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).

picnajnin
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.
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