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At least 91 records · Page 5Linked to original sources
PubMed1990

Panoramic zonography of the midface: a clinician's guide to vertical magnification.

Surgical procedures involving the maxilla require details regarding the topography of the nasal floor and that of the maxillary sinus. Panoramic zonography can provide this information with a minimum of radiation exposure and may be used in certain instances when other imaging modalities cannot. Spatially dependent variations in vertical magnification inherent to panoramic zonography using the midface program were determined. Radiographic lengths of marker wires attached at seven locations along the maxillary arch of dry skulls were measured. Radiographs were obtained at 0, -10 and -20 mm shifts. Analysis of variance and ranking did not reveal any significant differences in magnification between skulls. However, a dichotomy between the anterior and posterior portions of the maxillary arch in terms of vertical magnification was found at all three shift positions. In addition, at all marker locations, the magnification diminished as a function of decreasing shift position. These variations are due to the fact that teeth within the arch are not all located at the central plane of the focal trough generated by the unit. The results provide the clinician with vertical magnification factors from which actual measurements can be derived from zonographs.

Alveolar Process↗
PubMed1986

Magnetic resonance angiography in the head and neck.

Magnetic resonance angiography (MRA) provides a means to visualize vessel structure without the use of contrast agents and consequently without the risks and discomfort associated with contrast media. We have developed a flow encoding procedure which exploits the spatial dependence in the phase of spin magnetization generated by a gradient pulse. This technique yields image information for only those spins having macroscopic motion (i.e. flowing blood). Excellent suppression of non-moving spins can be obtained despite the overwhelming abundance of stationary spins. Data are presented in a projection format similar to that of traditional subtraction angiography.

Blood Vessels↗
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↗
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↗
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↗
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↗
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↗
PubMed1978

Mapping the ATP-binding site in the catalytic subunit of adenosine-3':5'-monophosphate-dependent protein kinase. Spatial relationship with the ATP site of the undissociated enzyme.

A set of 24 ATP analogs modified at various positions of the ATP molecule was used for mapping the ATP-binding site in the free catalytic subunit (C) of cAMP-dependent protein kinase (type I). Ki values for these analogs (of which 23 were shown to be competitive with ATP) were measured and compared with Ki values previously obtained for the same set of analogs upon binding to the undissociated form of the enzyme (R2C2). It was found that modifications at the adenine part of ATP bring about a considerable reduction in affinity between C and the resulting analog. The other parts of the ATP molecule play a less important, though definite, role in the binding of this nucleotide to C. By measuring the effect of each given modification in ATP on its binding to C, and comparing the effect of this modification on the binding of the same analog to R2C2, it was possible to obtain 'specificity profiles' for both forms of the kinase. Using such profiles it is shown that the adenine-binding subsite in C may well coincide with the adenine-binding subsite in R2C2. Two plausible models describing the spatial relationship between the ATP sites in C and R2C2 are proposed.

Adenosine Triphosphate↗
PubMedDate not supplied

[The dependence of spatial-motor asymmetry on the nature of the reinforcement].

Spatial-motor asymmetry (SMA) of rats goal-directed running was studied in condition of numerous alternative choices in complex T-maze. In condition of free choices SMA in trained rats depended on reinforcement biological significance, volume and disposition scheme. In condition of non-free choices a change was possible of asymmetry of sequence of goal-directed motor acts adequate to situation, i.e. SMA regulation. Examples of the presence of properties of ego- and allocentric systems in SMA are given. The results of experiments testify to SMA dependence on the motivation of running, to rightfulness of SMA interpretation as behavioural tactics and to non-coming of SMA of goal-directed running to a simple rotation of animals.

Animals↗
PubMed1992

Visual hemispheric asymmetries depend on which spatial frequencies are task relevant.

