Biomedical subjects

Tobias Buehren

Publications and source records attributed to Tobias Buehren.

10 recordsLinked to original sources

PubMed2006

Retinal image quality, reading and myopia.

Analysis was undertaken of the retinal image characteristics of the best-spectacle corrected eyes of progressing myopes (n = 20, mean age = 22 years; mean spherical equivalent = -3.84 D) and a control group of emmetropes (n = 20, mean age = 23 years; mean spherical equivalent = 0.00 D) before and after a 2h reading task. Retinal image quality was calculated based upon wavefront measurements taken with a Hartmann-Shack sensor with fixation on both a far (5.5 m) and near (individual reading distance) target. The visual Strehl ratio based on the optical transfer function (VSOTF) was significantly worse for the myopes prior to reading for both the far (p = 0.01) and near (p = 0.03) conditions. The myopic group showed significant reductions in various aspects of retinal image quality compared with the emmetropes, involving components of the modulation transfer function, phase transfer function and point spread function, often along the vertical meridian of the eye. The depth of focus of the myopes (0.54 D) was larger (p = 0.02) than the emmetropes (0.42 D) and the distribution of refractive power (away from optimal sphero-cylinder) was greater in the myopic eyes (variance of distributions p < 0.05). We found evidence that the lead and lag of accommodation are influenced by the higher order aberrations of the eye (e.g. significant correlations between lead/lag and the peak of the visual Strehl ratio based on the MTF). This could indicate that the higher accommodation lags seen in myopes are providing optimized retinal image characteristics. The interaction between low and high order aberrations of the eye play a significant role in reducing the retinal image quality of myopic eyes compared with emmetropes.

Accommodation, Ocular↗

PubMed2006

Factors influencing lid pressure on the cornea.

PURPOSE: To investigate the effects of eyelid pressure on corneal topography in different angles of gaze and under different eye movement conditions. METHODS: Ten young subjects with healthy eyes were recruited for the study. Corneal topography of the right eye was measured with a videokeratoscope before and after four 15-minute visual tasks (downward gaze of 25 degrees or 45 degrees , both with no eye movements or 1 Hz eye movements). The four tasks were conducted on four separate mornings. RESULTS.: All subjects showed significant regions of topographic change after each of the four tasks. In general, eye movements were found to cause greater topographic changes than tasks without eye movements, and the larger angle of downward gaze led to greater changes in topography. The topographic changes were located close to the lid margin position during each task. CONCLUSIONS: The pressure exerted by the eyelid is capable of altering corneal topography. These changes are enhanced when the eye looks downward and when the eye moves laterally.

Adult↗

PubMed2006

Corneal optics after reading, microscopy and computer work.

PURPOSE: To compare lid-induced changes in corneal optics following reading, microscopy and computer work. METHODS: Nine subjects with normal ocular health were recruited for the study. Five subjects were myopic, two were emmetropic, one was astigmatic and one was hyperopic. Corneal topography was measured before and after 60 mins of reading a novel, performing a blood cell counting task on a microscope and Internet searching. Corneal topography data were used to derive the corneal wavefront Zernike coefficients up to the fourth order. A meridian analysis of instantaneous corneal power along the upper 90-degree semi-meridian was performed to examine local changes caused by eyelid pressure. Digital photography was used to capture body posture and eyelid position during the tasks. RESULTS: Each of the three tasks showed systematically different effects on both the characteristics and location of corneal topography changes. Reading and microscopy generally exhibited larger and more centrally located changes compared with the computer task. Differences in wavefront aberration characteristics between the three tasks were apparent in both lower and higher order aberrations. The location of corneal distortions differed significantly between microscopy and computer work, with microscopy causing distortions to occur closer to the videokeratoscope measurement axis compared with computer work (p = 0.015). CONCLUSIONS: Reading, microscopy and computer work have different effects on corneal aberrations. The results are in agreement with the hypothesis that lid-induced corneal aberrations may play a role in myopia development.

Adult↗

PubMed2005

Accommodation stimulus-response function and retinal image quality.

