparities, we have measured the upper limit for stereopsis, "Dmax", and assessed its dependence on carrier frequency and overall envelope size. The results differ markedly from the stereoacuity data. Dmax for stereopsis is primarily dependent on the size of the envelope of the Gabor patch, and is relatively independent of its carrier frequency. These results support the proposition that stereopsis is achieved at large disparities by way of non-linear processing (envelope extraction).

Contrast Sensitivity↗

PubMed1991

Spatial frequency difference thresholds depend on stimulus area.

Spatial frequency difference thresholds were estimated for vertical, high contrast 5.0 cpd sinewave gratings that had been multiplied by a two dimensional Gaussian window function of contrast. A range of different elliptical window configurations were tested that varied in their relative height and width. Spatial frequency acuity improved as the area of the target was increased. The data are compatible with a model of spatial frequency discrimination that is based on statistical averaging of spatial intervals interpolated between local retinal signs.

Adult↗

PubMed1971

Configuration dependence of scotopic spatial summation.

1. Psychophysical experiments were done under conditions of complete dark adaptation, 7 degrees in the human temporal retina, using blue-green flashes brief enough to be within the range of complete temporal summation.2. Frequency-of-seeing experiments were done for two 3.4' squares separated by 20' or 23' and for each square separately. The results were consistent with the hypothesis that the squares acted independently in determining the absolute threshold. There was no observable summation occurring between them.3. The absolute threshold (50% seeing) for two 3.4' squares (the centre-to-centre separation varied from 3.4' to 56') was measured relative to the absolute threshold for just one such square. It required more photons at the cornea at threshold for two spots than for one such spot, except when they were actually touching.4. The same subject, in the same retinal region and under otherwise identical test conditions, showed complete spatial summation at absolute threshold for circular spots up to 30' in diameter.5. Similar experiments were done on other subjects using 3.4' squares and lines of width 3.4'. All subjects showed complete summation at absolute threshold for lines 34' long. All showed lack of spatial summation for two squares unless the two squares touched to form a rectangle.6. The absolute threshold for N small spots was compared to the absolute threshold for 1 spot, where N varied from 2 to 25. Again complete spatial summation broke down even when all the spots lay within the Ricco area.7. It was shown that the results are inconsistent with the hypothesis of a linear output and a linear summation of excitations.8. Various hypotheses are examined and it is shown that no simple hypothesis can explain all the results. The conclusion is that spatial summation at absolute threshold is configuration dependent.

Adaptation, Ocular↗

PubMed1999

Visual evoked potentials in migraine patients: alterations depend on pattern spatial frequency.

Visual information is conducted by two parallel pathways (luminance- and contour-processing pathways) which are thought to be differentially affected in migraine and can be investigated by means of pattern-reversal visual evoked potentials (VEPs). Components and habituation of VEPs at four spatial frequencies were compared between 26 migraineurs (13 without aura, MO; 13 with aura, MA) and 28 healthy volunteers. Migraineurs were recorded in the headache-free interval (at least 72 h before and after an attack). Five blocks of 50 responses to chequerboards of 0.5, 1, 2 and 4 cycles per degree (c.p.d.) were sequentially averaged and analysed for latency and amplitude. Differences in VEPs were dependent on spatial frequency. Only when small checks were presented, i.e. at high spatial frequency (2 and 4 c.p.d.), was the latency of N2 significantly prolonged in MA and did it tend to be delayed in MO subjects. Habituation behaviour was not significantly different between groups under the stimulating conditions employed. Prolonged N2 latency might be explained by the lack or attenuation of a contour-specific component N130 in migraineurs, indicating an imbalance of the two visual pathways with relative predominance of the luminance-processing Y system. These results reflect an interictally persisting dysfunction of precortical visual processing which might be relevant in the pathophysiology of migraine.

Adult↗

PubMed2006

Retrosplenial cortex lesions impair water maze strategies learning or spatial place learning depending on prior experience of the rat.

