Biomedical subjects

P Artal

Publications and source records attributed to P Artal.

At least 19 recordsLinked to original sources

PubMed2001

Polarization and retinal image quality estimates in the human eye.

We have previously studied how polarization affects the double-pass estimates of the retinal image quality by using an imaging polarimeter [Opt. Lett. 24, 64 (1999)]. A series of 16 images for independent combinations of polarization states in the polarimeter were recorded to obtain the spatially resolved Mueller matrices of the eye. From these matrices, double-pass images of a point source for light with different combinations of incoming (first-pass) and outcoming (second-pass) polarization states were reconstructed and their corresponding modulation transfer functions were calculated. We found that the retinal image or, alternatively, the ocular aberrations, are nearly independent of the state of polarization of the incident light (in the first pass). This means that a significant improvement in the ocular optics by using a specific type of polarized light could not be achieved. However, quite different estimates of the retinal image quality are obtained for combinations of polarization states in both the first and the second passes in the double-pass apparatus.

Humans↗

PubMed2001

Dynamics of the eye's wave aberration.

It is well known that the eye's optics exhibit temporal instability in the form of microfluctuations in focus; however, almost nothing is known of the temporal properties of the eye's other aberrations. We constructed a real-time Hartmann-Shack (HS) wave-front sensor to measure these dynamics at frequencies as high as 60 Hz. To reduce spatial inhomogeneities in the short-exposure HS images, we used a low-coherence source and a scanning system. HS images were collected on three normal subjects with natural and paralyzed accommodation. Average temporal power spectra were computed for the wave-front rms, the Seidel aberrations, and each of 32 Zernike coefficients. The results indicate the presence of fluctuations in all of the eye's aberration, not just defocus. Fluctuations in higher-order aberrations share similar spectra and bandwidths both within and between subjects, dropping at a rate of approximately 4 dB per octave in temporal frequency. The spectrum shape for higher-order aberrations is generally different from that for microfluctuations of accommodation. The origin of these measured fluctuations is not known, and both corneal/lenticular and retinal causes are considered. Under the assumption that they are purely corneal or lenticular, calculations suggest that a perfect adaptive optics system with a closed-loop bandwidth of 1-2 Hz could correct these aberrations well enough to achieve diffraction-limited imaging over a dilated pupil.

Accommodation, Ocular↗

PubMed2000

Corneal wave aberration from videokeratography: accuracy and limitations of the procedure.

A procedure to calculate the wave aberration of the human cornea from its surface shape measured by videokeratography is presented. The wave aberration was calculated as the difference in optical path between the marginal rays and the chief ray refracted at the surface, for both on- and off-axis objects. The corneal shape elevation map was obtained from videokeratography and fitted to a Zernike polynomial expansion through a Gram-Schmidt orthogonalization. The wave aberration was obtained also as a Zernike polynomial representation. The accuracy of the procedure was analyzed. For calibrated reference surface elevations, a root-mean-square error (RMSE) of 1 to 2 microm for an aperture 4-6 mm in diameter was obtained, and the RMSE associated with the experimental errors and with the fitting method was 0.2 microm. The procedure permits estimation of the corneal wave aberration from videokeratoscopic data with an accuracy of 0.05-0.2 microm for a pupil 4-6 mm in diameter, rendering the method adequate for many applications.

Cornea↗

PubMed2000

Analysis of the performance of the Hartmann-Shack sensor in the human eye.

A description of a Hartmann-Shack sensor to measure the aberrations of the human eye is presented. We performed an analysis of the accuracy and limitations of the sensor using experimental results and computer simulations. We compared the ocular modulation transfer function obtained from simultaneously recorded double-pass and Hartmann-Shack images. The following factors affecting the sensor performance were evaluated: the statistical accuracy, the number of modes used to reconstruct the wave front, the size of the microlenses, and the exposure time.

Computer Simulation↗

PubMed2000

Optical aberrations of the human cornea as a function of age.

