E. Hita

Effect on laser-ablation algorithms of reflection losses and nonnormal incidence on the anterior cornea

In this paper, we provide an analytic expression for an adjustment factor in ablation algorithms for photorefractive laser surgery which take into account reflection losses and the nonnormal incidence on the cornea. We evaluate the influence of this factor on certain ocular parameters, calculating alterations in the radius of anterior cornea and corneal asphericity when we use the algorithm given by Munnerlyn’s paraxial formula. Our data indicate that this adjustment factor should be considered in the ablation algorithms that are currently being proposed in customized corneal ablation and that need great accuracy for the correction of eye aberrations.

Are We Able to Distinguish Color Attributes

The perception and understanding of the three color attributes have been analyzed from two experiments using pairs of Munsell samples, where only one of the three color attributes were changed/unchanged (Experiment I/II) at a time. In each experiment, 36 pairs with color differences of 3 different sizes (average values of 15.8 and 21.7 CIELAB units for Experiments I and II, respectively) were assessed under standardized conditions by 40 normal observers, 20 of them with previous knowledge and experience in colorimetry. At a 95% confidence level, the results from the two experiments were not significantly different, indicating that color attributes were not easily distinguished: for example, for experienced observers, the percentage of correct answers for identifying the color attribute responsible for a color difference was only 72.4%, the random probability being 33.3%. There were no significant differences between the results found by men and women. The worst distinguished attribute was Chroma, that is, the least frequent confusion was between Hue and Value or vice versa. Value differences were more easily detected for achromatic than for chromatic pairs, both for experienced and inexperienced observers. With respect to the size of the color differences, we observed that large hue differences were more easily identifiable than smaller ones, and a constant Hue was more identifiable when the entire color difference was small.

Influence of the Luminance and Opponent Chromatic Channels on Stereopsis with Randomdot Stereograms

The present work examines the relationship between random-dot stereograms (via the disparity range parameter) and color-vision mechanisms (via the luminance channel and red-green and tritan directions at isoluminance). The results clearly indicate that the variations in the stereograms along red-green confusion lines contribute to stereopsis. Stereoscopic perception depends on spatial information for stereograms generated with variations along tritan confusion lines. For observers who perceive stereopsis via tritan directions, the results show a gradation in the disparity range, with the disparity range for stereograms generated by luminance variations being greater than for stereograms generated in red-green directions; the latter range is, in turn, greater than for stereograms generated along tritan directions.

Evaluation of color-discrimination ellipsoids in two-color spaces

We obtained the color-differential thresholds for 20 stimuli and 3 observers within 2 color-representation spaces. First, in the CIE-1931 color space we compare the results of our experiments, which followed chromatic-discrimination criteria, with those obtained by other authors that followed color-matching criteria. Second, the representation of our data in the cone-excitation space shows that all the discrimination ellipses are nearly oriented along the S-cone-variation axis. In our experiments the variations in the threshold of each color channel according to its level of excitation are similar to those found by other authors using different experimental conditions and methods, but certain discrepancies appear when the influence that one color channel may exert on the other is analyzed. The characteristics of luminance discrimination are shown to be independent of the chromaticity of the stimuli.

Changes in Stereoscopic Depth Perception Caused by Decentration of Spectacle Lenses

The effect that decentered spectacle (ophthalmic) lenses exert on depth perception has been studied, evaluating stereopsis through the disparity range (maximum horizontal disparity) for random-dot stereograms (RDS). The results show that variations in fusional convergence due to increments of decentration can diminish the stereopsis in observers, reducing the region within which stereoscopic correspondence can take place. Decreases in disparity ranges were found for vertical and horizontal prismatic effects, although the prismatic effect necessary for this was less in the vertical case. A decreased disparity range has also been confirmed with figural-stimuli stereograms and using prisms for generating the prismatic effects.

