Tag Archives: refraction

Eye Growth and Sign of Defocus

The following article is presented to you by Najnin Sharmin
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
  • Human Eye Growth

During birth to adulthood the human eye grows very little. The eye of a newborn is around 70% of the size of an adult and the growth is approximately 7.6 mm from birth to adulthood. The “Eye Socket” also grows with the eyeball. Different kind of variations can occur during the eyeball growth and this can cause optical errors shifting the location of the best focus within the eye. If the eye is too short in length, it will focus images behind the retina. This case is known as “Hyperopia” or far-sightedness. Difficultly in reading, headaches, eye strain, fatigue are some consequences of Hyperopia. On the other hand, If the eyeball grows too long, it will focus images in front of the retina. This case is known as “Myopia” or near-sightedness. Myopia also causes headaches, eye strain and squinting if not treated.

What is Myopia?

  • Accommodation

The human eye changes the optical power by altering the shape of its lens to focus objects at various distances, this mechanism is known as “Accommodation”. Young people can change the optical power by up to 15 dioptres by changing the ciliary body. Their eye can change focus from infinite distance to just 6.5 cm from the eye. But accommodation cannot shift images back in focus on the retina in myopic eyes. For a relaxed eye, the accommodation level is zero, when the power of the eye is 60 D. Accommodation and eye growth are intricately linked but not the same.

Risk Factors 

  • What is Sign of defocus

Like any other optical system, human eyes also suffer from aberrations.There are different kind of optical aberrations e.g. defocus, tilt, spherical aberration, astigmatism, coma, distortion etc. In optics, defocus is one kind of aberration in which an image is simply out of focus. High levels of axial aberration (spherical aberration) is responsible for night myopia. Moreover, low-order aberrations cause Myopia (positive defocus) and Hyperopia (negative defocus). One of many common techniques to measure eye aberrations is the Hartmann-Shack wavefront sensor (HS-WFS). It is comprised of a camera with an array of microlenses called “lenslets” mounted in or near to the camera.

The sign of defocus is very important for the rapid control of accommodation and also for regulating the slower long-term growth of the eye (1). Human eyes typically have a positive Spherical aberration (SA) when accommodation is relaxed. The amount of positive SA falls when the eye accommodates, vanishes with about 2 or 3 diopters (D) of accommodation, and grows steadily more negative with further accommodation of eye (2,3,4-8) because of the changes in eyeball shape (2,9) and refractive index distribution of the crystalline lens (10).

  • Retina alone detect the sign of defocus?

A fundamental question in emmetropisation (ideal vision) is – whether the retina by itself can perform the image processing necessary to derive the sign of defocus without any help from the brain?

An experiment on chicks shows that eye growth can be locally stimulated by local degradation of the retinal image, even after the optic nerve was cut. So, it was clear that the retina has at least the complete machinery to convert image features into growth signals (11).

When we talk about light absorption in the retina, we tend to consider the retina as a single surface. In general, the retina is a multilayered surface. According to the antenna model of outer-segment pigments, those retinal layers can be considered as layered circular discs (12). Each disc has around 4,000 (1 µm) to 4,000,000 (5 µm) pigment molecules that can absorb light. Each outer segment has approximately 1000 lamellae and the interspacing between the lamellae is approximately 20 nm. (12).

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Fig.1 Photoreceptor outer-segment model.

In this retinal model, defocus symmetry is broken by the light acceptance of layered membrane infoldings –

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Fig 2. Role of defocus seen in a cross section through the middle of the outer segments consisting an array of 19 hexagonally packed outer segments, when each segment has 1000 layers each containing 12 dipoles. The figure shows the light incidence near the (a) upper entrance and (b) far-end exit of a single outer segment (13).

The role of defocus for accommodation can be noticed from Fig. 2 suggesting that the best focus is obtained once the amount of light within the outer segment is maximized (13). Moreover, those stacked pigments may determine the sign of defocus.


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Myopia in Science!

  • References.
1) Thibos LN, Bradley A, Liu T, and Lo´pez-Gil N. Spherical Aberration and the Sign of Defocus. Optom Vis Sci 2013; 90:1284 –1291.

2) Young T. The Bakerian Lecture: on the mechanism of the eye. PhilTrans R Soc Lond 1801;91:23 – 88.

3) Tscherning MH. Physiologic Optics, 3rd ed. Philidelphia, PA:Keystone Publishing; 1920.

4) Ivanoff A. On the influence of accommodation on spherical aberration in the human eye, an attempt to interpret night myopia. J Opt Soc Am 1947;37:730.

5) Atchison DA, Collins MJ, Wildsoet CF, Christensen J, Waterworth MD. Measurement of monochromatic ocular aberrations of human eyes as a function of accommodation by the Howland aberroscope technique. Vision Res 1995;35:313–23.

