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Biomedical subjects

S Marcos

Publications and source records attributed to S Marcos.

At least 19 recordsLinked to original sources
PubMed2000

On the symmetry between eyes of wavefront aberration and cone directionality.

There are two optical processes that control the retinal image sampled by the photoreceptor array: aberrations of the ocular optics and cone directionality (Stiles-Crawford effect). The shape of wavefront aberration and Stiles-Crawford functions are known to vary markedly across subjects. In this study we investigate in twelve subjects the symmetry between right and left eyes of wavefront aberration (measured using a spatially resolved refractometer) and cone directionality (measured using an imaging reflectometric technique). The pattern of aberrations is in general non-symmetric, suggesting that the development of aberrations follow independent paths in many right and left eye pairs. Cone directionality is in most cases mirror-symmetric (with one case of direct symmetry), suggesting some systematic process underlying cone orientation. Except in two subjects, symmetry in these two functions seems to be unrelated. Cone directionality apodization improves optical quality, but not optimally in all eyes, and it does not tend to increase symmetry in the optical performance of left and right eyes.

Adult↗
PubMed2000

Monochromatic aberrations in the accommodated human eye.

The wave-front aberration of the human eye was measured for eight subjects using a spatially resolved refractometer (a psychophysical ray-tracing test). The eyes were undilated and presented with accommodative stimuli varying from 0 to -6 diopters. Monochromatic wave-front aberrations tend to increase with increasing levels of accommodation, although there are substantial individual variations in the actual change in the wave-front aberration. While spherical aberration always decreased with increasing accommodation, it did not change from positive to negative for every observer. The direction and amount of change in fourth order aberrations varied between observers. Aberrations with orders higher than fourth are at a minimum near the resting state of accommodation. The accommodation induced change in wavefront aberration was not strongly related to the total amount of aberration in the eight eyes studied.

Accommodation, Ocular↗
PubMed2000

One-week therapy with pantoprazole versus ranitidine bismuth citrate plus two antibiotics for Helicobacter pylori eradication.

AIM: A combination of omeprazole plus amoxycillin (Amo) and clarithromycin (CIa) for 7 days has been studied extensively. However, the role of other proton pump inhibitors, such as pantoprazole (Pan), in this therapy is not well known. On the other hand, ranitidine bismuth citrate (RBC) also seems to be effective when combined with Amo and CIa. Our aim was to evaluate and to compare these two novel short-term triple therapies (Pan+Amo+Cla and RBC+Amo+Cla) for treatment of Helicobacter pylori. METHODS: In a randomized clinical trial 150 consecutive patients (38 with duodenal ulcer, 112 with non-ulcer dyspepsia) infected by H. pylori were studied prospectively. Exclusion criteria were: previous H. pylori eradication therapy, gastroerosive drug use, gastric surgery, and associated diseases. One of two regimens was given for 7 days: Pan (40 mg b.i.d.), Amo (1 g b.i.d.), Cla (500 mg b.i.d.) (group Pan+Amo+Cla, n = 75); or RBC (400 mg b.i.d.), Amo (1 g b.i.d.), Cla (500 mg b.i.d.) (group RBC+Amo+Cla, n = 75). All drugs were administered together after meals. Compliance was evaluated by return tablet count. Data were analysed by univariate (chi2) and multivariate (multiple logistic regression) analysis. Eradication was defined as a negative 13C-urea breath test 1 month after completing therapy. RESULTS: The distribution of studied variables (age, gender, smoking, duodenal ulcer/non-ulcer dyspepsia) was similar in both therapy groups. Per-protocol eradication was achieved in 48/71 (68%) in group Pan+Amo+Cla, and in 61/70 (87%) in group RBC+Amo+Cla (P= 0.01). Intention-to-treat (ITT) eradication was achieved in, respectively, 48/ 75 (64%) and in 61/75 (81%) (P= 0.03). The RBC+ Amo+Cla regimen was more effective than Pan+Amo+Cla in non-ulcer dyspepsia patients (ITT, 84% vs 58%; P = 0.005), but statistically significant differences were not demonstrated in duodenal ulcer patients (72% vs 80%). In the multivariate analysis the odds ratio for the effect of the type of therapy on H. pylori eradication in patients with non-ulcer dyspepsia was 3.8 (95% Cl, 1.6-9.3; P = 0.003). No relevant adverse effects were reported with any regimen. CONCLUSION: A RBC+Amo+Cla regimen for only 1 week is a promising therapy for H. pylori infection, due to its high efficacy, simple posology, and excellent tolerability. Combination of Pan with Amo and Cla, although effective in duodenal ulcer patients, but in non-ulcer dyspepsia has not achieved the favourable results previously reported with other proton pump inhibitors.

