https://escholarship.org/content/qt43k6z8gp/qt43k6z8gp.pdf?t=onkgvd

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

Optical blur in the peripheral retina is known to be highly anisotropic due to nonrotationally symmetric wavefront aberrations such as astigmatism and coma. At the neural level, the visual system exhibits anisotropies in orientation sensitivity across the visual field. In the fovea, the visual system shows higher sensitivity for cardinal over diagonal orientations, which is referred to as the oblique effect. However, in the peripheral retina, the neural visual system becomes more sensitive to radially-oriented signals, a phenomenon known as the meridional effect. Here, we examined the relative contributions of optics and neural processing to the meridional effect in 10 participants at 0°, 10°, and 20° in the temporal retina. Optical anisotropy was quantified by measuring the eye's habitual wavefront aberrations. Alternatively, neural anisotropy was evaluated by measuring contrast sensitivity (at 2 and 4 cyc/deg) while correcting the eye's aberrations with an adaptive optics vision simulator, thus bypassing any optical factors. As eccentricity increased, optical and neural anisotropy increased in magnitude. The average ratio of horizontal to vertical optical MTF (at 2 and 4 cyc/deg) at 0°, 10°, and 20° was 0.96 ± 0.14, 1.41 ± 0.54 and 2.15 ± 1.38, respectively. Similarly, the average ratio of horizontal to vertical contrast sensitivity with full optical correction at 0°, 10°, and 20° was 0.99 ± 0.15, 1.28 ± 0.28 and 1.75 ± 0.80, respectively. These results indicate that the neural system's orientation sensitivity coincides with habitual blur orientation. These findings support the neural origin of the meridional effect and raise important questions regarding the role of peripheral anisotropic optical quality in developing the meridional effect and emmetropization.

Optical and neural anisotropy in peripheral vision.

The contrast sensitivity function (CSF) has shown promise as a functional vision endpoint for monitoring the changes in functional vision that accompany eye disease or its treatment. However, detecting CSF changes with precision and efficiency at both the individual and group levels is very challenging. By exploiting the Bayesian foundation of the quick CSF method (Lesmes, Lu, Baek, & Albright, 2010), we developed and evaluated metrics for detecting CSF changes at both the individual and group levels. A 10-letter identification task was used to assess the systematic changes in the CSF measured in three luminance conditions in 112 naive normal observers. The data from the large sample allowed us to estimate the test-retest reliability of the quick CSF procedure and evaluate its performance in detecting CSF changes at both the individual and group levels. The test-retest reliability reached 0.974 with 50 trials. In 50 trials, the quick CSF method can detect a medium 0.30 log unit area under log CSF change with 94.0% accuracy at the individual observer level. At the group level, a power analysis based on the empirical distribution of CSF changes from the large sample showed that a very small area under log CSF change (0.025 log unit) could be detected by the quick CSF method with 112 observers and 50 trials. These results make it plausible to apply the method to monitor the progression of visual diseases or treatment effects on individual patients and greatly reduce the time, sample size, and costs in clinical trials at the group level.

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Evaluating the performance of the quick CSF method in detecting contrast sensitivity function changes.

The contrast sensitivity function (CSF) provides a fundamental characterization of spatial vision, important for basic and clinical applications, but its long testing times have prevented easy, widespread assessment. The original quick CSF method was developed using a two-alternative forced choice (2AFC) grating orientation identification task (Lesmes, Lu, Baek, & Albright, 2010), and obtained precise CSF assessments while reducing the testing burden to only 50 trials. In this study, we attempt to further improve the efficiency of the quick CSF method by exploiting the properties of psychometric functions in multiple-alternative forced choice (m-AFC) tasks. A simulation study evaluated the effect of the number of alternatives m on the efficiency of the sensitivity measurement by the quick CSF method, and a psychophysical study validated the quick CS method in a 10AFC task. We found that increasing the number of alternatives of the forced-choice task greatly improved the efficiency of CSF assessment in both simulation and psychophysical studies. The quick CSF method based on a 10-letter identification task can assess the CSF with an averaged standard deviation of 0.10 decimal log unit in less than 2 minutes.