We investigated how the optical aberrations associated with the anterior surface of the human cornea change with age in a normal population. Aberrations were computed for a central part of the cornea (4, 5, and 6 mm in diameter) from the elevation data provided by a videokeratographic system. Measurements were obtained in 59 normal healthy, near-emmetropic [spherical equivalent lower than 2 diopters (D)] subjects of three age ranges: younger (20-30 years old), middle-aged (40-50 years old), and older (60-70 years old). The average corneal radius decreased with age and the cornea became more spherical. As a consequence, spherical aberration was significantly larger in the middle-aged and older corneas. Coma and other higher-order aberrations also were correlated with age. The root mean square of the wave aberration exhibited a linear positive correlation (P < 0.003) with age for the three ranges of pupil diameter. Despite a large intersubject variability, the average amount of aberration in the human cornea tends to increase moderately with age. However, this increase alone is not enough to explain the substantial reduction previously found in retinal image quality with age. The change in the aberrations of the lens with age and the possible loss of part of the balance between corneal and lenticular aberrations in youth may be the main factors responsible for the reduction of retinal image quality through the life span.

Adult↗

PubMed1999

Off-axis monochromatic aberrations estimated from double pass measurements in the human eye.

Off-axis monochromatic aberrations in the human eye impose limits on peripheral vision. However, the magnitude of the aberrations off-axis, and in particular coma, has not been yet completely determined. We have developed a procedure to estimate third order aberrations in the periphery of the human eye. The technique is based on recording series of double pass retinal images with unequal entrance and exit pupil diameters (Artal, Iglesias, López-Gil & Green (1995b). J. Opt. Soc. Am. A, 12, 2358-2366.) which allows the odd asymmetries in the retinal image be assessed. The procedure that is described provides accurate estimates of the main off-axis aberrations: astigmatism, defocus and coma. We have measured these aberrations in four normal subjects. For a given eccentricity, the measured amount of coma and astigmatism are relatively similar among subjects, because the angular distance from the axis is the dominant factor in determining the magnitude of these aberrations. However, we found considerable variability in the values of peripheral defocus, probably due to a complicate combination of off-axis aberrations and fundus shape. The final off-axis optical performance of the eye for a given object location is determined by a particular mixture of defocus, astigmatism, coma and higher order aberrations.

Astigmatism↗

PubMed1999

Microglial cells during the lesion-regeneration of the lizard medial cortex.

The lizard medial cortex (lizard fascia dentata) is capable of neural regeneration after being lesioned by the anti-metabolite 3-acetylpyridine (3AP). This study was aimed at detecting microglial behaviour during the medial cortex lesion-regeneration process using tomato lectin histochemistry to label microglia (both with light and electron microscopy) and proliferating cell nuclear antigen (PCNA) immunocytochemistry to label proliferating cells. As expected, 1-2 days post-injection lectin-labelled microglia cells could not be observed in the medial cortex plexiform layers, but later (7 days post-injection) abundant lectin-labelled microglia cells re-populated the regenerating medial cortex. Abundant PCNA-immunolabelled nuclei were detected both in the subjacent ependymal neuroepithelium (neuroblasts, maximum at 2 days post-injection) as well as in some parenchymal cells which were also lectin-labelled (microglia, maximum at 7-15 days post-injection). Re-invasive microglia were also detected in the vicinity of ventricular ependymal lining, blood vessels and meninges. The electron microscope demonstrated that these microglial cells participate in cell debris removal, especially of neural granular cell somata. Other cell types related to microglia (mast cells, peri-vascular cells and meningeal cells) were also present during the scavenging process. Significant numbers of microglial cells remained in close relationship with the ependymal proliferative areas, even in control non-lesioned animals. This is indirect evidence for the working hypothesis that microglia are not only implicated in cell debris removal, but also in the regulation of newly generated neuroblast incorporation onto the cortical areas. Whether they phagocytose immature neuroblasts or induce cell death in them or even prevent their migration onto the principal layer areas are likely possibilities that remain to be proven.

Animals↗

PubMed1999

Average optical performance of the human eye as a function of age in a normal population.

PURPOSE: To determine the average optical performance of the human eye, in terms of the modulation transfer function (MTF), as a function of age. METHODS: An apparatus was constructed to measure the ocular MTF, based on the recording of images of a green, 543-nm laser-point source after reflection in the retina and double pass through the ocular media. MTFs were computed from the average of three 4-second-exposure double-pass images recorded by a slow-scan, cooled charge-coupled device camera. The ocular MTF was measured for three artificial pupil diameters (3 mm, 4 mm, and 6 mm) with paralyzed accommodation under the best refractive correction in 20 subjects for each of three age categories: young subjects aged 20 to 30 years, middle-aged subjects aged 40 to 50 years, and older subjects aged 60 to 70 years. The selected subjects passed an ophthalmologic examination, excluding subjects with any form of ocular or retinal disease, spherical or cylindrical refractive errors exceeding 2 D, and corrected visual acuity lower than 1 (0.8 in the older age group). RESULTS: The average MTF was determined for each age group and pupil diameter. A two-parameter analytical expression was proposed to represent the average MTF in each age group for every pupil diameter. The ocular MTFs declined as age increased from young to older groups. The SD of the MTF results within age groups was lower than the differences between the mean for each group. CONCLUSIONS: The average optical performance of the human eye progressively declines with age. These MTF results can serve as a reference for determining mean ocular optics according to age.

