The visual system concepts and definitions

Prof. Kent Caslow, Elia Professional Director.
Eye structure


  • Transparent dome-like structure
  • The first and outer layer of the eye
  • Transmits light rays from the object into the eye

Iris –Iris

    • Located behind the cornea and in front of the lens.
    • Gives the characteristic color to the eye
    • In the center is the pupil.
    • Responsible for the contraction and dilation of the pupil.

The iris regulates the amount of light entering and contributes to visual acuity (eyelet phenomenon).

Pupil – Pupil

  • A hole located in the center of the iris. It seems to us black because light does not come out of it
  • Determines the amount of light that enters the eye and reaches the retina.
  • Its function is similar to the camera aperture:
  • In the dark the pupil dilates.
  • The pupil light shrinks.

Lens – lens

  • Located behind the pupil.
  • Made of transparent elastic fabric.
  • Its function is to further refract the light rays that pass through the lens and thus concentrate on one focus that falls on the retina.
  • The lens is dynamic and intensifies for near vision and returns to zero mode for far vision

Retina – Retina

  • The inner part of the eye, consists of cells (receptors) and nerve fibers – their job is to absorb the light rays that have passed through all parts of the eye and transmit the information to the brain.
  • Light energy becomes signals of electricity that pass through the optic nerve to the brain.

 The receptors
The main cells in the retina are called receptors, there are two types, rods in the shape of a reed and cones in the shape of a cone.
The receptors pick up the light and send an electrical message to the brain (not images).
The most important difference between them is:

    • Cones can report details, central vision, colors and day vision

The Herods report peripheral (general) vision and night vision.


  • The macula is responsible for central vision and its center is called the phobia. Physically it is about 2% of the entire retina
  • This area is responsible for sharp vision, color vision, detail vision, reading and face recognition.
  • Outside the macula in the retinal area – responsible for peripheral vision and night vision. This area is very much related to orientation in space

Main Eye Track – Visual Mind
At least 40% of the brain is directly involved in the vision process, with new studies indicating the involvement of about 90% of the brain.

Why is it important for the visual system to mature quickly?

  • The sense of sight leads us farther and more accurately than any other sense
  • To enable the development of visually supervised motor movements
  • Damage to the sense of sight affects all of the baby’s motor development in addition to affecting visual perception.

The connection between the brain and the eye develops and is strengthened following the quality of the message that the eye conveys. At the beginning of the development of the visual system, the reporting to the brain is of each eye individually – there are cells that receive reporting from the right eye and cells that receive reporting from the left eye of equal quality. Later the reporting of two eyes merges into cells that receive reports from both eyes together (“two-eyed” cells). In a state of equality between the eyes, the “competition” between them ends in a “draw” and the two eyes are connected to the “two-poor” cells equally.This is the basis for binocular vision and three-dimensional vision
In a state of visual impairment, when the quality of vision in one eye (or both) is impaired, the message passing from the eye to the brain is not high quality, the connection is weakened and therefore there will be an effect on communication and connection to the brain.
For example, if the message from one eye is incorrect due to blurred vision, a lazy eye (amblyopia) may develop, which is poor vision even if the cause of the blurring (such as a cataract) is removed and if glasses are appropriate.
If the message from the eye is incorrect due to strabismus, there is a chance of two problems:

  • Difficulty in binocular vision: In brain competition for “binocular” cells, one eye will win, “conquer” most (or all) of the binocular cells.
  • Lazy eye: poor vision in the same eye even if the defect is removed / corrected

In both cases, although the problem starts with an eye phenomenon, the real problem is in the brain-eye communication.
Lens accommodation
Accommodation – focusing the lens of the eye by changing the curvature of the lens as a result of the contraction of the muscles surrounding it.
Eye lens accommodation allows a person to see sharply as he looks at different distances.
When looking at a distant object, the focus of the rays on their way to the retina is mostly done in the cornea, so the lens can be relatively flat. But when we look at a close object, further refraction and focus by the lens is needed.

Farsightedness Hyperopia
When the rays of light come from a nearby point, they are not concentrated on the retina but behind it, so that the person cannot clearly see close objects.
Distant objects are relatively clearly visible.
Myopia may be caused by a short eyeball, either due to the non-curvature of the cornea or due to poor lens shape. This is usually a birth defect.
A state of defect in the refraction of light rays caused by the inability of the lens to concentrate the light on the retina.
Myopia is manifested by focusing the bone observed in front of the retina. Thus close objects are clearly visible, while distant objects appear blurred.
The force of our eye is supposed to be symmetrical, so many people with glasses have a “number” that corrects them (such as +2.00 or -4.00). In astigmatism the eye has two forces on different planes (sometimes comparing it to the difference between a sphere and an egg, even though the eye never looks like an egg). Fix it by fixing having two forces on different planes (cylinder) and then there is a prescription for more complex glasses, such as + 2.00-1.00X180 Eye movements
These movements reach a good level very early in the baby’s life.

  • Vision or Pursuit Tracking (Pursuit)

– The purpose of this movement is to “capture” the character with the center of vision to get more content and understanding
– Begins to exist at the age of 3 months and improves greatly until the age of 4 months when observed in 80% of the capacity of an adult.

  • Saccades – : Movements used to “scan” space and also in reading

– Starts around a month
– Reaches 70% mature ability at the age of six months
These two movements are functions of the central visual system.
binocular vision

  • The ability to combine two eyes together starts around the age of 3 months.
  • The most sensitive period for the development of three-dimensional vision ends around age 3
  • In many cases it is difficult to determine that there is strabismus before the age of six months (there is still the possibility of independent eye operations in normal newborns)
  • Any squint determined up to the age of 12 months is defined as the newborn squint.

 What is the field of view?

  • The amount of peripheral vision around the center of view
  • The field of view is measured in degrees and is angular
  • There is an overlap in the field of view of two eyes

<€ Visual acuity
Visual acuity is angular and there are different types of visual acuity:

  • Light perception
  • Light Projection
  • Discrimination
  • Form vision or recognition
  • Resolution,
  • Localization
  • Abilities on

Visual acuity develops rapidly and already at the age of six months it is very mature and therefore there is a high importance of therapeutic intervention very early in the baby’s life.

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