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