Adult↗

PubMed1998

Retinal image quality in the human eye as a function of the accommodation.

The changes in the retinal image quality with accommodation in the human eye were studied by using a near-infrared double-pass apparatus. A slightly better modulation transfer function (MTF) in the unaccommodated eye with respect to the accommodated eye was found when using an artificial pupil with a fixed diameter. The technique allows the estimation of the MTF of the accommodated eye discounting the effect of the accommodative defocus error. Most of the reduction found in the MTF with accommodation could be explained in terms of the accommodative defocusing error. However, the shape of the retinal images clearly changes with accommodation, indicating that other aberrations are also altered with accommodation. In general, the double-pass image for the accommodated eye tends to be more symmetric than that of the unaccommodated eye. This is probably due to either a decrease in the amount of coma-like aberrations with accommodation or to an increase of other symmetric aberrations, such as defocus or spherical aberration, that hide the asymmetries present in the retinal image of the unaccommodated eye.

Accommodation, Ocular↗

PubMed1998

Reconstruction of the point-spread function of the human eye from two double-pass retinal images by phase-retrieval algorithms.

In the double-pass technique used to measure the optical performance of the eye, the double-pass image is the cross correlation of the input spread function with the output spread function [J. Opt. Soc. Am. A 12, 195 (1995)]. When entrance and exit pupil sizes are equal, the information on the point-spread function is lost from the double-pass image, although the modulation transfer function of the eye is obtained. A modification of the double-pass technique that uses unequal-sized entrance and exit pupils allows a low-resolution version of the ocular point-spread function to be recorded [J. Opt. Soc. Am. A 12, 2358 (1995)]. We propose the combined use of these two double-pass measurements as input in a phase-retrieval procedure to reconstruct the ocular point-spread function. We use an adapted version of the iterative Fourier-transform algorithm consisting of two steps. In the first step, error-reduction iterations with expanding weighting functions in the Fourier domain yield an estimation of the phase that serves as an initial guess for the second step, which consists of cycles of hybrid input-output iterations. We tested the robustness and limitations of the retrieval algorithm by using simulated data with and without noise. We then applied the procedure to reconstruct the point-spread function from actual measurements of double-pass retinal images in the living eye.

Algorithms↗

PubMed1998

Estimates of the ocular wave aberration from pairs of double-pass retinal images.

We apply a computational technique to retrieve the wave aberration of the eye from the point-spread function obtained from pairs of double-pass retinal images. The method consists of an adapted pyramidal version of a nonlinear least-squares fitting procedure to a wave aberration expressed as an expansion in Zernike polynomials. Although the procedure provides accurate estimates of the wave aberration, it presents several drawbacks that are discussed in detail. In particular, since a great deal of computational time is necessary to retrieve a single wave aberration, this technique is not useful for real-time applications. We present results of wave aberrations in five normal subjects in the fovea for a 4-mm-pupil diameter. In every case there is a clear presence of comalike aberrations, while the third-order spherical aberration is usually smaller than previous estimates. The root-mean-square error in the retrieved wave aberration, when defocus and astigmatism were corrected, ranges from 0.24 to 0.5 wavelength. The particular values of the aberration coefficients present a large intersubject variability.

Adult↗

PubMed1998

Correction of the aberrations in the human eye with a liquid-crystal spatial light modulator: limits to performance.

We evaluated the performance of a liquid-crystal spatial light modulator for static correction of the aberrations in the human eye. By applying phase-retrieval techniques to pairs of double-pass images we first estimated the wave aberration of the eye to be corrected. Then we introduced the opposite phase map in the modulator, which was placed in a plane conjugated with the eye's pupil, and we recorded double-pass images of a point source before and after correction of the aberrations. In a slightly aberrated artificial eye a clear improvement was obtained after correction, and, although diffraction-limited performance was not achieved, the results were close to the theoretical predictions. In the two living eyes that we studied some benefit also appeared in the correction, but the performance was worse than that expected. We evaluated possible explanations for the relatively poor performance that was obtained in the human eye: an incorrect estimate of the ocular aberration, the limited spatial resolution of the modulator, and the dynamic changes in the ocular aberrations. Based on the results in the artificial eye, the first problem was not considered to be a major source of error. However, we showed that the spatial resolution of the liquid-crystal spatial light modulator limits the maximum correction to be attained. In addition, the changes in the ocular optics over time also impose a limit in the performance of static corrections.