6) Plainis S, Ginis HS, Pallikaris A. The effect of ocular aberrations on steady-state errors of accommodative response. J Vis 2005;5:466–77.

7) Lopez-Gil N, Fernandez-Sanchez V, Legras R, Montes-Mico R,Lara F, Nguyen-Khoa JL. Accommodation-related changes in mono-chromatic aberrations of the human eye as a function of age. Invest Ophthalmol Vis Sci 2008;49:1736–43.

8) Cheng H, Barnett JK, Vilupuru AS, Marsack JD, Kasthurirangan S, Applegate RA, Roorda A. A population study on changes in wave aberrations with accommodation. J Vis 2004;4:272–80.

9) Lopez-Gil N, Fernandez-Sanchez V. The change of spherical aberration during accommodation and its effect on the accommodation response. J Vis 2010;10:12.

10) Navarro R, Palos F, Gonza´lez LM. Adaptive model of the gradient index of the human lens. II. Optics of the accommodating aging lens. J Opt Soc Am (A) 2007;24:2911–20.

11) Schaeffel F. Can the retina alone detect the sign of defocus? Ophthalmic Physiol Opt 2013;33,362–367.

12) J. J. Wolken. Light detectors, photoreceptors, and imaging systems in nature. (New York, Oxford University Press, 1995).

13) Vohnsen B. Directional sensitivity of the retina: A layered scattering model of outer-segment photoreceptor pigments. BOE 2014;5:1569–1587.

Peripheral vision and myopia

The following article is presented to you by Petros Papadogiannis
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
  • What is peripheral vision

Peripheral vision is the part of our vision that is outside the center of our gaze, and it is the largest portion of our visual field. For both eyes the combined visual field is 130°–135° vertical and 200°–220° horizontal with 180-200 degrees comprising the peripheral vision. It is weaker in humans than in many other species, and this disparity is even greater where it concerns our ability to distinguish color and shape. This is due to the density of the receptor cells on the retina and the enlargement of optical errors in the periphery. As a result, reduced visual acuity and contrast sensitivity occurs.

  •  Retinal shape and myopia

Myopic eyes have multiple variations on their retinal shape. This phenomenon is related to the potential models of retinal stretching that occurs during axial elongation. The picture below represents the 4 models of retinal stretching that can occur in myopia. The solid circles represent the shape of the retina of an emmetropic eye, the dashed shapes represent the myopic retinas, and the arrows indicate the regions of stretching. (1,2)
It was found that despite the existence of myopia in both the central and peripheral retina, myopic error in the periphery is smaller. (1)
Also, in 2009 Tabernero and Schaeffel found that myopes (even those with medium refractive error) appear to have more irregular shape than emmetropes, on the peripheral retina. (8)

eyeshape
Eye expansion.
Source: Eye shape and retinal shape, and their relation to peripheral refraction, OPO 2012
  •  What do animal studies show?

Animal studies have shown that the peripheral retina can trigger or stop the growth of the eye depending on the location of the peripheral image relative to the retina. When an image is focused on the central retina and for the peripheral retina, the image is focused behind, this results in a relatively hypermetropic periphery and a defocused image. This defocused image sends a growing signal to the eye and makes the eye myopic.
By their experiments in laboratory animals, Smith et all found that visual signals from the peripheral retina can dominate against the visual signals from the central retina in terms of regulation of eye’s refractive status. (3)
The concept that dominates is that cones are more involved than rods(they are located in the peripheral retina) in the detection of visual signals that contribute to eye growth. But a study of 2010 in mice, shows that rods are important for the detection of the signals that are involved in the procedure of emmetropization and the development of myopia.(4)

  •  Does peripheral refractive status affect the onset and progression of myopia?

A number of studies in humans, have shown that peripheral refractive errors are ante-dated to the onset of central myopia and can, therefore, be a risk factor for the onset and progression of myopia.
In a 1971 study in young trainee pilots, Hoogerheide found that emmetropes with peripheral hypermetropic refraction had greater possibilities to develop myopia, compared to emmetropes that appeared to have myopic astigmatism in the periphery. (5)
More recently, Schmid (2011) verified an important association between the greater steepness of the retina (more prolate eye shape) and the central myopic shift in children.(6)
On the other hand, Mutti in 2011 didn’t manage to verify the influence of peripheral hypermetropia in the onset of myopia. Particularly, despite the fact that he found a correlation between the magnitude of the peripheral hypermetropia and myopia progression, the total influence of peripheral hypermetropic state in central refraction was limited. (7)
To conclude with, although the hypothesis that a relatively hypermetropic periphery can drive the development of human myopia remains unproven, the existing research support the possibility of an interaction between the states of focus on axis and in the periphery.


