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K Knoblauch

Publications and source records attributed to K Knoblauch.

At least 19 recordsLinked to original sources
PubMed2001

Variation of chromatic sensitivity across the life span.

Thresholds were measured along three directions in color space for detecting an equiluminant color change of a set of bars embedded in a larger field of spatio-temporal achromatic noise for observers ranging in age from 3 months to 86 years. Pre-verbal observers were assessed with a forced-choice preferential-looking technique while older observers responded orally or manually. Over the life span, thresholds could be described along each color axis tested by a curve with two trends. Thresholds decreased with each doubling of age by nearly a factor of two until adolescence. Thereafter, thresholds increased by a factor of 1.4-2 with each doubling of age. Sensitivity to chromatic differences varied similarly along all three axes tested, suggesting uniformity in the sensitivity of chromatic mechanisms across the life span.

Adolescent↗
PubMed2000

Laminar distribution of neurons in extrastriate areas projecting to visual areas V1 and V4 correlates with the hierarchical rank and indicates the operation of a distance rule.

The directionality of corticocortical projections is classified as feedforward (going from a lower to higher hierarchical levels), feedback (interconnecting descending levels), and lateral (interconnecting equivalent levels). Directionality is determined by the combined criteria of the laminar patterns of the axon terminals as well as the cells of origins and has been used to construct models of the visual system, which reveals a strict hierarchical organization (Felleman and Van Essen, 1991; Hilgetag et al., 1996a). However, these models are indeterminate partly because we have no indication of the distance separating adjacent levels. Here we have attempted to determine a graded parameter describing the anatomical relationship of interconnected areas. We have investigated whether the precise percentage of labeled supragranular layer neurons (SLN%) in each afferent area after injection in either visual areas V1 or V4 determines its hierarchical position in the model. This shows that pathway directionality in the Felleman and Van Essen model is characterized by a range of SLN% values. The one exception is the projection of the frontal eye field to area V4, which resembles a feedforward projection. Individual areal differences in SLN% values are highly significant, and the number of hierarchical steps separating a target area from a source area is found to be tightly correlated to SLN%. The present results show that the hierarchical rank of each afferent area is reliably indicated by SLN%, and therefore this constitutes a graded parameter that is related to hierarchical distance.

Animals↗
PubMed1998

Spectral bandwidths for the detection of color.

The spectral properties of human color detection mechanisms were measured using a noise masking technique that minimizes the possibility of off-axis looking and artifactually narrow estimates of bandwidth. Observers were induced to use a single detection mechanism throughout a spectral bandwidth measurement by using sectored noise to mask a time-varying signal of fixed chromatic properties. Sectored noise draws samples from sectors of variable width in the color plane, centered on the signal axis. Contrast thresholds for equiluminant signals that appeared yellow, orange, red and violet were found to depend on the power of the noise, projected along the chromatic axis of the signal, but not on the sector width of the noise. These results are consistent with the activity of spectrally broadband, linear detection mechanisms that are tuned to the signal color directions tested.

Color Perception↗
PubMed1998

M- and L-cones in early infancy: I. VEP responses to receptor-isolating stimuli at 4- and 8-weeks of age.

A silent-substitution technique combined with measures of the visually-evoked potential (VEP) was used to determine whether M- and L-cones are functional in early infancy. Data were successfully collected from twenty six infants in response to three receptor-isolation conditions (rod, M- and L-cone isolation) and a luminance-modulation condition. The efficacy of the receptor-isolation conditions was first verified by measuring VEP responses from both dichromatic and color-normal adults to each of the receptor-isolation conditions. Both 4- and 8-week-old infants demonstrated VEP responses to the M- and L-cone isolating stimuli, though the amplitude of the the responses at 4-weeks were reduced compared to those at 8-weeks. These data suggest that the functioning of M- and L-cones can be differentiated as early as 4-weeks of age.

Adult↗
PubMed1998

M- and L-cones in early infancy: II. Action spectra at 8 weeks of age.

Field sensitivities were measured under conditions of M- and L-cone isolation for seven infants (8-12 weeks-old) and two adults, using silent-substitution and the visually evoked potential (VEP). The efficacy of the receptor-isolation conditions were first verified by measuring psychophysical and VEP-derived action spectra from two color-normal adults under conditions of M- and L-cone isolation. M- and L-cone action spectra obtained from the two methods were found to be similar to the Smith and Pokorny M- and L-cone fundamentals, respectively. The VEP-derived action spectra obtained from infants and adults were well fit by the Smith and Pokorny M- and L-cone fundamentals. These data, in conjunction with our previous study, confirm that M- and L-cones are operating by 8 weeks and possibly as early as 4 weeks of age.

Adult↗
PubMed1998

Dependence of color on context in a case of cortical color vision deficiency.

Color constancy depends on sensitivity to change in both illumination spectral properties and object position. We investigated this latter form of color constancy by asking a cerebral achromatopsic to name the colors of papers that were presented atop black, gray or white backgrounds under identical illumination. Comparison of color names across background conditions reveals poor constancy, characterized by a contrasting of foreground and background values that is not corrected by proper anchoring.