Adult↗

PubMed1997

Membranous glomerulonephritis associated with hepatitis C virus infection in renal transplant patients.

BACKGROUND: Hepatitis C virus (HCV) infection has been described in association with various types of glomerular diseases, usually type I membranoproliferative glomerulonephritis and rarely membranous glomerulonephritis (MGN). In this article, we describe the first series of MGN exhibited in renal transplant patients and associated with HCV infection. METHODS: From January 1980 to December 1994, 2045 kidney transplantations were performed in our renal transplant units. A retrospective analysis demonstrated an overall 20% prevalence of HCV virus-positive patients; 409 transplanted patients were HCV positive (ELISA and RIBA). RESULTS: Fifteen patients developed an allograft MGN (3.66%) 24 months after renal transplantation. MGN appeared in the form of significant proteinuria (>1.5 g/24 h) with stable renal function. In all cases, graft biopsy demonstrated a thickening of the capillary wall, subepithelial electron-dense deposits, and IgG and C3 diffuse granular deposits along the basal membrane. Ten cases were considered de novo, two cases were considered recurrent MGN, and three cases were considered undetermined because the primary renal disease was chronic glomerulonephritis. All patients showed negative antinuclear antibodies and cryoglobulins, normal complement, and negative rheumatoid factors. During follow-up (an average of 2 years), 12 patients developed a progressive worsening of renal function, with increased serum creatinine and persistent proteinuria; 8 of the 12 patients returned to dialysis. Of the remaining three cases, two patients showed partial remission of nephrotic syndrome after high doses of steroids, and one patient persisted with stable renal function and proteinuria (<2 g/24 h.). CONCLUSIONS: In summary, HCV is preferentially associated with MGN in renal transplant patients, rather than with membranoproliferative glomerulonephritis as in the normal adult population. MGN associated with HCV infection has a similar clinical picture and outcome to posttransplant idiopathic de novo MGN, with persistent massive proteinuria and progressive deterioration of renal function.

Adult↗

PubMed1997

Comparison of double-pass estimates of the retinal-image quality obtained with green and near-infrared light.

We compared retinal point-spread functions obtained by the double-pass method with two different wave-lengths, green (543 nm) and near-infrared (780 nm), in both cases under the best conditions of focus. The best refractive state at each wavelength was determined with two procedures: subjective refraction and analysis of the recorded double-pass images as a function of focus. Since the refraction results agree quite well, we assume that in both cases, green and near-infrared light, most of the light of the central core in the double-pass images comes from a layer close to that of the photoreceptors. The central spread of the double-pass images was also quite similar for the two wavelengths: a width of approximately 2-3 arcmin at half-intensity relative to the peak. However, larger differences were found in the tails of the images, with the infrared images presenting a larger scattering halo, probably as a result of a more important contribution of retinal and choroidal scattering for that wavelength. By using the central core in the double-pass images and ignoring the tails, we can use the near-infrared data to predict the modulation transfer function measured with the use of green light. These results raise the possibility of using near-infrared illumination the double-pass method to estimate the optical performance of the human eye.

Adult↗

PubMed1996

Off-axis optical quality and retinal sampling in the human eye.

Using the double pass procedure, Navarro et al. (1993; Journal of the Optical Society of America A, 10, 201-212) measured the monochromatic modulation transfer function (MTF) of the human eye as a function of retinal eccentricity. They chose conditions as similar as possible to those encountered in natural viewing. We report new measurements obtained with conditions chosen instead to optimize retinal image quality: we paralyzed accommodation, used a 3 mm pupil, and corrected defocus and oblique astigmatism at each retinal location. MTFs were estimated at the tangential focus, circle of least confusion, and sagittal focus produced by oblique astigmatism. Though optical blur is well-known to have little effect on peripheral visual acuity, it can nonetheless substantially reduce aliasing by receptoral and post-receptoral spatial sampling.

Adult↗

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

Refraction, aliasing, and the absence of motion reversals in peripheral vision.

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd.m-2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial.(ABSTRACT TRUNCATED AT 250 WORDS)

Discrimination, Psychological↗