April 29th
2010

Eye Physiology  ⁄  Accommodation and Convergence

Accommodation and convergence allow us to see objects clearly both near and far without diplopia (double vision). There is a direct relationship between the effort of accommodation and the effort of convergence. The effort of accommodation is measured in diopters, and the effort of convergence is measured in degrees. There are three main types of eye refraction: emmetropia (normal-sightedness), myopia (nearsightedness), and hyperopia (farsightedness). Emmetropia is the normal condition of perfect vision, in which parallel light rays are focused on the retina without the need for accommodation. When an emmetropic person is looking at a distant object, accommodation and convergence are resting. The axes of the eyes are parallel, and there is no effort of convergence (the angle of intersection is zero degrees). When the emmetropic (normal sighted) person is looking at a nearby object, some effort of accommodation and convergence is necessary.

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February 23rd
2010

Eye Physiology  ⁄  Less Area of Active Vision Means Better Eyesight

Myopia or nearsightedness is a refractive defect of the eye. Those with myopia can see clearly at a close distance, but far away objects appear blurred.

There are two important questions related to nearsightedness,

“Why do nearsighted people try to see things close to their eyes?”

“Why do they have mental strain when gazing at an object?”

I am sure your answer in both cases is, “Because their far-distance eyesight is not sharp”. Yes, this is the most popular answer, but I do not accept it because of my empirical observations. I examined normal-sighted children with a habit of seeing close. Also they were gazing with mental strain. In preschool, they had 20/20 visual acuity, but subsequently in grade school, myopia developed as a consequence of wrong habits. Undoubtedly, the wrong visual behavior is the cause, and progression of myopia is the effect.

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August 1st
2009

Eye Physiology  ⁄  Binocular Vision

Binocular vision is an incorporated activity of the sensory and motor systems of both eyes. When the eyes are looking at an object, the images appear on the retinas of both eyes. Two monocular images fuse to create the single visual image in a process called fusion. Binocular vision increases the brightness of the visual object and visual acuity by about 20%.

When the medial parts of the visual fields of each eye fuse, a common visual field appears where binocular fixation is possible. In the diagram below, the visual field of the left eye is shown with horizontal lines, and the visual field of the right eye is shown with vertical lines. The middle common part is the binocular visual field. The width of the binocular field is about 115 degrees.

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May 13th
2009

Eye Physiology  ⁄  Function of Extraocular Muscles

When I observe a person with nearsightedness, he reminds me of the driver at the steering wheel of a supercar, waiting for the car to start going by itself wherever its owner wishes. When his desire is not executed, he thinks that his car is not like the others. In this article, I will tell you about our eyes’ capabilities, about the quick and exact actions they can perform. I hope, having read it, that you will feel respect for your eyes and that you will want to learn to operate them to release all their potential.

Extraocular muscles are cross-striated but are different than skeletal muscles. They consist of two types of fibers – extrafusal and intrafusal. Extrafusal fibers create the force that drives the movement and fixation of the eyes. Intrafusal fibers control the frequency of the pulsed discharge from the neuromuscular spindles (which sense muscle length).

There are two types of extrafusal fibers: fast phasic (80%) and slow tonic (20%). So eyes can make fast movements (saccade, microsaccade and tremor) and slow ones (smooth pursuit eye movement and drift).

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