Observers classified sine-wave and square-wave gratings on the basis of fundamental frequency (Are the bars wide or narrow?) or on the basis of higher harmonic frequencies (Are the bars sharp or fuzzy?). Stimuli were presented in either the left (LVF) or right (RVF) visual field. When the classification was made on the basis of the fundamental frequencies (1 or 3 c/deg), there was a LVF/right hemisphere advantage. However, when the classification was on the basis of a sharp/fuzzy distinction which involved searching for the higher harmonic frequencies, then a RVF/left hemisphere advantage was found.

Adult↗
PubMed1987

Some task and signal dependent rules for spatial vision.

In this paper we consider the types of computational processes which may be involved in solving a variety of perceptual problems from the detection of signals in the presence of others, to texture discrimination, and some aspects of pattern recognition. These processes centre around the involvement of correlational computations, the transduction of their input/output values, and the apparent involvement of selective filtering mechanisms. Our results suggest that even if fixed detectors (in tuning characteristics) are involved in low-level vision, the human observer apparently employs much more adaptive (variable tuning characteristic) filters and nonlinear mechanisms in more complex spatial tasks.

Humans↗
PubMed1985

A complex-cell receptive-field model.

The time course of the response of a single cortical neuron to counterphase-grating stimulation may vary as a function of stimulation parameters, as shown in the preceding paper (19). The poststimulus-time histograms of the response amplitudes against time are single or double peaked, and where double peaked, the two peaks are of equal or unequal amplitudes. Furthermore, the spatial-phase dependence of cortical complex-cell responses may be a function of spatial frequency, so that the receptive field appears to have linear spatial summation at some spatial frequencies and nonlinear spatial summation at others (19). In the first part of this paper, we analyze a model receptive field that displays this behavior, and in the second part experimental data are presented and analyzed with regard to the model. The model cortical receptive field in its simplest form contains (two rows) of geniculate X-cell-like, DOG (difference-of-Gaussians)-shaped, center-surround antagonistic, circular-input subunits. We propose nonlinear summation between these two subunits, by introducing a half-wave rectification stage before pooling. The model is tested for the responses it predicts for the application of counterphase-grating stimulation. This simple model predicts the appearance of three response forms as a function of counterphase-stimulation parameters. At periodic spatial frequencies the expected-response histogram has a single peak, whose amplitude has a sinusoidal dependence on spatial phase. At spatial frequencies halfway between these, the expected-response histogram has two equal peaks whose amplitudes have a full-wave rectified sinusoidal dependence on spatial phase. At all intermediate spatial frequencies the expected-response histogram has a "mixed" form; the histogram appears sometimes with one peak, sometimes with two equal peaks, and generally with two peaks of unequal amplitude, as a function of spatial phase. Null responses are expected to appear at specific spatial phases only for the periodic spatial frequencies that give "pure" response time courses as in paragraph 5 above, and not in the more common mixed response case of paragraph 6. The analysis procedure described in the preceding paper (19) is used, separating the odd and even Fourier components of the response histograms reflecting the receptive-field intrasubunit linear summation and intersubunit nonlinear summation, respectively. We propose that this model may be used as a working hypothesis for the analysis of these aspects of the various cortical receptive-field types. Experimental data are described and discussed in terms of the model.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Nonlinear directionally selective subunits in complex cells of cat striate cortex.

