Biomedical subjects

H Vaitkevicius

Publications and source records attributed to H Vaitkevicius.

7 recordsLinked to original sources

PubMed2006

Almost complete colour constancy achieved with full-field adaptation.

A successive asymmetric colour-matching task was used to study the changes in colour appearance of simulated Munsell samples. Colour shifts were induced with two Planckian illuminants, standard illuminant A (u'=0.256, v'=0.524) and illuminant S (u'=0.174, v'=0.392). Measurements were conducted with a 20 degrees field and a 120 degrees field. Adaptation period varied from 1 to 30s with the smaller field and from 1 to 60s with the larger field. Colour shifts were specified in terms of a modified Brunswik ratio (BR). Higher values of BR were associated with longer adaptation periods but only when the larger background was used. Supplementary experiments showed that the changes in colour appearance were related to a slight shift in the perceived colour of the background. The timing of the colour shifts are modelled in terms of cone opponent responses. High values of BR correspond to almost complete von Kries adaptation in all three cone types.

Adaptation, Ocular↗

PubMed2006

Cone contrast computations: physical versus perceived background and colour constancy.

The light reflected from an object depends both on the object's surface and on the illuminant. Visual systems attempt to resolve this intrinsic ambiguity by comparing the light reflected from the object with respect to the background by computing the difference between the object-background light sampled by three cones. The cone-contrasts for the sample-background stimulus under the test illumination should correspond to the cone-contrasts for samples matched in appearance under the standard background (C). The validity of this cone-contrast rule (Whittle, 2003) and its possible link with stability of perceived colour was studied here using six test illuminants. A successive asymmetric colour-matching task with 40 simulated Munsell samples (value 7, chroma 4) on a neutral background (N7) was used. The subject adjusted the sample under standard illuminant C to match the colour appearance of the test sample under one of the test illuminants. Brunswik Ratio (BR) was used as an index of stability of colour appearance. When computed with respect to the reference illuminant C, the cone contrast rule was violated (particularly for S-cone-contrast). However, if a new reference point based on the perceived colour of the neutral background under the test illumination was used, the cone contrast rule applied. That is, when cone contrasts of the matching samples are computed with respect to this perceived background, they correspond to cone contrasts of the test stimuli. This represents a form of discounting the illuminant for the purpose of determining an object's cone-contrast against the background, which does not vary with background illumination. These cone contrasts, however, do not provide any information about the colour appearance of objects under particular viewing conditions, unless calibrated against a standard by allowing subjects to learn particular colours.

Adaptation, Ocular↗

PubMed2001

Sequencing and characterization of the human thiazide-sensitive Na-Cl cotransporter (SLC12A3) gene promoter.

The thiazide-sensitive Na-Cl cotransporter SLC12A3 displays expression restricted to distal convoluted tubule cells where it catalyzes the uptake of sodium and chloride through the apical membrane. We sequenced 1959 bp of the 5' flanking region of human SLC12A3, located the area of transcription initiation, and used deletion constructs of the flanking region to determine areas that affect reporter gene expression in two cell lines, MDCT and CHO. Amplification of the 5' end of SLC12A3 cDNA from an adapter-ligated human kidney cDNA library demonstrated that transcription initiation is confined to an area from -18 to -6 bp upstream of the translation start codon. Maximum promoter activity (9.815 +/- 0.864 times control) was observed in MDCT cells using a promoter containing 1019 bp of the 5' flanking region. A promoter containing only 134 bp of the 5' flanking region upstream of the translation initiation codon maintained reporter gene expression at levels equal to 75% of that maximally observed (7.375 +/- 0.533 times control). Sequence analysis of this minimal promoter responsible for most of the SLC12A3 promoter activity revealed a TATA element, two Sp binding sites, a potential E box, and a potential binding site for NF-1/CTF or NY-I/CP-I. This promoter, and all other promoter constructs from SLC12A3, displayed repressor activity in CHO cells. A construct containing sequence 94 bp upstream of the initiation codon with two potential Sp binding sites was required for this repression. Protein-DNA interactions between the 182 bp region immediately upstream of the start codon and the nuclear proteins from rat kidney cortex and HeLa cells were examined to further clarify the role of the putative binding sites for SLC12A3 expression. Physiological studies investigating the effects of osmolarity, pH, and mineralocorticoid steroid on promoter activity demonstrated that the promoter activity was repressed by acidification, whereas no effects of increased osmolarity or deoxycorticosterone acetate addition were observed.

