Visual Perception Disorders
What happens when your visual system is damaged?
Your right and left visual fields make up the area that you see in front of you. At a basic level, each eye’s field can be divided into 4 quadrants (see the page on "Vision" for detailed information). Depending on where damage occurs along the visual system, a person will experience different types of blind spots or deficits in perception.
Visual Field Cuts
In the right panel of the figure below, pink lox shows which visual field quadrants have blindness based on where damage occurs in the right hemisphere's visual system (demonstrated with vegetables). The white krab shows quadrants that are still intact despite this damage.
Damage to the optic nerve in the right brain hemisphere (cucumber) results in complete blindness to the right eye.
Full Eye Blindness
Bitemporal Heteronomous Hemianopsia
Damage at the optic chiasm (orange pepper) results in blindness in each eye's outer visual field quadrants.
Left Homonomous Hemianopsia
Damage at the right hemisphere optic tract (red pepper) results in blindness in the left parts of each eye's visual field.
Left Superior Quadrantanopsia
Damage at the right hemisphere optic radiation (mushroom) results in blindness in the upper left visual field quadrants.
Left Homonomous Hemianopsia with Macular Sparing
Damage at the right hemisphere visual cortex (yellow pepper) results in blindness to the left hemi-fields with intact vision in the right hemi-fields and focally.
What number do you see in the image below? For most people, the number 5 easily pops out. For others, it can be much harder to perceive that there is a number at all. Why does this happen?
The world around us is full of waves. No, not the kind you find at the beach, but waves from electric and magnetic fields (e.g., radio waves, microwaves, ultraviolet waves). Although we are unable to see most of these waves naturally with our eyes, there is a portion of this "electromagnetic spectrum" that is visible to us in the form of visible light. However, not all visible light is equal in wavelength.
Color perception is our ability to distinguish among different light wavelengths in the world around us based on how they reflect off of objects and in to our eyes. We perceive blue colors from visible
The Basis for Color Vision
light with shorter wavelengths, green colors from visible light with medium wavelengths, and red colors from visible light with longer wavelengths. However, many factors can influence how we perceive these wavelengths, including genetics, damage to the visual system, and even language/culture. For example, one study found that native Greek speakers were better at distinguishing between two different shades of blue associated with specific Greek words (i.e., ghalazio and ble) than native English speakers who did not have specific words for those shades.
Intact Color Vision
When all goes well in the visual system, the first step in color perception happens when light reflected off of an object enters our eyes. Parts of our eyes work to focus this light into a portion of our eye called the retina. The retina is located at the back of the eye and is made up of millions of cells called "photoreceptors." There are two types of photoreceptors that are named based on their shape (i.e. "cones" and "rods"). They differ in their sensitivity to light and the types of pigment molecules, called "photopigments" that are activated by light exposure.
Cones respond to bright lights and have three types of photopigments. They are typically activated in daytime vision and are sensitive to long, medium, and short light wavelengths. Rods respond to dim lights and have one type of photopigment. They are much more abundant in our eyes and are useful for night vision. Because humans have three types of photopigments, we are said to have "trichromatic" vision. Other species have different numbers of photopigments, causing them to perceive colors more or less sensitively than humans do.
Once reflected light enters our eyes and activates cones and rods, this signal is sent through the rest of the visual system to interact with memory, emotion, and language brain regions in order to produce our perception of the color associated with the wavelength. Because of this complex interaction among brain regions, color perception might vary slightly from person to person. So maybe the red that I see when looking at an apple is ever so slightly different from the red that you see when looking at that same apple. We'll never truly know.
So what does a person with color blindness see, and how does this happen? Let's start by talking about the how before we talk about the what.
Typically, color blindness is genetic, meaning if a parent carries a gene for color blindness, there is a chance that their child will be color blind. Specifically, color blindness genes are associated with the X chromosome. But, an X chromosome without a color blindness gene can override an X chromosome with a color blindness gene. This means the person becomes a carrier for color blindness (i.e., can pass it down to kids), but does not show symptoms. Because men have XY and women have XX, it is more likely that men both inherit and show symptoms of a color blindness gene than women. When a person expresses a color blindness gene, the photopigments in his/her cones are impacted. People with color blindness experience reduced ability to sensitively detect wavelengths because of the affected photopigments. These people tend to have limited or a complete loss of function in photopigments.
The most common types of color blindness happen when photopigments activated by long or medium wavelengths (i.e., red and green colors) are affected. That's why some people might have difficulty seeing the number 5 in the image above; their ability to distinguish between green and red is reduced. This image is modeled after an actual test for color blindness called the "Ishihara Color Test." Depending on the type of color blindness gene, people with red-green color blindness can experience symptoms ranging from a just a dulling of red, orange, yellow, and green colors, all the way to seeing reds as brown/yellow and green as beige.
Aside from red-green color blindness, there is also blue-yellow color blindness, which happens when photopigments activated by short wavelengths are affected. This type of color blindness is rarer than red-green color blindness. Symptoms can range from difficulty distinguishing between blue-green and red/yellow-pink to yellows appearing as violet or grey and blues appearing as green.
The rarest and most severe form of color blindness is complete color blindness. This happens when either two or all of the types of photopigments do not work. People with this type of color blindness have symptoms ranging from difficulty distinguishing among all colors or completely seeing the world in black, white, and grey.
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