2-Pyridinylmethylsulfinylbenzimidazoles↗
PubMed1999

The depth-of-field of the human eye from objective and subjective measurements.

The depth-of-field (DOF) measured through psychophysical methods seems to depend on the target's characteristics. We use objective and subjective methods to determine the DOF of the eye for different pupil diameters and wavelengths in three subjects. Variation of image quality with focus is evaluated with a double-pass technique. Objective DOF is defined as the dioptric range for which the image quality does not change appreciably, based on optical criteria. Subjective DOF is based on the accuracy of focusing a point source. Additional DOFs are obtained by simulation from experimental wavefront aberration data from the same subjects. Objective and subjective measurements of DOF are only slightly affected by pupil size, wavelength and spectral composition. Comparison of DOF from double-pass and wavefront aberration data allows us to evaluate the role of ocular aberrations and Stiles-Crawford effect.

Adult↗
PubMed1999

A new approach to the study of ocular chromatic aberrations.

We measured the ocular wavefront aberration at six different visible wavelengths (between 450 and 650 nm) in three subjects, using a spatially resolved refractometer. In this technique, the angular deviation of light rays entering the pupil at different locations is measured with respect to a target viewed through a centered pupil. Fits of the data at each wavelength to Zernike polynomials were used to estimate the change of defocus with wavelength (longitudinal chromatic aberration, LCA) and the wavelength-dependence of the ocular aberrations. Measured LCA was in good agreement with the literature. In most cases the wavefront aberration increased slightly with wavelength. The angular deviations from the reference stimulus measured using a magenta filter allowed us to estimate the achromatic axis and both optical and perceived transverse chromatic aberration (TCA), (including the effect of aberrations and Stiles-Crawford effect). The amount of TCA varied markedly across subjects, and between eyes of the same subject. Finally, we used the results from these experiments to compute the image quality of the eye in polychromatic light.

Adult↗
PubMed1999

Seven-day 'rescue' therapy after Helicobacter pylori treatment failure: omeprazole, bismuth, tetracycline and metronidazole vs. ranitidine bismuth citrate, tetracycline and metronidazole.

BACKGROUND: Eradication therapy with omeprazole (O), amoxycillin (A) and clarithromycin (C) is used extensively, although it often fails. A 'rescue' therapy with a quadruple combination of O, bismuth (B), tetracycline (T) and metronidazole (M) has been recommended. AIM: : To assess ranitidine bismuth citrate (Rbc) instead of O and B for treatment failure. METHODS: Sixty consecutive patients (13 duodenal ulcer, 47 non-ulcer dyspepsia) in whom a previous eradication trial with O, A and C had failed were randomized to receive one of two regimens for 7 days: O (20 mg b.d.), B (120 mg q. d.s.), T (500 mg q.d.s.) and M (250 mg q.d.s.) (group OBTM, n=30); or Rbc (400 mg b.d.), T (500 mg q.d.s.) and M (250 mg q.d.s.) (group RbcTM, n=30). Eradication was defined as a negative 13C-urea breath test 1 month after completing therapy. RESULTS: Mean age +/- s.d. was 45 +/- 12 years, 47% were males. Distribution of studied variables (age, sex, smoking, duodenal ulcer/non-ulcer dyspepsia) was similar in both therapeutic groups. Per protocol eradication was achieved in 17 out of 29 patients (59%) in group OBTM and in 25 out of 29 patients (86%) in group RbcTM (P < 0.05). Intention-to-treat eradication was achieved, respectively, in 17 out of 30 (57%) and in 25 out of 30 (83%) (P < 0.05). In the multivariate analysis the variables which influenced on H. pylori eradication were the type of therapy (odds ratio, OR=3.9; 95%CI: 1.02-15; P < 0.05) and diagnosis (duodenal ulcer/non-ulcer dyspepsia) (OR=0.1; CI: 0.02-0.4). Adverse effects were infrequent and mild with both regimens. CONCLUSION: Therapy with RbcTM is a promising option after H. pylori eradication failure with OCA, achieving a higher efficacy than quadruple therapy with OBTM.

Adult↗
PubMed1999

Cone spacing and waveguide properties from cone directionality measurements.

