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PubMed1980

Reconstruction of an arbitrary cross section from the serial drawings of a stereotaxic brain atlas.

A method is described for the reconstruction of arbitrary cross section from the serial line drawings of a stereotaxic brain atlas. Mathematically, the problem reduces to the intersection of a plane with a line pattern lying in another plane. However, the calculation yields a cloud of points, often confusing and unusable for the surgeon. Because it is impossible to link correctly all the calculated points in all cases [1,2], we propose a method where the number of points is drastically reduced: only regions of interest are depicted, i.e., the area where the electrode point is situated and the neighbouring regions. Instead of points, symbols are displayed, each symbol representing a different region. The efficiency of the method is shown using an atlas simulation of four intersecting spheres in the three dimensional space, Although the procedure is useful for all stereotaxic brain atlasses, a practical example is given where the poor quality of the reconstructed images does not allow good interpretation.

Analog-Digital Conversion↗

PubMed1996

Femtosecond kinetics of electron transfer in the bacteriochlorophyll(M)-modified reaction centers from Rhodobacter sphaeroides (R-26).

Formation of the vibronic wavepacket by 90-fs excitation of the primary electron donor P in bacteriochlorophyll(M)-modified reaction centers is shown to induce nuclear motions accompanied by (1) oscillation of the stimulated emission from excited primary electron donor P* and (2) wavepacket motions leading to electron transfer at 293 K from P* to bacteriochlorophyll (B(L)) and then to bacteriopheophytin (H(L)). The latter motions have low frequency (about 15 cm-1) and are related to protein-nuclear motions which are along the reaction coordinate. When the wavepacket approaches the intersection of the reactant (P*B(L)) and product (P+B(L)-) potential energy surfaces (approximately 1.5 ps delay), about 60% of P* is converted to the P+B(L)- state. The P+H(L)- state formation is delayed by approximately 2 ps with respect to that of P+B(L)-. It is suggested that the wavepacket is transferred to and moves also slowly on the P+B(L)- potential energy surface and approaches the intersection of the surfaces of P+B(L)- and P+H(L)- within approximately 2 ps (approximately 8 cm-1), indicating the electron transfer to H(L).

Bacteriochlorophylls↗

PubMed1989

The heliocentric evolution of cometary infrared spectra: results from an organic grain model.

Observations of Comets Halley and Wilson reveal an emission feature peaking near 3.4 micrometers, characteristic of C-H stretching in hydrocarbons. We have previously (Chyba and Sagan 1987a, Nature (London) 330, 350-353) fit this feature with a simple two-component thermal emission model for dust in the cometary coma (one component corresponding to large, cool, optically thick particles, the other due to smaller, hotter, organic grains) by employing laboratory spectra of the organic residue produced by the irradiation of carbon-bearing ices. This procedure yields optical depths in agreement with limits from spacecraft data. One remarkable result of such modeling is that at approximately 1 AU emission features at wavelengths longer than 3.4 micrometers are largely overwhelmed (or "diluted") by continuum emission. The large particle optical depth is approximately 10(2) times that of the emitting organics, so that, relative to the continuum, only near the continuum minimum can the emitting organics make a significant contribution. At approximately 1 AU, the 3.4-micrometers feature is the sole feature near that minimum, lying at the intersection of the curves for particle thermal emission and scattered sunlight. Thus, since as a comet moves away from perihelion the intersection of the scattered solar spectrum and the comet's thermal emission spectrum will move to longer wavelengths, we predicted (Chyba and Sagan 1987a) that the 3.4-micrometers feature is diluted while those at longer wavelengths are progressively revealed--so long as the comet retains its coma. We now quantitatively develop this model and find agreement with observational data for Comet Halley for certain plausible values of optical constants. Thus the observed heliocentric evolution of the 3.4-micrometers feature provides information on the composition, and perhaps structure, of the organic grains in Comet Halley. In addition, we argue that the heliocentric evolution of organic features will differ in the cases of thermal emission from small grains and gas-phase fluorescence. Therefore observations of cometary spectral evolution can in principle distinguish between solid or gas-phase origins for these features.

Cosmic Dust↗

PubMed1995

A polygon matching algorithm and its applications to verification of radiation field placement in radiotherapy.