Using 10AFC to further improve the efficiency of the quick CSF method

In the field of objective image quality assessment (IQA), Spearman’s $\rho$ and Kendall’s $\tau$ , which straightforwardly assign uniform weights to all quality levels and assume that each pair of images is sortable, are the two most popular rank correlation indicators. These indicators can successfully measure the average accuracy of an IQA metric for ranking multiple processed images. However, two important perceptual properties are ignored. First, the sorting accuracy ( SA ) of high-quality images is usually more important than that of poor-quality images in many real-world applications, where only top-ranked images are pushed to the users. Second, due to the subjective uncertainty in making judgments, two perceptually similar images are usually barely sortable, and their ranks do not contribute to the evaluation of an IQA metric. To more accurately compare different IQA algorithms, in this paper, we explore a perceptually weighted rank correlation indicator, which rewards the capability of correctly ranking high-quality images and suppresses the attention toward insensitive rank mistakes. Specifically, we focus on activating a “valid” pairwise comparison of images whose quality difference exceeds a given sensory threshold ( ST ). Meanwhile, each image pair is assigned a unique weight that is determined by both the quality level and rank deviation. By modifying the perception threshold, we can illustrate the sorting accuracy with a sophisticated SA-ST curve rather than a single rank correlation coefficient. The proposed indicator offers new insight into interpreting visual perception behavior. Furthermore, the applicability of our indicator is validated for recommending robust IQA metrics for both degraded and enhanced image data.

A Perceptually Weighted Rank Correlation Indicator for Objective Image Quality Assessment

Measuring the contrast sensitivity function (CSF) in the periphery of the eye is complicated. The lengthy measurement time precludes all but the most determined subjects. The aim of this study was to implement and evaluate a faster routine based on the quick CSF method (qCSF) but adapted to work in the periphery. Additionally, normative data is presented on neurally limited peripheral CSFs. A peripheral qCSF measurement using 100 trials can be performed in 3 min. The precision and accuracy were tested for three subjects under different conditions (number of trials, peripheral angles, and optical corrections). The precision for estimates of contrast sensitivity at individual spatial frequencies was 0.07 log units when three qCSF measurements of 100 trials each were averaged. Accuracy was estimated by comparing the qCSF results with a more traditional measure of CSF. Average accuracy was 0.08 log units with no systematic error. In the second part of the study, we collected three CSFs of 100 trials for six persons in the 20° nasal, temporal, inferior, and superior visual fields. The measurements were performed in an adaptive optics system running in a continuous closed loop. The Tukey HSD test showed significant differences (p < 0.05) between all fields except between the nasal and the temporal fields. Contrast sensitivity was higher in the horizontal fields, and the inferior field was better than the superior. This modified qCSF method decreases the measurement time significantly and allows otherwise unfeasible studies of the peripheral CSF.

Quick contrast sensitivity measurements in the periphery.

The purpose of this study was to measure the transverse chromatic aberration (TCA) across the visual field of the human eye objectively. TCA was measured at horizontal and vertical field angles out to ±15° from foveal fixation in the right eye of four subjects. Interleaved retinal images were taken at wavelengths 543 nm and 842 nm in an adaptive optics scanning laser ophthalmoscope (AOSLO). To obtain true measures of the human eye's TCA, the contributions of the AOSLO system's TCA were measured using an on-axis aligned model eye and subtracted from the ocular data. The increase in TCA was found to be linear with eccentricity, with an average slope of 0.21 arcmin/degree of visual field angle (corresponding to 0.41 arcmin/degree for 430 nm to 770 nm). The absolute magnitude of ocular TCA varied between subjects, but was similar to the resolution acuity at 10° in the nasal visual field, encompassing three to four cones. Therefore, TCA can be visually significant. Furthermore, for high-resolution imaging applications, whether visualizing or stimulating cellular features in the retina, it is important to consider the lateral displacements between wavelengths and the variation in blur over the visual field.

Transverse chromatic aberration across the visual field of the human eye.

Blur adaptation: contrast sensitivity changes and stimulus extent.

Peripheral visual evaluations are important in many aspects of vision care and research. One instance is central visual field loss, where the visual evaluation has to be performed in the eccentric preferred retinal locus.1–3 Peripheral evaluation is challenging because of reduced retinal function and large optical errors. Care must therefore be taken when designing the psychophysical procedure to avoid additional uncertainties in the estimated threshold. Spatial visual acuity in the periphery is known to vary with task (resolution or detection),4–6 field loci (eccentricity and meridian), 7–9 and stimulus properties (orientation of the stimulus).7–11 For accurate visual evaluation, all three factors should be taken into consideration. However, little is known about the interaction between task and stimulus orientation. In this study, we therefore quantify the difference in detection and resolution thresholds for gratings of different orientations in the peripheral visual field.