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Myopia in Science!

  • References.
1) Pavan K Verkicharla,Ankit Mathur,Edward AH Mallen,James M Pope,David A Atchison. Eye shape and retinal shape, and their relation to peripheral refraction. OPO 2012; 32: 184–199
2) Strang NC, Winn B & Bradley A. The role of neural and optical factors in limiting visual resolution in myopia. Vision Res 1998; 38: 1713–1721.
3) Earl L. Smith. The Charles F. Prentice Award Lecture 2010: A Case for Peripheral Optical Treatment Strategies for Myopia Optom Vis Sci. 2011 September ; 88(9): 1029–1044
4) S. B. Jabbar; A. E. Faulkner; G. F. Schmid; F. Schaeffel; J. Abey; P. M. Iuvone; M. T. Pardue. Rod Photoreceptor Contributions to Refractive Development and Form Deprivation Myopia in Mice. Investigative Ophthalmology & Visual Science 2010; 51: 1726
5) Hoogerheide J. · Rempt F. · Hoogenboom W.P.H. Acquired Myopia in Young Pilots. Ophthalmologica 1971;163:209–215
6) Schmid GF. Association between retinal steepness and central myopic shift in children. Optom Vis Sci. 2011 Jun;88(6):684-90.
7) Donald O. Mutti; Loraine T. Sinnott; G. Lynn Mitchell; Lisa A. Jones-Jordan; Melvin L. Moeschberger; Susan A. Cotter; Robert N. Kleinstein; Ruth E. Manny; J. Daniel Twelker; Karla Zadnik Relative Peripheral Refractive Error and the Risk of Onset and Progression of Myopia in Children Investigative Ophthalmology & Visual Science.2011;52:199-205
8) Juan Tabernero; Frank Schaeffel. More Irregular Eye Shape in Low Myopia Than in Emmetropia. Investigative Ophthalmology & Visual Science.2009;50:4516-4522.

Refraction Fluctuations in the Eye

The following article is presented to you by Dmitry Romashchenko
Disclaimer: The following text may content specific terms, requiring more in deep knowledge in the field.
  • Refraction

As it was said previously, the mechanisms for the myopia onset are currently not completely understood. That makes every difference in static or dynamic behavior of emmetropic and myopic eye of particular interest. Refraction, or optical power of the eye, (compared to its length) is the main criterion by which the judgment about ammetropia (myopia or hypermetropia (far-sightedness)) is made. Refraction is the reciprocal (1/value) of the distance to the plane on which the eye is focused. For the relaxed (not accommodating) emmetropic (healthy) eye refraction is 0D. That means that infinitely far objects (1/infinity =  0) will be in focus on the retina. Relaxed myopic eyes have refraction 0. The signs are showing the position of the plane in focus (negative – forward, in front of the eye, positive – backwards, behind the eye) and the number is showing the amount of image blur: the bigger is the absolute refraction value the more the image is unfocused.

  • Refraction fluctuations

When talking about the ‘refraction’ of the eye the ‘mean refraction value’ is meant as the value is not completely constant over time (see the fig.1). These changes can clearly be seen on the figure 1.

Fluctuations_Dmitri
Fig. 1 Dynamics of the eye refraction

This microfluctuations in the eye optical power can be categorized in 2 groups by their frequency: lower (below 1 Hz) and higher (above 1 Hz) frequency domains. The second group fluctuations are lower than those of the first one (2). The low-frequency group is believed to be responsible for ‘physiological control’ of the eye refractive state (2). In other words, when looking at any object, the eye ‘checks’ if the refractive power of the eye is still optimal for viewing the particular object. It is represented by slow fluctuations of the optical power from the mean value.
Comparing myopic and emmetropic eyes showed that myopic patients have larger refraction fluctuations for far and near targets than emmetropic ones (1). This is one of the clues to the theory idea that in myopic eyes the whole accommodation mechanism (change of the optical power depending on the distance to the object) is working differently than in emmetropic eyes. Since the loop ‘accommodation mechanics + neurological control’ is not fully understood as well, this difference can play a major role in myopia onset processes by itself or be a significant part of it. On the other hand, this observed changes can be not the cause but the result of the myopia development in the eye. In both cases it gives a better understanding of the myopic and emmetropic eyes dynamics, creates new and answers previously arisen questions on the matter.


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Myopia in Science!

References.

1) Seidel; N.C. Strang; L.S. Gray; E.A. H. Mallen. The Influence of Target Vergence Upon the Magnitude of the Accommodative Microfluctuations in Emmetropia, Early–Onset Myopia and Late–Onset Myopia, 2005

2) Ronald B. Rabbets, Edward E. A. Mallen, 2007. Clinical Visual Optics, 4th edn., p. 125 – 149. References to the chapter:

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