Reflectometric techniques estimate the directionality of the retinal cones by measuring the distribution of light at the pupil plane of light reflected off the bleached retina. The waveguide-scattering model of Marcos et al. [J. Opt. Soc. Am. A 15, 2012 (1998)] predicts that the shape of this intensity distribution is determined by both the waveguide properties of the cone photoreceptors and the topography of the cone mosaic (cone spacing). We have performed two types of cone directionality measurement. In the first type, cone directionality estimates are obtained by measuring the spatial distribution of light returning from the retina with a single-entry pupil position (single-entry measurements). In the second type, estimates are obtained by measuring the total amount of light guided back through the pupil as a function of entry pupil position (multiple-entry measurements). As predicted by the model, single-entry measurements provide narrower distributions than the multiple-entry measurements, since the former are affected by both waveguides and scattering and the latter are affected primarily by waveguides. Measurements at different retinal eccentricities and at two different wavelengths are consistent with the model. We show that the broader multiple-entry measurements are not accounted for by cone disarray. Results of multiple-entry measurements are closer to results from measurements of the psychophysical Stiles-Crawford effect (although still narrower), and the variation with retinal eccentricity and wavelength is similar. By combining single- and multiple-entry measurements, we can estimate cone spacing. The estimates at 0- and 2-deg retinal eccentricities are in good agreement with published anatomical data.

Adult↗
PubMed1999

Comparison of cone directionality determined by psychophysical and reflectometric techniques.

We measured the directionality of the cones with both a psychophysical (Stiles-Crawford I) technique and an optical technique. The two sets of measurements were made in the same subjects, with stimuli as similar as possible used. The two types of measurements gave similar estimates of the location in the pupil toward which the cones were optimally aligned. However, the two measurements gave quite dissimilar estimates of the width of the directional sensitivity. On average, optical measurements were half as broad as psychophysical measurements in the fovea, but there were substantial individual differences. At 2-deg retinal eccentricity the difference between techniques was even more marked.

Adult↗
PubMed1998

Model for cone directionality reflectometric measurements based on scattering.

Reflectometric measurements provide an objective assessment of the directionality of the photoreceptors in the human retina. Measurements are obtained by imaging the distribution at the pupil plane of light reflected off the human fundus in a bleached condition. We propose that scattering as well as waveguides must be included in a model of the intensity distribution at the pupil plane. For scattering, the cone-photoreceptor array is treated as a random rough surface, characterized by the correlation length T (related to the distance between scatterers, i.e., mean cone spacing) and the roughness standard deviation sigma (assuming random length variations of the cone outer-segment lengths that produce random phase differences). For realistic values of T and sigma we can use the Kirchhoff approximation for computing the scattering distribution. The scattered component of the distribution can be fitted to a Gaussian function whose width depends only on T and lambda. Actual measurements vary with experimental conditions (exposure time, retinal eccentricity, and lambda) in a manner consistent with the scattering model. However, photoreceptor directionality must be included in the model to explain the actual location of the peak of the intensity distribution in the pupil plane and the total angular spread of light.

Computer Simulation↗
PubMed1998

Measurement of the wave-front aberration of the eye by a fast psychophysical procedure.

We used a fast psychophysical procedure to determine the wave-front aberrations of the human eye in vivo. We measured the angular deviation of light rays entering the eye at different pupillary locations by aligning an image of a point source entering the pupil at different locations to the image of a fixation cross entering the pupil at a fixed location. We fitted the data to a Zernike series to reconstruct the wave-front aberrations of the pupil. With this technique the repeatability of the measurement of the individual coefficients was 0.019 micron. The standard deviation of the overall wave-height estimation across the pupil is less than 0.3 micron. Since this technique does not require the administration of pharmacological agents to dilate the pupil, we were able to measure the changes in the aberrations of the eye during accommodation. We found that administration of even a mild dilating agent causes a change in the aberration structure of the eye.

Accommodation, Ocular↗
PubMed1997

Foveal cone spacing and cone photopigment density difference: objective measurements in the same subjects.

Foveal cone spacing was measured in vivo using an objective technique: ocular speckle interferometry. Cone packing density was computed from cone spacing data. Foveal cone photopigment density difference was measured in the same subjects using retinal densitometry with a scanning laser ophthalmoscope. Both the cone packing density and cone photopigment density difference decreased sharply with increasing retinal eccentricity. From the comparison of both sets of measurements, the computed amounts of photopigment per cone increased slightly with increasing retinal eccentricity. Consistent with previous results, decreases in cone outer segment length are over-compensated by an increase in the outer segment area, at least in retinal eccentricities up to 1 deg.

Adult↗
PubMed1997

Determination of the foveal cone spacing by ocular speckle interferometry: limiting factors and acuity predictions.

