Tag Archives: refraction

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)

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.

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!


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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