There has been debate whether lesions strictly limited to retrosplenial (RS) cortex impair spatial navigation, and how robust and reliable any such impairment is. The present study used a detailed behavioral analysis with naive or strategies-pretrained rats given RS lesions and trained in a water maze (WM). Naive RS lesioned rats failed to acquire the required WM strategies throughout training. Strategies-pretrained RS lesioned rats were specifically impaired in spatial place memory without a WM strategies impairment. Additional training overcame the spatial memory impairment. Thus the behavioral consequences of the lesion depend on the specific previous experience of the animal. The use of appropriate training and testing techniques has revealed experience-dependant dissociable impairments in WM strategies learning and in spatial memory, indicating that RS cortex is involved in both forms of learning.

Animals↗

PubMed1991

The ability of cationic amphiphilic compounds to depress the transition temperature of dipalmitoylphosphatidic acid liposomes depends on the spatial arrangement of the lipophilic moiety.

The hypothesis was tested with the help of model compounds that the ability of cationic amphiphilic drugs to depress the phase-transition temperature Tt of dipalmitoylphosphatidic acid (DPPA) liposomes depends on the spatial arrangement of the lipophilic moiety. The main structure of the compounds with identical cationic side chain was 1-dimethylamino-3-phenylpropane (compound I). A further phenyl ring was introduced either at C3 of the propane chain (compound II) to broaden the lipophilic moiety, or in para-position of the phenyl ring (compound III) to elongate it. As shown by differential scanning calorimetry, the reduction of Tt (control 64 degrees) amounted for compound I to 29 degrees, for compound II to 28 degrees and for compound III to 53 degrees. In order to assess the binding affinity of the compounds to DPPA, their inhibitory effect on 45Ca(2+)-binding to DPPA films was measured. The IC50 values were 2100 microM for compound I, 40 microM for compound II, and 9 microM for compound III. Thus, binding affinity corresponded with the hydrophobicity of the compounds. In contrast, the depressing effect on the transition temperature was only augmented by the additional phenyl ring when substituted in the elongating position.

Calcium↗

PubMed2004

Direct visualization of electroporation-assisted in vivo gene delivery to tumors using intravital microscopy - spatial and time dependent distribution.

BACKGROUND: Electroporation is currently receiving much attention as a way to increase drug and DNA delivery. Recent studies demonstrated the feasibility of electrogene therapy using a range of therapeutic genes for the treatment of experimental tumors. However, the transfection efficiency of electroporation-assisted DNA delivery is still low compared to viral methods and there is a clear need to optimize this approach. In order to optimize treatment, knowledge about spatial and time dependency of gene expression following delivery is of utmost importance in order to improve gene delivery. Intravital microscopy of tumors growing in dorsal skin fold window chambers is a useful method for monitoring gene transfection, since it allows non-invasive dynamic monitoring of gene expression in tumors in a live animal. METHODS: Intravital microscopy was used to monitor real time spatial distribution of the green fluorescent protein (GFP) and time dependence of transfection efficiency in syngeneic P22 rat tumor model. DNA alone, liposome-DNA complexes and electroporation-assisted DNA delivery using two different sets of electric pulse parameters were compared. RESULTS: Electroporation-assisted DNA delivery using 8 pulses, 600 V/cm, 5 ms, 1 Hz was superior to other methods and resulted in 22% increase in fluorescence intensity in the tumors up to 6 days post-transfection, compared to the non-transfected area in granulation tissue. Functional GFP was detected within 5 h after transfection. Cells expressing GFP were detected throughout the tumor, but not in the surrounding tissue that was not exposed to electric pulses. CONCLUSIONS: Intravital microscopy was demonstrated to be a suitable method for monitoring time and spatial distribution of gene expression in experimental tumors and provided evidence that electroporation-assisted gene delivery using 8 pulses, 600 V/cm, 5 ms, 1 Hz is an effective method, resulting in early onset and homogenous distribution of gene expression in the syngeneic P22 rat tumor model.

Animals↗

PubMed2006

Spatial and force dependency of mechanomyographic signal features.