An automated method for quantitative evaluation of geometric accuracy of radiation field placement during radiation therapy treatments has been developed and is presented in this article. The method is based on matching polygons representing the boundaries of the prescribed and treated radiation fields. An efficient algorithm for finding the intersection of two arbitrary simple polygons has been developed for that purpose. The process of polygon matching is performed by iterative optimization of the goodness-of-match measure derived from the area of the intersection between the two polygons to be matched. The optimization is done with respect to the translation and rotation in the plane that are applied to the polygon representing the treated field. The optimum translation and rotation define the adjustments in the position of the treated field, relative to the patients anatomy, which are necessary in order to bring the treated field into the best possible agreement with the prescription. The radiation field matching scheme was applied to a clinical dataset consisting of prescription-treatment field pairs extracted from simulation and portal images for 40 patients. It is concluded that the observed accuracy and efficiency of the method make it very suitable for applications in radiation therapy verification.

Algorithms↗

PubMed1985

Moiré patterns: an accurate technique for determination of the locus of the centres of rotation.

This study describes an accurate technique for the determination of the centre of rotation of small angles. The moiré fringe method localizes the centre of rotation by defining two primary fringes, each of which is found by the intersection of three lines. The primary fringes intersect at the centre of rotation at 90 degrees to each other, the angle least likely to produce an error in measurement. By utilizing joints with known centres of rotation, we have found that the method is extremely accurate and reproducible to within 2 mm of the real centre for angular changes as small as 3 degrees. This technique is useful in evaluating whether a joint is a simple hinge, i.e. rotating about a single axis of rotation or whether the joint moves about a changing axis of rotation referred to as a locus or centrode.

Arthrography↗

PubMed1990

An anatomy-based coordinate system for the description of the kinematic displacements in the human knee.

The effects and interaction of the anatomical displacements in the human knee are a prerequisite to an accurate assessment and communication of the kinematic data. For the kinematic information to be used to improve diagnosis and treatment, and for better prosthetic design and installation, there must be clear, concise, and universal definitions of the displacements. In general, the displacements are defined as three translations and three rotations. In this paper, anatomic landmarks on the femur and on the tibia are used to define the locations and orientations of the six displacement axes; i.e. three translational and three rotational displacement axes. The most commonly accepted kinematic representation of the knee joint, in the literature, is a special geometry three-cylindric open chain in which the axes of the cylindric joints are defined according to the rotational displacement axes. The sequentially adjacent joint axes are assumed to not only intersect but to intersect at right-angles. The open chain permits a total of six degrees of freedom between a Cartesian reference frame attached to the femur and a Cartesian reference frame attached to the tibia. In this paper, the three rotational axes are shown to be skewed and off-set from each other, therefore, a three-cylindric open chain with skewed joint axes is proposed to measure the six displacements between the two reference frames. The authors believe that the proposed open chain is the most general to date and provides a more realistic representation of the displacements in the knee. To illustrate the significance of the reference frames on the interpretation of measured data, the anterior/posterior drawer is plotted against per cent gait cycle for three existing open chains and the proposed open chain.

Biomechanical Phenomena↗

PubMed1993

Direct calculation of the surface-to-volume ratio for human cancellous bone.

There are many diseases which cause detrimental changes in the trabecular structure of cancellous bone, leading to mechanical failure of the tissue. One approach to understanding the mechanisms of these diseases is to create idealized models that recreate the morphology of the tissue. This paper presents a partial development of such a model. Further histological methods must be developed before a complete definition of morphologically valid models is possible. In a histological section of cancellous bone, the orientation and length of the trabecular surfaces determine how a line drawn across the bone section will intersect the bone-marrow interface. The distribution of the average length between intersections for a set of parallel lines is defined as the mean intercept length distribution. In this paper, the average surface morphology and volume of the average structure of cancellous bone is determined from an examination of the mean intercept length. The average structure of cancellous bone contains a repeated structural element (SE). As a result, the basic bone structure is analogous to a brick wall made from many similar bricks. For a group of 107 specimens, a strong relationship between structural element volume (SE.V) and bone volume fraction (BV/TV) is demonstrated, SE.V = 0.017 kappa (BV/TV)-2.05 mm3, R2 = 0.93, with kappa a model-dependent constant. For the same specimens, the structural element surface (SE.S) showed the relationship, SE.S = 0.144 kappa (BV/TV)-1.35, R2 = 0.92. As a result of the inverse square dependence of structural element volume on bone volume fraction, it is predicted that cancellous bone strength is inversely proportional to structural element volume.

Adult↗

PubMed1996

A controlled study of facial mobility treatment in Parkinson's disease.