Contribution to the experimental review of the colorimetric standard observer

The color-matching functions of six observers have been measured in the range of 400–700 nm in intervals of 10 nm using two modified Donaldson-type colorimeters at a luminance of 4.5 cd/m. A statistical study has been carried out to determine the interobserver variability (in our results), and the results of other authors using various representational color systems have been compared with ours. The most important findings are that, although a common mathematical mean cannot be found, the shape of the curves for the color-matching functions obtained in our laboratory for all the observers is very close to that of other authors, thereby implying that the overall color response of the human visual system is similar in all observers. In addition, the results show that, for the long wavelengths (around 600 nm), a great amount of desaturant primary (460 nm) is necessary, indicating possible rod intrusion under our experimental conditions.© 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 377–388, 1999

Towards a colorimetric characterization of the human iris

We propose a new approach to the quantitative determination of the iris colour through the spectroradiometric measurement of a set of 72 artificial irises under controlled illumination and geometry. The spread of these samples in the CIELAB colour space indicates no clearly distinguished groups within the set analysed. Colour measurements for different points of these irises and for the overall irises (pupil and specular image of the light source included) are well correlated. Complementary colour measurements were also performed for the irises of 25 ocular prostheses and 40 real observers. All these colour measurements appear to be quite consistent, the average standard deviation of any of the three a*, b*, L* co‐ordinates being lower than 1.0 units. In a visual experiment where 11 normal observers matched the 25 ocular prostheses using the 72 artificial irises, the results indicate that a simple criterion based on the lowest colour‐difference agreed with the observers’judgements in only 18% of the cases. This result suggests that, although colour appears to be a major parameter in the overall characterization of the human iris, additional factors such as patterns or textures should also be considered.

Chromaticity-discrimination thresholds with aperture and object colors: experimental results and predictions of some recent color-difference formulas.

Threshold-discrimination ellipses were obtained by three normal observers, at five color centers, by the use of two experimental devices, which provided light-surface colors, and the same method and experimental conditions. The ellipses obtained for each center in both devices were quite similar when the sample distributions were appropriate and slightly smaller for object color than for aperture color. The ellipses predicted by three recent color-difference formulas based on the CIE uniform color space (CIELAB) were compared with those experimentally obtained by us as well as by other researchers who analyzed a greater number of color centers. The color-difference formula proposed by the CIE Technical Committee 1-29 [Color Res. Appl. 18, 137 (1993)] provides the best prediction of the semiaxis relationship for all the experimental datasets used, confirming their good performance in previous works [Appl. Opt. 33, 8069 (1993)], although the differences with respect to the experimental results are higher than those attributable to the interobserver variability.

Amoxicillin-clavulanic acid hepatotoxicity in children.

Objectives: The objectives of the present study were to determine the importance of amoxicillin-clavulanic acid (AC) hepatotoxicity in the paediatric population and to characterise the episodes identified and potential host-specific factors. This was addressed via a prospective observational study in which 8 Spanish hospitals participated. Methods: A total of 11 cases were examined. For each patient included in the study, a structured and codified data-collection protocol was complied with, taking note of patient demographics, characteristics of the treatment assumed to provoke the reaction, concomitant medication, course and outcome of the episode, and laboratory variables during the reaction. The latter were determined every 6 months from the outset to the eventual resolution of the case. Results: A total of 11 cases of AC hepatotoxicity were detected, affecting 9 boys and 2 girls, ages 1 to 11 years. Causality criteria were assessed using the Council for International Organizations of Medical Sciences scale. Conclusions: We conclude that the introduction of hepatotoxicity record systems in paediatric care, together with the continuing study and development of existing systems, would contribute to improving our epidemiological knowledge about the harmful effects of drugs on the liver.

Spatial and chromatic dependencies on visual reaction time

The visual reaction time has been analyzed as a function of two experimental parameters : size of the field and chromaticity of the stimulus. With regard to the first one, field sizes of 0.72, 1.25, 2, 3.5, 5 and 7.5 degrees were studied. The results show that reaction time decreases when the field size increases, also for values above the limits of Riccos law. On the other hand, the reaction time was measured for chromatic stimuli with dominant wavelengths at 472, 500, 547, 571, 602 and 634 nm. In this case, no correlation between reaction time and chromaticity was found. The fact that these results were in agreement with those obtained for achromatic stimuli at the same luminance was analyzed in terms of color vision models, especially the opponent-colors theory.