We have developed a high-resolution imaging technique, based on speckle interferometry, for the objective determination of the cone spacing in the living human fovea. The spatial resolution attained with this technique is theoretically diffraction limited by the pupil size. However, the highest frequency that we measure varies greatly among subjects, especially for fully dilated pupils. We have conducted several experiments (determination of the cutoff frequency of ocular speckle interferometry, the double-pass modulation transfer function, and the Stiles-Crawford effect) that indicate that, as expected, the resolution is not limited by the incoherent modulation transfer function. We found, though, a high correlation between the cutoff frequency and the width of the eye's Stiles-Crawford function. This implies that the resolution depends on the structural properties of the cone mosaic itself. In addition, we have compared the Nyquist frequency of the cone mosaic, determined objectively by our technique, with the grating visual acuity measured in the same eyes at the same foveal eccentricities. For our subjects, visual resolution nearly matches the Nyquist frequency within the fovea, except at the foveal center, where the optical transfer function of the eye attenuates the contrast of frequencies close to the Nyquist limit to a value below threshold.

Cell Count↗
PubMed1996

Optical modulation transfer and contrast sensitivity with decentered small pupils in the human eye.

Human observers experience a large decrement in visual acuity when a small artificial pupil is displaced from the center to the edge of the dilated natural pupil. This decrement in visual resolution, called the Campbell effect, has been attributed to the retina, the ocular optics, or a combination of the two. Given the uncertainty about the relative magnitudes of these two components over the range of spatial frequencies used in normal vision, we have obtained objective measurements of the retinal image quality and psychophysical measurements of visual performance, with decentered pupils. The contributions of monochromatic aberrations were determined by using double pass measurements of the modulus of the optical transfer function (MTF). For all of the observers, there was a substantial decrement in the MTF with decentering, showing that even when using a 1.5 mm pupil and appropriate spherical/cylindrical refractive corrections, there is a considerable contribution of monochromatic aberrations to the effect. We have compared these optical MTFs with the psychophysical contrast sensitivity functions (CSFs) measured under exactly the same conditions using green gratings generated on the screen of a color monitor. At the low and intermediate spatial frequencies considered (2-16 c/deg), we find the fall in the CSF is much greater than the fall in the monochromatic MTF, with the difference becoming greater as the spatial frequency increases. We show that this discrepancy can be mostly attributed to the effect of transverse chromatic aberration due to the bandwidth of the green stimulus used for the CSF measurements. In conclusion, the combination of the ocular transverse chromatic aberration and monochromatic aberrations accounts for the loss in visual sensitivity found with a decentered small pupil at low and intermediate spatial frequencies.

Color↗
PubMed1996

Coherent imaging of the cone mosaic in the living human eye.

A new system for the recording of high-resolution images of the cone mosaic in the living room fovea has been developed. The experimental method is inspired by stellar speckle interferometry, used in astronomy to resolve binary stars. Series of short-exposure images of small areas of the fovea are registered under coherent illumination. These images show speckle patterns that have some correlation with the topography of the cone mosaic and retain high-resolution information. Such correlation is better revealed in the power spectrum (square modulus of the Fourier transform). The signal-to-noise ratio is increased, without loss of high frequencies, by averaging the power spectra of a number of such speckle patterns. The average power spectra show, in most of the cases, an elliptical ring (or hexagon), whose mean radius corresponds to the characteristic spatial frequency of the cone mosaic (or the inverse of the mean row-to-row cone spacing) at a given retinal location. Good results are obtained in the five normal observers tested, at various retinal eccentricities, up to 1 visual degree, including the center of the fovea for two eyes. We find a decrease in the spatial frequency of the mosaic with the eccentricity and an important intersubject variability, in agreement with anatomical studies.

Adult↗
PubMed1995

Odd aberrations and double-pass measurements of retinal image quality.

We investigated the formation of the aerial image in the double-pass method to measure the optical quality of the human eye. We show theoretically and empirically that the double pass through the eye's optics forces the light distribution in the aerial image to be an even-symmetric function even if the single-pass point-spread function is asymmetric as a result of odd aberrations in the eye. The reason for this is that the double-pass imaging process is described by the autocorrelation rather than the autoconvolution of the single-pass point-spread functions, as has been previously assumed. This implies that although the modulation transfer function can be computed from the double-pass aerial image, the phase transfer function cannot. We also show that the lateral chromatic aberration of the eye cannot be measured with the double-pass procedure because it is canceled by the second pass through the eye's optics.

Humans↗