Silvestre Manzanera

Neural compensation for the eye’s optical aberrations

A fundamental problem facing sensory systems is to recover useful information about the external world from signals that are corrupted by the sensory process itself. Retinal images in the human eye are affected by optical aberrations that cannot be corrected with ordinary spectacles or contact lenses, and the specific pattern of these aberrations is different in every eye. Though these aberrations always blur the retinal image, our subjective impression is that the visual world is sharp and clear, suggesting that the brain might compensate for their subjective influence. The recent introduction of adaptive optics to control the eyes aberrations now makes it possible to directly test this idea. If the brain compensates for the eyes aberrations, vision should be clearest with the eyes own aberrations rather than with unfamiliar ones. We asked subjects to view a stimulus through an adaptive optics system that either recreated their own aberrations or a rotated version of them. For all five subjects tested, the stimulus seen with the subjects own aberrations was always sharper than when seen through the rotated version. This supports the hypothesis that the neural visual system is adapted to the eyes aberrations, thereby removing somehow the effects of blur generated by the sensory apparatus from visual experience. This result could have important implications for methods to correct higher order aberrations with customized refractive surgery because some benefits of optimizing the correction optically might be undone by the nervous systems compensation for the old aberrations.

Adaptive optics with a programmable phase modulator: applications in the human eye.

Adaptive optics for the human eye has two main applications: to obtain high-resolution images of the retina and to produce aberration-free retinal images to improve vision. Additionally, it can be used to modify the aberrations of the eye to perform experiments to study the visual function. We have developed an adaptive optics prototype by using a liquid crystal spatial light modulator (Hamamatsu Programmable Phase Modulator X8267). The performance of this device both as aberration generator and corrector has been evaluated. The system operated either with red (633nm) or infrared (780nm) illumination and used a real-time Hartmann-Shack wave-front sensor (25 Hz). The aberration generation capabilities of the modulator were checked by inducing different amounts of single Zernike terms. For a wide range of values, the aberration production process was found to be linear, with negligible cross-coupling between Zernike terms. Subsequently, the modulator was demonstrated to be able to correct the aberrations of an artificial eye in a single step. And finally, it was successfully operated in close-loop mode for aberration correction in living human eyes. Despite its slow temporal response, when compared to currently available deformable mirrors, this device presents advantages in terms of effective stroke and mode independence. Accordingly, the programmable phase modulator allows production and compensation of a wide range of aberrations, surpassing in this respect the performance of low-cost mirrors and standing comparison against more expensive devices.

Visual effect of the combined correction of spherical and longitudinal chromatic aberrations

An instrument permitting visual testing in white light following the correction of spherical aberration (SA) and longitudinal chromatic aberration (LCA) was used to explore the visual effect of the combined correction of SA and LCA in future new intraocular lenses (IOLs). The LCA of the eye was corrected using a diffractive element and SA was controlled by an adaptive optics instrument. A visual channel in the system allows for the measurement of visual acuity (VA) and contrast sensitivity (CS) at 6 c/deg in three subjects, for the four different conditions resulting from the combination of the presence or absence of LCA and SA. In the cases where SA is present, the average SA value found in pseudophakic patients is induced. Improvements in VA were found when SA alone or combined with LCA were corrected. For CS, only the combined correction of SA and LCA provided a significant improvement over the uncorrected case. The visual improvement provided by the correction of SA was higher than that from correcting LCA, while the combined correction of LCA and SA provided the best visual performance. This suggests that an aspheric achromatic IOL may provide some visual benefit when compared to standard IOLs.

Effect of optical correction and remaining aberrations on peripheral resolution acuity in the human eye.

Retinal sampling poses a fundamental limit to resolution acuity in the periphery. However, reduced image quality from optical aberrations may also influence peripheral resolution. In this study, we investigate the impact of different degrees of optical correction on acuity in the periphery. We used an adaptive optics system to measure and modify the off-axis aberrations of the right eye of six normal subjects at 20 degrees eccentricity. The system consists of a Hartmann-Shack sensor, a deformable mirror, and a channel for visual testing. Four different optical corrections were tested, ranging from foveal sphero-cylindrical correction to full correction of eccentric low- and high-order monochromatic aberrations. High-contrast visual acuity was measured in green light using a forced choice procedure with Landolt Cs, viewed via the deformable mirror through a 4.8-mm artificial pupil. The Zernike terms mainly induced by eccentricity were defocus and with- and against-the-rule astigmatism and each correction condition was successfully implemented. On average, resolution decimal visual acuity improved from 0.057 to 0.061 as the total root-mean-square wavefront error changed from 1.01 mum to 0.05 mum. However, this small tendency of improvement in visual acuity with correction was not significant. The results suggest that for our experimental conditions and subjects, the resolution acuity in the periphery cannot be improved with optical correction.