Brain Diseases↗
PubMed1998

M- and L-cones in early infancy: III. Comparison of genotypic and phenotypic markers of color vision in infants and adults.

Genetic analyses were performed on five male children (approximately 3 years), two suspect color-normals and three suspects for congenital color vision deficiencies. These classifications were based on visually-evoked potential (VEP) responses to M- and L-cone-isolating stimuli obtained in a previous study when each subject was either 4- or 8-weeks old. The present analyses were performed in a blind study to characterize the genotypes of these subjects. Four male adults with various color vision phenotypes were also tested as a control. DNA was isolated using a non-invasive technique followed by polymerase chain reaction (PCR) amplification and restriction enzyme analysis to examine the genomic DNA of each subject. The genetic analyses confirmed the VEP identification of two color defective infants, and were consistent with the diagnosis of two other infants as color normal. A third infant was predicted by VEP analysis to have a protan defect, but he did not have a gene array typically found in protan observers.

Child, Preschool↗
PubMed1997

Color transparency.

Observation suggests that the chromatic changes which elicit an impression of transparency include translations and convergences in color space. Neither rotations nor shears in color space lead to perceived transparency. Results of matching experiments show that equiluminous translations, which cannot be generated by episcotister or filter models, give rise to the perception of transparency. This implies that systematic luminance change is not needed for transparency to be perceived. These results were used for the development of a method for detecting a transparent overlay within a color image and for separating the overlay from the underlying surfaces. The method tests for the coherence of chromatic change along contours through X-junctions to help detect the contour of a transparent region. The algorithm tests locally for translation and convergence to detect a transparent region. It estimates globally the chromatic parameters of the transparent overlay in order to separate the overlay from the underlying surfaces.

Color Perception↗
PubMed1996

Testing the indeterminacy of linear color mechanisms from color discrimination data.

It have previously been reported that, for some choices of the fixed spatial and temporal characteristics of test stimuli, it was possible to estimate the spectral sensitivities of chromatic mechanisms from chromatic discrimination data alone. If mechanism sensitivities could be reliably estimated for any choice of test stimuli characteristics, the influence of spatial and temporal factors on chromatic discrimination performance could be directly measured. Previous studies, using test stimuli with other spatio-temporal characteristics, have found equi-discrimination contours whose ellipsoidal shapes seem to preclude estimation of mechanisms. Since there is no commonly-accepted method for testing the adequacy of ellipsoidal fits of chromatic equi-discrimination contours, it is possible that alternative psychophysical procedures combined with more powerful statistical tests could detect the pattern of deviations from ellipticality reported previously. In this paper, we described psychophysical tests and statistical analyses that, taken together, provide a more powerful test of the indeterminacy of mechanisms than previous methods. We develop a method based on analysis of residuals for detecting the pattern of deviations from ellipticality. We apply these tests under fixed experimental conditions similar to those in which other researchers have found ellipsoidal equi-discrimination contours. For these conditions, for any of the tests performed, we do not reject the hypothesis that equi-discrimination surfaces are ellipsoidal.

Color Perception↗
PubMed1996

Detection and identification of mirror-image letter pairs in central and peripheral vision.

Reading performance is poorer in the peripheral than in the central visual field, even after size-scaling to compensate for differences in visual acuity at the different eccentricities. Since several studies have indicated that the peripheral retina is deficient with respect to spatial phase discrimination, we compared the psychometric functions for detection (D) and identification (I) of size-scaled, mirror-symmetric letters (i.e. letters differing in the phase spectra of their odd symmetric components) at three inferior field eccentricities (0, 4, and 7.5 deg) using a two-alternative, temporal, forced-choice procedure and retinal image stabilization to control retinal locus. Each subject's data were fit with Weibull functions and tested for goodness-of-fit under several hypotheses. This analysis revealed that while the psychometric functions were of constant shape across eccentricity for the respective tasks, they showed statistically significant variations in the D/I threshold ratios. However, these variations were so small that poorer reading outside the fovea is unlikely to be due to reduced letter discriminability that might occur secondary to a loss of peripheral field phase sensitivity.

Adult↗
PubMed1995

Discrimination of binocular color mixtures in dichromacy: evaluation of the Maxwell-Cornsweet conjecture.

We tested the Maxwell-Cornsweet conjecture that differential spectral filtering of the two eyes can increase the dimensionality of a dichromat's color vision. Sex-linked dichromats wore filters that differentially passed long- and middle-wavelength regions of the spectrum to each eye. Monocularly, temporal modulation thresholds (1.5 Hz) for color mixtures from the Rayleigh region of the spectrum were accounted for by a single, univariant mechanism. Binocularly, univariance was rejected because, as in monocular viewing by trichromats, in no color direction could silent substitution of the color mixtures be obtained. Despite the filter-aided increase in dimension, estimated wavelength discrimination was quite poor in this spectral region, suggesting a limit to the effectiveness of this technique.

Adult↗
PubMed1994

Activation of complement by cold agglutinins.