The aim of this study was to evaluate the effects of orofacial physiotherapeutic treatment (OPT) on the facial mobility of Parkinson's disease (PD) sufferers. Sixteen participants with PD were allocated randomly to either the Treatment group or the Control group. A short interview between the physiotherapist and each subject was videotaped, and 10 random frames of the videotape were selected to be used in the facial expression assessment. The quantification of facial expressions was achieved by using an objective microcomputer-based measurement system, based on a mathematical model of the face (FACEM). A facial outline is obtained, as well as 12 facial measures, which represent distances between key facial landmarks. The facial assessment was performed on 3 separate occasions, that is, baseline (pretreatment), posttreatment, and follow-up (4 weeks later). A repeated measures analysis of variance (MANOVA) revealed a significant main effect of Time and a significant interaction effect between Time and Group for the Mouth-Opening Measure, suggesting that after treatment, members of the Treatment group opened their mouths to a greater degree than members of the Control group. Within the Treatment group, significant differences between pretreatment and posttreatment scores (MANOVA) were found for Mouth-Opening Measure and Mid-Top-Lip Measure. Similarly, Mouth-Opening Measure, Mid-Top-Lip Measure, Lower-Lip Thickness Measure, Top Eyelid/Iris Intersect Measure and Lower Eyelid/Iris Intersect Measure were significant across time from baseline to follow-up in the Treatment group only. No significant differences were found on any of the facial measures during the same period for members of the Control group. These findings suggest that OPT can improve facial movement and that this benefit extends in time, beyond the period of OPT itself. Such an increase in facial mobility can be expected to modify the "Parkinsonian facies" and facilitate the display of facial expressions.

Aged↗

PubMed1986

The kinetic mechanism of the anterior pituitary progesterone 5 alpha-reductase.

An analysis of the kinetic mechanism of the microsomal NADPH-linked progesterone 5 alpha-reductase obtained from female rat anterior pituitaries was performed. Initial velocity, product inhibition and dead-end inhibition studies indicate that the kinetic mechanism for the progesterone 5 alpha-reductase is equilibrium ordered sequential. Analysis of the initial velocity data resulted in intersecting double reciprocal plots suggesting a sequential mechanism [apparent Km(progesterone) = 88.2 +/- 8.2 nM; apparent Kia(NADPH) = 7.7 +/- 1.1 microM]. Furthermore, the plot of 1/v vs 1/progesterone intersected on the ordinate which is indicative of an equilibrium ordered mechanism. Additional support for ordered substrate binding was provided by the product inhibition studies with NADPH versus NADP and progesterone versus NADP. NADP is a competitive inhibitor versus NADPH (apparent Kis = 7.8 +/- 1.0 microM) and a noncompetitive inhibitor versus progesterone (apparent Kis = 9.85 +/- 2.1 microM and apparent Kii = 63.2 +/- 12.5 microM). These inhibition patterns suggest that NADPH binds prior to progesterone. In sum, these kinetic studies indicate that NADPH binds to the microsomal enzyme in rapid equilibrium and preferentially precedes the binding of progesterone.

Animals↗

PubMed1989

The kinetic mechanism of the hypothalamic progesterone 5 alpha-reductase.

The kinetic mechanism of the hypothalamic NADPH-linked progesterone 5 alpha-reductase from female rats was determined to be equilibrium ordered sequential by initial velocity, product inhibition and dead-end inhibition studies. Analysis of the initial velocity data resulted in intersecting double reciprocal plots indicating a sequential mechanism (apparent Km (progesterone) = 95.4 +/- 4.5 nM; apparent Kia(NADPH) = 9.9 +/- 0.7 microM). The plot of 1/v vs 1/progesterone intersected on the ordinate which is consistent with an equilibrium ordered mechanism. Ordered addition of the substrates was also supported by product inhibition studies with NADP versus NADPH and NADP versus progesterone. NADP is a competitive inhibitor versus NADPH (apparent Kis = 4.3 +/- 1.3 microM) and a noncompetitive inhibitor versus progesterone (apparent Kis = 31.9 +/- 1.4 microM and apparent Kii = 145.4 +/- 15.5 microM). These inhibition patterns show that NADPH binds prior to progesterone. Taken together, these analyses indicate that the cofactor, NADPH, binds to the enzyme in rapid equilibrium and preferentially precedes the binding of progesterone.

3-Hydroxysteroid Dehydrogenases↗

PubMed1975

Some morphometric methods for the central nervous system.