Liquid crystal Adaptive Optics Visual Simulator: Application to testing and design of ophthalmic optical elements

The concept of Adaptive Optics Visual Simulation applies to the use of an Adaptive Optics system to manipulate ocular aberrations in order to perform visual testing through a modified optics. It can be of interest both to study the visual system and to design new ophthalmic optical elements. In this work, we describe an apparatus based on a liquid crystal programmable phase modulator and explore its capabilities as a tool in the early stages of the design of ophthalmic optical elements with increased depth of field for presbyopic subjects. To illustrate the potential of the instrument, we analyze the performance of two phase profiles obtained by a hybrid optimization procedure. The liquid crystal Adaptive Optics Visual Simulator can be used to experimentally record the point spread function for different vergences in order to objectively measure depth of focus, to perform different psychophysical experiments through the phase profile in order to measure its impact on visual performance, and to study the interaction with the eyes particular aberrations. This approach could save several steps in current procedures of ophthalmic optical design and eventually lead to improved solutions.

Impact of scattering and spherical aberration in contrast sensitivity

We investigated the impact in spatial visual performance of the combined presence of different amounts of spherical aberration and intraocular scattering in the eye. In a group of subjects, contrast sensitivity at 6 cycles per degree was measured when viewing through holographic diffusers to produce different levels of scattering and with their spherical aberration simultaneously controlled using an adaptive-optics visual simulator. For elevated levels of scattering, the addition of small amounts of spherical aberration either does not decrease, or even may slightly increase, contrast sensitivity under some conditions. This seems to be due to an optical effect also demonstrated in an artificial eye. Although the visual effect is quite small, this finding could suggest a balancing mechanism where larger spherical aberration could keep relatively stable the retinal image quality under the presence of elevated scattering. This is actually the situation in older eyes with both spherical aberration and intraocular scatter being higher than in young eyes.

Are optical aberrations during accommodation a significant problem for refractive surgery

PURPOSE To study the limits to a perfect ideal customized wavefront correction due to the change of aberrations during accommodation. METHODS. We measured the dynamic changes of ocular aberrations during accommodation in normal eyes with a real-time Hartmann-Shack wavefront sensor. Those results were used in computer simulations to predict the benefit of a perfect customized correction. RESULTS Due to the continuous changes of the aberrations over time, an ideal perfect static correction will not provide stable aberration-free optics. For example, when the eye accommodates to near objects, due to the changing aberrations, the eye will become aberrated again. An alternative correction using the aberration pattern for a slightly accommodated condition could provide a better-correction in a larger accommodative range, although at the cost of non-perfect correction for far vision. CONCLUSIONS Due to the dynamic nature of ocular optics, a static perfect correction, for instance performed in customized refractive surgery, would not remain perfect for every condition occurring during normal accommodation.

Hybrid adaptive-optics visual simulator

We have developed a hybrid adaptive-optics visual simulator (HAOVS), combining two different phase-manipulation technologies: an optically addressed liquid-crystal phase modulator, relatively slow but capable of producing abrupt or discontinuous phase profiles; and a membrane deformable mirror, restricted to smooth profiles but with a temporal response allowing compensation of the eyes aberration fluctuations. As proof of concept, a phase element structured as discontinuous radial sectors was objectively tested as a function of defocus, and a correction loop was closed in a real eye. To further illustrate the capabilities of the device for visual simulation, we recorded extended images of different stimuli through the system by means of an external camera replacing the subjects eye. The HAOVS is specially intended as a tool for developing new ophthalmic optics elements, where it opens the possibility to explore designs with irregularities and/or discontinuities.

A wavelength tunable wavefront sensor for the human eye

We have designed and assembled an instrument for objective measurement of the eyes wave aberrations for different wavelengths with no modifications in the measurement path. The system consists of a Hartmann-Shack wave-front sensor and a Xe-white-light lamp in combination with a set of interference filters used to sequentially select the measurement wavelength. To show the capabilities of the system and its reliability for measuring at different wavelengths, the ocular aberrations were measured in three subjects at 440, 488, 532, 633 and 694 nm, basically covering the whole visible spectrum. Even for the shortest wavelengths, the illumination level was always several orders of magnitude below the safety limits. The longitudinal chromatic aberration estimates and the wavelength dependence of coma and spherical aberration, as examples of higher-order aberration terms, were compared to the predictions of a chromatic eye model, with good agreement. To our knowledge, this is the first report of a device to objectively determine the spectral fluctuations in the ocular wavefront.

Adaptive optics for vision: the eye's adaptation to point spread function.

PURPOSE Despite the fact that ocular aberrations blur retinal images, our subjective impression of the visual world is sharp, which suggests that the visual system compensates for subjective influence. If the brain adjusts for specific aberrations of the eye, vision should be clearest when looking through a subjects typical wave aberration rather than through an unfamiliar one. We used adaptive optics techniques to control the eyes aberrations in order to evaluate this hypothesis. METHODS We used adaptive optics to produce point spread functions (PSFs) that were rotated versions of the eyes typical PSF by angles in 45 degrees intervals. Five normal subjects were asked to view a stimulus with their own PSF or with a rotated version, and to adjust the magnitude of the aberrations in the rotated case to match the subjective blur of the stimulus to that seen when the wave aberration was in typical orientation. RESULTS The magnitude of the rotated wave aberration required to match the blur with the typical wave aberration was 20% to 40% less, indicating that subjective blur for the stimulus increased significantly when the PSF was rotated. CONCLUSION These results support the hypothesis that the neural visual system is adapted to an eyes aberrations and has important implications for correcting higher order aberrations with customized refractive surgery or contact lenses. The full visual benefit of optimizing optical correction requires that the nervous system compensate for the new correction.