OBJECTIVE: To review recent reports on the interaction of cold agglutinins with the complement system and its relevance to cold agglutinin disease. DATA SOURCES: Review articles and original papers have been selected for this contribution. SELECTION CRITERIA: The report focuses on experimental data available from in vitro studies as well as clinical findings regarding the mechanisms of cold agglutinin-induced complement activation. RESULTS: Despite the observation that only few cold agglutinins (almost exclusively IgM molecules with anti-I specificity) induce in vitro hemolysis of human red blood cells with human serum (homologous system), the vast majority of these autoantibodies are able to initiate the classical pathway sequence with the fixation of C1 and to a lesser degree of C4. The ability of IgM cold agglutinins to activate, in principle, complement is demonstrated by their hemolytic efficiency in the presence of animal serum as a source of heterologous complement. In addition to the thermal amplitude of cold agglutinin binding, a possible interference with membrane regulatory proteins may render certain cold agglutinins hemolytically active in a homologous system. CONCLUSION: Despite a hemolytic inefficiency, cold agglutinin-induced fixation of early complement components up to C3 leads to an accelerated clearance of red cells from the circulation by hepatic sequestration. However, it is not yet clear, to what degree these cells are eliminated by the reticuloendothelial system or whether they return to the circulation. Dependent on the amount of membrane-bound C3 fragments these cells may even be protected against further cold agglutinin-induced complement attack.

Agglutinins↗
PubMed1993

Theory of wavelength discrimination in tritanopia.

Many theories of color discrimination predict a discontinuity in the wavelength-discrimination function of a tritanope at the point in the spectrum at which the rate of change of the visual signal constrained to an equiluminant plane passes through zero (near 460 nm). The predicted discontinuity follows from the use of a first-order approximation for which the reciprocal of the slope of the response function that generates the visual signal is proportional to the discrimination limen. In view of the good discrimination shown by such observers elsewhere in the spectrum, however, such a singularity is impossible. I show that the inclusion of the higher-order terms produces a finite value in the 460-nm region that falls in the range of values from the literature that have been obtained experimentally.

Color Perception↗
PubMed1991

Effects of chromatic and luminance contrast on reading.

Reading performance was measured for drifting text defined by chromatic contrast with various amounts of luminance contrast present. With 0.12 luminance contrast added, reading performance was unaffected by the presence of chromatic contrast over a range of character sizes varying 30-fold. When luminance contrast was reduced to near the threshold for reading, chromatic contrast sustained reading rates of nearly 300 words per minute, almost as high as those found with high luminance contrasts. Low-pass filtering of chromatic text had a proportionately greater effect on small characters, as would be predicted from the lower bandwidth of chromatic visual channels. Arguments are presented suggesting that reading rates for equiluminant text are sustained by luminance transients introduced by transverse chromatic aberrations of the eye.

Color Perception↗
PubMed1990

Reading with fixed and variable character pitch.

We compared the effects of fixed and variable (proportional) spacing on reading speeds and found variable pitch to yield better performance at medium and large character sizes and fixed pitch to be superior for character sizes approaching the acuity limit. The data indicate at least two crowding effects at the smallest sizes: one that interferes with individual character identification and one that interferes with word identification. A control experiment using rapid serial visual presentation suggests that it is the greater horizontal compression and consequently reduced eye-movement requirements of variable pitch that are responsible for its superiority at medium and large character sizes.

Eye Movements↗
PubMed1987

A retinal smear technique for varying grating contrast.

We present here a technique for modulating contrast in gratings seen in Maxwellian-view or on a projection screen. The method can be used with achromatic or chromatic grating stimuli. It is based on a high frequency oscillatory displacement of the grating image, which in turn produces a smearing of the image on the observer's retina. The waveform of the displacement and its amplitude determine the appearance and contrast of the image produced on the retina.

Form Perception↗
PubMed1987

[Meptazinol, a new analgesic. Hemodynamic and respiratory effects].

In a prospective, randomized double-blind study the hemodynamic and respiratory effects of i.v. meptazinol, a new synthetic narcotic agonist-antagonist analgesic given in 2 dosages (2 mg/kg and 4 mg/kg), were compared over 15 min to a control group that received 0.9% saline solution as placebo. In a total of 80 patients scheduled for elective aortocoronary bypass surgery, the study was performed before and during general anesthesia and during extracorporeal circulation (ECC). A dose-dependent increase in mean arterial pressure (MAP) and total systemic resistance (TSR) were the predominant hemodynamic effects of meptazinol before and during anaesthesia (Tables 1 and 2). Within the study period no change in heart rate (HR) or cardiac index (CI) could be observed. Directly measured left ventricular parameters revealed a significant increase in left ventricular pressure (LVEDP, and dp/dtmax (Fig. 1). During ECC meptazinol injection was followed by a dose-dependent increase in perfusion pressure, indicating a direct vasoconstrictive effect; no influence on the capacitance system ("venous pooling") could be observed (Fig. 2). In the spontaneously breathing, premedicated patients (morphine hydrochloride 0.15 mg/kg and flunitrazepam 0.03 mg/kg) paO2 increased and paCO2 decreased, especially after injection of the higher dosage of meptazinol; there was no influence on intrapulmonary right-to-left shunting (Qs/Qt).(ABSTRACT TRUNCATED AT 250 WORDS)

Azepines↗