Peripheral resolution acuity is well known to be dependent on the orientation of the visual stimuli—known as the meridional effect. Several psychophysical and functional magnetic resonance imaging studies have found that stimuli oriented radially along the meridian are better resolved than stimuli oriented in other directions. 7–11 It has also been reported that the meridional preference for resolution is not caused by refractive errors or other aberrations, hence supporting neural origin.8,9 The physiological cause for the meridional preference would be that retinal ganglion cell dendritic fields are radially oriented.12–15 A neural selectivity for radial grating orientations also in cortical level has been confirmed by Schall et al.16 who found that the orientation columns in the visual cortex of cats are wider for the radial orientation than for other orientations. This neural orientation preference will increase the uncertainty and variability of threshold estimations if the alternatives of a forced-choice paradigm are orientated parallel and perpendicular to the meridian. In addition, there might be a response bias when one of the orientations is more easily seen than the others. Peripheral vision is very important for daily activities, such as orientation, movement, and driving. Peripheral vision evaluation is very important in cases such as macular degenerations, myopia development, and progression research. To avoid uncertainties and inaccuracies in the peripheral acuity measurements, appropriate stimuli selection is vital.

Previous studies on the meridional effect have mainly focused on resolution acuity, and little is known about orientation preferences for peripheral detection acuity. Unlike central vision, there is a difference between resolution and detection acuity for peripheral vision: high-contrast resolution is relatively unaffected by optical errors because it is neurally limited,5,17 whereas detection is affected by optical errors.5,6 This means that a stimulus with a spatial frequency above the neural sampling limit cannot be resolved. However, the undersampled stimulus undergoes aliasing and can be perceived through its moire pattern with a lower spatial frequency, a lower contrast, and a different orientation.4,18 The zone of aliasing is dependent on the optical errors in the periphery. Correction of peripheral optical errors enhances the contrast, which improves detection acuity and thereby widens the aliasing zone. Because of the influence of optical errors on peripheral detection acuity, a meridional preference for detection tasks may be a combination of optical and neural orientation sensitivities. The amount and direction of asymmetric optical errors like astigmatism, coma, and transverse chromatic aberration (TCA) can favor the detection of certain orientations over others. The peripheral image quality is dominated by off-axis astigmatism and coma oriented radially, which could lead to poorer detection of perpendicular gratings and hence a larger meridional effect. To reveal neural limitations and evaluate whether the meridional effect is also present for detection tasks, correction of these optical errors is necessary. Therefore, the present study uses a combination of trial lenses, an adaptive optics system, and monochromatic stimuli to correct for the existing monochromatic and chromatic optical errors. In a peripheral detection study by Cheney et al.,19 the optical errors were surpassed by using circular windowed interferometric stimuli. The reported detection cutoff frequencies were high, and a large meridional preference was reported in white light and was suggested to be purely optical in origin, caused by TCA, as the effect disappeared in green light. However, these results may be caused by the special properties of the interferometric stimuli (such as high retinal image contrast and the presence of TCA in the absence of longitudinal chromatic aberrations), and the meridional preference for detection of noninterferometric stimuli could therefore be very different.

In the present study, we evaluate the peripheral resolution and detection acuities for different orientations in different visual field meridians with optical correction. The detection measurements are also performed in monochromatic light to elude the effects of chromatic aberrations.

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Choice of Grating Orientation for Evaluation of Peripheral Vision.

Objective measurements of transverse chromatic aberration (TCA) between two or more wavelengths with an adaptive optics scanning laser ophthalmoscope (AOSLO) are very accurate, but frequent measurements are impractical in many experimental settings. Here, we demonstrate a pupil tracker that can accurately measure relative changes in TCA that are caused by small shifts in the pupil relative to the AOSLO imaging beam. Corrections for TCA caused by these shifts improve the measurement of TCA as a function of eccentricity, revealing a strong linear relationship. We propose that pupil tracking be integrated into AOSLO systems, where robust and unobtrusive control of TCA is required.

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Simon Winter

Papers

Quick contrast sensitivity measurements in the periphery.

Transverse chromatic aberration across the visual field of the human eye.

Blur adaptation: contrast sensitivity changes and stimulus extent.

Choice of Grating Orientation for Evaluation of Peripheral Vision.

Eye-tracking technology for real-time monitoring of transverse chromatic aberration.