Aspects of vision


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While doctors often focus on visual acuity as a measure of vision, there’s more to vision than just that acuity number.  In addition to acuity, activities of daily living such as reading, cooking, using a computer, walking the streets and recognizing faces rely on visual components such as contrast sensitivity, visual field, color perception, stereoscopic acuity, dark adaptation, fixation and more.

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Reduced contrast sensitivity can significantly affect vision quality, including affecting one’s acuity! Someone with low contrast sensitivity may have difficulty navigating his or her environment, even if they have good acuity. Disruptions to the visual field can affect one’s vision even with “good” acuity. In order to understand your current vision, it is invaluable to understand the many factors that affect how you are seeing.

“Your vision is 20/25.”

For more than 100 years the measurement and description of a person’s vision has been tied to the results of the Snellen acuity test. But the letter chart test measures only visual acuity and measures it under almost ideal contrast conditions (deep black letters on an empty white background), an environment that does not represent the world we actively inhabit.

Three of the most important aspects of vision are:

  • Detail vision – the ability to see small details.
  • Contrast vision – the ability to perceive small differences in brightness.
  • Surround vision – the ability to respond to stimuli outside the area of central vision.

 

 

Detail visionContrast visionSurround vision

The sharpness of vision – often referred to as visual acuity –  denotes the ability to recognize small details.

light_foveaThe center of gaze — where we are looking — is through the fovea, which provides the greatest possible visual acuity. Measuring visual acuity, which is often equated with vision, assesses the function of a small area of the retina – less than 1 degree in diameter.

Outside the center of gaze, we actually have quite low visual acuity, yet we remain unaware of this because we instinctively direct our center of gaze to where we are looking, moving our eyes, head and our body as necessary.

Contrast is the difference in brightness that separates an object from its background. It is the difference in brightness and/or color that makes an object (either real or an image) distinguishable from other objects and from its surroundings.

Contrast sensitivity is a person’s ability to see these differences. The better able one is to detect objects of low contrast, the higher one’s contrast sensitivity.

contrast1In seeing everyday objects, our eyes collect several bits of information using various visual channels. These visual channels collect information about size, shape, contrast, color and motion. Each channel collects and feeds this information individually to our eye/brain system.

The Snellen letter chart tests only one of these visual channels.; it measures one’s ability to see well-defined black letters on a white background, which is a very high contrast environment. While most objects we see are larger than the letters on a letter chart, they are much lower in contrast.

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Many people with normal or near-normal high-contrast acuity have low contrast sensitivity. Low contrast is common among the elderly, as it is a common characteristic of the aging eye.

Low contrast sensitivity often goes un-diagnosed, in part because a contrast sensitivity test is often not administered as part of a routine eye exam.In everyday life you encounter high contrast situations, low contrast situations and everything in between. So loss of contrast sensitivity can have a greater impact on the activities of daily living than the loss of visual acuity alone.

Even in normal lighting situations reduced contrast sensitivity affects how and what you see.

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If you have normal visual acuity but poor contrast sensitivity you might see the high contrast tree line in the foreground of the picture above yet have difficulty seeing the shapes of the mountains against the sky in the background (low contrast).

Loss of contrast sensitivity – either from aging or eye conditions – can ____ly affect performance of activities of daily living such as walking down poorly lit stairs.

stairs

 

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Your visual field is limited; you don’t see 360 degrees. You don’t have “eyes in the back of your head.” You can see the edges of your visual field by focusing your sight on a point in front of you and then moving a hand, wiggling a finger, further and further to one side. If you keep your focus on the point in front of you, you will see your finger disappear outside the border of your visual field. The human visual field is usually a little more than 180 degrees, or 90 degrees on each side from your central fixation gaze.

Peripheral vision

While central (foveal) vision allows you to see the fine details of objects you are looking at directly, peripheral vision is used to find objects and detect threats and is important for general orientation and balance. When you move through your environment and around obstacles, go up or down stairs, reach for something, ride a bicycle, or do any physical activity, you’re using peripheral vision.

When your vision becomes diminished on one side, you may find yourself bumping into door frames or furniture, and driving can be dangerous when you cannot see traffic in the next lane or cars entering from side streets.

Peripheral vision is also important for tasks such as reading. If, for example, you have reduced vision on the right side if may be difficult for you to read a line of text, as your ability to quickly see the next word will be diminished. You might not be able to even read long words. With poor vision on the left side you might have difficulty finding the beginning of the next line.

Most of our peripheral vision is subconscious. Driving, walking down the street, eating a meal, reaching for something all use visual information in our visual field that is processed sub-consciously, never reaching consciousness.
Because much of this process is subconscious we are not spontaneously aware of vision loss in our visual field, and this as well makes testing more difficult.

As well, peripheral vision is an essential part of detail vision, as it is by scanning the periphery that we see what we are looking for and then we turn our gaze to it. When we “find” what we are looking for, the salient peripheral stimuli send a signal to the brain that asks for fixation and engagement of the central system for conscious recognition. reword

For more on aspects of vision including problems associated with each and potential solutions, see  Aspects of vision: problems and solutions

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