The aim was to investigate with a novel technique the spatial inhomogeneity in surface mechanomyographic (MMG) response to muscle contraction at varying force levels. MMG signals were detected over the dominant tibialis anterior muscle of 10 volunteers using a 5 x 3 grid of accelerometers. The subjects performed 3 s long isometric contractions at forces ranging from 0% to 100% of the maximal force (10% increment) in a randomised order. From the two-dimensional MMG recordings, maps of absolute and normalized temporal and spectral MMG descriptors were obtained. The centroid and entropy of these maps were computed to describe the spatial centre of activity and degree of homogeneity, respectively. MMG absolute amplitude did not depend on location over the muscle while normalized amplitude did and the centroid shifted with increasing force. Amplitude increased with force and its entropy decreased. Absolute and normalized spectral variables depended on location over the muscle and their centroid shifted with increasing force. In addition, the dependency of absolute and normalized spectral variables on force was affected by location. These results highlight limitations when using single-channel MMG features for the assessment of motor unit control strategies, due to a substantial effect of position on the relation between force and MMG characteristics.

Acceleration↗

PubMed1988

Spatial frequency-dependent abnormalities of the pattern electroretinogram and visual evoked potentials in a parkinsonian monkey model.

The pattern visual evoked potential (PVEP) and pattern electroretinogram (PERG) were studied in 5 cynomolgus monkeys before and during the development of a parkinsonian syndrome induced by MPTP. The stimuli were vertical bars of four spatial frequencies (0.5, 1.2, 2.5 and 3.5 cycles/degree (cpd) modulated at temporal rates of 1, 4, 6, and 8 Hz. Following MPTP administration, all monkeys developed parkinsonian signs accompanied by changes in the amplitude and latency of the PVEP and PERG. Sinemet L-dopat carbi olopa administration produced temporary recovery of both PVEP and PERG. Two of the monkeys were followed for a prolonged period: 30-40 days after MPTP, the parkinsonian signs showed partial recovery; the PVEP latency and amplitude to 2.5 and 3.5 cpd stimuli and the latency to 1.2 cpd showed improvement but remained abnormal. The latencies of PERGs were normal, but the amplitudes were significantly reduced when stimuli of 2.5 and 3.5 cpd were used. Both PVEP and PERG to 0.5 cpd stimuli returned to normal. No further modifications were seen in the recordings performed 6 months and 1 year later. This study demonstrates (1) that spatial frequency-dependent electrophysiological abnormalities occur in the MPTP-treated monkey, a result previously found in human Parkinson's disease, and (2) that dopamine has a specific function in neurotransmission in the visual system of primates.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗

PubMed2003

The hippocampus mediates glucocorticoid-induced impairment of spatial memory retrieval: dependence on the basolateral amygdala.

Previous studies have indicated that stress-activated glucocorticoid hormones induce temporary memory retrieval impairment. The present study examined whether adrenal steroid receptors in the hippocampus mediate such glucocorticoid effects on spatial memory retrieval. The specific glucocorticoid receptor (GR) agonist 11beta, 17beta-dihydroxy-6,21-dimethyl-17alpha-pregna-4,6-trien-20yn-3-one (RU 28362; 5 or 15 ng) infused into the hippocampus of male Sprague-Dawley rats 60 min before water-maze retention testing, 24 h after training, dose-dependently impaired probe-trial retention performance, as assessed both by time spent in the training quadrant and initial latency to cross the platform location. The GR agonist did not affect circulating corticosterone levels immediately after the probe trial, indicating that RU 28362 infusions did not influence retention by altering glucocorticoid feedback mechanisms. As infusions of the GR agonist into the hippocampus 60 min before training did not influence water-maze acquisition or immediate recall, the findings indicated that the GR agonist-induced retention impairment was induced selectively by an influence on information retrieval. In contrast, pretest infusions of the GR agonist administered into the basolateral complex of the amygdala (BLA; 2 or 6 ng) did not alter retention performance in the water maze. However, N-methyl-d-aspartate-induced lesions of the BLA, made 1 week before training, blocked the memory retrieval impairment induced by intrahippocampal infusions of RU 28362 given 60 min before the retention test. These findings indicate that the effects of glucocorticoids on retrieval of long-term spatial memory depend on the hippocampus and, additionally, that neuronal input from the BLA is critical in enabling hippocampal glucocorticoid effects on memory retrieval.