It is the quantitative study of the central nervous system to enumerate neurone populations. Since neurones are generally arranged in non-random patterns the conventional method of counting with a square lattice graticule in the ocular of the microscope is severely restricted. Appropriately tailored methods of (a) counting, or (b) estimating the density of a neurone population must therefore be used. Four types of neuronal distribution pattern are discussed and morphometric methods adapted to them are presented, as follows: (1) Where the whole population is circumscribed, as in the anterior horn motor neurones of the cord, direct counting is feasible. Only those cells whose nucleoli are visible should be included; this diminishes the bias in favour of large cells, and if one corrects for the fact that larger neurones have larger nucleoli the bias can be eliminated completely. (2) Where neurones are situated on an "interface", as Purkinje cells are, the length of its profile (in this instance the boundary between the molecular and granular zones) is estimated by superimposing parallel lines on the microscopical image and counting intersections as these lines cross the interface. The number of Purkinje cells in each particular field is counted at the same time. A "linear dinsity" (PL) is then calculated from the formula pl = 2n/pid where n is the quotient of (a) the number of cells counted along (b) an estimated length of profile, and d is the distance between the parallel lines of the graticule. For comparative studies between cerebella of different sizes a correction factor is easily introduced. Those who prefer to work on photomicrographs can use a mapping wheel to measure a length of profile and then proceed as before. (3) Where neurones are organised in a curving band, as in the dentate nucleus, another form of cell density can be established. The microscope is focused on a neurone at random, and the distance from this cell to its nearest neighbour is measured, preferably by means of a screw-micrometer eyepiece. The mean value r of a number of such measurements is substituted in a formula which gives the "areal density" of the neurone population: pa = 1/4r2. This method can be applied to photomicrographs by using a pair of dividers and/or an accurate rule to measure the set of r values. (4) In large collections of neurones, e.g., in the thalamus, three methods are available: (a) the nearest neighbour method; (b) a conventional squared graticule count, and (c) a count of cells intersected by a line probe as in Haug's (1972) technique (fig. 5), or a modified form of Strong's (1966) transect method.

Brain↗

PubMed1989

Perceived diagonals in grids and lattices.

Diagonal lines are seen running between the intersections in repetitive grid patterns, but not in lattices of dots. We show that these lines, which are of reverse contrast to the grid, cannot be explained by global Fourier components in the pattern, or by low-pass orientation-specific filtering. The lines depend upon local diagonal components at the intersection, and we suggest that the output of local detectors are combined by collector units receiving their input from different visual field positions. Relevant neurophysiological evidence is discussed, and the model is extended to subjective contours and the Fraser twisted cord.

Form Perception↗

PubMed1990

Effect of contrast on the perceived direction of a moving plaid.

We performed a series of experiments examining the effect of contrast on the perception of moving plaids. This was done to test the hypothesis put forth by Adelson and Movshon (1982) that the human visual system determines the direction of a moving plaid in a two-staged process: decomposition into component motion followed by application of the intersection of constraints rule. Although there is recent evidence that the first tenet of their hypothesis is correct, i.e. that plaid motion is initially decomposed into the motion of the individual grating components (Movshon, Adelson, Gizzi & Newsome, 1986; Welch, 1989), the nature of the second-stage combination rule has not as yet been established. We found that when the gratings within the plaid are of different contrast, the perceived direction is not predicted by the intersection of constraints rule. There is a strong (up to 20 deg) bias in the direction of the higher-contrast grating. A revised model, which incorporates a contrast-dependent weighting of perceived grating speed as observed for 1-D patterns (Thompson, 1982), can quantitatively predict most of our results. We discuss our results in the context of various models of human visual motion processing and of physiological responses of neurons in the primate visual system.

Adolescent↗

PubMed1993

Isolating the effect of one-dimensional motion signals on the perceived direction of moving two-dimensional objects.

A considerable body of evidence suggests the existence of a two-stage mechanism for the detection of global motion. In the first stage the motion of elongated contours is extracted and then at the second stage these one-dimensional (1D) motion signals are combined. What is the nature of the computation carried out in combining the 1D motion signals towards forming a global motion percept? We devised a set of stimuli that differentiate between different possible computations. In particular, they distinguish between a velocity-space construction (such as intersection of constraints) and a linear computation such as vector averaging. In addition, these stimuli do not contain two-dimensional (2D) motion signals such as line intersections, that allow unambiguous determination of global velocity. Stimuli were presented in uncrossed disparity relative to the aperture through which they were presented, to reduce the effect of line terminator motion. We found that subjects are unable to detect the veridical global direction of motion for these stimuli. Instead, they perceive the stimulus pattern to be moving in a direction which reflects the average of its 1D motion components. Our results suggest that the visual system is not equipped with a mechanism implementing a velocity-space computation of global motion.

