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  • 1
    ISSN: 1432-1106
    Keywords: Amblyopia ; Human ; Psychophysics ; Edge-detection ; Subthreshold addition ; Contrast sensitivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Detection of broadband, aperiodic stimuli (edges) was investigated in normal observers, and in observers with abnormal visual experience which resulted in amblyopia. The spatial properties of the mechanisms used to detect an edge were investigated by a method of subthreshold addition. The method involved the determination of the threshold contrast for detecting an edge in the presence of a subthreshold line at various distances from the edge. In normal eyes, the one dimensional sensitivity profile of the edge detecting mechanism was: (1) approximately antisymmetric, (2) very localized, with sensitivity changes restricted to ±6'–8' on either side of the edge, and (3) phase dependent, showing an abrupt change in sign between ±1.5'. The sensitivity profiles of the amblyopic eyes were also approximately antisymmetric and showed the same steep rate of change from plus to minus as the fellow (nonamblyopic) eyes. However, in every case, the spatial extent of the profile was much broader than that of the nonamblyopic eyes. In normal eyes, the narrowest edge sensitivity profile was associated with the fovea; however, in two amblyopes with eccentric fixation, the narrowest edge sensitivity profile coincided with the locus of eccentric fixation. Moreover, the grating sensitivity function of the edge detecting mechanism of the amblyopic eye was similar to that of the non-amblyopic eye, but was shifted toward lower spatial frequencies. Control experiments show that these results are not accounted for on the basis of optics, eccentric fixation, or abnormal eye movements. The findings are discussed in terms of current models for the detection of aperiodic stimuli, and in the context of animal models of amblyopia.
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  • 2
    ISSN: 1432-119X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Alveolar type II like cells (ALT II) represent a small subpopulation of alveolar type II cells, which is able to proliferate, can be passaged and possess many characteristics of differentiated adult type II cells. A correlation was found between the growth and development of ALT II cells in culture and their alkaline phosphatase activity. Unlike alveolar type II cells, which lose the activity in culture, ALT II cells regain the activity and maintain it for a long culture period. Quantitative histochemical analysis of the stained cells indicate that 80% of the cells at days 15–20 in culture are alkaline phosphatase positive. Inhibition studies indicate that alkaline phosphatase from ALT II cells and freshly isolated type II cells were similar. The inhibition of ALT II alkaline phosphatase byl-levamisole and its heat stability are similar to that of the bone enzyme and differ from the intestinal enzyme. Alkaline phosphatase expression is considered part of the differentiated phenotype of these cells. Therefore, the presence of this enzyme in ALT II cells adds support to the notion that these cells maintain many aspects of mature alveolar type II cells.
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  • 3
    ISSN: 1432-1106
    Keywords: Visually evoked potential ; Amblyopia ; Pseudorandom modulation ; Temporal frequency ; Spatio-temporal interactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The visually evoked potential was recorded in response to pseudorandom modulation of a uniform field and sine-wave gratings in humans with naturally occurring amblyopia. Analysis in the frequency domain showed similar temporal tuning when the nonamblyopic and amblyopic eye were stimulated with a uniform field and with spatial stimuli. Although most observers showed some reduction in the cortical response to stimulation of the amblyopic eye with uniform field modulation, all observers showed reductions in the response obtained for stimulation of the amblyopic eye with spatial stimuli. This decrease in the cortical response to stimulation of the amblyopic eye for sine-wave gratings was present at either the low and middle temporal frequencies or over the range of temporal frequencies tested, and was greater than that observed in those amblyopes who also showed reductions with uniform field modulation. Latency measures in the time domain showed increases in the response of the early components when the amblyopic eye was stimulated with a uniform field and with sine-wave gratings above 2 c/deg. These electrophysiological results confirm the results obtained psychophysically which suggest that although amblyopia is primarily a spatial anomaly, the response is influenced by the temporal attributes of the stimulus as well.