5' Untranslated Regions↗

PubMed1998

Mechanism of epinephrine-induced platelet aggregation.

We report that a genetic polymorphism of the alpha2-adrenergic receptor (A2AR) encoded by chromosome 10 is associated with hypertension and an increase in epinephrine-mediated platelet aggregation in humans. The mechanism responsible for this heritable contrast in sensitivity to epinephrine is unknown. We tested our hypothesis that epinephrine-induced platelet aggregation is mediated by activation of chloride transport. We measured epinephrine-mediated increases in optical density of gel-filtered platelets suspended in a bicarbonate-buffered physiological salt solution. Compared with platelets incubated in the control buffer (130 mmol/L NaCl), platelets incubated with either bumetanide, a Na/K/2Cl cotransport inhibitor; anthracene-9-carboxylic acid, a chloride channel blocker; or acetazolamide, an agent that blocks ATP-dependent chloride transport had significantly decreased aggregation responses to epinephrine. When measured fluorometrically, epinephrine significantly increased intraplatelet chloride concentrations. Chloride-dependent modifications of epinephrine-induced platelet aggregation were not attributable to changes in A2AR ligand binding characteristics or to the concentration of platelet cAMP. Finally, subthreshold concentrations of epinephrine also potentiated thrombin-induced platelet aggregation, and blockade of chloride transport diminished this synergistic action of epinephrine on thrombin-stimulated platelet aggregation. Heritable differences in epinephrine-mediated platelet aggregation may be attributable to genetic differences in chloride transport in platelets. Furthermore, because we observed a necessary role for chloride transport in epinephrine-mediated platelet aggregation, pharmacological agents that block chloride transport, such as diuretics, may provide salutary protection against vascular thrombosis in patients with hypertension independent of the effect of these drugs on blood pressure.

Adrenergic alpha-Antagonists↗

PubMed1997

Colour constancy as a function of hue.

The effects of colour categories on colour constancy were studied under two illuminants and two neutral grey backgrounds using Munsell chips and colour matching. It was found that the categorical colours: red, yellow, green and blue, which are processed by basic colour-opponent mechanisms, show relatively better colour constancy than intermediate colours. The dominant wavelength of these categorical colours are closely related to the typical hues obtained in experiments with narrow-band spectral hues.

Color Perception↗

PubMed1983

A model for the monocular line orientation analyzer.

A model for monocular line perception by human Ss is based on three basic assumptions: (a) the line's inclination is coded by the maximally excited orientation detector's number; (b) the inclination of the perceived line is equivocally determined by the excitation vector in the subjective space; (c) the analyzer has a maximum differential sensitivity over the whole range of the line inclinations. This simple model for the line inclination analyzer, taking into account the optimization of its sensitivity, provides incomplex explanations for a wide range of psychophysical and neurophysiological data obtained from human and animal experiments.

Animals↗

PubMedDate not supplied

Cone contrasts do not predict color constancy.

A successive, asymmetric color-matching paradigm was used to investigate the link between cone contrast and the stability of perceived colors. We measured the perceived color shifts of 10 Munsell samples, induced by test illuminant A, simulated in u'v' color space. The capacity of the visual system to resist these shifts, otherwise known as color constancy, is measured in terms of the Brunswik ratio, BR. Cone contrasts are calculated with respect to either the physical or perceived background. Subjective cone contrasts show a better fit to the von Kries law than those based on the physical background. Complete cone adaptation occurs when color constancy is high. However we show conditions where cone adaptation seems complete but color constancy is poor.

Color↗