Karen M. Hampson

Adaptive optics and vision

It has long been recognised that the optical quality of the human eye is far from diffraction limited. This affects our visual acuity and severely limits the resolution at which images of the living retina can be obtained. Adaptive optics is a technique that can correct for the eyes aberrations and provide diffraction limited resolution. The origins of the technique lie in astronomy, but it was successfully adapted to the human eye just over 10 years ago. Since then there have been rapid developments in the field of adaptive optics and vision science. In vivo images of the retina can now be routinely achieved with unprecedented resolution. Sophisticated experiments can be performed to gain a deeper knowledge of the interaction of neural retinal architecture and visual perception. This article presents the theory behind adaptive optics for the human eye and reviews the developments in this field to date.

Role of ocular aberrations in dynamic accommodation control

Background:  Accommodation control is mediated by a number of cues, including blur, chromatic aberration and target proximity. Data from wavefront measurements have shown clear shifts in ocular aberrations during increasing accommodative demand, most notably a negative shift in spherical aberration. Work in adaptive optics, where aberrations have been corrected, has suggested a role for aberrations in the control of accommodation for some individuals. This study aimed to determine the relative effects of aberration correction and inversion on closed‐loop stepwise accommodation responses to small increases and decreases in stimulus vergence.

Effect of correction of ocular aberration dynamics on the accommodation response to a sinusoidally moving stimulus

We used an adaptive optics system to correct the aberration dynamics of five subjects while they fixated on a monochromatic stimulus undergoing sinusoidal vergence changes between 1.5 and 2.5 D, at a temporal frequency of 0.2 Hz. The aberrations were measured at 20 Hz using a Shack-Hartmann sensor and corrected using a 37-actuator deformable mirror. The accommodation response (AR) was analyzed in terms of the gain and phase lag. Manipulation of aberrations significantly affected the gain of the AR for only one subject when the odd-order aberrations were corrected. The predictability of the sinusoidal stimulus could account for the lack of an effect in the remaining subjects and conditions.

Binocular Shack–Hartmann sensor for the human eye

During steady-state fixation the aberrations of the human eye display dynamic behaviour. It has been suggested that the fluctuations in focus are correlated between both eyes. However, nothing is known about the dynamic correlation between the aberrations other than focus. We have developed an open-view binocular Shack–Hartmann sensor which measures the ocular wavefront aberrations simultaneously in both eyes at 25 Hz. A single sensor and laser source are used to reduce system cost and complexity. Speckle is reduced using a rotating diffuser in a plane conjugate to the retinae. We measured the wavefront dynamics of two subjects and decomposed the data into Zernike aberration terms up to and including fifth radial order. Coherence function analysis was used to determine the correlation between aberrations in the frequency domain. The correlations were dependent upon subject, frequency and aberration type.

Vision science and adaptive optics, the state of the field.

Adaptive optics is a relatively new field, yet it is spreading rapidly and allows new questions to be asked about how the visual system is organized. The editors of this feature issue have posed a series of question to scientists involved in using adaptive optics in vision science. The questions are focused on three main areas. In the first we investigate the use of adaptive optics for psychophysical measurements of visual system function and for improving the optics of the eye. In the second, we look at the applications and impact of adaptive optics on retinal imaging and its promise for basic and applied research. In the third, we explore how adaptive optics is being used to improve our understanding of the neurophysiology of the visual system.

Binocular correlation of ocular aberration dynamics.

Fluctuations in accommodation have been shown to be correlated in the two eyes of the same subject. However, the dynamic correlation of higher-order aberrations in the frequency domain has not been studied previously. A binocular Shack-Hartmann wavefront sensor is used to measure the ocular wavefront aberrations concurrently in both eyes of six subjects at a sampling rate of 20.5 Hz. Coherence function analysis shows that the inter-ocular correlation between aberrations depends on subject, Zernike mode and frequency. For each subject, the coherence values are generally low across the resolvable frequency range (mean 0.11), indicating poor dynamic correlation between the aberrations of the two eyes. Further analysis showed that phase consistency dominates the coherence values. Monocular and binocular viewing conditions showed similar power spectral density functions.

Multifractal nature of ocular aberration dynamics of the human eye

Ocular monochromatic aberrations display dynamic behavior even when the eye is fixating on a stationary stimulus. The fluctuations are commonly characterized in the frequency domain using the power spectrum obtained via the Fourier transform. In this paper we used a wavelet-based multifractal analytical approach to provide a more in depth analysis of the nature of the aberration fluctuations. The aberrations of five subjects were measured at 21 Hz using an open-view Shack-Hartmann sensor. We show that the aberration dynamics are multifractal. The most frequently occurring Hölder exponent for the rms wavefront error, averaged across the five subjects, was 0.31 ± 0.10. This suggests that the time course of the aberration fluctuations is antipersistant. Future applications of multifractal analysis are discussed.

Effect of temporal location of correction of monochromatic aberrations on the dynamic accommodation response

Dynamic correction of monochromatic aberrations of the eye is known to affect the accommodation response to a step change in stimulus vergence. We used an adaptive optics system to determine how the temporal location of the correction affects the response. The system consists of a Shack-Hartmann sensor sampling at 20 Hz and a 37-actuator piezoelectric deformable mirror. An extra sensing channel allows for an independent measure of the accommodation level of the eye. The accommodation response of four subjects was measured during a +/− 0.5 D step change in stimulus vergence whilst aberrations were corrected at various time locations. We found that continued correction of aberrations after the step change decreased the gain for disaccommodation, but increased the gain for accommodation. These results could be explained based on the initial lag of accommodation to the stimulus and changes in the level of aberrations before and after the stimulus step change. Future considerations for investigations of the effect of monochromatic aberrations on the dynamic accommodation response are discussed.

Dual wavefront sensing channel monocular adaptive optics system for accommodation studies

Manipulation of the eyes aberrations using adaptive optics (AO) has shown that optical imperfections can affect the dynamic accommodation response. A limitation of current system designs used for such studies is an inability to make direct measurements of the eyes aberrations during the experiment. We present an AO system which has a dual wavefront sensing channel. The corrective device is a 37-actuator piezoelectric deformable mirror. The measurements used to control the mirror, and direct measurements of the eyes aberrations, are captured on a single Shack-Hartmann sensor. Other features of the system include stroke amplification of the deformable mirror and a rotating diffuser to reduce speckle.We demonstrate the utility of the system by investigating the impact of aberration dynamics on the control of steady-state accommodation on four subjects.

Coherence function analysis of the higher-order aberrations of the human eye

We measured the wavefront aberrations of the eyes of five subjects with a Shack-Hartmann sensor sampling at 21.2 Hz and decomposed the measurements into Zernike aberration terms up to and including the fifth radial order. Coherence function analysis was used to determine the common frequency components between the aberrations within subjects. We found the results to be highly subject dependent. The coherence values were typically <0.4. Possible reasons for this are discussed. Coherence function analysis is a useful tool that can be used in future investigations to determine correlations between the aberration dynamics of the eye and other physiological mechanisms.