Accommodation stimulus-response function (ASRF) and its relationship to retinal image quality were investigated using a modified wavefront sensor. Ten subjects were presented with six vergence stimuli between 0.17 D and 5 D. For each vergence distance, ocular wavefronts and subjective visual acuity were measured. Wavefronts were analysed for a fixed 3-mm pupil diameter and for natural pupil sizes. Visual Strehl ratio computed in the frequency domain (VSOTF) and retinal images were calculated for each condition tested. Subjective visual acuity was significantly improved at intermediate vergence distances (1D and 2D; p < 0.01), and only decreased significantly at 5 D compared with 0.17 D (p < 0.05). VSOTF magnitude was associated with subjective visual acuity and VSOTF peak location correlated with accommodation error. Apparent accommodation errors due to spherical aberration were highly correlated with accommodation lead and lag for natural pupils (R(2) = 0.80) but not for fixed 3-mm pupils (R(2) < 0.00). The combination of higher-order aberrations and accommodation errors improved retinal image quality compared with accommodation errors or higher order aberrations alone. Pupil size and higher order aberrations play an important role in the ASRF.

Accommodation, Ocular↗

PubMed2005

Regression of lid-induced corneal topography changes after reading.

PURPOSE: The purpose of this article is to investigate the magnitude of lid-induced corneal topography changes as a function of time spent reading and the subsequent time course of regression of these changes. METHODS: Six young subjects, five myopes and one emmetrope with normal ocular health, participated in the study. Corneal topography of one eye was measured with videokeratoscopy before four reading sessions of 10, 30, 60, and 120 min duration performed on four separate mornings. Corneal topography was again measured at fixed intervals up to 180 min after each reading session. A control trial without reading was also performed on a separate morning. RESULTS: All six subjects showed significant changes in corneal topography directly after each of the reading sessions. Longer reading periods caused larger corneal changes. Local instantaneous power changes reached up to 5.95 D (+/- 2.80 D), whereas refractive power changes in the superior cornea ranged up to 1.26 D (+/- 0.44) for the 120-min reading trial. The duration of regression of corneal changes showed slower recovery times after longer reading periods. The pattern of regression was similar for all reading times, showing a rapid recovery within the first 10 minutes followed by a slower regression period. CONCLUSIONS: The length of time spent reading has a cumulative effect on the period over which corneal topography remains altered as a result of lid forces.

Adult↗

PubMed2004

Near work induced wavefront aberrations in myopia.

We undertook a detailed analysis of the wavefront aberrations of the eyes of 20 young progressing myopes (mean age=22 years; mean spherical equivalent=-3.84 D, range -1.00 to -7.5 D) and twenty young age matched emmetropes (mean age=23 years; mean spherical equivalent=-0.00 D, range +0.25 to -0.25 D). A wavefront sensor was used to measure the ocular wavefront and a videokeratoscope was used to measure corneal topography. The corneal wavefront was subsequently calculated and the difference between the corneal and ocular wavefront was derived to give the internal wavefront component of the eye. Ocular and corneal wavefronts were measured before and after a 2-h reading task. At the baseline measurements, the myopes showed greater levels of some high order ocular wavefronts than the emmetropes. These differences between the groups became larger following 2 h of reading. Ocular higher order wavefront RMS was (baseline RMS: myopes=0.21 microm, emmetropes=0.16 microm, difference p=0.05 and after 2 h reading was RMS: myopes=0.27 microm, emmetropes=0.17 microm, difference p=0.02). The differences between the groups are primarily due to changes in the corneal wavefront associated with a narrower lid aperture during reading for the myopes. These differences are enhanced by longer periods spent reading, larger pupils and consequently low light levels. We suggest lid induced corneal changes caused by reading in downgaze provides a theoretical framework that could explain the known features of myopia development. The inherited characteristics of facial and lid anatomy would provide a mechanism for a genetic component in the genesis of myopia.

Adult↗

PubMed2004

A refined bootstrap method for estimating the Zernike polynomial model order for corneal surfaces.

Following our previous work on optimal modeling of corneal surfaces with Zernike polynomials, we have developed a refined bootstrap-based procedure which improves the accuracy of the previous method. We show that for normal corneas, the optimal number of Zernike terms usually corresponds to the fourth or fifth radial order expansion of Zernike polynomials. On the other hand, for distorted corneas such as those encountered in keratoconus or in surgically altered cases, the estimated model was found to be up to three radial orders higher than for normal corneas.