Humans↗

PubMed1994

A role for a low level mechanism in determining plaid coherence.

A number of recent studies have suggested that the "intersection of constraints" model of two dimensional motion perception, put forward by Adelson and Movshon [(1982) Nature, 300, 523-525], is incomplete. Evidence has been mounting that there is a second two-dimensional motion sensitive mechanism which is monocular and which appears to respond directly to the movement of the intersections (or "blobs") in a two-dimensional image. The current study extends these findings by demonstrating that the perceived coherence of a drifting plaid is largely under the control of a monocular mechanism. Prior exposure to a similarly drifting grating or plaid substantially raises the coherence threshold of a test plaid only if the same eye is adapted and tested. The threshold elevation is much more modest if the test plaid is presented to the unadapted eye, suggesting that coherence judgements are primarily based on the activity level of a monocular process--possibly the "blob tracking mechanism". The results of Expt 2 suggest the possibility that this monocular mechanism is inhibited by binocular exposure.

Adaptation, Ocular↗

PubMed1994

On the role of second-order signals in the perceived direction of motion of type II plaid patterns.

Second-order Type I and Type II plaids were constructed by combining two random-dot gratings. Each component consisted of a dynamic random-dot field, the contrast of which was modulated by a drifting sinusoidal grating. Orienting the two components suitably and interleaving at 120 Hz allowed us to produce a two-dimensional plaid pattern made from one-dimensional second-order components. The perceived direction of motion of both Type I and Type II plaids was measured as a function of stimulus duration. Type I plaids had a perceived direction close to the intersection of constraints/vector sum solution (which only coincide for these patterns) at all durations. Type II plaids had a perceived direction that moved away from the vector sum and toward the intersection of constraints solution as the duration of presentation increased. These results are similar in form to those found for plaids made from first-order (luminance-defined) components [Yo & Wilson (1992), Vision Research, 32, 135-147]. This suggests that a delay which operates specifically on second-order signals cannot be the sole cause for the change in perceived direction of Type II plaids made from first-order components [Wilson, Ferrera & Yo (1992), Visual Neuroscience, 9, 79-97].

Humans↗

PubMed1996

Confusion points and constant-luminance planes for trichromats, protanopes and deuteranopes.

The confusion points of dichromats are derived from the constant-luminance planes of trichromats, protanopes and deuteranopes experimentally defined by heterochromatic-flicker photometry: (1) the zero-luminance planes of the observers considered in this experiment intersect almost exactly in a line that crosses the plane of the chromaticity diagram in the tritanopic-confusion point and confirm that the short-wavelength sensitive cones can be considered to have no contribution to luminance; (2) protanopic- and deuteranopic-confusion points are taken as being defined by the intersection of the tangent line to the long-wavelength region of the spectrum locus and the zero-luminance plane for protanopes and deuteranopes, respectively.

Adaptation, Ocular↗

PubMed1990

Longitudinal Gompertzian analysis of amyotrophic lateral sclerosis mortality in the U.S., 1977-1986: evidence for an inherently susceptible population subset.

Age-adjusted mortality rates for amyotrophic lateral sclerosis (ALS) for men and women in the United States from 1977 to 1986 were determined and subjected to longitudinal Gompertzian analysis. The exponential decline in the rate of increase of age-adjusted ALS mortality rates after age 55 years is most consistent with the existence of an inherently susceptible population subset that is decreasing faster than the general population. In the U.S. between 1977 and 1986, annual age-adjusted ALS mortality rate distributions were determined by a common fixed intersect point (for men, the death rate at age 46.38 years was 0.91/100,000; for women, the death rate at age 45.85 years was 0.46/100,000); and an environmental factor that varied erratically during the decade by a factor of 7.21 for men and 11.64 for women. On the average, the U.S. environment during this period was 4.33 times more conducive to mortality from ALS in men than in women. Overall ALS mortality between 1977 and 1986 increased 46% and 49% for men and women, respectively. The common fixed intersect point in mortality rate distributions suggests that these increases were real and not merely the result of improved diagnosis and/or better reporting. The overall increase in ALS mortality most likely results from an effective increase in the susceptible population subset due to increasing life expectancy, rather than to environmental factors. That is, as life expectancy increases, more of the susceptible population subset live long enough to express the disease.

Adult↗