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Eye and Vision

Among our senses, vision is our primary sensory system which we use most. With it, we experience the outside world-we see people, houses, cars, water, mountains, trees, flowers, animals and we also see ourselves in a mirror. Our vision is highly developed and extremely efficient. We can quickly determine the nature of an object we see, its distance and movement and within a split second recognize the gender, age, familiarity and expression of a face. Vision is essential and indispensable to many parts of our daily lives, our work, free time or pleasures. Vision consists of the complex combination of visual acuity, color sense, the ability to distinguish contrasts, and ability to evaluate the location of objects in the environment (space). Most older people experience normal changes in their eyes that are associated with the aging process. In addition, there are four age related eye conditions that may result in visual impairment. In general, environmental conditions such as adequate lighting, elimination of glare, and the use of color contrast are more significant for the visual functioning of older persons than of younger persons. Many of these helpful environmental modifications are minor and inexpensive; they can be made quite easy within the homes of older persons who are visually impaired, as well as in public environments. Demonstration of human eye is given below:

Structure of the Human Eye

 

Visual Acuity

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Visual acuity is the ability to see objects clearly, and is measured by the high contrast acuity charts typically found in an eye care professional's office. A visual acuity of 20/20 means that an individual reading the chart at 20 feet can see what the average person can at that distance; likewise, visual acuity of 20/40 means that the individual can see at 20 feet what the average individual can see at 40 feet. Visual acuity generally declines modestly beyond age 60. For example, a reduced visual acuity of 20/30 or 20/40 with the best possible correction by eyeglasses, compared to a prior visual acuity of 20/20, is typical for an individual age 60 or older. 

Absolute Sensitivity

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Almost all the information needed in order to know both the resolving power and the absolute sensitivity of eyes is available from simple anatomical measurements of the eye and its receptors. Given knowledge of the optical principle on which the eye is based, the eyes of different species can then be compared, using as criteria their resolving power and sensitivity.
In the case of hearing, sensitivity is defined as the capacity of a sense organ to detect a stimulus and is quantified by the determination of threshold of audibility or threshold of detection of change. There are at least two kinds of sensitivity, absolute and differential. Absolute sensitivity pertains to the capacity of the auditory system to detect faint sound. Differential sensitivity pertains to the capacity of the auditory system to detect differences or changes in intensity, frequency or some other dimension of a sound. Hearing sensitivity most commonly refers to absolute sensitivity to faint sound. In contrast, hearing acuity most accurately refers to the differential sensitivity, usually to the ability to detect differences in signals that differ in the frequency domain. 

Polarization Sensitivity of Retinula Cells

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The light sensitive cells that contain the visual pigment are called "retinula cells". They are primary receptors, which simply means that their axons project into the optic lobe of the brain before synapsing. By way of contrast, the rods and cones in the human eyes are secondary receptors because their axons synapse several times before reaching the brain. Typically there are eight retinula cells arranged in a rough circle in each ommatidium. Sometimes the compound eye is divided into dorsal and ventral parts; adult dragonflies have four retinula cells in each ommatidium in the dorsal part of the eye and six in the ventral part. Usually the eighth cell, or in honeybees the ninth cell, is short and extends only in the more proximal region of an ommatidium. Retinula cells vary greatly in length.

Optical Properties of Rhabdomeric Membrane

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The optical properties of photoreceptors, example, arrangement, orientation, shape, size, refractive index and membrane properties, influence absorption and establish many specialized functions. Furthermore, photoreceptor optics are closely related to the study of optical waveguides, the theory of which is now well developed. 

Waveguide Properties of Visual Photoreceptors

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Light is less effective in stimulating the retina when it enters the eye obliquely to the optical axis. This affects both the brightness and color of the light. This effect was a result of waveguide properties of cone photoreceptors. It is interesting to speculate on why the visual system evolved two classes of photoreceptors with different waveguide properties. One possibility is that only cone photoreceptors need directional sensitivity because they attinuate the effects of longitudinal chromatic aberration. Another possibility is that scattered light gives an independent estimate of overall light intensity and that the visual system can calculate scattered light by comparing directional sensitive (cone) photoreceptors to rods. This information can be used to calibrate the visual system's lightness scale, ensuring that gray levels do not change with illuminants.

Focusing and Accommodation

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Accommodation occurs unconsciously, so you are usually unaware that the lens is constantly changing its focusing power so you can see clearly at different distances. This unconscious focusing process works so efficiently that most people assume that everything, near and far, is always in focus.
You can demonstrate that this is not so by holding a pencil point up, at arm's length, and looking at an object that is at least 20 feet away. As you look at the faraway object, move the pencil point toward you without actually looking at it. The pencil will probably appear blurred. Then move the pencil closer, while still looking at the far object, and notice that the point becomes more blurred and appears double. When the pencil is about 12 inches away, focus on the pencil point. You now see the point sharply, but the faraway object you were focusing on before has become blurred. Now, bring the pencil even closer until you can't see the point sharply no matter how hard you try. Notice the strain in your eyes as you try unsuccessfully to bring the point into focus. 

Dark Adaptation

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Dark adaptation is defined as the dramatic increase in retinal sensitivity to light that occurs when a person enters  to the dark. The causes of dark adaptation is given below. Both rods and cones consists of light sensitive visual pigments. When a light is incident to these pigments, that pigments undergo chemical break down or bleaching. The images produced after looking at a flash tube is the result of this bleaching. To regain the visual sensitivity, these visual pigments have to recombine, this process takes time. Increase in the rhodopsin that is the rod pigments causes the night vision. When completely dark adapted, the human eye is almost as sensitive to light as the eye of an owl.

Blind Spot

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The region where the nerves and blood vessels enter and exit the eyeball is known as the optic disc. It contains no receptors and so objects projected by the lens onto the optic disc are not detected. For this reason the optic disc is also known as the blind spot.
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