Geometrical scaling of the developing eye and photoreceptors and a possible relation to emmetropization and myopia

The effect of multifocal contact lenses on the dynamic accommodation step response

PURPOSE

To measure the dynamic accommodation response (AR) to step stimuli with and without multifocal contact lenses (MFCLs), in emmetropes and myopes.

METHODS

Twenty-two adult subjects viewed alternating distance (0.25D) and near (3D) Maltese crosses placed in free space, through two contact lens types: single vision (SVCL) or centre-distance multifocal (MFCL; +2.50D add). The AR level was measured along with near to far (N-F) and far to near (F-N) step response characteristics: percentage of correct responses, magnitude, latency, peak velocity and duration of step response.

RESULTS

There was no difference between N-F and F-N responses, or between refractive groups in any aspect of the accommodation step response dynamics. The percentage of correct responses was unaffected by contact lens type. Through MFCLs, subjects demonstrated smaller magnitude, longer latency, shorter duration and slower peak velocity steps than through SVCLs. When viewing the near target, the AR through MFCLs was significantly lower than through SVCLs. When viewing the distance target with the MFCL, the focal points from rays travelling through the distance and near zones were approximately 0.004D behind and 2.50D in front of the retina, respectively. When viewing the near target, the respective values were approximately 1.89D behind and 0.61D in front of the retina.

CONCLUSION

The defocus error required for accommodation control appears not to be solely derived from the distance zone of the MFCL. This results in reduced performance in response to abruptly changing vergence stimuli; however, these errors were small and unlikely to impact everyday visual tasks. There was a decrease in ocular accommodation during near tasks, which has previously been correlated with a reduced myopic treatment response through these lenses. With MFCLs, the estimated dioptric myopic defocus was the largest when viewing a distant stimulus, supporting the hypothesis that the outdoors provides a beneficial visual environment to reduce myopia progression.

ON and OFF receptive field processing in the presence of optical scattering

The balance of ON/OFF pathway activation in the retina plays a role in emmetropization. A new myopia control lens design uses contrast reduction to down-regulate a hypothesized enhanced ON contrast sensitivity in myopes. The study thus examined ON/OFF receptive field processing in myopes and non-myopes and the impact of contrast reduction. A psychophysical approach was used to measure the combined retinal-cortical output in the form of low-level ON and OFF contrast sensitivity with and without contrast reduction in 22 participants. ON responses were lower than OFF responses (ON 1.25 ± 0.03 vs. OFF 1.39 ± 0.03 log(CS); p < 0.0001) and myopes showed generally reduced sensitivities (myopes 1.25 ± 0.05 vs. non-myopes 1.39 ± 0.05 log(CS); p = 0.05). These findings remained unaffected by contrast reduction (p > 0.05). The study suggests that perceptual differences in ON and OFF signal processing between myopes and non-myopes exist but cannot explain how contrast reduction can inhibit myopia development.

Emmetropization and nonmyopic eye growth

Most eyes start with a hypermetropic refractive error at birth, but the growth rates of the ocular components, guided by visual cues, will slow in such a way that this refractive error decreases during the first 2 years of life. Once reaching its target, the eye enters a period of stable refractive error as it continues to grow by balancing the loss in corneal and lens power with the axial elongation. Although these basic ideas were first proposed over a century ago by Straub, the exact details on the controlling mechanism and the growth process remained elusive. Thanks to the observations collected in the last 40 years in both animals and humans, we are now beginning to get an understanding how environmental and behavioral factors stabilize or disrupt ocular growth. We survey these efforts to present what is currently known regarding the regulation of ocular growth rates.

Myopia: why the retina stops inhibiting eye growth

In myopia, the eye grows too long, and the image projected on the retina is poorly focused when subjects look at a distance. While the retina normally controls eye growth by visual processing, it seems to give up during myopia development. But what has changed? To determine whether the sharp image is in front or behind the retinal plane, a comparison of image sharpness in red and blue would provide a reliable cue because focal planes are about 1.3 D apart due to longitudinal chromatic aberration (LCA). However, up to now, it could not be demonstrated that the retina does, in fact, such a comparison. We used a new approach: movies were digitally filtered in real time to present either the blue channel of the RGB color format unfiltered while green and red were blurred ("blue in focus"), or the red channel was unfiltered while green and blue were blurred ("red in focus") accordingly to the human LCA function. Here we show that, even though filtered movies looked similar, eyes became significantly shorter when the movie was sharp in the red plane but became longer when it was presented sharp in the blue plane. Strikingly, the eyes of young subjects who were already myopic did not respond at all-showing that their retina could no longer decode the sign of defocus based on LCA. Our findings resolve a long-standing question as to how the human retina detects the sign of defocus. It also suggests a new non-invasive strategy to inhibit early myopia development: keeping the red image plane on a computer screen sharp but low pass filtering the blue.

Visual Axis and Stiles-Crawford Effect Peak Show a Positional Correlation in Normal Eyes: A Cohort Study

Purpose

The purpose of this study was to locate the visual axis and evaluate its correlation with the Stiles-Crawford effect (SCE) peak.

Methods

Ten young, healthy individuals (20 eyes) were enrolled. An optical system was developed to locate the visual axis and measure SCE. To locate the visual axis, 2 small laser spots at 450 nm and 680 nm were co-aligned and delivered to the retina. The participants were asked to move a translatable pinhole until these spots were perceived to overlap each other. The same system assessed SCE at 680 nm using a bipartite, 2-channel (reference and test) Maxwellian-view optical system. The peak positions were estimated using a two-dimensional Gaussian fitting function and correlated with the visual axis positions.

Results

Both the visual axis (x = 0.24 ± 0.35 mm, y = -0.16 ± 0.34 mm) and the SCE peak (x = 0.27 ± 0.35 mm, y = -0.15 ± 0.31 mm) showed intersubject variability among the cohort. The SCE peak positions were highly correlated in both the horizontal and vertical meridians to the visual axes (R2 = 0.98 and 0.96 for the x and y coordinates, respectively). Nine of the 10 participants demonstrated mirror symmetry for the coordinates of the visual axis and the SCE peak between the eyes (R2 = 0.71 for the visual axis and 0.76 for the SCE peak).

Conclusions

The visual axis and SCE peak locations varied among the participants; however, they were highly correlated with each other for each individual. These findings suggest a potential mechanism underlying the foveal cone photoreceptor alignment.