1. We have analyzed receptive fields (RFs) of directionally selective (DS) complex cells in the striate cortex of the cat. We determined the extent to which the DS of a complex cell depends on spatially identifiable subunits within the RF by studying responses to an optimally oriented, three-luminance-valued, gratinglike stimulus that was spatiotemporally randomized. 2. We identified subunits by testing for nonlinear spatial RF interactions. To do this, we calculated Wiener-like kernels in a spatial superposition test that depended on two RF positions at a time. The spatial and temporal separation of light and dark bars at these two positions varied over a spatial range of 8 degrees and a temporal range of +/- 112 ms in increments of 0.5 degree and 16 ms, respectively. 3. DS responses in complex cells cannot be explained by their responses to single light or dark bars because any linear superposition of responses whose time course is uniform across space shows no directional preference. 4. Nonlinear interactions between a flashed reference bar that is fixed in position and a second bar that is flashed at surrounding positions help explain DS by showing multiplicative-type facilitation for bar pairs that mimic motion in the preferred direction and suppression for bar pairs that mimic motion in the null direction. Interactions in the preferred direction have an optimal space/time ratio (velocity), exhibited by elongated, obliquely oriented positive domains in a space-time coordinate frame. This relationship is inseparable in space-time. The slope of the long axis specifies the preferred speed, and its negative agrees with the most strongly suppressed speed in the opposite direction. 5. When the reference bar position is moved across the RF, the spatiotemporal interaction moves with it. This suggests the existence of a family of nearly uniform subunits distributed across the RF. We call the subunit interaction, as averaged across the RF, the "motion kernel" because its spatial and temporal variables are those necessary to specify the velocity, the only parameter that distinguishes a moving image from a temporally modulated stationary image. The nonlinear interaction shows a spatial periodicity, which suggests a mechanism of velocity selectivity for moving extended images.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
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↗
PubMed1992

Responses of cat horizontal cells to sinusoidal gratings.

The spatiotemporal properties of cat horizontal (H-) cells were studied by recording the intracellular responses in the optically intact, in vivo, eye to sinusoidal gratings at a photopic mean illumination level. In order to investigate the linearity of spatial summation a "null test" was performed in which the responses to contrast reversal gratings were measured at different positions of the grating relative to the receptive field. Spatial and temporal transfer functions were measured using drifting sinusoidal gratings of variable spatial and temporal frequencies. The amplitudes of cat H-cell responses to contrast reversal gratings modulated with a square wave time-course showed a sinusoidal dependence on spatial phase. When zero crossings of the grating were lined up with the receptive field center, as defined by the maximum of the measured line weighting function, contrast reversal produced no response modulation. This result did not depend on the spatial frequency of the grating or the temporal frequency of contrast modulation over substantial ranges. The response waveform was found not to depend on the spatial phase of the grating. The spatial transfer function of cat H-cells has low-pass characteristics with a cut-off frequency in the range of about 0.4-1.5 c/deg. The shape of the spatial transfer function was roughly the same for temporal frequencies ranging from 3 to 10 Hz. The temporal transfer function exhibited band-pass characteristics with a maximum response amplitude at 3-6 Hz. The amplitude fall-off for low and high temporal frequencies was independent of the spatial frequency of the grating. The results obtained with sine gratings were found not to agree with the receptive field profiles measured with narrow slits flashed at different positions in the receptive field.

Animals↗
PubMed1988

Components of the location probability effect in visual search tasks.

In visual search tasks, targets are detected more rapidly when they appear in locations that commonly contain a target than when they appear in locations that rarely contain a target. Five experiments were conducted to investigate two specific properties of this location probability effect: its dependence on spatial location versus relative position and its dependence on or independence of target identity. In Experiment 1 spatial location of a stimulus row was varied to determine whether high location probability facilitates target detection in a particular location in visual space or a particular relative position within the row. Both were facilitated to approximately the same extent. In Experiment 2 an inducing target occurred with high probability in one of four display locations, and a test target occurred with equal probability in all four locations. Both targets were found more quickly in the high-probability location than in the other locations, but the advantage associated with targets in the high-probability location was larger for the inducing target than for the test target. In Experiments 3-5 the correspondence between the components observed in Experiments 1 and 2 was examined. The overall pattern of results was compatible with a model in which the location probability effect is produced partly by an attentional spotlight, which facilitates processing of any stimulus appearing in a particular location in visual space, and partly by a network of position-specific letter detectors, which facilitates detection of a particular letter in a particular relative position within a display. Models with flexible scanning strategies were also considered.

Attention↗