Algorithms↗

PubMed2003

Corneal topography and accommodation.

PURPOSE: To investigate whether there are significant changes in corneal topography during accommodation in normal corneas and corneas that are pathologically thinner due to keratoconus. METHODS: A videokeratoscope was modified to present an accommodation stimulus that was coaxial with the instrument's measurement axis. Six subjects with normal corneas and four subjects with keratoconus were studied. Eighteen videokeratoscope measurements of one eye of each subject were taken at 0 diopter (D) accommodation demand and six for both of 4 D and 9 D accommodation demand. The effects of ocular micromovements on multiple topography maps were minimized using software algorithms. Average maps for the 4 D and 9 D accommodation demands were calculated and subtracted from the average map of the 0-D accommodation demand. A t test was applied at each point location within the topography maps to analyze the statistical significance of change (p < 0.001) within the difference maps. RESULTS: In the initial analysis, we found that a number of the subjects showed significant changes in corneal topography as accommodation changed. However, further analysis showed a significant group mean excyclotorsion of the topography maps of 1.6 +/- 1.1 degrees (p < 0.03) for the 4-D stimulus and 2.0 +/- 1.3 degrees (p < 0.01) for the 9-D stimulus compared with the 0-D stimulus. When we accounted for the excyclotorsion, we did not find clear evidence of statistically significant changes in corneal topography as a result of accommodation, either for the normal corneas or the keratoconic corneas. CONCLUSIONS: It appears unlikely that changes occur in central corneal shape during accommodation up to a level of 9 D in normal or keratoconic corneas. A small ocular excyclotorsion typically accompanies accommodation, and this changes the relative orientation of the topography of the cornea. This has significant implications for the interpretation of the optical characteristics of eyes during near viewing conditions.

Accommodation, Ocular↗

PubMed2003

Corneal aberrations and reading.

PURPOSE: To investigate the effects of eyelid pressure on corneal shape and corneal aberrations during reading. METHODS: Twenty young subjects with normal ocular health were recruited for the study. The experiment was conducted early in the morning, with subjects instructed not to perform any prolonged reading before the experiment. Corneal topography of one eye was measured with a videokeratoscope before reading and then again after a 60-min reading task. The natural position of the eyelids was photographed in primary gaze and during the reading task. RESULTS: Twelve of the 20 corneas showed significant changes in central topography immediately after reading. The location of the changes corresponded closely to the position and angle of the subject's eyelids during reading. The change in shape was best described as a wave-like distortion that significantly altered some corneal wavefront Zernike coefficients. There was a significant correlation between the changes in primary vertical coma and trefoil (along 30 degrees). Within the central 6 mm of the cornea, there were significant changes in the root mean square error, overall refractive power, and astigmatism. CONCLUSIONS: The changes we observed in corneal topography appear to be directly related to the force exerted by the eyelids during reading. Because the cornea is the major light-refracting surface of the eye, the optical characteristics of some eyes can be significantly changed during reading by the force of the eyelids. These findings may have important implications for the definition of refractive status and may also aid in the understanding of the relationship between reading and the development of refractive errors.

Adult↗

PubMed2002

Ocular microfluctuations and videokeratoscopy.

PURPOSE: We aimed to minimize the effect of ocular microfluctuations during videokeratoscopy to improve the qualitative and quantitative analysis of corneal topography. METHODS: A technique was developed to measure the tilt, displacement, and cyclotorsion in multiple videokeratographs from the same cornea. This information was used to reposition each videokeratograph according to the average position of a sample of multiple measurements. The corneal topography of 10 subjects was measured 20 times each, using videokeratoscopy. RESULTS: The root mean square error calculated from difference between single videokeratographs and the average videokeratograph decreased by an average of 24.6% for the 10 subjects' data. The standard deviation of some terms of Zernike polynomials fitted to the cornea was significantly reduced after the procedure. CONCLUSION: The method that we propose can improve the precision performance of videokeratoscopy in multiple measurements of corneal topography.

Adult↗