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  • 4
    ISSN: 1573-9333
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Physics
    Notes: Abstract We identify the Painlevé Lax pairs with those corresponding to stationary solutions of non-isospectral flows, both for partial differential equations and differential-difference equations. We discuss symmetry reductions of integrable differential-difference equations and show that, in contrast with the continuous case, where Painlevé equations naturally arise, in the discrete case the so-called “discrete Painlevé equations” cannot be obtained in this way. Actually, symmetry reductions of integrable differential-difference equations naturally provide “delay Painlevé equations.”
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  • 5
    ISSN: 1573-9333
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics , Physics
    Notes: Abstract We review the Levi-Civita theory, which reduces the study of the irrotational flow in a one-dimensional channel or the solution of a non-linear differential-functional partial differential equatin for the velocity potential. We show how, by considering small perturbations in a shallow water channel, we can reduce the non-linear differential-functional equation to a complex Korteweg-de Vries equation which, for almost horizontal flow and for initial conditions indepedent of the vertical variable, reduces to the usual one.
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  • 6
    ISSN: 1573-2622
    Keywords: Binocular interactions ; Psychophysics ; Amblyopia and strabismus ; Dichoptic masking ; Binocular vision ; Contrast sensitivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Binocular interactions for grating patterns were investigated in humans with normal binocular vision and in humans with abnormal binocular visual experience due to strabismus and/or amblyopia via 1) comparison of monocular and binocular contrast thresholds; 2) interocular transfer of the threshold elevation aftereffect; and 3) dichoptic masking. Whereas the normal observers showed improved binocular over monocular contrast sensitivity (i.e., binocular summation) and substantial interocular transfer of the threshold elevation aftereffect, the abnormal observers showed an absence of binocular summation and no significant interocular transfer. The dichoptic masking experiments showed that a suprathreshold masking grating presented to one eye elevated the contrast threshold for gratings presented to the fellow eye, within a narrow range of spatial frequencies (about 1 octave wide at half height) and orientations, centered about the spatial frequency and orientation of the mask. The magnitude and bandwidth of this masking effect was similar in subjects with normal and abnormal binocular vision, occurring even when the masking grating was presented to the amblyopic eye. These effects depend upon the contrast of the masking grating. In individuals with normal binocular vision, a grating with subthreshold contrast presented to one eye reduces the contrast threshold for detection of gratings of similar spatial frequency and orientation presented to the fellow eye. No such subthreshold summation is evident in the amblyopic observers. We conclude that while strabismus and/or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections were not disrupted.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Documenta ophthalmologica 55 (1983), S. 213-236 
    ISSN: 1573-2622
    Keywords: amblyopia ; suprathreshold ; contrast magnitude estimation ; contrast matching ; reaction-time ; psychophysics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Three experiments were conducted to investigate suprathreshold contrast perception of a group of strabismic and anisometropic amblyopes. In the first experiment, simple reaction-time was measured as a function of the contrast of sinusoidal gratings. At all contrast levels, the amblyopic eyes showed prolonged reaction-times compared to the non-amblyopic eyes. In the second experiment, the perceived contrast of suprathreshold sinusoidal gratings was measured using an interocular successive matching paradigm. The third experiment compared perceived contrast of the amblyopic and non-amblyopic eye by contrast magnitude estimation. The results of the matching and magnitude estimation experiments showed that while amblyopic observers show large losses of contrast sensitivity at threshold, at suprathreshold contrast levels perceived contrast is essentially normal or near normal in the amblyopic eye. Taken together, these experiments suggest that: 1) the amblyopic eye has a higher contrast gain than the non-amblyopic eye, and 2) the amblyopic eye shows prolonged response latencies. Similar results can be obtained in normal observers under conditions of peripheral viewing and noise masking.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Journal of elasticity 24 (1990), S. 295-300 
    ISSN: 1573-2681
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Invariance of the nonlinear elasto-static system descriptive of the plane-strain deformation of a neo-Hookean material is determined under Lie group transformations which accommodate change in stretch. The associated finite transformations are constructed and the results are set in the context of previous work in the area.
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