Adrenal Cortex Hormones↗

PubMed2001

Hemispheric asymmetry and interhemispheric transfer in pointing depend on the spatial components of the movement.

The purpose of the present study was to compare the asymmetry and transfer in 3 pointing movements with increasing spatial requirements. The triggering signal was one of four visual targets appearing on the right or left of a central fixation point (FP). The first task consisted in simply removing the arm from the starting platform; the second was a pointing movement towards the FP, and the third was a classical pointing task towards one of the four lateral targets. 20 right-handers (Rhrs) and 20 left-handers (Lhrs) participated in this experiment. In the classical pointing task (task 3), the reaction times were shorter in the Rhrs using their left hand. No such hand-related difference was observed in the Lhrs. No hand asymmetry was observed in the other tasks. In addition, the responses were faster in the uncrossed than in the crossed conditions, in task 3 only. It was concluded that in pointing tasks, both the hemispheric asymmetry and the interhemispheric transfer depend on the spatial requirements of the movement.

Adult↗

PubMedDate not supplied

The spatial and energy dependence of bremsstrahlung production about beta point sources in H2O.

A Monte Carlo simulation was performed to characterize the spatial and energy distribution of bremsstrahlung radiation from beta point sources important to radioimmunotherapy (RIT). Using the EGS4 Monte Carlo code, the isotropic emission and transport of monoenergetic 0.1-, 0.5-, 1.0-, 2.0-, and 3.0-MeV electrons and 32P and 90Y beta particles was simulated in an infinite, homogeneous H2O phantom. The probability of bremsstrahlung production for each Monte Carlo-simulated electron step was accumulated in energy intervals not exceeding 5 keV and stored as a function of radial position. To validate this scheme, the EGS4 code was tested in the continuous slowing down approximation (csda) mode, with resulting radiation yields seen to agree with values in ICRU Report No. 37 (ICRU, Bethesda, MD, 1984) to better than 1.6%. The radiation yield calculated with the simulation of secondary particles is seen to be 3%-5% greater than the csda yield. The photon energy distributions are characterized by a typically broad bremsstrahlung spectrum with the probability of photon generation decreasing with radial distance. In the energy range 0.05-0.511 MeV, the probability for bremsstrahlung production from 90Y (2.76 x 10(-2) decay-1) is twice that from 32P (1.35 x 10(-2) decay-1). When passed through 10 cm of H2O and put upon a standard NaI scintillation camera, count rates of 2.3 x 10(-6) and 1.2 x 10(-6) counts s-1 Bq-1 are estimated from point sources of 90Y and 32P. These results predict the inherent spatial resolution limitation and provide the initial data required for modeling and analyzing the scatter, attenuation, and image formation processes in quantitative imaging of bremsstrahlung for RIT dosimetry.

Humans↗

PubMed2000

Spatial frequencies in short-term memory for faces: a test of three frequency-dependent hypotheses.

Spatial frequency manipulations have been shown to have significant utility in ascertaining the various types of information that might be important in object identification and recognition tasks. This utility suggests that, given some mapping between ranges of spatial frequencies and different types of psychological information, it should be possible to examine the roles of these different types of psychological information by way of spatial frequency manipulations. One potential problem, however, is that there is no well-specified, unambiguous mapping between the distinctions in the frequency domain and the distinctions in the informational domain. Three experiments provide tests of three general hypotheses regarding the ways in which different spatial frequencies might map to different information in facial perception and memory tasks: (1) the low-frequency dominance hypothesis, which proposes that low-frequency information should be superior (to high-frequency information) as a cue to perception and memory; (2) the distinct informational roles hypothesis, which holds that high spatial frequencies should carry featural information while low spatial frequencies should carry information about the configuration of those features; and (3) the task-dependent information hypothesis, which suggests that high-frequency information should be best suited to discrimination tasks while low-frequency information should be best suited for recognition tasks. Results generally contradict the first two of these hypotheses, while providing support for the third. Implications with regard to the various issues related to the mapping between spatial frequencies and the informational content of faces, as well as the need to consider important interactions among perceptual and memory processes, are discussed.

Adult↗